POLITECNICO DI MILANO

School of Industrial and Information Engineering

Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”

Master of science in

Materials Engineering and Nanotechnology

PEDEFERRI DIAGRAMS OF STAINLESS STEEL:

STUDY OF THE EFFECT OF TEMPERATURE AND pH

BY MEANS OF DESIGN OF EXPERIMENT

Supervisor:

Prof. Fabio Maria BOLZONI

Co – supervisor:

Ing. Andrea BRENNA

Umberto BELLÈ 858617

Academic Year 2016 – 2017

V

Contents

Table of Figures .............................................................................................................................. VII

Table of Tables ................................................................................................................................. XI

Sommario ....................................................................................................................................... XIII

Abstract ........................................................................................................................................... XV

Introduction ........................................................................................................................................ 1

Chapter 1. Corrosion of stainless steel ............................................................................................. 3

1.1 Stainless steel families .......................................................................................................... 3

1.2 General aspects of corrosion................................................................................................. 4

1.2.1 Thermodynamic aspects of corrosion ...................................................................... 6

1.2.2 Kinetic aspects of corrosion ..................................................................................... 7

1.3 Pitting corrosion of stainless steel ...................................................................................... 12

1.3.1 Mechanism of pitting corrosion ............................................................................. 13

1.3.2 Pitting potential and protection potential ............................................................... 14

1.3.3 Metastable pitting ................................................................................................... 15

1.3.4 Influence of metallurgical factors .......................................................................... 17

1.3.5 Influence of environmental factors ........................................................................ 21

1.4 Crevice corrosion of stainless steel .................................................................................... 28

1.4.1 Mechanism of crevice corrosion ............................................................................ 29

1.4.2 Relationships between pitting and crevice corrosion ............................................. 30

Chapter 2. Pedeferri diagram of stainless steel ............................................................................. 33

2.1 Pedeferri diagram. An overview ......................................................................................... 33

2.2 Pedeferri diagram of stainless steel. The experimental approach ...................................... 34

Chapter 3. Materials and methods ................................................................................................. 41

3.1 Statistical approach ............................................................................................................. 41

3.1.1 Design of experiment ............................................................................................. 41

3.1.2 Design of experiment. The experimental conditions ............................................. 45

3.2 Experimental procedure ...................................................................................................... 48

3.2.1 Sample characterization and preparation ............................................................... 48

3.2.2 Solution preparation ............................................................................................... 51

3.2.3 Preconditioning time tests ...................................................................................... 51

3.2.4 Electrochemical cell ............................................................................................... 52

3.2.5 Cyclic potentiodynamic polarization tests ............................................................. 54

VI

Chapter 4. Results and discussion .................................................................................................. 55

4.1 Preconditioning time tests .................................................................................................. 55

4.2 Cyclic potentiodynamic polarization tests .......................................................................... 66

4.3 Pedeferri diagrams .............................................................................................................. 76

4.4 Regression equation. The statistical analysis ..................................................................... 78

4.5 Regression equation. Comparison with the experimental points ....................................... 85

4.6 Regression equation. Comparison with the literature ......................................................... 88

Conclusions ....................................................................................................................................... 93

Appendix A. Comparison between the bars .................................................................................. 95

References ......................................................................................................................................... 99

VII

Table of Figures

Figure 1.1 – Schaeffler diagram [1] ..................................................................................................... 3

Figure 1.2 – Compositional and property correlations of different stainless steels [4] ....................... 4

Figure 1.3 – Schematic representation of an electrochemical mechanism [4] ..................................... 5

Figure 1.4 – a) Cathodic characteristic curve of oxygen reduction process;

b) Anodic characteristic curve of stainless steel ............................................................. 7

Figure 1.5 – Anodic characteristic curve of stainless steel – Detailed representation [4] ................... 8

Figure 1.6 – Evans diagram of stainless steel in chloride – free aerated solutions .............................. 9

Figure 1.7 – Anodic characteristic curves of stainless steel as the chloride content rises [2] ........... 10

Figure 1.8 – Evans diagram of macrocell mechanism in case of pitting attack ................................. 10

Figure 1.9 – Pourbaix diagram of chromium [4] ............................................................................... 11

Figure 1.10 – Different morphological aspects of pit [10] ................................................................ 12

Figure 1.11 – Schematic representation of a macrocell mechanism [4] ............................................ 14

Figure 1.12 – Cyclic anodic polarization curve of an active – passive material in

halide – containing environments [7] .......................................................................... 15

Figure 1.13 – Magnified section of a potentiodynamic polarization curve of AISI type 304L

in 1000 ppm chloride solution .................................................................................... 16

Figure 1.14 – Schematic representation of a pit with its porous cover [13] ...................................... 17

Figure 1.15 – Effect of chromium (a) and nickel (b) content on pitting potential [2] ....................... 18

Figure 1.16 – Effect of different alloying elements on anodic characteristic curve (a) and on

pitting resistance (b) in chloride – containing environments [2] ................................ 18

Figure 1.17 – Pourbaix diagram of MnS – H2O – Cl– [18] ................................................................ 19

Figure 1.18 – a) Pitting potential versus chloride concentration (in logarithmic scale) in aerated

solutions at different temperatures; b) Pitting potential versus inductiontime at

different temperatures and chloride concentrations (white squares refer to

experiments carried out with different Cl– content at 20, 40, 60 and

80 °C; black squares refer to experiments carried out with different

Cl– content at 150 and 200 °C) [30] ............................................................................ 22

Figure 1.19 – Effect of pH on pitting potential of different stainless steels in

3% NaCl solutions [2] ................................................................................................. 23

Figure 1.20 – Potentiodynamic polarization curves of AISI type 304 in 10% NaCl solutions

at different pH values [32] .......................................................................................... 23

Figure 1.21 – Effect of temperature on pitting potential of different stainless steels in 3% NaCl

solutions [2]................................................................................................................. 24

Figure 1.22 – Schematic representation of pitting potential versus temperature diagram [5] ........... 25

VIII

Figure 1.23 – Effect of temperature on pitting potential of AISI type 304 in two

different aerated solutions containing respectively 100 and

1000 ppm of chlorides [30] ......................................................................................... 26

Figure 1.24 – Potentiodynamic polarization curves of AISI type 304 in 0.1 M NaCl solutions

at different temperatures [38] ...................................................................................... 26

Figure 1.25 – a) Pitting potential versus temperature diagram obtained with potentiodynamic

polarization tests; b) Current density versus temperature diagram obtained with

potentiostatic polarization tests [39] ........................................................................... 27

Figure 1.26 – a) Schematic representation of a possible nucleation site for crevice corrosion

attack [5]; b) Case study of a developed crevice corrosion attack [3] ........................ 28

Figure 1.27 – Schematic representation of crevice corrosion propagation mechanism [2] ............... 29

Figure 1.28 – Different polarization curves in halide – containing solutions [42] ............................ 30

Figure 2.1 – Corrosion conditions of carbon steel rebars in concrete as function of potential

and chloride content [41] .............................................................................................. 33

Figure 2.2 – a) Sample holder with configuration 1; b) Sample holder with configuration 2 [48] ... 37

Figure 2.3 – Pedeferri diagram for different chloride concentrations. The graph shows the

results of both potentiodynamic and potentiostatic polarization tests [49] ................... 38

Figure 3.1 – Geometrical representation of a three – factor full factorial experiment [53] ............... 43

Figure 3.2 – Geometrical representation of a CCD for three variables at two levels,

showing full factorial (F), axial (A) and center (C) points [56] .................................... 45

Figure 3.3 – A sample just after the cutting (a) and a polished one (b) ............................................. 50

Figure 3.4 – a) Stereoscopic imaging of an AISI type 304L polished sample;

b) Optical imaging of the same sample ......................................................................... 50

Figure 3.5 – Four electrolytic cells for the evaluation of the preconditioning time .......................... 52

Figure 3.6 – a) Components of the sample holder; b) Working electrode in the assembled

sample holder ................................................................................................................ 52

Figure 3.7 – Electrochemical cell with the three electrodes and the sample holder. The red cable,

the blue one and the black one link respectively WE and its sample holder,

RE and CE to the potentiostat ....................................................................................... 53

Figure 3.8 – Two assembled electrochemical cells, with the corresponding vertex thermoregulator

and hot plate stirrer, during a cyclic potentiodynamic polarization test.

The potentiostat is capable to perform two tests at the same time ................................ 54

Figure 4.1 – Open circuit potential versus time diagrams performed in different conditions

of pH and chloride content ............................................................................................ 57

Figure 4.2 – a) Stereoscopic imaging of an AISI type 304L sample after the preconditioning

test in solutions at pH 1.4; b) Optical imaging of the same sample .............................. 58

Figure 4.3 – a) Stereoscopic imaging of an AISI type 304L sample after the preconditioning

test in solutions at pH 2; b) Optical imaging of the same sample ................................. 58

Figure 4.4 – a) Optical imaging of an AISI type 304L sample after the preconditioning test in

NaCl – free solutions at pH 5.5; b) Optical imaging of an AISI type 304L

sample after the preconditioning test in solutions at pH 5.5 containing

43000 ppm of chlorides................................................................................................. 59

Figure 4.5 – Ecor mean value versus pH diagram ............................................................................... 59

IX

Figure 4.6 – a) Evans diagram of stainless steel in solutions at pH 1.4; b) Evans diagram

of stainless steel in solutions at pH equal or greater than 3 .......................................... 61

Figure 4.7 – Effect of pH (a) and chloride content (b) on the anodic and cathodic

characteristic curves ...................................................................................................... 62

Figure 4.8 – a) Cyclic potentiodynamic polarization curves in solutions at pH 2 + 1000 ppm

of chlorides, 40 °C; b) Cyclic potentiodynamic polarization curves in

solutions at pH 3 + 10000 ppm of chlorides, 60 °C ...................................................... 64

Figure 4.9 – Cyclic potentiodynamic polarization curves in solutions at pH 6 + 43000 ppm

of chlorides, 20 °C ........................................................................................................ 64

Figure 4.10 – Cyclic potentiodynamic polarization curves for all the conditions provided

by the CCC design ...................................................................................................... 68

Figure 4.11 – Crevice attack which has developed close to the O – ring/specimen contact area ...... 70

Figure 4.12 – Ecor mean value versus pH diagram – Comparison between the data obtained

through preconditioning tests and the ones obtained through the

potentiodynamic polarization tests.............................................................................. 71

Figure 4.13 – Two cyclic potentiodynamic polarization curves in solutions at pH 8 at room

temperature with increasing chloride content. The triangle refers to Epit, the

rhombus specifies Ecor whereas the square shows Epro ............................................... 72

Figure 4.14 – Pitting potential versus pH diagram for solutions containing 100 ppm (a) and

10000 ppm (b) of chlorides ......................................................................................... 72

Figure 4.15 – Protection potential versus pH diagram for solutions containing 100 ppm (a) and

10000 ppm (b) of chlorides ......................................................................................... 72

Figure 4.16 – Pitting potential versus temperature diagram .............................................................. 73

Figure 4.17 – a) Corrosion potential versus temperature diagram; b) Protection potential

versus temperature diagram ........................................................................................ 74

Figure 4.18 – Schematic representation of a potentiodynamic polarization curve. The red line

represents the range evaluated for the analysis of the passive current distribution .... 74

Figure 4.19 – Effect of temperature (a) and chloride content (b) on the cumulative distribution

of the passive current density ...................................................................................... 75

Figure 4.20 – Effect of temperature on the pitting behavior and on the passive current density ...... 75

Figure 4.21 – Pedeferri diagrams in different conditions in terms of pH and temperature ............... 77

Figure 4.22 – Fisher distribution for a generic couple of degree of freedom. Setting a F – value

(red line), the corresponding α – value is defined as the size of the displayed

orange area. On the other and, the size of the green area is equal to the

difference between 1 and α – value ............................................................................. 80

Figure 4.23 – Residual plots .............................................................................................................. 82

Figure 4.24 – a) Pareto chart of the standardized effects; b) Normal plot of the standardized

effects .......................................................................................................................... 84

Figure 4.25 – Comparison between experimental data and the regression model by means of

Pedeferri diagrams ...................................................................................................... 86

Figure 4.26 – Comparison between experimental data and the regression model by means of Epit

versus pH diagrams ..................................................................................................... 87

Figure 4.27 – Comparison between experimental data and the regression model by means of Epit

versus T diagrams ....................................................................................................... 87

X

Figure 4.28 – Pedeferri diagrams with different plotted regression equations, each for a

specific temperature, in pH 6 solutions (a – [44,48]; b – [49]; c – [61];

d – [62];e – [60]; f – [2]; g – [63]) .............................................................................. 89

Figure 4.29 – Pedeferri diagram with different plotted regression equations for different

evaluated conditions (a – [69]; b – [70]; c – [38]) ...................................................... 90

Figure 4.30 – Pedeferri diagram with different plotted regression equations, each for a

specific pH value, at 25 °C (a – [61]; b – [64]; c – [27]; d – [65];

e – [66]; f – [67]; g – [68]; h – [2]; i – [8]) ................................................................. 91

Figure 4.31 – Pedeferri diagram with the plotted regression equation at 25 °C and pH 9.3

(a – [71]; b – [72]) ....................................................................................................... 92

Figure A.1 – Microstructure of sample A (a) and sample B (b) ........................................................ 95

Figure A.2 – Cyclic potentiodynamic polarization curves in solutions containing 100 ppm and

1000 ppm of chlorides.................................................................................................. 96

Figure A.3 – Pedeferri diagram of sample A and sample B .............................................................. 97

XI

Table of Tables

Table 3.1 – Minimum and maximum values for each studied factor. ............................................... 46

Table 3.2 – Point coordinates for each studied factor. ....................................................................... 46

Table 3.3 – Experimental conditions for each run. ............................................................................ 47

Table 3.4 – Chemical composition (in weight %) and PREN value of an AISI type 304L

sample used for the potentiodynamic polarization tests. ............................................... 49

Table 3.5 – Chemical composition (in weight %) of three different inclusions. All the

elements were analyzed and normalized through cross – section EDS. ........................ 49

Table 3.6 – Grain size number obtained through planimetric grain analysis and the

corresponding number of grains. The magnification of the microscope

was 100X, as suggested by ASTM E112 standard [59]. ................................................ 49

Table 3.7 – Summary of all the different solutions (in terms of chloride content and pH) used for

preconditioning tests. The terms in brackets represent the amount of chlorides in

NaCl – free solutions (values different from 0 are related to the presence of

hydrochloric acid in acidic solutions). ........................................................................... 51

Table 4.1 – Original minimum and maximum values for each studied factor. .................................. 56

Table 4.2 – Original point coordinates for each studied factor. ......................................................... 56

Table 4.3 – Mean value and standard deviation of the open circuit potential measured in

24 hours for each condition and for each replication. .................................................... 60

Table 4.4 – Preconditioning time for all the evaluated conditions in terms of chloride content

and pH of the solution. ................................................................................................... 65

Table 4.5 – Evaluated conditions for the 40 cyclic potentiodynamic polarization tests. ................... 66

Table 4.6 – Potential values of Ecor, Epit and Epro for each repetition of the evaluated conditions.

The terms specified with “*” could not be determined. ................................................. 69

Table 4.7 – Summary of the properties of the chosen CCC 23 design. .............................................. 78

Table 4.8 – ANOVA of the elaborated design. .................................................................................. 79

Table 4.9 – Model summary. ............................................................................................................. 81

Table 4.10 – Coded coefficients of the regression equation. ............................................................. 83

Table A.1 – Chemical composition (in weight %) and PREN value of a sample from bar A

and one from bar B........................................................................................................ 95

XIII

Sommario

Gli acciai inossidabili sono ampiamente utilizzati nelle applicazioni industriali grazie alla loro

resistenza alla corrosione; tuttavia, in presenza di cloruri possono subire un attacco localizzato, come

la vaiolatura (pitting) e la corrosione in fessura, che possono portare alla rottura degli impianti

industriali. Per questo motivo, gli ingegneri hanno bisogno di strumenti pratici che permettano di

selezionare materiali per ambienti specifici a seconda dei parametri ambientali quali il contenuto di

cloruri, la temperatura e il pH.

Lo scopo di questo lavoro è la messa a punto di diagrammi potenziale – concentrazione di cloruri per

diverse condizioni in termini di pH e temperatura, insieme all'analisi di un’equazione empirica in

grado di predire il potenziale di pitting in diversi ambienti industriali e naturali. Il diagramma

potenziale – concentrazione di cloruri, chiamato diagramma di Pedeferri, consente di prevedere i

range dei potenziali di pitting e di protezione, fornendo in tal modo uno strumento utile per la

selezione dei materiali e la protezione catodica.

È stato studiato il comportamento dell’acciaio inossidabile AISI 304L in soluzioni saline a diversi

valori di pH e temperatura al fine di valutare il potenziale di vaiolatura, Epit, il potenziale di

protezione, Epro e il potenziale di corrosione, Ecor.

I dati ottenuti attraverso i test di polarizzazione potenziodinamica sono stati utilizzati per

l’elaborazione del diagramma di Pedeferri; i risultati hanno mostrato una diminuzione del potenziale

di pitting all’aumentare del contenuto di cloruri, della temperatura e dell’acidità della soluzione, in

accordo con la teoria.

Per questa indagine è stato utilizzato il design of experiment. Questo approccio statistico ha permesso

di studiare contemporaneamente gli effetti dei tre parametri esaminati, ovvero la temperatura, il pH

della soluzione e la concentrazione di cloruri, e di elaborare l’equazione empirica che mette in

relazione Epit e i tre fattori. Il confronto tra il modello ed i punti sperimentali ha mostrato una discreta

corrispondenza.

XV

Abstract

Stainless steels are widely used in industrial applications thanks to their corrosion resistance;

nevertheless, in presence of chlorides they may suffer localized corrosion attack, such as pitting and

crevice, which can lead to industrial plant failure. For this reason, engineers need practical tools that

allow to select materials for specific environments depending on environmental parameters such as

the chloride content, the temperature and the pH.

The aim of this work is the setting – up of potential versus chloride content diagrams for different

conditions in terms of pH and temperature, together with the analysis of an empirical equation capable

of predicting the pitting potential in different industrial and natural environments. Potential versus

chloride content diagram, called Pedeferri diagram, allows to predict pitting and protection potential

ranges, thereby providing a useful tool for material selection and cathodic protection.

The behavior of AISI type 304L stainless steel in NaCl solutions at different pH and temperature

values was investigated in order to evaluate the pitting potential, Epit, the protection potential, Epro,

and the corrosion potential, Ecor.

The data achieved through potentiodynamic polarization tests were used for the setting – up of

Pedeferri diagram; the results showed a decrease in pitting potential with increasing chloride content,

temperature and solution acidity, in accordance with the theoretical explanation.

Design of experiment was used for this investigation. This statistical approach allowed to study at the

same time the effects of the three examined parameters, namely temperature, solution pH and chloride

concentration, and to elaborate the empirical equation which relates Epit and the three factors. The

comparison between the model and the experimental points showed a fair match.

1

Introduction

Stainless steels find applications in a wide variety of industrial applications due to their resistance to

general corrosion, mechanical properties, good workability and weldability. However, these family

of steels is susceptible to some forms of localized corrosion attacks, such as pitting and crevice

corrosion, stress corrosion cracking and intergranular corrosion, which must be avoided to prevent

plant failures.

The susceptibility of stainless steels to pitting corrosion depends on many factors, such as the

chemical composition of the steel, its surface finishing, or the environmental conditions.

Among the different environmental factors that affect the pitting corrosion resistance, the most

significant are the chloride concentration, the temperature and the pH. Chlorides are detrimental to

film stability, and lead to pit initiation and propagation, with a very high corrosion rate (up to 10

millimeters per year) that may lead to the perforation of the stainless steel structure.

The corrosion behavior of stainless steels in a certain environment can be evaluated by using Pedeferri

diagrams. These diagrams relate the potential with the chloride content of the studied environments,

providing a useful tool for material selection and for cathodic protection.

In a Pedeferri diagram, it is possible to identify potential ranges within the material is corroding,

passive or immune. Passivity can be perfect or imperfect, according to Pourbaix: perfect passivity

defines an area where the corrosion process can neither initiate nor propagate, whereas imperfect

passivity is the region in which corrosion process cannot initiate but it can propagate if already started.

These two regions are separated by the protection potential, whereas pitting potential divides the

corrosion zone from the imperfect passivity zone.

The first aim of this work is the elaboration of Pedeferri diagrams for different conditions in terms of

pH and temperature.

In this thesis work, the effects of these parameters on pitting potential and protection potential were

evaluated by means of cyclic potentiodynamic polarization tests. The tested material was AISI type

304L, the most used austenitic stainless steel in industrial applications thanks to its relatively low cost

and the already mentioned mechanical properties and corrosion resistance.

The second aim is the development of an empirical equation capable of predicting the pitting potential

in different environments in terms of chloride content, pH and temperature.

The statistical approach used for this thesis work is the design of experiment. This method, which

was born for industrial purposes, allows to study at the same time the effects of different parameters,

providing an accurate analysis of the results based on a limited number of experiments. Furthermore,

design of experiment allowed the elaboration of the empirical equation that relates Epit and the three

factors.

The thesis is organized in five chapters.

In the first one the phenomenon of stainless steel localized corrosion, together with the parameters

that influence it, are described.

The second chapter shows the experimental approach used for the elaboration of Pedeferri diagrams.

2

The third chapter provides an explanation of the design of experiments methodology together with a

description of the tested material and the adopted experimental methods.

Chapter four is focused on the discussion of the results and on the elaboration of Pedeferri diagrams;

furthermore, the empirical equation was compared with experimental results of the current research

and literature data.

Lastly, the fifth chapter shows the conclusions of this work and provides some claims for future

researches.

3

Chapter 1

Corrosion of stainless steel

1.1 Stainless steel families

According to the American Iron and Steel Institute (AISI) and European Norm (EN) 10088 standards,

stainless steels are iron – based alloys containing a minimum chromium content of 10.5%. This is the

minimum content needed to obtain the property of stainlessness, in other words the capacity to cover

itself with a chromium – rich passive layer able to self – heal in a wide variety of aggressive

environments [1–3].

Today more than 180 different alloys belong to this family of steels, and the addition of different

elements makes possible the improvement of different properties. For example, chromium, nickel,

nitrogen and molybdenum provide corrosion resistance; carbon, molybdenum, nitrogen, titanium,

aluminum and copper lead to a higher strength; sulfur is added in order to obtain a better machinability

and nickel contributes to the enhancement of the toughness [1,2].

The most common stainless steels can be divided into three groups: austenitic (with a face – centered

cubic structure), ferritic (with a body – centered cubic structure) and martensitic (with a

body – centered tetragonal or cubic structure). Ferritic and martensitic steels have excellent

mechanical properties but a low ductility, whereas austenitic steels have an excellent ductility.

Another important group gathers together duplex stainless steels, which have a mixed

austenitic – ferritic structure and show higher mechanical properties and an improved corrosion

resistance with respect to the other three families [1,2].

The Schaeffler diagram relates the metallurgical structure and the composition of different stainless

steels, and it is schematized in Figure 1.1.

Figure 1.1 – Schaeffler diagram [1].

4

In order to identify different stainless steels, AISI has provided a designation based on the

microstructure: austenitic stainless steels are identified by numbers in the 200 and 300 series, while

ferritic and martensitic stainless steels are identified by numbers in the 400 series [2].

The 300 series includes 18 – 8 stainless steels, that is stainless steels having 18% of chromium, 8%

of nickel and other elements, such as molybdenum or nitrogen, in order to improve pitting and crevice

corrosion resistance [2].

A simplified scheme provided by Figure 1.2 summarizes all the compositional and property

correlations of different stainless steels.

Figure 1.2 – Compositional and property correlations of different stainless steels [4].

1.2 General aspects of corrosion

Corrosion is a natural phenomenon which can be defined as the interaction between a metal and its

environment that can adversely affect different metal properties, leading to a chemical and physical

degradation. Any approach to this phenomenon must consider the features of the metal, the nature of

the environment and the reactions that occur at the metal – environment interface. In case of wet

corrosion, i.e. in presence of water, the corrosion mechanism is electrochemical, and the process

involves both the oxidation of the metal, i.e. the anodic process, and the reduction of a chemical

species existing in the environment, i.e. the cathodic process, which is the oxygen reduction in natural

environments and the hydrogen evolution in acidic solution [5,6].

According to Figure 1.3, the electrochemical mechanism can be considered as the sum of four partial

processes in series: the anodic process which releases electrons, the cathodic process which consumes

the electrons made available by the anodic reaction, the transport of current in the metal and the

transport of ionic current in the electrolyte [6].

5

The generic anodic process is:

(1.1) M ↔ Mz+ + ze–

where M is a metal, Mz+ is the metal ion that passes into solution, z is the valence of the metal and e–

designates the electron; if the metal gives rise to oxides or hydroxides, the reaction can be written as:

(1.2) M + zH2O ↔ M(OH)z + zH+ + ze–

where M(OH)z indicates hydroxides and H+ designates hydrogen ions [6].

The cathodic reactions which have a practical interest in corrosion science are mainly oxygen

reduction (1.3) and hydrogen evolution (1.4) [6]:

(1.3) O2 + 2H2O + 4e– ↔ 4OH

–

(1.4) H+ + 2e– ↔ H2

In natural environments, the prevalent reaction is the reduction of the molecular oxygen dissolved in

water, whose concentration typically varies from 0 to 12 ppm [5,6].

Figure 1.3 – Schematic representation of an electrochemical mechanism [4].

Since electroneutrality has to be maintained, i.e. the number of electrons provided by the anodic

process must equalize the number of electrons consumed by the cathodic process, both reactions must

occur simultaneously and with the same speed, so that the cathodic current Ic is equal to the anodic

one Ia [2–4].

Besides the anodic and the cathodic processes, even the transport of electrons inside the metal Im and

the transport of electrons in the electrolyte through ions Iel have to be taken into account for the

description of the complete corrosion process (Figure 1.3); in other words, all these complementary

processes must occur at the same rate represented by Icor (or Icorr). The slowest of the four partial

corrosion processes will determine the global corrosion rate [4,7].

6

1.2.1 Thermodynamic aspects of corrosion

Corrosion is in most of the cases a spontaneous process which tends to bring metals in the most stable

thermodynamic state through the combination with other elements [6].

A generic corrosion reaction for a metal occurs only if the variation of the Gibbs free energy

associated to the reaction ΔG is negative:

(1.5) ∆G = – zF∆E > 0

where F is the Faraday constant (96485 C·mol–1) and ΔE is the thermodynamic driving voltage;

otherwise, if the variation of the Gibbs free energy is positive, the corrosion process cannot take place

and the condition of thermodynamic immunity occurs [6].

Since in a spontaneous reaction ΔE must be positive, the thermodynamic necessary condition to have

corrosion is:

(1.6) ∆E = Ec – Ea > 0

(1.7) Eeq,c > Eeq,a

where Eeq,c and Eeq,a are respectively the equilibrium potentials of the cathodic and the anodic

processes given by Nernst equation.

For the anodic reaction of metal dissolution (1.1), the equilibrium potential is given by:

(1.8) Eeq,a (V vs. SHE) = Eo +

0.059

z∙log[Mz+]

where Eo is the standard potential of the metal, defined as the equilibrium potential when the metal

ion concentration is equal to 1 M, 0.059 is the constant valid at 298 K and [Mz+] is the molar

concentration of metal ions in the electrolyte (considered equal to the activity); it is also important to

point out that these potential values are measured with respect to the standard hydrogen electrode

(SHE) [4].

For the two main cathodic processes, i.e. oxygen reduction and hydrogen evolution, the equilibrium

potential depends on pH:

(1.9) Eeq,c (V vs. SHE) = 1.229 – 0.059·pH oxygen reduction

(1.10) Eeq,c (V vs. SHE) = – 0.059·pH hydrogen evolution

These relationships are obtained by means of Nernst equation and they are valid at 298 K supposing

both water and hydrogen activities equal to 1.

The thermodynamic conditions of stability of a metal, its metal ions and its passivation layers are

represented in the potential versus pH diagram, called Pourbaix diagram after its introduction in 1945

by Marcel Pourbaix [4,5]. Nevertheless, these diagrams, available for each metal, do not provide any

kind of information about the kinetics of the corrosion process: the mere knowledge of the

electromotive force variation does not allow to predict the corrosion rate of the metal [4].

7

1.2.2 Kinetic aspects of corrosion

The availability of a driving force is a necessary condition for corrosion. Nevertheless, corrosion rate

can be low or negligible even in presence of a positive driving voltage: kinetic factors such as the

reaction resistances localized at the metal surface and in the electrolyte can radically affect a corrosion

process that can be slowed down or practically stopped [4,6].

The kinetic analysis of a corrosion system is provided by Evans diagram; this diagram was proposed

by Ulick Evans in 1945 and it reports the potential versus the current density (E – Log i). The anodic

and cathodic characteristic curves can be represented in this diagram, and their interception allows to

define the working point of the system, i.e. the corrosion potential Ecor (or Ecorr) and the corrosion

current density icor (or icorr) [3–6].

Evans diagram of stainless steel in chloride – free aerated solutions

In natural environments, as water, the main cathodic process occurring on stainless steels is oxygen

reduction, whose cathodic characteristic curve is schematized in Figure 1.4a.

Figure 1.4 – a) Cathodic characteristic curve of oxygen reduction process; b) Anodic characteristic curve of

stainless steel.

Starting from the equilibrium potential described by relation (1.9), the overvoltage η, which is

negative for a cathodic process and which can be evaluated through the equation (1.11), consists of

two contributions: the activation overvoltage, defined by the Tafel law (1.12), and the concentration

polarization overvoltage, defined by the oxygen diffusion limiting current density iL [4].

(1.11) η = E – Eeq

(1.12) E = a – b∙logi

(1.13) 2H2O ↔ O2 + 4H+ + 4e–

In Figure 1.4a, the equilibrium potential is represented together with the exchange current density at

the equilibrium i0: Eeq defines the thermodynamic aspects of an electrochemical reaction, whereas the

exchange current density defines the kinetic ones [4].

a b

8

On the other side, oxygen evolution reaction (1.13) can take place only for potential values higher

than the equilibrium potential of oxygen reduction defined by equation (1.9).

Regarding the anodic behavior, stainless steels in chloride – free aerated solutions exhibit an

active – passive behavior, as shown in Figure 1.4b; the anodic curve provides both the potential value

of a metal when it exchanges a certain current and the current density value when it is brought to a

certain potential [3].

A more detailed representation of such that anodic characteristic curve is given in Figure 1.5.

Figure 1.5 – Anodic characteristic curve of stainless steel – Detailed representation [4].

Five zones can be distinguished [4]:

Immunity zone (E < Eeq): the metal is in immunity condition and corrosion is not possible

from a thermodynamic point of view.

Active zone (Eeq < E < Epp): the metal behaves as an active metal, so that it is dissolves with

a corrosion rate rising exponentially as the potential increases. The potential range is included

between the equilibrium potential of the metal Eeq and the primary passivation potential Epp

reached at the critical current density for passivation icp.

Active – passive transition zone (Epp < E < Ep): the current density decreases as the potential

increases up to the passivation potential Ep. This region does not correspond to a stable

condition of the metal.

Passivity zone (Ep < E < Etr): the potential range of the region is included between the

passivation potential Ep and the transpassivity potential Etr. In this potential range, the current

density is very low and constant, and it is called passive current density ip. In this region, the

anodic process is passive film formation.

Transpassivity (or pitting) zone (E > Etr): in the absence of chlorides, in the transpassive zone

oxygen evolution can occur if the potential of the metal is higher than the equilibrium potential

(1.9). In other words, an anodic reaction takes place on the metal surface, but this reaction

does not correspond to metal dissolution or passive film growth. Nevertheless, if the acidity

produced by oxygen evolution reaction (1.13) is sufficiently high, the oxide layer will be no

more protective, and dissolution can take place.

9

Figure 1.6 shows the Evans diagram of a stainless steel in an aerated aqueous solution. Eeq,SS and i0,SS

are respectively the equilibrium potential and the exchange current density at the equilibrium for the

stainless steel.

Figure 1.6 – Evans diagram of stainless steel in chloride – free aerated solutions.

The interception between the two curves, i.e. the anodic characteristic (red) and cathodic one (blue),

provides the corrosion current density, which coincides with the passive current density, and the

corrosion potential.

For an austenitic stainless steel in a free – NaCl aqueous aerated solution at room temperature, the

corrosion potential is in a range 0 to 0.1 V vs. silver/silver chloride electrode (SSC), whereas the

corrosion current density is in a range 0.1 to 1 mA·m–2. If an AISI type 304 stainless steel with these

current density values is considered, its corrosion rate, which will be in a range 0.1 to 1 μm·year–1,

can be considered negligible [4–6].

This low corrosion rate is related to the development of a protective layer on the metal surface which

prevents corrosion: such phenomenon, called passivation, allows the protection of the stainless steel

in different environments.

Evans diagram of stainless steel in chloride – containing aerated solutions

In presence of both an aerated solution, i.e. in sufficiently oxidizing conditions, and chlorides, pitting

attack can take place on the surface of the stainless steel exposed to the aggressive environment.

Pitting is a form of localized corrosion that occurs when chlorides or other aggressive ions that

promote the depassivation of the stainless steel are dissolved in the solution, leading to the formation

of cavities called pits. In this situation, the highest potential value above which the passive film breaks

down is called pitting potential Epit, and it is less noble with respect to the transpassivity potential

previously described [2–4,6].

As the chloride concentration of the solution increases, two effects on the anodic characteristic curve

of stainless steel are easily identifiable, namely the decrease in pitting potential and the increase in

10

passive current density (Figure 1.7); this implies that an increase in chloride concentration leads to a

weakening of the passive conditions.

Figure 1.7 – Anodic characteristic curves of stainless steel as the chloride content rises [2].

Pitting corrosion attack causes the increase in corrosion current density, and so the rise in corrosion

rate, and it propagates with a macrocell mechanism, as shown in Figure 1.8: the pit behaves as anode

characterized by a high local current density (point B), whereas the surrounding area remains passive

and acts as cathode (point A). It must be pointed out that the formation of this macrocell mechanism,

and so the breakdown of the passive film, is possible only if the pitting potential decreases up to

equalize the corrosion potential.

Figure 1.8 – Evans diagram of macrocell mechanism in case of pitting attack.

While in passive condition the corrosion current density current density is about 1 mA·m–2, which

can be considered a negligible value, in presence of pitting attack the corrosion rate can be very high

(up to 10 mm·year–1), leading to the perforation of the stainless steel [4,5].

11

Passivity of stainless steel

As said before, the low corrosion rate of stainless steel is related to the formation of a passive layer

that hinders corrosion phenomena and protect the metal in different environments.

Pedeferri [3] describes passivity as the condition in which metals are covered by a protective layer,

typically 1 to 10 nm thick, made of oxide or other compounds mixed with chemical species coming

from the aggressive environment and able to reduce the current density up to negligible values.

Passivity is always preceded by the intervention of passivation phenomenon, i.e. the growth of a film,

but this one not necessarily leads to passive conditions, as in case of porous film or layers that do not

cover completely the metal surface [4].

Passivity is a peculiarity of very reactive metals as stainless steels, which give rise to a protective

layer in environments where carbon steels or low – alloyed steels suffer corrosion, or reinforcements

in concrete, in which steel is passivated due to the alkalinity of the hydrated cement mixture [3,5].

In contrast to iron in basic environments, whose passive layer consist only of magnetite Fe3O4 and

maghemite γ – Fe2O3 with cubic spinel – type structure, the chemical composition of oxide films on

stainless steels is more complex. However, it must be pointed put that the corrosion resistance of

these materials is attributed to the protective nature of chromium hydroxide and chromium oxide, so

that the analysis of the passivation field of a stainless steel can be done by means of Pourbaix diagram

of chromium (Figure 1.9) [5,8].

Figure 1.9 – Pourbaix diagram of chromium [4].

Considering the composition of air – formed oxide films on stainless steels, it changes continually

with depth. According to some authors [2,8], at the air – oxide interface the film has a small chromium

content and it appears rich in oxygen or lacking in iron. Immediately beneath the surface layer,

chromium content increases up to a content three times higher than the alloy one and then decreases

to the level of the bulk stainless steel.

In case of aqueous environment, iron dissolves selectively from the surface of the alloy, enriching the

chromium concentration at the surface up to a level in which the film can cover the surface

completely. Such passive film consists of two layers: an outer layer of chromium hydroxide Cr(OH)3

and an inner one of chromium oxide Cr2O3 and iron oxide. Further analyses have also demonstrated

that chloride ions cannot penetrate the inner layer, but they can penetrate the outer one leading to a

porous and amorphous morphology characterized by channels through which anions can move

[2,5,8].

12

According to Okamoto [9], the passive film of stainless steels in aqueous environment contains bound

water, so that the protective layer is a hydrated oxide film having a gel – like structure. Where

conditions allow, metal ions give rise to intermediates denoted as MOH+; these are captured by

surrounding water molecules and then they bond with the gel – like film.

These passive films typically have a thickness in the range 2 to 4 nm, and they allow to achieve a

good corrosion protection while maintaining the mechanical properties of the steel. However, the

passive behavior can be modified by both metallurgical factor (alloying elements, second phases,

carbides, etc.) and the environmental conditions in which the film develops. The chemical species

dissolves in the electrolyte, in fact, can be incorporated into the film as it grows, taking part in the

film development, or may hinder the formation of oxide layers, making them defective, as in case of

crevice corrosion and pitting [4, 8].

1.3 Pitting corrosion of stainless steel

Corrosion of stainless steels is mainly localized, taking place locally on the metal surface. The most

important corrosion forms of stainless steel in natural and industrial environment are pitting, crevice

corrosion and stress corrosion cracking that occurs in particular combination of tensile stress and

environmental conditions. General corrosion can also occur but only in very aggressive environments,

in particular when the use of high temperatures and very acidic pH is involved, with the dissolution

of the passive film [3]. Despite of the importance of each corrosion forms, only pitting and crevice

corrosion of stainless steels will be accurately described, being object of this thesis work.

Pitting is a form of localized corrosion and it is defined as the local dissolution of the passive film

that leads to the development of cavities, called pits. This kind of corrosion concerns active – passive

metals or alloys that are exposed to aqueous solution containing aggressive anions, particularly

chlorides. These cavities can have different morphologies, as schematized in Figure 1.10, and,

according to the American Society for Testing and Materials (ASTM) standards, they can be defined

as: wide and shallow, narrow and deep, elliptical, undercutting, subsurface, vertical or horizontal. The

shape of pits depends on chemical composition of stainless steels, features of the aggressive solution

(pH, dissolves chemical species, etc.) and exposure time [3–6,8].

Figure 1.10 – Different morphological aspects of pit [10].

13

The pitting attack on passivated materials is one of the most insidious form of corrosion since

corrosion rates are very high, especially in high – conductive environments, due to the presence of a

macrocell mechanism. Furthermore, the dangerousness of a pit is worsened due to the fact that its

position on the surface is not always predictable, and both its small diameter and the presence of

different precipitates can easily veil its existence. Pitting attack often lead to perforation of pipes with

considerable damage, such as mixing of fluids inside heat exchangers or release of toxic or flammable

materials. In other cases, pitting may result in a loss of appearance, which is of greater importance

when the metal is used for decorative architectural purposes [4,5].

1.3.1 Mechanism of pitting corrosion

Two different phases of pitting corrosion can be distinguished: initiation and propagation. As a

general description, the formation of a corrosion attack on the metal surface produces a current flow

between the anodic area (the pit) where the corrosion attack takes place, and the surrounding one

where oxygen reduction reaction (or reduction processes of other oxidizing species) takes place, i.e.

the cathodic zone. This current flow gives rise to a macrocell mechanism, with very severe attack in

the pit. Due to this mechanism and the very high cathodic – to – anodic surface ratio, the pitting attack

tends to penetrate inside the cavity instead of extending on the surface [3,4,7].

Pitting initiation

Initiation time, or incubation time, is the time necessary for the local breakdown of the film, whenever

the concentration of specific aggressive chemicals exceeds a threshold value that depends on both

metal properties and the environmental parameters. In presence of chloride ions, this threshold value

is called critical chloride content [Cl–]th, and it represent the highest content of chlorides compatible

with the conditions of passivity [3–7].

Actually, a unique model able to describe initiation step is still missing and a lot of theories have been

proposed to overcome this lack of explanations. Even though no complete agreements exist on the

essential causes of pitting mechanism, all the models of literature conclude that metal surfaces contain

numerous inclusions, second phases and compositional heterogeneities which make easier the

passage of ions through the oxide film, so that the pitting initiation phase can be identified with

structural heterogeneities at the surface [2,5,8].

Moreover, it must be considered that pit nucleation is a stochastic phenomenon: even if the

metallurgical and environmental conditions are very controlled, the pitting potential, the incubation

time and the nucleation site distribution are dispersed over a wide range [11].

Pitting propagation

After the pit initiation the corrosion attack propagates with a macrocell mechanism, schematized in

Figure 1.11.

The anodic reaction occurs inside the pit, where the metal dissolves, and the cathodic reaction (oxygen

reduction) takes place around the pit in a region defined by the throwing power, which measures the

length of the area affected by the corrosion attack.

The high concentration of metal ions within the pit leads to the migration of chloride ions inside the

cavity, through the macro couple current, with the consequent generation of metal chlorides M+Cl–.

These are hydrolyzed by water yielding to metal hydroxide MOH and hydrochloric acid H+Cl–. Due

to this hydrolysis reaction and the increase in chloride concentration in the pit moved by the macrocell

14

current, the anolyte, i.e. the electrolyte in the cavity, becomes more and more aggressive: at the

bottom of the pit, pH can be reduced to values lower than 3 and the concentration of chloride ions

can strongly increase creating severe corrosion conditions [2].

In presence of this macrocell mechanism, the corrosion rate is defined by:

(1.14) Crate = iL∙Sc + Sa

Sa

where iL is the oxygen limiting current density, Sc is the surface of the cathodic area and Sa is the

surface of the pit. In presence of chlorides, Sc is generally high due to the high conductivity of the

solution, i.e. high throwing power, and the corrosion rate can be significant (some mm·year–1).

Figure 1.11 – Schematic representation of a macrocell mechanism [4].

1.3.2 Pitting potential and protection potential

The susceptibility to pitting can be estimated by [8]:

Determination of characteristic potentials, i.e. pitting potential and protection potential.

Determination of critical chloride content.

Determination of critical pitting temperature.

Measurement of number of pits per unit area, weight loss and size and depth of pits developed

in a standard solution.

Among the different way used for the analysis of the susceptibility, pitting potential and protection

potential deserve particular attention.

Pitting potential, called also “Breakdown potential”, is considered as one of the most important

features characterizing the pitting resistance of a given metal in a solution containing aggressive

anions (Cl–, Br–, etc.), and it is defined as the potential value at which the anodic current density

begins to increase continuously and drastically after the passive range. As discussed in §1.2.2, the

presence of aggressive ions is crucial: in case of their absence, the metal retains its passivity up to the

transpassivity potential [8]. Therefore, pitting can develop only if the corrosion potential of stainless

steel is higher than pitting potential, i.e. Ecor > Epit; vice versa the metal remains in a passive condition.

15

It follows that the pitting corrosion resistance is related to the stability of the passive film, and so to

the difference between the pitting potential and the free corrosion potential. Ecor depends on the

oxidizing power of the solution in contact with the stainless steels, so that it rises as the content of

oxygen or other oxidizing species increases, on the presence of bacterial activity which ennobles the

corrosion potential and on the possible galvanic contacts with other metals [4,8,12]. Pitting potential

depends on several parameters related to the metal and the environmental conditions; they are

discussed in the following subparagraphs.

Another fundamental parameter that is considered both for the analysis of pitting susceptibility and

for the protection of structures is the protection potential Epro.

The protection potential is the potential at which the macrocell current is zero and corrosion stops.

Three conditions can be distinguished, according to Pourbaix [4,8,12]:

E > Epit: pitting corrosion starts and propagate.

Epit > E > Epro: pitting corrosion does not start but it propagates if initiated (imperfect

passivity).

E < Epro: pitting corrosion does not start and it stops if initiated (perfect passivity).

Figure 1.12 shows the three conditions described on an anodic characteristic curve. As pitting

potential, protection potential is influenced by different metallurgical and environmental factors, such

as chemical composition of stainless steel, pH, temperature, chloride concentration, etc. [7,8].

Figure 1.12 – Cyclic anodic polarization curve of an active – passive material in halide – containing environments [7].

1.3.3 Metastable pitting

In the previous subparagraph, pitting potential has been defined as the potential value at which the

passive film breaks, and the anodic current density increases continuously and drastically during an

anodic potential scan.

Nevertheless, considering an anodic potentiodynamic scan, current oscillations can occur before

reaching pitting potential: these are related to the formation of pits which cannot propagate because

not stable, so that they repassivate. This phenomenon is called metastable pitting [8].

Figure 1.13 shows a magnified section of a potentiodynamic polarization test carried out on an AISI

type 304L specimen at room temperature in an electrolytic solution achieved by dissolution of 1000

16

ppm of chlorides in distilled water (pH 6). It may be noted that the frequency with which this

phenomenon occurs rises as the potential increase, and that the maximum current density value

increases as the potential levels up, but without reaching a condition of stable increase; this last one

can be obtained only for potential values higher than Epit.

Figure 1.13 – Magnified section of a potentiodynamic polarization curve of AISI type 304L in 1000 ppm

chloride solution.

Therefore, it is possible to claim that for potentials lower than Epit both film breakdown and metal

dissolution can occur, but each pit precursor cannot growth until pitting potential is reached. This

indicates that film breakdown is a necessary but non – sufficient condition for pitting attack to occur.

It has been verified that metastable pit propagation rate is controlled by the diffusion of metal cations

from the cavity into the bulk electrolyte, so that the anodic reaction occurring for metastable pits is

the same of the one occurring for pits [8,13–15].

Among the great deal of theories capable to explain the metastable pitting, Frankel et al. [13] have

provided a detailed analysis of this phenomenon. During the metastable growth a porous cover, which

is a remnant of the passive film, sets up over the mouth of the pit (Figure 1.14); considering that it

tends to limit diffusion phenomena, it must be sufficiently porous to permit the electrolyte to flow

into the pit and metal ions to flow out of the pit.

The decrease in current density up to passive current value, which is pointed out in Figure 1.13, arises

from the rupture of the cover: when an opening appears in the porous cover, the anolyte dilutes so

that bulk electrolyte reaches the bottom of the pit and a passive film nucleates, leading to the

repassivation of the metastable pit and stopping the metal dissolution [13,15].

Pit stability is also related to the precipitation of a salt film made of corrosion products on the bottom

of the growing pit; these films are able to control the kinetics of pitting by blocking repassivation of

the surface. If the porous cover breaks, both the anolyte and the salt film precipitate dilute leading to

pit repassivation; however, if a salt film precipitates on the pit surface and it reaches stability before

the cover breakdown, pit will not repassivate. This is possible only if the pit depth is enough big to

0

0.1

0.2

0.3

0.4

0.5

0.1 1 10 100 1000

E v

s. A

g/A

gC

l/K

Cl s

at.

(V)

i (mA·m-2)

AISI 304L

pH 6, 1000 ppm chlorides

20 C

17

act as a diffusion barrier [13–15]. The critical condition for this transition is determined by the

magnitude of the pit stability product, defined as the product between the pit current density i and the

pit depth a. All the pits initially grow in the metastable condition, and the pit stability product

increases linearly with time. Once a critical value is reached, generally 3 mA·cm–1, pit growth

becomes stable [5,14,15].

Figure 1.14 – Schematic representation of a pit with its porous cover [13].

1.3.4 Influence of metallurgical factors

Effect of stainless steel chemical composition

The presence of various alloying elements plays a significant role on the resistance to pitting attack.

Elements such as chromium and nickel, which are the most important alloying elements of an

austenitic stainless steel, have beneficial effects on pitting resistance, shifting Epit to more noble

values (as shown in Figure 1.15), expanding the passivation potential range and reducing the passive

current density [2,8].

Generally, the highest pitting corrosion resistance is found in super – austenitic, duplex and

super – duplex stainless steels, whereas it decreases passing from austenitic to ferritic and to

martensitic ones [2,3].

Considering stainless steels in chloride solutions, both chromium (vital and irreplaceable element for

passive film development) and molybdenum are capable to reduce susceptibility to pitting attack

thanks to a decrease in pit development rate and an increase in Epit, whereas nickel and manganese

exert a smaller influence on the pitting susceptibility. In particular, as shown in Figure 1.15b, every

10% increase in nickel content involves an increase in only 5 mV of pitting potential [2,3,8].

It is well known that the addition of molybdenum improves the passive properties and the resistance

to pitting in austenitic stainless steels. According to Pardo et al. [16,17], molybdenum increases the

corrosion potential and decreases corrosion current density through the presence of

molybdenum – containing ionic compounds in the passive layer that act as a barrier against pitting

attack and, in case of acidic media, through the formation of a protective molybdenum oxide MoO3

layer.

Nevertheless, the amount of molybdenum that can be added to austenitic stainless steels is limited by

the onset of sigma phase precipitation, which embrittles the alloys and reduces pitting resistance [2,8].

Further increase in pitting resistance can be obtained by introducing in the alloy nitrogen, whose

beneficial effect is significantly enhanced if molybdenum is present. The addition of nitrogen

improves pitting resistance and promotes passivity through complex mechanism which involves the

18

enrichment of chromium below the passive film, the development of a nitride layer at the film – metal

interface and the formation of a ferrous molybdate layer in the outer regions of the passive film. It

has to be mentioned that, as the nitrogen level rises, manganese content must increase in order to

ensure the solid solubility of nitrogen and to prevent the precipitation of both chromium nitrides and

carbo – nitrides. However, manganese is affine to sulfur and it gives rise to manganese sulfides MnS

which are preferential sites for pit initiation, so that its effect on pitting phenomenon is still

controversial [2,8,16,17].

Figure 1.15 – Effect of chromium (a) and nickel (b) content on pitting potential [2].

The effects of various elements on the anodic polarization curve and on the pitting resistance in

chloride solution are shown in Figure 1.16. It is possible to observe that vanadium, silicon and

tungsten improve the pitting resistance, whereas boron has a variable effect, due to the fact that it is

beneficial in solid solution but it is detrimental if it precipitates as an intermetallic compound [2].

Figure 1.16 – Effect of different alloying elements on anodic characteristic curve (a) and on pitting resistance (b)

in chloride – containing environments [2].

a b

a b

19

A compositionally derived index, called pitting resistance equivalent number (PREN), can be used in

order to select the proper stainless steel for specific environmental conditions; generally, the higher

is the PREN the higher will be the pitting potential and also the higher will be the chloride threshold

value [3]. PREN is so calculated [6]:

(1.15) PREN = % Cr + 3.3·(% Mo + 0.5% W) + 16% N

Even though this parameter is often used, it does not consider other important parameters that affect

the pitting resistance, such as the surface finish, so that the selection of a material by means of this

index is not sufficient.

Effect of stainless steel microstructure

Commercial materials often have considerable amounts of impurities and non – metallic inclusions;

for example, sulfur in stainless steels can be considered as an impurity or it can be added to improve

machinability but, due to its low solubility, it gives rise to the precipitation of sulfides. In presence of

impurities, inclusions or other defects, the metal is more susceptible to localized corrosion [2,8].

In this context, sulfides are recognized to be inclusions acting as pit nucleation sites. In particular,

manganese sulfides have been intensively studied due to their abundance and weakness in stainless

steels [8]. The role of manganese sulfides as favored sites for pitting attack has been investigated by

Eklund [18] through the analysis of Pourbaix diagram of MnS – H2O – Cl– shown in Figure 1.17.

Due to the fact that MnS is outside its domain of thermodynamic stability for potentials higher than

– 100 mV vs. SHE, and considering that the corrosion potential of stainless steels in aqueous chloride

solutions is in the range 0 to 200 mV vs. SHE, it is possible to conclude that at potential values equal

or higher than the corrosion potential of stainless steel the sulfide will tend to dissolve, confirming

that manganese sulfides are active anodic sites at stainless steel surface in presence of aqueous

chloride solutions. Also morphology can influence their effectiveness as pit initiators: for a given

type of inclusion, spheroidized particles are less susceptible to corrosion than elongated plastically

deformed ones, whereas voids developed between MnS and the metal matrix during rapid cooling

seems to increase pitting susceptibility [8,19,20].

Figure 1.17 – Pourbaix diagram of MnS – H2O – Cl

– [18].

20

Some authors [2,20–23] have observed a decrease in corrosion rate as the stainless steel purity

increases, so that different techniques for the inclusion removal, such as potentiostatic polarization

treatments or laser melting, have been developed, and they have demonstrated to be effective for the

improvement of pitting resistance.

Effect of metalworking

As far as it concerns metalworking, in this section the effects of cold working, heat treatments and

welding are discussed.

Studies focusing on cold working have found no significant effect on pitting potential; however this

process affects both the morphology of MnS inclusions, particularly their deformation and their

fracture, and the coherence between MnS and the metal matrix [2,8].

Heat treatments, by contrast, not only affects MnS – metal matrix coherence but also affects the

thickness of the oxide film, and it takes part in the precipitation of sigma phase [8].

Sigma is introduced into the microstructure due to the exposure of delta phase at elevated

temperatures for a sufficiently long period of time, and it is detrimental to pitting corrosion due to the

depletion of chromium and molybdenum in the surrounding matrix. Furthermore, delta ferrite, which

is characterized by the presence of fine precipitated carbides, causes problems during hot working.

Due to this reason, nickel is added in the 300 series of austenitic stainless steel, so that both delta

ferrite formation and sigma precipitation is minimized. For this purpose, addition of elements, rapid

cooling or other specific heat treatments can be used [2,8,22].

Welding treatments lead not only to the effects of heat treatments but also to sensitization, with the

precipitation of metal carbides M23C6 and sigma phase; in particular, carbides that precipitate at grain

boundaries triple points are preferential sites for pit initiation and for other forms of localized

corrosion. However, in Mo – free austenitic stainless steels the weld metal often shows better pitting

resistance than the other alloys due to the lack of molybdenum – chromium depleted zoned and due

to the redistribution of MnS inclusions during welding process [2,8].

Finally, it is important to consider the effects of these processes on the austenitic phase grain size.

Different studies [24–26] have revealed no dependence of corrosion potential on the austenite grain

size, whereas grain refinement lead to both an increase in pitting potential and a decrease in corrosion

rate and current density. However, some authors [27] have reported that refinement has no effect on

pitting potential value.

Actually, no complete agreement exists on the role of grain size and further analyses are required.

Effect of surface finish

Another key factor which affects pitting potential is the surface finishing of the material. The more

homogeneous the surface is, both chemically and physically, the better the resistance to pitting attack.

It is generally accepted that surface with higher roughness, and therefore a more pronounced peaks

and valleys structure, can increase the number of pitting nucleation sites due to a chloride ions

accumulation on the grooves of the protective oxide film. Even the presence of discontinuity and

defects may act as preferential sites for pitting initiation [2,8].

As an example, Manning et al. [19] have studied the effect of surface roughness on pitting potential,

and they discovered that Epit obtained for specimens polished with diamond past is 60 mV higher with

respect to the pitting potential of a specimen polished with 80 grit paper.

Different chemical surface treatments can improve the pitting resistance of stainless steels.

21

For example, pickling is based on immersion in a 10% nitric acid HNO3 + 2% hydrofluoric acid HF

bath and it can be used to remove chromium – depleted layer, oxides produced by heat treatments or

welding, embedded iron particles from machining operations and manganese sulfide inclusions

exposed by grinding operations, so that the pitting potential increases after this treatment [2].

Similar to pickling, a passivation treatment can be performed by means of immersion in a 20% HNO3

bath; this chemical treatment not only has the same benefits of pickling, but also allows to increase

the chromium content of the passive film [2].

Alonso – Falleiros et al. [28] have analyzed the behavior of an AISI type 304L specimen in 3.5%

sodium chloride NaCl solution. They have discovered that both pitting and corrosion potential are

affected by surface treatments.

The main conclusions given by the authors are that different surface treatments affect the protection

degree of the passive layer and that passivation seems to provide the most homogeneous and

protective film against pitting attack.

1.3.5 Influence of environmental factors

Effect of chloride ions

Chloride concentration in the solution is a very important parameter which affects pitting corrosion

of stainless steels, causing passive film breakdown. Chlorides have a special importance in pitting

corrosion because they are present in several environments in which stainless steels are used, such as

seawater and brackish waters [2,29].

As discussed in §1.2.2, in presence of chlorides two effects on the anodic characteristic curve of

stainless steel are recognized: the decrease in pitting potential and the increase in passive current

density. In other words, in presence of chlorides, passive condition is weakened [2–4,6].

The relationship between Epit and chloride concentration [Cl–] can be expressed through a linear

dependence (in logarithmic scale):

(1.16) Epit = A + B∙log[Cl–]

where A and B are constants which depend on environmental and metallurgical factors; for example,

higher chromium and molybdenum contents in austenitic stainless steels involve a higher chloride

level that can be tolerated [2–4,6,12].

Due to the fact that incubation time decreases as chloride concentration rises [22,30], Wang et al.

[30] have found the following empirical relationship for AISI type 304L:

(1.17) Epit = E + F∙logti

where E is a constant which depends on different factors, F is a constant that depends on the

temperature (0.25 in the range 20 to 80 °C, and 0.32 at 150 and 200 °C) and ti is the incubation time.

Both the dependence of pitting potential on chloride concentration and the empirical relationship

between Epit and ti are shown in Figure 1.18.

Therefore, for each potential of the steel there is a chloride concentration below which pitting attack

cannot be induced.

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This value is called critical chloride content [Cl–]th, i.e. the minimum amount of chloride ions required

to initiate pitting corrosion or, alternatively, the highest content of chlorides compatible with

conditions of passivity.

The definition of this parameter is quite complex due to the stochastic nature of pitting and the

influence of environmental and metallurgical factors that affect this phenomenon [4,6,12].

Figure 1.18 – a) Pitting potential versus chloride concentration (in logarithmic scale) in aerated solutions at different

temperatures; b) Pitting potential versus induction time at different temperatures and chloride concentrations

(white squares refer to experiments carried out with different Cl– content at 20, 40, 60 and 80 °C;

black squares refer to experiments carried out with different Cl– content at 150 and 200 °C) [30].

Even more, it should be noted that other anions can induce pitting, such as bromide, fluoride or iodide

ions, and that some ions can rise up (sulfurs, thiosulfates, etc.) or reduce (sulfates, perchlorates,

acetates, etc.) pitting tendency [2,4,6,8,22].

Effect of pH

The pH of the solution affects not only the cathodic processes of oxygen reduction and hydrogen

evolution, as explained in §1.2, but also the kinetics of the anodic processes through solubility of the

corrosion products and their protective properties [31].

In general, pitting potential of stainless steels increases as the pH increases, as shown in Figure 1.19.

In particular, in alkaline environment the passivity is strongly strengthened and the pitting potential

is significantly moved in the noble direction because of the high concentration of OH– ions which

allow the incorporation of hydroxide groups in the stainless steel passive layers [2,22,31–33]. From

Figure 1.19 it can also be noted that:

The lower the PREN is the higher the effect of the pH on the pitting potential will be.

There is always a strengthening of the passivity appears for basic pH, independently of the

PREN index of the stainless steel.

The effects of basic solutions of the pitting potential have been evaluated also by Freire et al. [32].

the authors have performed potentiodynamic polarization tests, one of the most used electrochemical

a b

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method for the evaluation of the pH effect on pitting potential, by means of 10% NaCl solutions at

different pH values (9, 11 and 13).

Figure 1.19 – Effect of pH on pitting potential of different stainless steels in 3% NaCl solutions [2].

The results are shown in Figure 1.20: as the pH increases, the pitting potential strongly rises, passing

from 0.04 V vs. saturated calomel electrode (SCE) for pH 9 solution to 0.52 V vs. SCE for pH 13

solution, in accordance with the theory. Furthermore, it can also be noted a decrease in corrosion

potential value with the increase in solution pH, whereas no significant differences have been

observed in the general evolution of the curves, suggesting that the kinetics of process is not

significantly affected by the solution pH.

Figure 1.20 – Potentiodynamic polarization curves of AISI type 304 in 10% NaCl solutions at different pH values [32].

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Regarding the acidic solutions, Ramana et al. [34] have analyzed the effects of acidic pH on the

pitting potential of AISI type 316L. They have noted a decrease in pitting potential as the acidity of

the solution increases, in accordance with the theory. Moreover, they have noted that there is not a

big difference in pitting potential between pH 5 and pH 2.2 (less than 20 mV), whereas a sharp

decrease in Epit has been noted passing from pH 2 to pH 1.66 (more than 60 mV).

It has also been noted that only in very acidic solutions the type of attack changes to severe general

corrosion. In particular, Tzaneva [31] has analyzed the corrosion behavior of 304 stainless steel in

acidic solution. The results show that the steel show active behavior at pH equal to 1, with a high

corrosion current density and an active – passive transition under the anodic polarization, while at pH

values above 1 the stainless steel is passivated. Even more, the author has noted that the corrosion

potential decreases as the pH of the solution decreases, particularly for very aggressive solutions (pH

1).

Effect of temperature

Another important parameter that influences the protective properties of a passive film is the

temperature. Generally, both pitting potential and protection potential values decrease with the

increase in solution temperature [2,30].

The decrease in pitting potential and the increase in pit number at high temperatures can be explained

by a stronger agglomeration and chemisorption of chloride ions on the metal surface, causing an

easier breakdown of passivity. An increase in temperatures also accelerates the transport of reagents

and reaction products, even if it should be pointed out that the effect of temperature on transport rate

is considerably less than the temperature effect on both the chemisorption of chloride ions and the

ionization of metal, so that any change in the anolyte concentration feels more the effect of the initial

current density in pits. It has also been noted that, as the temperature rises, both the porosity of the

passive film and the density of vacancies or voids in the oxide film increase, leading to a higher

probability of pit nucleation [8,22,30].

The effect of temperature above pitting behavior depends also on stainless steel chemical

composition. As an example, it is possible to observe in Figure 1.21 that pitting potential of AISI type

316 becomes independent on temperature above 70 °C, so that, as the PREN of the material increases,

the dependence of the pitting potential on the temperature is less evident.

Figure 1.21 – Effect of temperature on pitting potential of different stainless steels in 3% NaCl solutions [2].