Pitting Corrosion 304

7/23/2019 Pitting Corrosion 304

1/12

Pitting corrosion of 304ss nanocrystalline thin film

Chen Pan, Li Liu , Ying Li, Fuhui Wang

State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Rd., Shenyang 110016, China

a r t i c l e i n f o

Article history:

Received 5 November 2012

Accepted 25 March 2013

Available online 6 April 2013

Keywords:

A. Sputtered film

B. AFM

C. Pitting corrosion

a b s t r a c t

Pitting corrosion behavior of coarse crystalline (CC) 304ss and its nanocrystalline (NC) thin film have

been investigated by electrochemical measurement and in situ AFM observation in 3.5% NaCl solution.

Results show two effects of nanocrystallization on pitting corrosion behavior: (1) more frequent occur-rence of metastable pits, but with lower probability of transition to stable pits, which is attributable to

differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of NC

thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger

pit cavities, and modifies the morphology of stable pit cavity.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Three hundred and four stainless steels have been extensively

used as good corrosion resistant materials. However, resistance

to pitting corrosion of 304ss in solutions containing Cl is not good

enough and adversely influences the service life and integrity ofstructures made of this material. Hence, there is need to improve

the corrosion resistance of 304ss. Some investigations have shown

that nanocrystallization can significantly enhance the corrosion

resistance of stainless steels[13]. Among several nanocrystalliza-

tion methods, magnetron sputtering technique has attracted con-

siderable attention. Through the magnetron sputtering technique,

a homogeneous thin film, having the same composition as the tar-

get, but with a smaller grain size, can be deposited on a material.

The sputtered thin film has the same chemistry with the substrate,

which ensures good adhesion to the thin film on the substrate[4].

In addition, the sputtered nanocrystalline thin films have been

found to possess better corrosion resistance than the correspond-

ing conventional coarse crystalline alloys[57]and have been used

successfully in high-temperature applications[810].

It is well known that the corrosion behavior of 304 stainless

steel mainly includes passive and pitting behavior. A previous

study [11] has demonstrated that nanocrystallization changed

the nucleation mechanism and the growth structure of the passive

film on rolled coarse crystalline (CC) 304ss and also accelerated the

growth rate of the passive film, thereby enhancing the passivation

ability of the material. On the other hand, the influence of nano-

crystallization on the pitting corrosion mechanism of CC 304ss is

not clearly known; therefore, it is significant to study the influence

of nanocrystallization on the pitting corrosion behavior of CC

304ss.

Pitting corrosion is a complicated phenomenon, which is largely

dominated by random parameters [12]. The pit generation event

has been widely considered to be stochastic in nature, and pit ini-

tiation processes have been investigated via stochastic analysis[13,14]. It is believed that both pit initiation rate and pit growth

probability influence the pitting corrosion resistance. If a material

exhibits high pit initiation rate, metastable pits would spread over

the surface, and the pit would easily become a larger cavity, pro-

vided the material has a high pit growth rate. This stochastic ap-

proach could indicate the frequency of metastable pit events and

the probability of stable pits being formed, which deeply exposes

the stochastic nature of pitting corrosion[14].

In this work, a stochastic approach and in situ AFM observation

were employed to study the characteristic features of both the pit

initiation and the pit growth processes of the CC 304ss and its

sputtered thin film in 3.5% NaCl solution. Our goal is to understand

the effect of nanocrystallization on the pitting process.

2. Experimental

2.1. Materials preparation

The composition (in wt.%) of CC 304ss was as follows: 8.054%

Ni, 17.10% Cr, 0.091% Mo, 1.280% Mn, 0.277% Cu, 0.003% As,

0.006% Sn, 0.387% Si, 0.045% C, 0.026% P, 0.002% S, and the rest

Fe. The NC thin film was deposited on one side of a glass substrate

using the SBH-5115D DC magnetron sputtering system with CC

304ss as target. A glass substrate, much unlike a stainless steel sub-

strate, will not likely interfere with the electrochemical responses

of the thin film. The magnetron sputtering chamber was evacuated

0010-938X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.03.022

Corresponding author. Tel./fax: +86 24 2392 5323.

E-mail address:[email protected](L. Liu).

Corrosion Science 73 (2013) 3243

Contents lists available at SciVerse ScienceDirect

Corrosion Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i
http://dx.doi.org/10.1016/j.corsci.2013.03.022mailto:[email protected]://dx.doi.org/10.1016/j.corsci.2013.03.022http://www.sciencedirect.com/science/journal/0010938Xhttp://www.elsevier.com/locate/corscihttp://www.elsevier.com/locate/corscihttp://www.sciencedirect.com/science/journal/0010938Xhttp://dx.doi.org/10.1016/j.corsci.2013.03.022mailto:[email protected]://dx.doi.org/10.1016/j.corsci.2013.03.022


2/12

to 5 103 Pa, then filled with Ar, and maintained at 0.2 Pa. The

temperature of the substrate glass was approximately 200C.

The DC power was 1800 W and the sputtering duration was 2 h.

The CC 304ss samples (10 mm 10 mm 10 mm) were ground

to 1000 grit SiC paper and degreased in acetone. The CC 304ss and

NC thin film were either embedded in epoxy resin or paraffin-resin,

leaving an exposed working area.

2.2. Materials characterization

The microstructure of the CC 304ss was characterized by optical

metallography. The grain size of the NC thin film was characterized

by transmission electron microscopy (TEM) (JEM-2000EXII).

Transmission electron microscope, equipped with a high-angle

angular-dark-filed (HAADF) detector and X-ray energy-dispersive

spectrometer (EDS) system, was used for electron diffraction,

HAADF imaging, and composition analysis. The cross-section of

the CC 304 was observed by scanning electron microscopy (SEM)

(XL30FEG). The phases of the two materials were analyzed by

X-ray diffraction (XRD) analysis.

2.3. Electrochemical experiments

All electrochemical measurements were performed using an

Autolab Electrochemical Measurement System (EG&G) in a con-

ventional three-electrode cell, with a large platinum plate as the

counter electrode and a saturated calomel electrode (SCE) as the

reference electrode. All potential values reported in this paper

are with reference to SCE in saturated KCl solution whose potential

value vs. SHE is 0.2438 V. The aggressive medium used in all exper-

iments was 3.5% NaCl solution prepared from reagent grade chem-

icals and distilled water. The test solution was degassed with

nitrogen for 1.5 h before experiment. A water bath was used to

maintain the solutions at 30 1C during testing.

Prior to all electrochemical measurements, the specimens wereinitially reduced potentiostatically at 1 VSCE for 2 min to remove

air-formed oxides on the surface and then kept in solution until a

stable corrosion potential was attained.

For the polarization measurements, the specimens were kept in

the NaCl solution until a stable corrosion potential was attained

and then scanned in 0.333 mV/s. For the induction time measure-

ment, a potentiostatic technique (0.25 VSCE and 0.9 VSCE for CC

304ss and NC thin film, respectively) was used to measure the ano-

dic current trace. The current response to the applied potential was

recorded within a data-sampling interval of 0.2 s. A sudden in-

crease was the result of the pit corrosion, which was confirmed

by morphology observation. The time interval for this sudden cur-

rent increase is defined as the pit induction time.

2.4. Pit diameter and pit depth measurements

For CC 304ss, the radius of pit mouth (a) was determined from

photomicrography by measuring the area of the pit mouth with a

planimeter in the microscope. The estimated error in a is 5%. Pit

depths, h, were measured by applying the Fine Focus Technique

[15], where the distances required shifting the optical objective be-

tween the focal points on the original surface of sample and on the

bottom of the pit are compared. The estimated error inh is 1 lm.

For the NC thin film, the radius of pit mouth a and pit depth h

was obtained by AFM observation. The AFM resolution is 1.4 nm

inXYscan size and 0.5 nm in Zscan range. The AFM observation

was replicated severally on different samples. The results shown inthe paper are the average sizes.

2.5. In situ AFM measurements

For AFM measurements, the NC thin film was cut into coupons

of dimensions 20 mm 20 mm 2 mm and fixed in a Teflon elec-

trochemical cell with an O-ring to prevent the liquid from leaking

out. The measurements were carried out with a three-electrode

setup: the working electrode was the sample on the bottom of

the cell, the counter electrode was a Pt ring line with 0.25 mmdiameter around the cell, and an Ag line with 0.51 mm diameter

was used as the reference electrode. All the AFM experiments were

carried out in contact mode. In situ AFM scanning was performed

under anodic potential at 0.9 VSCE for NC thin film. To calibrate

the potential of the Ag reference electrode with respect to the

SCE used for polarization experiments, the OCP differential was

determined using both reference electrodes, and a corresponding

potential (anodic) was imposed on the samples during AFM

measurements.

3. Results

3.1. Microstructure

The microstructural characteristics of the CC 304ss and NC thin

film have been extensively described in a previous report [11]. The

grain sizes of the CC 304ss and the NC thin film were found to be

about 100 lm and 50 nm, respectively. The possible occurrence

of austenite to ferrite transition in the NC thin film is still being

investigated; nonetheless, such transition, for single phase struc-

tured sputtered thin films, has been shown to have negligible influ-

ence on corrosion resistance[16,17].

3.2. Potentiodynamic polarization measurements

The potentiodynamic polarization curves of the CC 304ss and

NC thin film in 3.5% NaCl solution are presented inFig. 1. It is obvi-

ous that the NC thin film possesses superior pitting resistance com-pared to CC 304ss. The pitting corrosion morphologies of the

samples after polarization are shown in Fig. 2a and b for the CC

304ss and NC thin film, respectively. In order to properly accom-

modate measurement errors, each experiment was replicated sev-

erally. Statistical analysis of the obtained results confirmed larger

size pits (with diameter of about 70 lm) on the CC 304ss, whereas

the pits formed on the NC thin film had diameter of about 10lm.

Fig. 1. Potentiodynamic polarization plots of CC 304ss and NC thin film in 3.5% NaClsolution.

C. Pan et al./ Corrosion Science 73 (2013) 3243 33


3/12

The above results imply that both materials had pitting corrosion

behavior within the studied polarization range. Therefore, 0.25

VSCE and 0.9 VSCE were, respectively, chosen as the applied anodic

polarization potential for CC 304ss and NC thin film to investigate

the differences in pitting corrosion behavior of the two materials.

3.3. Potentiostatic polarization measurement

The current traces for both samples under anodic potential areshown inFig. 3. The initial rapid decrease in current density corre-

sponds to the formation of passive films on the electrode surfaces.

After about 700 s of anodic polarization, the current density of CC

304ss shows a sudden rapid increase, which occurs after about

900 s for NC thin film. The time interval for this sudden current in-

crease is defined as the pit induction time. It is clear that the cur-

rent traces of the two materials both comprise of two distinctive

stages: metastable pit (before induction time) and stable pit (after

induction time).

Fig. 3shows the metastable pit current transients of CC 304ss

and NC thin film, which reflects the generation, growth, and

repassivation of the metastable pits. It is obvious that within

200 s, large metastable pits appeared on CC 304ss (inFig. 4a). How-

ever, metastable pit events on NC thin film (inFig. 4b) are signifi-cantly different from that of CC 304ss, with shorter transient

lifetime and lower spike height compared to CC 304ss.Fig. 4a also

displays the shape of a metastable pit current transient of CC

304ss: the slow rise of the current followed by a sharp decay of

the current corresponds to slow growth and rapid repassivation

of the metastable pits. The same metastable pitting events hap-

pened on the NC thin film (shown inFig. 4b), indicating that nano-

crystallization did not alter the formation mechanism of the

metastable pit.

4. Discussion

4.1. Effect of nanocrystallization on metastable pit process

4.1.1. Effect of nanocrystallization on pit location

Pit initiation has been reported to proceed via initial dissolution

of MnS at the MnCr2O4/MnS interface in the presence of salt water

[18]. Webb et al.[19]proposed that the shape, size, and distribu-

tion of inclusions influenced the pit initiation. The coarse crystal-

line CC 304ss usually undergoes hot-rolling or cold-rolling before

component-making, during which MnS inclusions were deformed

to be needle-shaped.Fig. 5depicts an SEM image showing the typ-ically needle-like MnS inclusions within the present stainless steel

50 m

(a)

10 m

(b)

Fig. 2. SEM morphologies of CC 304ss (a) and NC thin film (b) after corrosion.

Fig. 3. Potentiostatic polarization plots of CC 304ss (a) and NC thin film (b) in 3.5%

NaCl solution.

34 C. Pan et al./ Corrosion Science 73 (2013) 3243


4/12

specimen. However, the MnS inclusions in NC thin films have not

been previously characterized, at least not in terms of shape and

size. The NC thin film used in this experiment was prepared by

the magnetron sputtering technique which is a non-equilibrium

method employed to develop materials with fine crystalline struc-

ture that often exhibit high corrosion resistance. Therefore, the

shape and size of MnS inclusions in NC thin film may differ from

that in CC 304ss, and could be responsible for the observed differ-ences in metastable pitting events on the two materials.Fig. 6dis-

plays a high-angle angular-dark-field (HAADF) image (a) of NC thin

film and the EDS results (b) of a scan made along the red line in (a).

It is obvious that there are no noticeable inclusions embedded in

NC thin film (Fig. 6a). However, the EDS profile (inFig. 6b) obtained

from the line-scanning along the red line reveals that the compo-

nent element of MnS, sulfur and manganese, exists in NC thin film.

That is to say, sulfur and manganese may melt into the a-Fe phase

as atoms instead of forming MnS inclusion embedded in the NC

thin film. This seeming absence of MnS inclusions on the NC thin

film should be responsible for the difference in the metastable pit-

ting events compared with CC 304ss.

Since pit initiation in stainless steel has been confirmed to be

associated with MnS dissolution, and there is no MnS inclusion

in the NC thin film, it is necessary to identify the pit initiation pro-

cess for the NC thin film. In order to illustrate clearly the differ-

ences in pit initiation mechanisms of the two materials, the

following experiments were carried out. The potentiostatic polari-

zation of CC 304ss was performed under anodic polarization and

Fig. 4. Current transient features of CC 304ss (a) and NC thin film (b) in 3.5% NaCl

solution.

10 m

MnS

Fig. 5. SEM morphology of MnS inclusion embedded in CC 304ss.

(a)

(b) Fe

Ni

Cr

MnSTi

280nm

Fig. 6. (a) An HAADF image of NC thin film and (b) EDS results of a scan made alongthe red line in (a).



5/12

then stopped at the time when the current suddenly increased, the

time interval of which is defined as the pit induction time. Fig. 7

displays the morphology of CC 304ss after polarization (chemical

compositions of corresponding sites are shown). It is clear thatthe pit initiation sites are within the dark gray area. The light gray

area is comprised mainly of alloy elements, such as Fe, Cr, Ni, and

Mn (shown in spectrum A). However, the composition of the dark

gray area includes not only alloy elements but also S (shown in

spectrum B), which suggests that the dark gray area should be

MnS inclusions. Replicate experiments on several samples con-

firmed that the pit initiation site on CC 304ss was on the MnS

inclusion. Analysis of the pit initiation site on NC thin film was

by in situ AFM observation. Fig. 8ac illustrates the morphology

changes of the NC thin film with progressing anodic polarization.

Fig. 8a shows that a lot of particles piled up on the surface of the

NC thin film after 170 s of polarization. Also, there were many

voids in the boundaries of these observed particles piled up in dif-

ferent layers[20]. It is, however, difficult to determine the compo-sition of these particles because they quite minute and formed

directly in the solution. However, XPS analysis of the passive film

formed on NC thin film in acid NaCl solution in a previous study

[21]showed the film to be primarily composed of chromium oxide

and iron oxide.

In the subsequent 60 s, the void (in green1 circle) expanded and

deepened (Fig. 8b), which may indicate the growth of a metastable

pit. After that, the void became reduced at 340 s (shown inFig. 8c).

Thus, the pit initiation site of NC thin film could be said to be at the

boundaries of observed particles. Although 304 stainless steel has a

high C content (0.045%), the carbide was not observed clearly on

the surface of CC 304ss and NC thin film by SEM and TEM in this

work, respectively.

4.1.2. Effect of nanocrystallization on pit initiation process

The above observations indicate that metastable pit events on

NC thin film differed from that of CC 304ss. In order to elucidate

the metastable pit events of both materials in more detail, further

analysis of metastable pit transients was performed by counting

the number of current spikes as a function of time interval and

as a function of amplitude, and their corresponding distribution

using the method described by Burstein and co-workers [2224].

The analytical method counted the total number of spikes between

any two adjacent points in the data set whose amplitude differed

by an amount greater than or equal to an imposed threshold ampli-

tude. The imposed thresholds had been taken to values low enough

to include the background noise as well.Fig. 9shows the electro-

chemical noise data of both materials in 3.5% Na2SO4and 3.5% NaCl

solutions, respectively. Since the present study assumes that there

is no pitting corrosion on both materials in 3.5% Na2SO4 solution,the electrochemical noise results of CC 304ss and NC thin film in

this solution could be considered as background noise. Separation

of the background noise from the metastable pit current transients

can be made by detecting the point at which the graphs deviates

from one another (about 22 nA in this case). In order words, all

peaks of amplitude greater than 22 nA are metastable pit spikes,

while those of amplitude less than 22 nA were not considered

further.

Figs. 10 and 11depict the average frequency and distribution of

metastable pitting events with various transient lifetimes and with

different spike heights on CC 304ss and NC thin film, respectively.

The large error bars on the data points show that metastable pit

formation is a random process which is stochastic in nature[14].

For NC thin film, the average frequency of metastable pit eventswith shorter transient lifetime (25 s) is higher than that of CC

304ss, while that with longer transient lifetime (710 s) is a little

lower than that of CC 304ss (shown inFig. 10a).Fig. 10b indicates

that the higher percent of the events accumulate at shorter time

intervals, which implies that metastable pits occurred more fre-

quently on the NC thin film compared with CC 304ss. In addition,

CC 304ss exhibits a higher average frequency than NC thin film

at the range of larger spike height (Fig. 11a). Also, there is a greater

proportion of larger spikes height on CC 304ss (Fig. 11b). The larger

the spike height of the metastable pit, the more probable the tran-

sition from metastable pit to stable pit. Therefore, it can be ob-

served that there is an increased probability of occurrence of

stable pits which develop from metastable pits for CC 304ss.

In summary, the pit initiation site on NC thin film is at theboundaries of the oxide particles. Because of so many boundaries

on the surface, metastable pit events on NC thin film occur more

readily compared with CC 304ss, indicating that nanocrystalliza-

tion promotes the metastable pit process. However, the outstand-

ing repassivation ability of NC thin film results in a decrease in the

probability of stable pits developing from metastable pits. In other

words, nanocrystallization promotes the occurrence of metastable

pitting events and inhibits the transition from metastable pits to

stable pits.

4.2. Effect of nanocrystallization on stable pit process

4.2.1. Effect of nanocrystallization on stable pit formation mechanism

The time when current increased rapidly is called the inductiontime (s). Repetition of the same experiment yielded a number of

A

B

S

5m

Fig. 7. SEM morphology of CC 304ss after anodic polarization stopped at the time when current suddenly increased.

1 For interpretation of color in Fig. 8, the reader is referred to the web version ofthis article.



6/12

induction time values. The stable pit generation has been widely

considered as a stochastic process. Shibata and co-workers [25

29] proposed a stochastic theory of pitting corrosion, which as-

sumed various models including the series or parallel combination

of an elemental birth stochastic process as well as the death sto-

chastic process. The stochastic model could rationally explain sto-chastic distributions of induction time for pit formation. The

proposed theory recognizes two models:

Type A: Pure birth stochastic models, which only consider pit

generation events.

Type B: Birth and death stochastic models, which assume sto-

chastic pit generation and pit repassivation.

The expected equations for the survival probability (Psur) and

time for pit initiation formulated for each model are shown in

Table 1, and corresponding curves between ln (Psur) and time are

illustrated inFig. 12. The survival probability could be defined as:

Psur 1 i

1 N

1

39nm

(a ) 170s 230s

44nm

(b)

42nm

340s(c)

230s

Fig. 8. In situ AFM images of NC thin film (scale 2 lm 2 lm) in initial pitting stage under anodic polarization in 3.5% NaCl solution.

Fig. 9. Determination of the threshold current required to distinguish pit current

transients from background noise.



7/12

where the symbol i is the order in the total number, N, the total

number of measured induction time.

To define the exact model, the distribution of the induction time

has to be fitted to a specific model by numerical or graphic simu-

lation using the equations for an assumed model. Fig. 13displays

the logarithm of the survival probability (Psur) as a function of

induction time (s) for CC 304ss (a) and NC thin film (b), respec-tively. It is obvious that both plots exhibit linear behavior. This dis-

tribution type is the specific character of A2 model (a series birth

stochastic model of stable pit generation process), which is ex-

pressed in the equation:

Pt expmkt t0 2

where k is the stable pit generation rate, t0 is the incubation time,

andm is the number of incorporated processes. The stable pit gen-

eration rate k is calculated according to Eq.(2). From the results of

Fig. 13, two kinds of information can be obtained: Firstly, plots of

the distribution of induction time for CC 304ss and NC thin film dis-

play analogous shapes, which suggest that nanocrystallization has

no influence on the stable pit generation mechanism. A series of

stochastic birth models could be used to describe the stable pit gen-

eration mechanism of both materials. Secondly, the stable pit gen-

eration rate of NC thin film (2.40 103 s1) is lower than that of

CC 304ss (2.69 103

s1

), which means that nanocrystallizationinhibits the stable pit formation process of CC 304ss.

4.2.2. Effect of nanocrystallization on stable pit growth

Beyond the induction time, the current densities of both mate-

rials (Fig. 3) showed a sudden rapid increase, indicating passive

film break down and subsequent occurrence of stable pits. Pit

growth is usually modeled using a non-homogeneous Markov pro-

cess[30,31]. To achieve this, the theoretical foundations of extreme

value statistics have been employed. The solution of the Kolmogo-

rov forward equations[30,31], governing the growth of an individ-

ual pit, is in the domain of attraction of the Gumbel distribution[32]. In many applications, the Gumbel-type distribution has been

Fig. 10. (a) Average frequencies of metastable events with different transientlifetime for both materials and (b) transient lifetime distribution for the both

materials under anodic polarization in 3.5% NaCl solution.

Fig. 11. (a) Average frequency of metastable events with different spike height and

(b) spike height distribution for the both materials under anodic polarization in

3.5% NaCl solution. The height ranges are as follows: (A) 22100 nA, (B) 100

200 nA, (C) 200300 nA, (D) 300400 nA, (E) 400500 nA, (F) 500600 nA, (G) 600

700 nA, (H) 700800 nA, and (I) >800 nA.

Table 1

Analytical expressions of the survival probability function for various stochastic

models.

Model Survival probability function

Birth process

A1 simple P(t) = exp[k(t t0)]

A2 series P(t) = exp[mk(t t0)]

A3 parallel P(t) = 1 {1 exp[k(t t0)]}m

A4 combination P(t) = Rfi exp[ki (t t0)]

Birth and death process

B1 parallel P(t) =l/(k+ l) + k/(k+ l) exp[(k+l)(t t0)]B2 series P(t) = exp[ak(t sc) exp (lsc)]



8/12

found to account for the stochastic nature in the observed behavior

of pitting corrosion systems[3335].

The extreme value in the present study is defined as the largest

pit depth in each of the potentiostatic measurements. The largest

pit depth can be determined by considering the quantity of charge

passed from the largest current spike. This was achieved by inte-grating the current trace over time as illustrated schematically in

Fig. 14. The charge results from the formation of a stable pit and

can be related to the physical volume of the stable pit via Faradays

equation, Eq.(3), which is based on the correlation between optical

pit size and anodic current trace charge [36]. If the pits are as-

sumed to be hemispherical, then the pit radius/depth can be calcu-

lated, using Eq.(4).

V

CM

nFD 3

R

ffiffiffiffiffiffiffi3V

2p3

r ! 10;000 4

whereV(cm3) is the volume of largest stable pit, Cis the quantity of

charge passed from the largest current spike within each of the

potentiostatic measurements, M is average molecular mass of CC

304ss and NC thin film (55.35 g/mol), D is density of both materials

(7.93 g/cm3),Fis Faraday constant, andR (lm) is the radius of larg-

est stable pit.

Extreme value statistical analysis was employed according to

the following procedure[32]: (1) all calculated extreme value data

were arranged in ascending order from the smallest to the largest

value; (2) the probabilityF(Y) is defined as M/(N+ 1), where Misthe rank in the ordered extreme values (M= 1,2,3, . . ., M) and N

the total number of extreme value data. The reduced variant (Y)

can be calculated by the formula Y= ln{ln[F(Y)]}.

The largest stable pit size within each potentiostatic measure-

ment was calculated, and the values were subjected to extreme

value statistics analysis according to the above mentioned

procedure.Fig. 15 shows the relationship between values of the

reduced variant and the ordered pit sizes.The linearity of the points

demonstrates that the data actually did fit the Gumbel distribution.

The scale parametera and location parameterl for CC304ssand NCthin film were obtained by the linear fitting and are shown in

Table 2. These values describe the shape and center of the

probability distribution of the largest stable pit sizes expected from

electrodes identical to that used for the measurements and treatedin the same manner for the same period of time.

The probability that the largest pit depth is described by a

double exponent (Gumbel-type extreme value distribution) can

be calculated by the following equation[35]:

P 1 exp exp D l a ln S

a

5

where Pis probability of the pit depth,D is pit depth, l is the centralparameter (the most frequent value), Sis the ratio of the area over

Fig. 14. A scheme about the determination method of stable pit charge.

Fig. 13. Plots of survival probability (Psur), vs. time for CC 304ss and NC thin film in

3.5% NaCl solution.

Fig. 12. A schematic illustration of ln Psurvs. time for various stochastic models.



9/12

which prediction is to be made to the area over which the data was

measured, anda is the scale parameter which defines the width ofthe distribution.

The probabilities of a series of given pit depths were calculated

using Eq.(5)in terms of obtaineda andl values for CC 304ss andNC thin film, respectively, and the results are shown inFig. 16. It is

obvious that the smaller the depth of the stable pit, the higher the

probability. The probability of larger stable pits (>5lm) is much

higher for CC 304ss than NC thin film. The reciprocal of the proba-

bility of the given pit depth corresponds with the expected time for

this given pit depth to occur. According toFig. 16, the probability of

generating a 10 lm stable pit is larger on the CC 304ss than on the

NC thin film. That is to say, the expected time for generating a

10 lm stable pit is shorter on the CC 304ss. This implies that it isvery hard to form deeper stable pits on the NC thin film, unlike

for the CC 304ss, which is prone to formation deep stable pits in

the test solution.

The current transient inFig. 3 exhibits two straight lines with

different slopes beyond the induction time (s). The first and thesecond straight lines are associated with the periods of stable pit

growth[37]. The increase in current transient afters can be repre-sented by the EngellStolica equation for pitting corrosion[37]:

log I log B b log t 6

where b is a constant depending on the applied potential and the

concentration of NaCl and is given by the slope of the log i vs. log t

curve. The values ofb relates to the stable pit growth. It can be ob-

served that both first and second slopes of the straight lines of CC

304ss (shown inFig. 3a):b1= 39.65,b2= 3.28, are larger than those

of NC thin film (shown inFig. 3b):b1= 9.41,b2= 2.32. These results

suggest that the growth rates of stable pit on the CC 304ss are sig-

nificantly faster than those of the NC thin film. That is to say, nano-

crystallization indeed retards the growth process of stable pit

formed on CC 304ss.

4.2.3. Effect of nanocrystallization on pit morphology

The above experimental results demonstrate that nanocrystalli-

zation inhibits formation and growth of stable pits on CC 304ss.

However, it is uncertain how nanocrystallization influences the

morphology of the resulting stable pits. It has been suggested that

pit geometry could be described by the ratio of the radius of pit

mouth (a) to pit depth (h) (a/h)[38]:

a/h< 1, pit geometry exhibits deep-hole shape.

a/h= 1, pit geometry shows hemispherical shape.

a/h> 1, pit geometry displays shallow-disk shape.

In order to illustrate the morphology of stablepits on both mate-

rials, optical microscopy and AFM were employed to measure (a)

and (h) for stable pits on CC 304ss and NC thin film, respectively.

Fig. 17 shows the ratio of the radius of pitting mouth to pitting

depth for CC 304ss. Most of the values ofa/h distributed around11.5, indicates that the pitting geometry of stable pits is described

by shallow-disk shape. This result is in accordance with the results

reported elsewhere[39].Fig. 18illustrates the SEM micrographs of

the surface (a) and the cross-section (b) along the red line in (a) of

stable pit on CC 304ss, which demonstrates that the geometry of

Fig. 16. Probabilities of various diameters pits occurring for CC 304ss and NC thin

film in 3.5% NaCl solution.

Fig. 17. Pitting geometry of CC 304ss in 3.5% NaCl solution.

Fig. 15. Gumbel distribution parameters for CC 304ss and NC thin film in 3.5% NaCl

solution.

Table 2

Gumbel distribution parameters for CC 304ss and NC thin film in 3.5% NaCl solution.

Samples Scale parameter (a) Location parameter (l)

CC 304ss 11.57 33.95

NC thin film 3.56 5.98



10/12

stable pits on CC 304ss is shallow-disk indeed.Fig. 19displays the

a/h ratio for NC thin film, which was obtained by measuring thewidth and depth of stable pits with the corresponding linear scan

plots of AFM images (shown inFig. 20). Compared with CC 304ss,

most of the values of a/h are much higher and fall within 35.

Hence, the geometry of stable pits on NC thin film is more shal-

low-disk than that on CC 304ss.In order to distinguish themorphol-

ogy of stable pits on the two materials, semi-elliptic shape and

shallow-disk shape are used to describe the geometry of stable pits

forCC 304ss and NC thin film, respectively. In addition, it canalso be

seen from Figs. 17 and 19 that the measured stable pits depth on CC304ss and the NC thin film are, respectively, of the same order of

magnitude with the calculated ones shown in Fig. 15. From the

300 m

(b)

300 m

(a)

Fig. 18. SEM geometry of stable pit for surface (a) and cross-section (b) of CC 304ss.

Fig. 19. Pitting geometry of NC thin film in 3.5% NaCl solution.

4.7 m28.7 m

Fig. 20. AFM image of stable pit on NC thin film.

Fig. 21. Lacy cover of stable pit grown on CC 304ss in 3.5% NaCl solution.



11/12

above results, it is obvious that the values of a/h did depend on

nanocrystallization and increased significantly with decreasing

grain size. Thus, nanocrystallization changes the morphology of sta-

ble pits from semi-elliptic shape to shallow-disk shape.

It is believed that stable pits are covered with a finely perfo-

rated cover, acting as a diffusion barrier which stabilizes pit

growth. In the present study, majority of the stable pits formed

on CC 304ss were enclosed with a perforated cover. AndFig. 21dis-plays the SEM micrograph of a stable pit on CC 304ss with a lacy

metal cover which helps to maintain the aggressive local chemistry

within the pit and stabilizes pit growth. However, it has not been

reported whether or not there is a pit cover on NC thin film. To gain

insight into the growth mechanism of stable pit on NC thin film,

in situ AFM was employed to record the entire pitting growth

process.

Fig. 22shows that stable pits appeared on the NC thin film with

prolonged potentiostatic polarization. According to in situ AFM

observation, the stable pit was initially about 650 nm wide and

140 nm deep after polarization 10 min, as determined from linear

scanning plots. Afterward, the particles dissolved gradually and

the pit propagated, until the dimensions of the stable pit were

about 750 nm wide and 141 nm deep. Within some minutes, the

width and depth of the stable pit was about 910 nm and 148 nm,

suggesting that the stable pit became larger with increasing polar-

ization time. From the in situ AFM results, two conclusions could

be drawn; firstly, the width of stable pit on NC thin film changes

more rapidly than the depth, which could explain the geometryof stable pits on NC thin film being shallow-disk shape; secondly,

no lacy cover existed during stable pit growth, which indicates that

the growth mechanism of stable pits on NC thin film is different

from that on CC 304ss.

The present study proposes that the absence of the lacy pit cov-

er on the NC thin film could be related to internal residual stress

induced by the nanocrystallization process [40]. Internal residual

stresses in nanocrystalline thin film prepared by magnetron sput-

tering technique are extremely high [41]. These stresses are nor-

mally comprised of two types of stress, namely thermal and

intrinsic stresses. The thermal contribution results from the differ-

140nm

650nm750nm

141nm

910nm

148nm

Fig. 22. In situ AFM images of NC thin film (scale 2 lm 2 lm) in stable pitting formation stage under anodic polarization in 3.5% NaCl solution.



12/12

ence in temperatures of deposition and stress measurements when

thin films and substrates exhibit different thermal expansion coef-

ficients. The intrinsic stress is introduced and built up in the films

during deposition. The occurrence of internal residual stress in NC

thin film may inhibit the formation of lacy cover during stable pit

growth and furthermore, change the growth mechanism of stable

pits on NC thin film. In other words, nanocrystallization retards

the nucleation and growth process of stable pits and changes thegeometry and growth mechanism of stable pits.

5. Conclusions

Nanocrystallization leads to the existence of the chemical ele-

ments, sulfur and manganese (in the uncombined state) in the

CC 304ss, leading to differences in the pit initiation sites on CC

304ss and NC thin film: the former is on the MnS inclusions, and

the latter is at the boundaries of observed particles. The pit initia-

tion site and outstanding repassive ability of NC thin film ensure

that metastable pit events occur more frequently, and the proba-

bility of stable pits developing from metastable pits is lower com-

pared with that of CC 304ss. Nanocrystallization retards the

formation and growth process of stable pits, changes the geometryand growth mechanism of stable pits, which enhance the pitting

corrosion resistance of CC 304ss.

Acknowledgements

The investigation was supported by the National Natural Sci-

ence Fund of China under the Contract No.50801063 and 512.

The authors acknowledge Dr. Tao Zhangs contributions to the dis-

cussion on stochastic analysis of metastable pit events and Emeka

E. Oguzie (Nigeria) for suggestions and modification of English.

References

[1] R.B. Intrui, Z. Szklarska-Smialowsska, Localized corrosion of nanocrystalline304 type stainless steel film, Corrosion 48 (1992) 398403.

[2] N. Li, Y. Li, S.G. Wang, F.H. Wang, Electrochemical corrosion behavior ofnanocrystallized bulk 304 stainless steel, Electrochim. Acta 52 (2006) 760765.

[3] X.Y. Wang, D.Y. Li, Mechanical and electrochemical behavior of nanocrystallinesurface of 304 stainless steel, Electrochim. Acta 47 (2002) 39393947.

[4] F.H. Wang, H.Y. Lou, Factors affecting adhesion between magnetron sputterdeposited stainless steel coating and a carbon steel substrate, Mater. Sci. Prog.3 (1989) 133137.

[5] W. Ye, Y. Li, F.H. Wang, Effects of nanocrystallization on the corrosion behaviorof 309 stainless steel, Electrochim. Acta 51 (2006) 44264432.

[6] M.P. Ryan, N.J. Laycock, H.S. Isaacs, R.C. Newman, Corrosion pits in thin films ofstainless steel, J. Electrochem. Soc. 146 (1999) 9197.

[7] G.Z. Meng, Y. Li, F.H. Wang, Electrochemical behaviour of Fe20Crnanocrystalline coatings, J. Soc. Corros. Prot. 26 (2006) 1118.

[8] H.Y. Lou, F.H. Wang, S.L. Zhu, B. Xia, L. Zhang, Oxide formation of K38Gsuperalloy and its sputtered micrograined coating, Surf. Coat. Technol. 63(1994) 105114.

[9] F.H. Wang, H.Y. Lou, S.L. Zhu, W.T. Wu, The mechanism of scale adhesion onsputtered microcrystallized CoCrAl films, Oxid. Met. 45 (1996) 3949.

[10] F.H. Wang, Oxidation resistance of sputtered Ni3(AlCr) nanocrystalline coating,Oxid. Met. 47 (1997) 247254.

[11] C. Pan, L. Liu, Y. Li, S.H. Wang, F.H. Wang, Passive film growth mechanism ofnanocrystalline 304 stainless steel prepared by magnetron sputtering anddeep rolling techniques, Electrochim. Acta 56 (2011) 7740.

[12] T. Shibata, T. Takeyama, Pitting corrosion as a stochastic process, Nature 260(1976) 315316.

[13] T. Shibata, Y. Zhu, The effect of film formation temperature on the stochasticprocesses of pit generation on anodized titanium, Corros. Sci. 36 (1994) 17351739.

[14] T. Zhang, Y.G. Yang, Y.W. Shao, G.Z. Meng, F.H. Wang, A stochastic analysis ofthe effect of hydrostatic pressure on the pit corrosion of Fe20Cr alloy,Electrochim. Acta 54 (2009) 39153922.

[15] ASTM Stand G46-94. Standard Guide for Examination and Evaluation of PittingCorrosion. ASTM International, West Conshohocken, PA, 2005.

[16] D.C. Liu, F.H. Wang, C.N. Cao, H.Y. Lou, The pitting corrosion resistance ofmicrocrystalline coatings of sputtered 321 stainless steel, Corrosion 46 (1990)

975982.[17] D.C. Liu, F.H. Wang, C.N. Cao, H.Y. Lou, Influence of phase composition on

pitting corrosion resistance of sputtered layer of 1Cr18Ni9Ti stainless steel,Acta Metall. Sin. 25 (1989) B299B302.

[18] S.J. Zheng, Y.J. Wang, B. Zhang, Y.L. Zhu, C. Liu, P. Hu, X.L. Ma, Identification ofMnCr2O4 nano-octahedron in catalyzing pitting corrosion of austeniticstainless steels, Acta Mater. 58 (2010) 50705085.

[19] E.G. Webb, T. Suter, R.C. Alkire, Microelectrochemical measurements of thedissolution of single MnS inclusions, and the prediction of the criticalconditions for pit initiation on stainless steel, J. Electrochem. Soc. 148 (2001)B186B195.

[20] L. Liu, Y. Li, F.H. Wang, Influence of nanocrystallization on pitting corrosionbehavior of an austenitic stainless steel by stochastic approach andin-situAFManalysis, Electrochim. Acta 55 (2010) 24302436.

[21] C. Pan, L. Liu, Y. Li, B. Zhang, F.H. Wang, The electrochemical corrosion behaviorof nanocrystalline 304 stainless steel prepared by magnetron sputtering, J.Electrochem. Soc. 159 (2012) C453C460.

[22] G. Ilevbare, G. Burstein, The role of alloyed molybdenum in the inhibition ofpitting corrosion in stainless steels, Corros. Sci. 43 (2001) 485513.

[23] G. Burstein, S. Vines, Repetitive nucleation of corrosion pits on stainless steeland the effects of surface roughness, J. Electrochem. Soc. 148 (2001) B504B516.

[24] G. Burstein, R. Soutob, Improvement in pitting resistance of stainless steelsurfaces by prior anodic treatment in metasilicate solution, J. Electrochem. Soc.151 (2004) B537B542.

[25] T. Shibata, Stochastic studies of passivity breakdown, Corros. Sci. 31 (1990)413423.

[26] T. Shibata, M. Ameer, Stochastic processes of pit generation on zirconium withan anodic oxide film, Corros. Sci. 33 (1992) 16331643.

[27] T. Shibata, Y. Zhu, The effect of film formation potential on the stochasticprocesses of pit generation on anodic titanium, Corros. Sci. 36 (1994) 153163.

[28] T. Shibata, Y. Zhu, A stochastic analysis of flow velocity effects on the pitgeneration process on anodized titanium, Corros. Sci. 37 (1995) 853862.

[29] T. Shibata, Statistical and stochastic approaches to localized corrosion,Corrosion 52 (1996) 813830.

[30] J. Provan, E. Rodriguez, Development of a Markov description of pittingcorrosion, Corrosion 45 (1989) 178188.

[31] P. Aziz, Application of the statistical theory of extreme value to the analysis ofmaximum pit, Corrosion 12 (1956) 495505.

[32] E.J. Gumbel, Statistics of Extremes, Columbia University Press, New York, 1957.[33] G. Engelhardt, D. Macdonald, Unification of the deterministic and statistical

approaches for predicting localized corrosion damage. I. Theoreticalfoundation, Corros. Sci. 46 (2004) 27552780.

[34] A. Valor, F. Caleyo, L. Alfonso, D. Rivas, J. Hallen, Stochastic modeling of pittingcorrosion: a new model for initiation and growth of multiple corrosion pits,Corros. Sci. 49 (2007) 559579.

[35] T. Zhang, Y. Yang, Y. Shao, G. Meng, F. Wang, A stochastic analysis of the effectof hydrostatic pressure on the pit corrosion of Fe20Cr alloy, Electrochim. Acta54 (2009) 39153922.

[36] S. Pride, J. Scully, J. Hudson, Metastable pitting of aluminum and criteria for thetransition to stable pit growth, J. Electrochem. Soc. 141 (1994) 30283040.

[37] H.H. Hassan, Effect of chloride ions on the corrosion behaviour of steel in 0.1 Mcitrate, Electrochim. Acta 51 (2005) 526535.

[38] D. Sun, Y. Jiang, Y. Tang, Q. Xiang, C. Zhong, J. Liao, J. Li, Pitting corrosionbehavior of stainless steel in ultrasonic cell, Electrochim. Acta 54 (2009) 15581563.

[39] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. I. Introductionand pit growth kinetics, Corros. Sci. 44 (2002) 927941.

[40] R. Daniel, K.J. Martinschitz, J. Keckes, C. Mitterer, The origin of stresses inmagnetron-sputtered thin films with zone T structures, Acta Mater. 58 (2010)26212633.

[41] N.R. Shamsutdinov, A.J. Bottger, F.D. Tichelaar, The effect of Cu interlayers ongrain size and stress in sputtered FeCu multilayered thin films, Scripta Mater.54 (2006) 17271732.


Documents

Pitting Corrosion 304