Corrosion is an irreversible phenomenon which results from the …bbaroux.free.fr/enseignements/CIB/CIB.pdf · 2006-10-16 · Corrosion is an irreversible phenomenon which results

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INPG. Oct. 2003 Corrosion in brief

Corrosion in brief

byBernard Baroux

Institut National Polytechnique de Grenoble + Arcelor R&D

• Introduction to Corrosion processes• Corrosion Electrochemistry • Passivity and Passive films• Stability and Breakdown of passivity• Pitting corrosion• Crevice corrosion• Intergranular corrosion of stainless steels• Concluding remarks

Corrosion is an irreversible phenomenon which results from the basic thermodynamic characteristics of the materials and the nature of their environment. In this lecture we will look at the water corrosion phenomena applicable to metals and alloys, but excluding both high temperature oxidation phenomena and corrosion of non-metallic materials.

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Experimental Evidences

• Formation of rust and other corrosion products

• Electrochemical dissolution M →M+++2e-

Metallurgical effects

⇒ loss of weight and pollution…

Corrosion phenomena may concern the most of the devices of our day to day life, when they are immersed insufficiently severe environments, or made from an insufficiently resistant material (which may happen for cost consideration, even when the appropriate solution is known, but expansive). Corrosion risks may also arise from inappropriate secondary processing conditions, such as welding or drawing. The conjunction The corrosion of metals takes two different forms: 1) Dissolution of the metal and therefore loss of matter. This consists of passage of the metal cations into an aqueous solution (anodic dissolution), the electrons p^roduced by this dissolution being consumed by a cathodic reaction. As such, the dissolution of metals differs greatly from ordinary dissolution such as when sugar is added to water. Aqueous corrosion is above all an electrochemical phenomenon. 2) The formation of rust, which is the common name given to certain iron oxides. In this case, the product of corrosion is not soluble but solid and often considered unsightly.

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The oxidation of metals(example of a divalent metal)

−++ +→+ eOHMOnHM n 2)( 22Anodic dissolution

−+ ++→+ eHMOOHM 222Passivation

Dry oxidation −−−

−++

→+

+→

OeO

eMM

221

2

2

MOOM →+ −−++

MOOM →+ 221

Precipitation of an oxideor a hydroxide (rust) ou

+++ +→+ HMOOHM 22

+++ +→+ HOHMOHM 2)(2 22

The corrosion of metals may take different forms: Oxidation in oxygen containing atmospheres at high temperatures (higher than the water dew point in this atmosphere). This produces a gain of matter (the oxygen trapped in the oxide) and will be investigated later. Morever, It is well known that most metal ores are found in their oxide form. However, the overall reaction involving formation of the oxide splits into several simple reactions including the dissociation of its cations and electrons. An oxide can also form by interaction with water (passivation reaction). In fact, this consists of both an oxidation reaction and an acid-base reaction which may include several steps. In certain conditions, metal cations are transferred to solution in hydrated form (anodic dissolution), resulting in a loss of matter. In aqueous corrosion, the electrons produced by the anodic dissolution reaction having to be consumed by a cathodic reaction. As such, the dissolution of metals differs greatly from ordinary dissolution such as when sugar is added to waterfor instance. Aqueous corrosion is above all an electrochemical phenomenon. Lastly, the cations anodically dissolved may cause hydrolysis, leading to the formation of a hydroxide or an oxide which can precipitate in the form of rust, which is the common name given to certain iron oxides and often considered unsightly. This 3 forms have in common to initiate from a chemical oxidation process , namely de-electronation , involving an oxidising element (for example oxygen, or ferric iron) which is able to trap the electrons from a reducing species (for instance oxhydrile ion, or ferrous ion). The tendency of a metal to be oxidized is related to its capacity to combine with the oxidizing agent.

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The role of water

The oxygen in the iron oxides forming the rustis drawn essentially from the molecules of water and not at all or only very slightly from the oxygen in the air, as is most often thought (the oxygen only encourages the cathodic reaction as will be explained later).

In temperate climates, atmospheric corrosion may be considered a water corrosion phenomenon due to the presence of a film of water on the surface of the metal which generally constitutes a cold surface encouraging the formation of condensation.

The predominant role of water in corrosion phenomena must be underlined. The oxygen in the iron oxides is drawn essentially from the molecules of water and not at all or only very slightly from the oxygen in the air, as is most often thought (the oxygen only encourages the cathodic reaction as will be explained later). In our temperate climates, atmospheric corrosion may be considered a water corrosion phenomenon due to the presence of a film of water on the surface of the metal which generally constitutes a cold surface encouraging the formation of condensation. Moreover, the example of very old iron parts found in hot and dry climates which have not rusted because the ambient moisture content is too low to cause condensation of water on the surface of a metal illustrates this point.

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Water assisted Corrosion and passivation processes

OH

H

OM

H

M

+ H+

OH

OH

M

M + H+ M

M

HO

O

M

OH

+ H+

+ H+

OM

He- + MOH+

M++ + OH-

Corrosion

Passivation

In common first step

e-

The dipolar water molecule adsorbs to metal surfaces due to either physical or chemical interactions. Adsorbed water can combine with the metal by loosing a proton and an electron, giving birth to a MOH type adsorbed complex. This second step (formation of the absorbed complex) is common between water corrosion and passivation mechanisms described below. In turn, the complex MOH may (or not) oxidise by loosing an electron. The oxidised form MOH+ is unstable and dissolves by giving up the cation to the water environment. This a typical water corrosion process (Bockris mechanism for divalent iron dissolution). 4) If, instead of oxidizing and dissolving, the MOH complex is again deprotonated, we observe the formation of a stable species closely linked to the metal and inhibitor of any subsequent aqueous corrosion (the water is no longer absorbed on the metal but on the MO oxide). This the typical water passivation process

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Formation of a passive film:ion hopping and water deprotonation

OM

H

MO

O

M

OM

OH

OH

M

OM

H

O

OM

(b) ion hopping and film growth

(a) Formation of the first oxide monolayer(b) Cation hopping and formation of a second layer.

(c) Vacancy migrates toward the metal(d) The process is repeated (Passive Film growth)

OM

H

MO

OM

M

OM

OH

OH

H

OH

H

O

OH

(a) First (hydrated) oxide layer

In ferrous alloys, the passive film growth proceeds from cation hopping and electromigration of the resulting vacancies toward the metal

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The developped passive film

MO

MO

M

OM

OM

OM

MO

MO

M

OM

OM

OM

MO

MO

M

OM

OM

OM

OH

H

MO

MO

M

OM

OM

OM

OH

M

IHP

metal

film

Electrolyte

few nm

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Chloride assisted corrosion process

M

OH

H

Cl-

MCle-

M++ + 2Cl-

MCl

e- MCl+

+ Cl-

• Adsorption of chloride ions, complexation, dissolution

In chloride containing environments, the adsorbed water can be replaced by a chloride ion. A similar mechanism to the one described for water corrosion is then possible but the MCl complex can only dissolve (by restituting the chloride ions in solution after dissolution of the cation) and cannot form a protective passivating compound. Finally, the chloride ion functions like a cation pump and the mechanism can be repeated. This mechanism is known as chloride dissolution.

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Oxidation reactions

223 222 HOHeOH +↔+ −+

metal

−− →++ OHeOOH 442 22

Examples of other oxidation processes

FeeFe ++−+++ →+ eONONO −− ++→223 2

1

Etc…

Water is not only an acid/base amphoter The hydrogen and oxygen evolution reactions are in fact two aspects of the oxydoreduction of water The deprotonated form of water (hydroxyl ion) behaves like a reducing agent, able to give up electrons with release of oxygen. Its protonated form (hydronium ion) behaves like an oxidizing agent, capable of trapping electrons with release Hydrogen These reactions are controlled by the solution pH and, in the presence of an electron reservoir (for example metal), by the difference of potential between this reservoir and the solution.

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The electrode potential :A consequence of the charge transfers

IHP OHP

V

Metal

ElectrolyteVH

Double layer (Helmoltz)

x

The Charge transfers (electrons and ions) between the metal and the aqueous solution produces a potential difference

The charge transfers between metal and solution lead to a difference of potential between the metal and the core of the solution. The figure has been drawn in the case of a net positive excess of charge on the metal surface and of a net negative excess of charge in the electrolyte. One has assumed that the solution excess of charge was concentrated at the immediate vicinity of the metal (outer Helmoltz plan), which is true for sufficiently conductive solutions. In this case , the interface behaves as a capacitor , the two plates of this capacitor beeing the metal surface (Inner Helmoltz plan) and the Outer Helmotz plan Both IHP and IHP form the so called Electrochemical double layer Remark: In the more general case, the charge transfered to the lectrolyte is not concentrated at the interface and form the so called « diffuse layer »

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Anodic (electron producing)

and cathodic (electron consuming)

reactions:an outlook

Ox +e- → Red

223 222 HOHeOH +↔+ −+

−− ++↔ eOOHOH 424 22

Fe+++ + e- → Fe++

M M++ + 2e-

e -Ox

Red

M MO + 2H++2e-(+H2O)

The anodic dissolution results in a transfer of cations (positive charges) from the metal to the solution From another hand, the metal forms a reservoir of electrons which it can exchange with other oxidizing species present in solution. This transfer is called "cathodic reaction". In an acid environment, the usual cathodic reaction is the one of so-called hydrogen evolution which reduces proton to hydrogen. In a neutral oxygenated environment, the oxygen is consumed (evolution of oxygen) thereby increasing the local pH. In ferric salt environments, the ferric ions can be transformed into ferrous salts. And so on … In each case, the cathodic reaction operates below the redox standard potential (above it is the opposite reaction which can take place, for instance release of oxygen by decomposition of the water).

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Corrosion equilibria

)log(2

3.2. cq

kTcstV +=

Assuming local equilibria, the mass action law writes

cst.(c) pH =+ log21

cst.pHq

kT.V =+ 32

At a metal electrode, several potential and pH dependent reactions occur

(1) Anodic dissolution:

M ↔M+++2e-

(2) Dissolution or Precipitation of the oxide

(3) PassivationM+++H2O ↔MO+2H+

M+H2O ↔ MO+2H++2e-

Where c is the concentration in dissolved cations near the metal interface.

C results from the balance between the dissolution rate and the diffusion flow J µ cfrom the metal surface to the solution bulk (where c=0)

The dissolution rate is then proportionnal to c

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Pourbaix representation

pH

Pot

entie

l

M++

M

MO

M+H2O ↔ MO+2H++2e-

M ↔M+++2e-

M+++H2O ↔MO+2H+

1

2

3

[ ]

cst.pHq

kT.V

cst.(c) pH

cq

kTcstV

Mc

=+

=+

+=

= ++

32 )3(

log21 )2(

)log(2

3.2. (1)

The typical reactions of metal-water interactions involve both electrons (oxydoreduction) , protons (acidobasicity), and solute cations (solution chemistry) Anodic disssolution: M M++ + e- Precipitation of rust M+++H20 MO + 2H+ Passivation: M+H20 MO + 2H+ + 2e-

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Pourbaix diagramof Aluminium

Al+++ (Al2O3,3H20) AlO2

-

Al

(5)(2)

(6)

(3) (4)

(1) Al+++ + 2 H2O ↔ AlO2- +4 H+

(2) Al + 3 H2O ↔ Al2O3 + 6 H+ + 6 e-

(3) 2 Al+++ + 3 H2O ↔ Al2O3 + 6 H+

(4) Al2O3 + H2O ↔ 2 AlO2- +2 H+

(5) Al ↔ Al+++ + 3 e-

(6) Al + 2 H2O ↔ AlO2- +4 H+ + 3 e-

NB: this diagram ise drawn for 1µM/l

concentrations in solute species

The actual Pourbaix Diagrams are more complex, due to the co-existence of several electrochemical reactions The Al diagramme shows 2 soluble species (3) log (Al+++) = 5,7 - 3 pH (5) V = -1,663 + 0.02 log (Al+++) (6) V = -1,2662 - 0.08 pH + 0,02 log (AlO2

-)

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Pourbaix diagram of Iron

Fe2O3

Fe+++

Fe++

HFeO2-Fe

For Iron, several forms of solute iron (Fe++, Fe+++ , HFeO2--, …) have to be considred

, defining several corrosion regions in the diagram…. (see figure).

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Corrosion kinetics and polarisation curves

Polarisation resistance (V=Vcor) : Rp=dV/di (Ω.cm2)

Vcor

icor iA

iK iG=iA+iK

- iK i

V

The anodic and cathodic reaction rates depend on the electrode potential

The intensity of the cathodic and anodic reactions is related to the difference of potential between the metal and the electrolyte. This is set using a potentiostat and the current density is measured; we obtain then a polarisation curve (see above). The overall measured current iG is in fact the sum of the anodic current iA (positive) and the cathodic current iK (negative). This overall current is nil at the free corrosion potential Vcor (no potential applied). The cathodic and anodic currents are then equal in absolute values to the corrosion current icor The corrosion potential and current correspond to the intersection of the individual polarisation curves for the anodic and cathodic reactions. The slope of the curve i(V) at the corrosion potential has a dimension which is the inverse of a resistance (by unit surface) called polarisation resistance. There are different techniques for measuring the polarisation resistance and deducing the corrosion current.

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The Tafel lawCU in H2SO4(0.5M) + CuSO4(0.075M)

In the cathodic domain: Proton reduction

In the anodic domain: Anodic dissolution

V = cst + b log iThe Tafel law: I = B.exp (2.3 V/ b)

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INPG. Oct. 2003 Corrosion in brief The Corrosion Current

Evans diagram (Tafel Laws)

i = icor(exp2.3η/ bA− exp-2.3η/ bK)

i = iA - iK = BA exp 2.3V/bA- BK exp –2.3 V/ bK

V=Vcor ⇒ η=O et iA=-iK=icor

Where η= V-Vcor is the overvoltage

η→0 ⇒ i ∼ i cor2.3 η(1/ b A+1/ b K) = η / Rp

1/Rp = 2.3 icor(1/bA+1/bK)

Stern-Geary law

Lorsque les deux réactions cathodique et anodique sont controlées par une loi de Tafel, le courant global résultant est une différence d ’exponentielles. On peut reformuler la loi composée ainsi obtenue en rapportant les potentiels au potentiel de corrosion (courant global nul), c ’est à dire en utilisant la surtension définie plus haut. Dès que l ’on s ’éloigne du potentiel de Corrosion, l ’une des deux réactions anodique ou cathodique devient dominante et l ’une des exponentielles peut etre négligée devant l ’autre. La surtension dépend alors logarithmiquement du courant global (Loi de Tafel). La Constuction d’ Evans permet alors de déterminer graphiquement le potentiel et le courant de corrosion Near the corrosion potential, the current varies linearly with the electrode potential. A polarisation resistance is defined. The Stern-Geary law gives then the corrosion current as a fonction of this resistance and Tafel coefficcients

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Galvanic coupling

• Coupling of 2 material with 2 different polarisation curves• Or of 2 zones of the same material exposed to different electrolyte compositions

A

B

V m

icor

V

LESS N O B LE

N O B LER

V m= m ixt potential

Icor= iA.S A = -iK .SK

i

2 materials electrochemically different exhibit different polarization curves when placed in tyhe same electrolyte. When these 2 materials are in contact each of the other, it results in an electrochemical coupling and the so called « galvanic corrosion ». Folowing the galvanic corrosion rules, the less noble material specialises in anodic reaction, then corrodes, while the nobler specialises in cathodic reaction, then is protected This is only a schematic view of galvanic corrosion problems and things may be more complex in real situations :We can give the example of corrosion due to contact between two different metals. In certain cases the most noble metal is protected but accelerates the corrosion of the less noble metal (galvanic corrosion). In other cases nothing happens, although the less noble metal remains passive in the considered environment, for instance aluminium joinery fixed by stainless-steel screws does not cause corrosion after disappearance of the protective polymer washer. We must also consider that bi-metallic corrosion is not limited to only the galvanic couple: an active metal in a given environment may lead to corrosion of a metal which should normally be passive in this environment, simply because the products of corrosion of the former acidify the solution!!!

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Galvanic Corrosion of Aluminium alloys

• Rest potentials measuredvs SCE in sea water (mV)

• Graphite +100 • SS 316 +10• titanium -100• Copper -150• Brass -200• Fe (Steel) -600• Al 2024-T4 -610• Al1050 -750• Al 7072 -880• Cd -800 • Zn -1100

3e-

Al FeAl 3+

3H+

3/2 H2

Cl-

1/2O 2+H 2O

2OH-

Lorsque deux métaux différents comme l ’Aluminium et l ’Acier sont mis en contact électrique dans un même milieu conducteur (ionique), ils forment une pile qui débite du courant en consommant le métal le plus anodique (voir schéma). Comme l ’Aluminium est anodique par rapport à la plus part des métaux usuels, il est souvent la victime de ces assemblages. La corrosion galvanique nécessite trois conditions : -deux métaux différents -un contact électrique(électronique) -un électrolyte conducteur (contact électrolytique) La suppression de l ’une de ces trois conditions supprime le phénomène. Bien que certains éléments comme le cuivre déplace le potentiel de l ’Aluminium vers les valeurs plus nobles, le choix de l ’alliage d ’Aluminium ne permet pas d ’éviter ce problème. Si l ’on s ’en tient a la série galvanique ci-dessus, on pourrait croire qu’il suffit de remplacer un assemblage aluminium (1050)/acier inoxydable par un assemblage aluminium (2024)/acier ordinaire, pour régler le problème. Malheureusement l ’expérience montre que si on diminue effectivement le courant de corrosion galvanique avec ce nouvel assemblage, on n ’augmente pas pour autant la pérennité de l ’assemblage. La solution utilisée remplace deux éléments résistants à la corrosion par deux éléments corrodables, ce qui est gagné sur la corrosion galvanique est perdu par l ’auto-corrosion des éléments du couple. L ’expérience montre que dans l ’air, l ’assemblage de tôles Aluminium par des vis en acier inoxydable est préférable (cf.. les assemblages de mâts en Aluminium avec vis inox. sur les voiliers). Par contre s ’il y a un électrolyte; cas des parties immergées par exemple, il faut trouver d ’autres solutions.

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The consequences of galvanic coupling

• The Galvanic corrosion– General rules: the electrochemical series– Examples– The effect of passivity– Breakdown of noble coatings. Local corrosion

• The Galvanic protection– Sacrificial anodes. Examples– Sizes and distances– Galvanic coatings

Remarks: 1) In most cases the most noble metal is protected but accelerates the corrosion of the less noble metal (galvanic corrosion). In other cases nothing happens, since the less noble metal remains passive in the considered environment, for instance aluminium joinery fixed by stainless-steel screws does not cause corrosion after disappearance of the protective polymer washer. 2) We must also consider that bi-metallic corrosion is not limited to only the galvanic couple: an active metal in a given environment may lead to corrosion of a metal which should normally be passive in this environment, simply because the products of corrosion of the former acidify the solution!!!

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Active and passive Iron

Passive (in H2SO4)Active (in HCl)

In some circumstances, the dissolution current drastically decreases in a potential range (passivity) whereas it remains large elsewhere (activity)

In very aggressive media (hydrochloric acid in the example), the anodic current (iron dissolution) increases continuously when the electrode potential increases In less aggressive media (sulphuric acid for instance) the polarisation curve of iron exhibit several different domains. First (active behaviour) the anodic current increases up to a maximum (The critical passivation current). Second, it decreases down to a very low intensity plateau (passive region) before increasing again for high potential (transpassive region). The onset of passivity in a potential range is due to the presence at the metal surface of a very thin but protective oxide layer (the passive film). This film is unstable at too high potentials (transpassive domain).

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Iron Chromium alloys

The figure above shows the typical behaviour of Fe-Cr alloys at different Cr contents in sulphuric acid (1M). The fall in passive current with the Chromium content increase is particularly noticeable up to 12% Cr which is generally considered as the practical limit for a steel to be stainless.

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The Passive film of stainless steelsThickness some 10 A° = some interatomic distances Typicallyt L=30 A° (for Fe,Cr, Ni, S.S...) Composition (oxi-hydroxide) oxidisables (Fe, Cr, etc...) Oxygen (O--) Water and its derived forms ( H20 , H30+ , OH--) Electrical properties F ~ 106 V/cm =10mV/Å Vf ~ 300 mV pour L=30 Å Ip ~1µA à 1nA/cm2

~10µm/an (si métal divalent)

The composition and the thickness (around a few nanometres) of a passive film can be measured by different surface analysis techniques. The film is rich in the most oxidizable elements of the alloy (in this case chromium). It is also hydrated and often heterogeneous (the parts furthest towards the outside being more hydrated than the inner part, closer to an oxide). In addition, the thickness and composition depends largely on the metallurgical history of the surface. They also evolve in time, generally ensuring an increasingly protective nature with regard to the different forms of corrosion.

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Certain materials, among which the chromium alloys, protect themselves by the formation of their own layer of corrosion in the form of a very thin protective film called a passive film.

These materials are described as "stainless", quite simply (but this paradox is only apparent) because their oxidation is fast enough and intense enough to inhibit any subsequent corrosion.

For these materials, the question of corrosion resistance no longer arises in terms of rate of dissolution, but in terms of the stability of the protective oxide.

Passivable materials: summary

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Some Remarks of practical interest

• Effect of the surface condition: Different surfaces conditions (2B, BA, mechanical polishing, chemical passivation,etc…) produce different film characteristics and corrosion resistance

• Aging of passive films. Passive films change with time (on periods of the order of days, weeks, months). Films generally grow, enrich in oxidisable elements and dehydrate (going from poorly protective hydroxides to more protective oxides). Localized corrosion resistance is generally improved

• Ion release troughout the film: it may significantly affect only the elements present in the film (Fe, Cr, in the case of stainless steel). The nickel of austenitic stainless steel can only be released in case of local film breakdown, leading to the corrosion of the base metal. This does not occur if the metal has been properly chosen (i.e. with a stable passive film).

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Destabilising elements and classification of corrosive environments

CORROSIONRISK

Quasi- neutral Acidic

Chloride free NO UNIFORMCORROSION

Chloridecontaining

LOCALISEDCORROSION

DANGER!!

MO

+ 2H+

H20

MCl+

+ Cl-

Acidity

Chlorides M++ + Cl-

The factors involved in the potential destabilisation of a passive film are the acidity (which consumes oxygen and hydroxil ions) and the chloride ions (or all other similar ions) which combine with the cations without forming a protective species. The chloridized acid environments are particularly dangerous. Neutral non-chloridized or quasi-neutral environments (by this we mean where the pH is greater than a critical pH called the depassivation pH) present no risk, the non-chloridized acid environments can lead to uniform corrosion of the surface by disappearance of the passive film if the critical current is higher than the capacity of the oxidising agent to consume the electrons produced. As we shall see later, the neutral chloridized environments (pH above the depassivation pH determined in the presence of chlorides) can lead to a local form of corrosion which only affects a small part of the surface, the remainder remaining passive. The relevant notion is the critical chloride content, but it is difficult to implement since many other parameters (such as the potential reached) may intervene and other quality criteria will generally be preferred.

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Passivity in acidic media:Typical polarisation curve

V

VtpVcrit

Passivity

Activity

Activity

Pre-passivity Transpassivity

icrit

i

On this typical polarisation curve, we can clearly distinguish 3 potential ranges: 1) The peak of anodic dissolution (also called activity range) with an anodic current which may reach several mA/cm²) 2) The passive range where the current is les than 1 µA/cm² and often of the order of 1 nA/cm² ) 3) The transpassive range where the dissolution current increases again. The passive domain corresponds to a quasi absence of dissolution, i.e. in practice non-oxidisability of the material (1 µA/cm² is approximately 10 µm/year). This slowing of the anodic dissolution is due to the presence of a thin film of oxide (a few nanometres) called passivating film or passive layer or PASSIVE FILM which considerably slows down the kinetics of the ionic transfer (by a factor often greater than 1000).

Diapositive 29

29


Passivation by an oxidising agent( e.g.: Fe+++→Fe++)

The electrochemical behaviour may differ in the presence of an oxidising agent other than the proton. In the example shown in the figure, the oxidising power is high enough for the intersection of the anodic and cathodic curves to be found in the passive range of potential (Point P). This situation occurs when the critical passivation current is below the current which can be supplied by the cathodic reaction. If not (for instance when oxidation is due to the single protons as described earlier), the system operating point is situated in the active domain and there is no protection by the passive film.

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30


The criticalpassivation current

A I

P

i

V

Cathodic

Anodic

Icrit

The left-hand figure shows how behave different alloys with increasing passivation currents, when immersed in an electrolyte containing an oxidising element Following the case, the cathodic and the anodic curves intersect in 1 or 3 points: For high critical currents, the 2 curves intersect in the active range (point A) which corresponds to a stable stationary state For low critical currents, they intersect in the passive range (point P) which also corresponds to a stable stationary state For intermediate critical currents, they intersect in 3 points: the active state (point A) which corresponds to a stable stationary state, the passive state (P) which corresponds to a metastable stationary state , and at point I, which represents an unstable state The critical passivation current represents therefore the capability of the steel to be to maintained steadily in the passive state . Finally, better than the corrosion current measured at free potential on the polarisation curve, the critical current constitutes the best quality criterion for the passivation aptitude of a stainless steel. The right hand figure shows the same thing that the left one, but for a single steel and different oxidising powers .

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31


Oxidising acids

• Nitric acid and other oxidising acids (such as phosphoric ..etc..)

encourages anodic reaction, but overall the cathodic one

• Then, adding HNO3 in sufficient amount in an electrolyte tends to

passivate and not to corrode the stainless steels

• This is why these acids are generally used for passivation

treatment of stainless steels

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32


The effect of pH (depassivation pH )(430 Ti in deaerated Na2SO4 1M)

pH0 2 4 6 8 10

-6.5

-5.5

-4.5

-3.5

-2.5 pHd

The effect of the pH can be measured by plotting the polarisation curves and by measuring the critical passivation currents in relation to pH. Above a certain pH, called the depassivation pH, the activity peak is no longer observed and the metal is therefore passive whatever the potential (provided it stays below the pitting potential as we shall see later, and of course provided we avoid the high potentials corresponding to the transpassive domain, or the decomposition of water, or the change in the degree of oxidation of certain cations).

Diapositive 33

33


Pitting in neutral chloride media(430 Ti in deaerated NaCl pH6.6)

In a chloride containig neutral environment, increasing the potential can lead to a sudden increase in the current long before the transpassivation potential range is reached. Micrographic examination after testing shows that local failure of passivity has arisen and that pitting has developed. The above polarisation curves have been obtained with a 430Ti steel in a NaCl aqueous solution with 6.6 pH. The pitting potential measured as described above diminishes linearly with the logarithm of the chromium content. It is also subject to wide scatter and this leads to the use of statistical methods. This scatter seems inherent in the pitting phenomenon, which appears to be probabilistic, at least insofar as initiation phase is concerned.

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34


Pitting in oxidising media I

K1 K2

AV

Vrest Vpit

Vredox1 Vredox2

The pitting potential is a quality criterion for the corrosion resistance

Si le milieu contient un oxydant permettant une réaction cathodique, le potentiel d ’abandon devrait s ’établir à lintersection de la courbe anodique déterminée à l ’aide du potentiostat et de la courbe cathodique (supposée connue) correspondant à la réduction de cette oxydant. Suivant le pouvoir oxydant de cette réaction (courbes cathodiques K1 ou K2), le potentiel d ’abandon s ’établira en dessus ou en dessous du potentiel de piqûration et il y aura ou non corrosion localisée. Le potentiel de piqûres est donc bien un critère de qualité pour la résistance à la corrosion par piqûres et les alliages ayant un potentiel de piqûre élevé sont ceux qui résistent le mieux à la piqûration.

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Acidic chloride containing media

• The fall in pH and the increase in the Cl- content reduces the passive range

Chloride acid environments cumulate trends to passivation failure. Diminishing the pH increases the critical current and increasing the content of chlorides brings down the pitting potential. This assembly considerably reduces the passivity range Such environments must really be considered constituting the most hazardous.

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36


Localized corrosion in neutral chloride containing solutions

solution

surface(passive) occluded

Corrosion zone

metal

A small local active region is surrounded by a large passive zone.This active region is partially occluded, then diffusion is slowered and

corrosion products accumulate. This strongly modify the local chemistry and agressiveness.

Local pH decreases continuously and Cl- concentration increases, which self maintains the active behaviour

CORROSIONRISK

Quasi- neutral Acidic

Chloride free NO UNIFORMCORROSION

Chloridecontaining

LOCALISEDCORROSION

DANGER!!

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The successive stages of localised corrosion

• Initiation (different specific modes)• Propagation (common mode)

– Chloride enrichment– Hydrolysis of dissolved cations– High propagation rate

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Chloride enrichment

• Les cations produits par la dissolution anodique sont peumobiles (car hydraté)s et restent confinés à l’intèrieur de la zone occluse, ce qui crèe un champ électrique orientévers l’extèrieur

• Entrainés par le champ électrique les anions majoritairesde la solution les plus mobiles migrent vers l’intérieur de la zone

• dans le cas d ’une solution chlorurée cela provoque un enrichissement local en Ions Cl- et dans certains cas la précipitation d ’un film salin (chlorure du métal de base)

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Hydrolysis of dissolved cations produces Local Acidification

This acidification is limited by the migration and the diffusion of reaction products outside the zone.The fall in pH and the increase in the Cl- content reduces the passive range

M++ + H20 → MOH+ + H+

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Large cathodic over anodic areas ratio producesHigh propagation rate

cathodic area SK cathodic area SK Dissolution Mn+

Anodic area SA<<SK

Anodic (A) and cathodic (K)Current intensity (I)and density (J=I/S)

IA +IK =0 IA =- IK = Icor

JA /JK =SK/SA >>1

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Localized corrosion:The different initiation modes

• The generic case: Pitting corrosion• Local modification of the environment: Crevice corrosion• Local modification of the material: Intergranular corrosion• Compound subjects: Stress corrosion cracking, Atmospheric

corrosion, others...

Although pitting corrosion constitutes the typical type of local corrosion, and certainly the moste investigated in the past, it is often not the major concern of the engineer when maintaining the integrity of a structure. This major concern is often associated with other factors which have an accelerating role in terms of corrosion priming. These factors are multiple and we give a few examples below: - geometrical factors: crevice corrosion, corrosion under deposits - mechanical factors: stress corrosion, fatigue corrosion, friction corrosion, etc., - metallurgical factors: intergranular corrosion (today considered controlled in most instances), etc… - complex situations where several phenomena interact and which often require the intervention of an expert if only to determine the various causes of corrosion.

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Instances of pittingCorrosion

in neutral chloride containing media

(AISI 304)

Observation of pits at 3 different scales. The last observation (right hand down corner) is performed using Scanning electron microscopy (X1000). It shows a typical view of a semi developped pit at the end of the mesoscopic stage:. A thin metallic film is still present and covers a part of the pit. Secondary pits are visible all around the main hole. The pit may either go on or repassivate if the thin "top" film breaks down

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The successive steps of the pitting process

• Initiation– Chloride ions adorption at the fs interface leading to

passive film breakdown (pit nucleation)• Metastable pitting

– Onset of local active conditions – Possible repassivation

• Stable pit growth – irreversible damage

A multistep mechanism for the onset of a stable pit (see fig.)

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The pitting potential as a criterion of quality

The figure above shows a typical potentiokinetic polarisation curve in a chloridized and deaerated neutral environment (i.e. without the possibility of consuming the electrons produced other than by the potentiostat). When we increase the metal-solution potential, we observe, in increasing order of potential: 1) A small anodic current corresponding to the stable passive condition. 2) A few current oscillations corresponding to metastable pitting (pitting is immediately repassivated after starting). 3) Above the pitting potential, a sudden increase in current corresponding to stable (and propagating) pitting. 4) If we stop potential sweeping from a certain anodic current and we then apply inverse sweeping, the return polarisation curve shows some hysterisis corresponding to the repassivation of the pitting which has already developed. The measured anodic current only returns to the measured passivation current on the outward sweep when the potential is largely below the pitting potential called repassivation potential. It is difficult to use the repassivation potential as a criteria for withstanding pitting since it depends to a large degree on the development of pitting which has repassivated, and therefore on the alloy and also on the experimental conditions (including the current for stopping the polarisation curves). If the environment contains an oxidizing agent allowing a cathodic reaction to take place, the abandon potential should be at the intersection of the anodic curve determined using the potentiostat and the cathodic curve (assumed known) corresponding to the reduction of this oxidising agent. Depending on the oxidising power of this reaction (K1 or K2 cathodic curve), the rest potential is established above or below the pitting potential, and local corrosion or otherwise could occur. The pitting potential is therefore clearly a quality criteria for pitting corrosion resistance and alloys which have high pitting potential are those which best withstand pitting.

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Relationship between the pitting potential and the number of pits

• The figure shows the correlation between the pitting potential and the number of pits

Close correspondence is observed between the potentiokinetically measured pitting potential and the number of pits after exposure to a chloride solution in identical and clearly defined conditions. Such a result confirms the role of pitting potential as a corrosion quality criterion in a chloride neutral environment.

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Pitting potentials of stainless steels

290 290330

400420

600

0

100

200

300

400

500

600

700

17Cr 17CrNb 17CrNbLS 17CrTi 304 316

V (m

V/E

CS)

The figure above shows typical pitting potentials measured on some stainless steels grades. It should be underlined that the pitting potential values are not inrinsic of the nature of the steels but strongly depend of the measurement conditions (electrolyte composition and temperature, scanning rate in the case of potentiokinetic experiment, specimen preparation and former ageing or passivation, etc…) The value above were obtained in the following conditions: Electrolyte : NaCl 0.02M pH = 6.6 Temperature : 23°C deaeration 2h ( bubbling 90% N2 + 10% d'H2) Electrochemical procedure: 15 minutes et rest potential, then potentiokinetic scan (100 mV/min) up to pitting reference = Saturated calomel electrode

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How to use Pitting potentials

Potentiels de piqûre des aciers inoxydables (dans NaCl désaéré sur surfaces polies)

= acier 430 (17%Cr) = acier 434 (17%Cr+1%Mo)

= acier 304 (18%Cr-10%Ni) ∆ = acier 316 (18%Cr+12%Ni +2%Mo)

É = acier très allié : 17%Cr+16%Ni+5%Mo+3%Cu)

The straight (red) line represents the rest potential of a sample immersed in aerated water. Alloys with poitting potential larger than this rest potential are acceptable. Others are not

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Metallurgical effects:The influence of the chromium content in an Fe - Cr alloy

• A: Steels with MnS (40 ppm S) • B:Steels without MnS (+0,4%Ti)

110

160

210

260

310

360

410

11 12 13 14 15 16 17 18

% Cr

V (m

V/E

CS)

B

A

It is not just the major oxidisable elements, such as Chromium, which have an effect on the pitting potential, via the protectiveness of the passive layer. For example, for an equal content in Cr, two FeCr steels have a pitting resistance which well depend enormously on their sulphur content (even in terms of just a few dozen ppm). This is due to the role played by the sulphur inclusions which encourage pitting initiation and possible passive film breakdown in their immediate neighbourhood. A steel containing manganese sulphides (relatively soluble in a chloride aqueous environment) has less pitting potential than the same steel to which we have added a few 0.1% Ti, thus leading to the formation of much less soluble Ti sulphides instead of the manganese sulphides.

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Scanning electron microscopy (x 3000)

Pitting on a manganese sulphide Pitting at the boundary of a Ti nitrideTi=0

Fe16Cr Mn=0.45% S= 40ppm

Ti=0.4%

Pitting and sulphide inclusions

The effect of sulphide inclusions on pitting is shown by the figure. It is also worth observing that the conditions needed for pitting to take place in a steel without Ti (therefore containing manganese sulphides) are much more severe (for example, in potentiostatic terms: higher potential).

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Pitting corrosion in practiceA few important comments

• Role of heterogeneities as corrosion initiation site

- Ranking of defects:geometrical faults,inclusions and precipitates.

• Effect of the metal surface condition

- production by an industrial fabrication process (surface geometry and chemistry, Possible Cr depletion….)

• Ageing effect of surfaces

- reinforcement of passivity

- detrimental effect of corrosion products

- dirt, cleaning, etc.

The compositional factors, although essential, are not the only determinant factors in pitting corrosion resistance. Geometrical defects (roughness, etc.) or mechanical defects (appearance of dislocations and slip planes) are doubtless in certain cases as important as the metallurgical heterogeneous factors such as inclusions and precipitates. In fact it can be considered that there is an entire hierarchy of defects, pitting intervening on the most severe of them or in its absence on the one ranked immediately beneath it. In addition, the surface condition as results from the fabrication process (mechanical polishing, 2D, BA, etc.) is extremely important, both due to the physico-chemical nature of the passive film which results and due to any modification in the underlying metal composition, even only its roughness. Lastly, the use conditions (in-service ageing) are also determinant. For the most severe environments, corrosion resistance of a stainless steel considerably improves with time, i.e. an ageing surface in subcritical conditions resists subsequent corrosion better. Nevertheless, in more severe environments, care must be taken to avoid the damaging effect of corrosive products, even external dirt.

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What is a crevice?

Corroded zone

Tige filetée

Ecrou de serrage

specimen

screw

wing

crevice

Artificial crevice system

Crevice corrosion takes place in a confined area (joint, anfractuosity, under an inert deposit, etc.) called crevices. The main characteristic of a crevice is the absence of easy convective exchanges with the outside. The corrosive solution penetrates in the crevice, possibly by capillarity. Due to the absence of convectiion and of sufficient diffusionin the crevice, the chemical composition can be very different in the crevice than outside In addition, it will be noted that the oxygen content (supposed here to be the majority oxidising agent in the environment) falls very quickly in the crevice leading to a differential aeration solution between the inside (anodic zone) and the outside (cathodic zone). Contrary to what occurs with ordinary steels, this coupling is insufficient to initiate crevice corrosion which occurs later by breakdown of the passive film (acid corrosion, when the pH falls below the depassivation pH, or pitting corrosion when the chloride content has become high enough). The left side figure shows an experimental device generating crevice corrosion. The resulting corrosion is shown on the right side figure

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Plate heat exchanger

Crevice corrosion under joint

Crevice corrosion at each contact pointbetween two plates

Grade : AISI 316 (1.4401) Using conditions : thermal exchange between molasses and chlorined juice Causes : there is an enclosed zone at each contact point between the two plates, and under the joint crevice corrosions appear. Solutions : avoid crevice configurations …. or use a grade more resistant to crevice corrosion (for example 1.4462 (Uranus 45N)).

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What happens in a

crevice?

M++→ H+Cl-

Cathodic reaction quickly exhaust oxygen in the crevice. Further cathodic reaction occurs then outside

Anodic dissolution across the passive surface, leads to an enrichment in dissolved cations

Chloride enrichment : Influx of the majority anion to compensate for the positive charges (electromigration)

Hydrolysis of cationsM++ + H20 → MOH+ + H+

⇒ Hydrochloric acidityThis acidification is limited by the migration and the diffusion of reaction products outside the zone.

The cations produced by slow dissolution through the passive film quickly enrich the crevice with positive electrical charges. An electrical field is then created from the crevice inside to the surrounding zone. The result of this is the electromigration of the cations from the crevice to the outside solution and of the majority negative species contained in the solution, i.e. the chloride ions, from the solution to the inside of the crevice. Finally, in the stationnary state, the crevice contains an amount of positive cations and of negative anions (chlorides) In addition, the cations are hydrolysed and the solution pH inside the crevice, regularly falls. The conditions are thus united for depassivation, either by acid corrosion in a hydrochloric medium when the pH falls below the depassivation pH Due to the increase in the chloride content pitting corrosion may also occur, when the piiting potential falls below the abandon potential. This pitting corrosion causes a catastrophic increase in the dissolution of cations in the crevice and there quickly occurs acid depassivation of all the film below the crevice.

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The Initiation of Crevice Corrosion

Acidic depassivationThe pH decreases with time inside the crevice. Passive

film breaks down when local pH becomes smaller than

the depassivation pH(uniform Corrosion)

Acidic Pitting Corrosion:The Chloride concentration increases with time, then the local pitting potential decreases

Passive film locally breaks down when Pitting potential becomes smaller than rest potential

pHdAcidity and chloride content progressively increase in the crevice, leading possibly to 2 depassivation mechanisms

Based on the geometry of the crevice, the kinetics involved in chloride enrichment and acidification can be calculated ( see figure ). An excellent correlation has been found in experimental conditions between the duration of actual incubation and the one deduced from the acidification kinetics. Invariably, the crevice pH falls as the cations content increases. This leads to corrosion starting after a certain period of time (in practice around a few days or months), a period which increases as the depassivation pH falls. The depassivation pH in a chloridized environment is therefore a quality criteria for the crevice corrosion resistance of stainless steels. To be complete, we should add a second criteria to take account of the possibilities of pitting beneath the crevice, i.e. the potential for pitting in acid chloridized environments, a notion which until now has been applied less.

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430

0,5

1

1,5

2

2,5

3

3,5 430439434304444316LURB645N52N

439434

304 444316L

URB645N 52N

pHd

Alloys

Depassivation pH of stainless steels

(in NaCl 2M)

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Pitting potentials of stainless steels in acidicmedia

NaCl 0.5M , pH 3

0

100

200

300

400

500

600

439 434 436 441 304 316 444

E1m

V/EC

S

23°C 50°C

439 = 1.4510 434 = 1.4113 436 = 1.4526 441 = 1.4509304 = 1.4301 316 = 1.4404 444 = 1.4521

23°C

23°C

23°C

23°C

23°C

23°C

23°C

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How to prevent crevice corrosion ?

- To use grades with appropriate depassivation pH and pitting potential in acidic media

- To clean up the installation with a periodicity smaller than the incubation time

- To use hydrophobic greases, galvanic protection, etc.. - But overall, to avoid crevices !!! The thinner are the

more hazardous

Since in an ideal crevice pH should irreversibly decrease and Cl- concentration irreversibly increase, the best way of preventing crevice corrosion consists in avoiding crevice corrosion conditions!! or if not at least dismantling the installations for cleaning at periodicities which are less than the depassivation time. However, in actual practice the situation is not as catastrophic as it might appear. Indeed, the severity of crevice corrosion (which governs the acidification kinetics) is associated with the possibility or otherwise of exchanges with the exterior. Fortunately, these exchanges always exist to a small extent (otherwise the water would not wet even the inside of the crevice and no corrosion would take place). Increasing the acidity and the chloride content are therefore limited by the diffusion factor. The result is that a material with a fairly low depassivation pH (and fairly high potential for pitting in a chloridized acid environment) will remain insensitive to crevice corrosion (for a given crevice geometry).

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Intergranular Corrosionof stainless steels

Carbide Precipitation and Chromium

depletion

The most usual occurrence of intergranular corrosion is due to the precipitation of Cr carbides at grain boundaries (location of preferential precipitation in the metals) as a result of heat treatment such as occurs for instance on cooling a metal product or in the neighbourhood of a weld. If this formation of Cr carbides is not followed by homogenization treatment, there will be a depletion of Cr for the formation of the carbides in the surrounding area (for instance in the event of cooling too fast to allow diffusion from the surrounding metal). This chromium depletion may lead to a very low content locally in the steel which is below 12%. Its resistance to all forms of corrosion then falls.

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Intergranular Carbides precipitation

and the chromium depletion

Intergranular corrosion occurs as a result of a change in the composition of the steel at the grain boundaries, a phenomenon which makes the steel more sensitive to several types of corrosion (pitting, stress, etc.). The most usual occurrence of intergranular corrosion is due to the precipitation of Cr carbides at grain boundaries (location of preferential precipitation in the metals) as a result of heat treatment such as occurs for instance on cooling a metal product or in the neighbourhood of a weld. If this formation of Cr carbides is not followed by homogenization treatment, there will be a depletion of Cr for the formation of the carbides in the surrounding area (for instance in the event of cooling too fast to allow diffusion from the surrounding metal). This chromium depletion may lead to a very low content locally in the steel which is below 12%. Its resistance to all forms of corrosion then falls.

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IGC Testing

Effect of the Cr content in acidic mediaThe different normalized tests

To qualified the intergranular corrosion resistance of a steel, we use tests in an acid environment in order to check whether or not there is a continuous chromium depleted zone at the grain boundaries. The acid environments do not of course correspond to the conditions encountered in actual use and the tests must always be realignment to correspond to actual experience. However, it may be considered that the results of the IGC standard tests now show excellent correlation with the chemical analysis of the material, to the extent that their use becomes practically pointless.

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Several secondary tubes are welded on a main tube

No post welding treatment was performed. Welds were then sensitised to intergranular corrosion, due to intergranular precipitation of chromium carbides in the heat affected zones

The corrosion looks locally« uniform »

all around thetube

weldments

Intergranular Corrosion of weldments : an example

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Heat Affected Zone

close to the weldments

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IGC:The remedies

• Heat Treatments– solution treatment– annealing after welding

(rarely used)• Low Carbon Steels:

C<0.03%– (304L, 316L , etc ..)

• Stabilised Steels– Stabilising elements : Ti,

Nb, Zr.– Ex: Ti stabilised steel

(grade 321) : Ti/C > 5 ?Maturation heat treatment

• Impossible solutions– Solution treatment– low carbon

• Possible solutions– annealing after welding– Stabilisation (Ti, Nb, Zr)

• Ex: Titanium stabilised steel

• Ti>0.15% +4(C+N)• Other effect: sulfur

trapping ⇒improvement of the pitting resistance

Austenitic Steels Ferritic Steels

There are several metallurgical solutions for overcoming the IGC (see table above). They cover either the composition of the steel or the heat treatments used after sensitisation.

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Stabilized Ferritic Steels(example of a Fe17CrTi steel)

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Stress corrosion cracking

The association of mechanical stress and corrosive conditions:may lead to damage which neither the mechanical stress nor the corrosion would have caused separately.

Conditions of occurrence:hot chloride containing environment.

Remedy: selection of appropriate grades. For instance, the use of austeno-ferritic or ferritic steels is preferable to FeCrNi austenitic steels.

Stress on corrosion is a complex phenomenon. Its concrete manifestation is the appearance of damage which neither the mechanics nor the science of corrosion taken on their own could have generated. In simple terms, it may be said that stress corrosion can appear in hot chloride containing environments, and that in situations where there is an element of risk, the use of ferritic stainless steels (body centred cubic), or austeno-ferritic steels, is much more preferable than the use of austenitic steels (face centred cubic). Several mechanisms are envisaged for taking account of the different observations. By simplifying to the extreme, it may be considered that stress causes mechanical failure of the passive film which then creates a zone of active dissolution which evolves like a pit. Stabilising this type of corrosion will then depend on this supposed competition between local failure of the film (emergence of the slip plane) and its re-passivation or otherwise in a chloridized environment. Hence, we can envisage an interaction between the failure generated by residual dissolution through the passive film and the dislocations present in the metal. Lastly, once local corrosion has begun, the behaviour at the bottom of the crack is doubtless driven by the evolution of the hydrogen produced by the cathodic reaction, or its interactions with the dislocations present in the metal.

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Examples of atmospheric Corrosion features

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Role of the corrosion expert

To identify the type corrosion- multiplicity of the initiation modes- actual systems and complex cases. Example of corrosion beneath deposits.

To analyse the in-service conditions- familiarity with the environment and the operating sequences- expertise, recommendations, prevention.

The need to understand- research into corrosion- correct and incorrect use of the corrosion tests

In this paper we have summarily described a limited number of ways in which local corrosion begins. In actual fact, there are many reasons for corrosion starting and they sometimes combine. For instance atmospheric corrosion, which from a distance resembles pitting corrosion, is in fact a very complex phenomenon. Similarly, corrosion beneath deposits (a reason for proper cleaning) is different depending on whether the deposit is chemically inert (in which case similar to crevice corrosion) or active. In all cases, careful analysis of the service conditions (composition of the corrosive environment, temperature, etc.) and the sequences involved in operation of the considered system are essential, including the phases which are often neglected such as cleaning, non-use of the installation, etc.) or long-term trends (climatic cycle in atmospheric corrosion, road or other urban use for a vehicle). Such analysis is essential when carrying out an assessment but also for any recommendations (an exhaustive examination of the predicted conditions of use is then necessary) and often permits preventive measures to be adopted. The critical analysis should also cover the tests used in the laboratory to qualify the solutions adopted and their representativity. This work is not always performed correctly. Indeed it is always necessary to question the relevance of the tests, even when accelerated. The absence of such a procedure will result in illusory and economically unreasonable solutions. Lastly, the constant and necessary reduction of industrial costs requires the choice of stainless products exactly matching a given use with minimum "over qualities". In order to avoid a concomitant reduction in safety, a better understanding of the corrosion mechanisms will always be needed. This justifies both the need for constant research into the science of corrosion and the existence of a body of corrosion engineers responsible for providing the link between this research and industrial practices.

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