10-crevice corrosion and pitting

Crevice Corrosion and Pitting

Corrosion Engineering

ChE 291

We will discuss the following topics

Why study corrosion? An introduction

Basic concepts of corrosion science and engineering

Charged interfaces

Review of thermodynamic principles for corrosion

Electrochemical cells and Galvanic corrosion

Poubaix Diagrams

Kinetics of Corrosion

Concentration Polarization and Diffusion

Passivity

- Introduction; Electrochemical Basis

- Theories of Passivity; Surface Analysis Techniques for the Examination of Passive Films

- Properties of Passive Oxide Films; Passivity in Binary Alloys; Passivity in Stainless Steels; Anodic Protection

Crevice Corrosion and Pitting

- Crevice Corrosion Initiation and propagation; Testing; Area Effects; Protection

- Pitting Critical pitting potential; Effect of chloride ions, inhibitors, Mechanism of pit initiation and propagation;

protection potential; metastable pits and repassivation; Effect of molybdenum and sulfide inclusions;

temperature

- Occluded corrosion cells (OCC) on iron, aluminum, and copper; differences between pitting and crevice

corrosion

- Detection of corrosion pits

What is crevice corrosion? Localized corrosion that occurs within narrow clearances or under shielded metal surfaces; can occur in geometrical clearances (e.g. under gaskets or seals; between overlapping metal sheets; etc Can also occur under deposits (corrosion products; dust particles; barnacles in seawater)

Alloy 625 after crevice corrosion testing at 65C in natural seawater (a) +200 mV vs. Ag/AgCl for 24 h(b) +400 mV vs. Ag/AgCl for 24 h

The crevice was provided with a compressed gasket

How does crevice corrosion initiate?

Crevice corrosion initiates due to the operation of a differential oxygen cell Oxygen reduction occurs both on the metal surface which is exposed to the bulk

electrolyte and also on the portion of the metal surface which is contained within the crevice


Metal exposed to the bulk electrolyte is in contact with an open supply of oxygen from the atmosphere, so as O2 is consumed near the external metal surface

When O2 molecules are consumed within the narrow clearance of the crevice, they are not easily replaced due to the long narrow diffusion path formed by the crevice


Variation of the oxygen concentration within crevices on a Cr-containing stainless steel

Oxygen becomes depleted within the crevice Oxygen concentration cell is formed between the metal surface outside the crevice and the metal surface within the crevice


Metal exposed to the lower concentration of oxygen has a more negative potential for oxygen reduction than does the metal exposed to a higher concentration of oxygen

Limiting current density iL for O2 reduction within the crevice is decreased relative to that for the bulk solution; limiting current density is proportional to concentration


Electrode potential for metal within the crevice becomes more negative with time before leveling off


Establishment of steady-state potentials for open iron and iron within crevices in 0.6 M NaCl

Electrode potential for metal within the crevice becomes more negative with time before leveling off


Limiting cathodic density is less for the creviced iron than for open iron

Limiting cathodic rate is suppressed to a constant value for crevice heights between 0.13 and 0.50 mm

Crevice height of 0.50 mm ~ thickness of oxygen diffusion layer near the metal surface

When crevice height ~ thickness of oxygen diffusion layer, diffusion of oxygen into crevice is impeded; rate of oxygen reduction is decreasedCathodic polarization curves for open iron and

iron in crevices in 0.06 M NaCl


Evans diagrams for the crevice corrosion of iron

If iron within the crevice were not coupled to external open iron (an isolated crevice) then the corrosion rate would be given by icorr,1

When the crevice iron is coupled to external iron, the initial corrosion rate is given by the larger value icorr,2

How does crevice corrosion propagate? Propagates by changes in the electrolyte composition within the crevice Crevice electrolyte will become acidic in nature and will also contain concentrated amounts of cations discharged from the metal or alloy

Expressions for the pH of various crevice hydrolysis reactions

A local internal pocket of acidity develops within the active crevice even though the external electrolyte has a neutral pH of 7

Example 1

Chromium contained within a crevice of 0.5

cm2 area and 0.2 mm in height corrodes at the

rate of 1 A/cm2. What is the pH within the

crevice after 10 days if Cr corrodes as Cr3+, and

Cr3+ ions hydrolyze to form Cr(OH)3? Assume

that all Cr3+ ions produced by crevice

corrosion remain within the crevice.

Example 1 Solution

Amount of Cr3+ ions produced

Volume of the crevice

Concentration of Cr3+ within the crevice

How does crevice corrosion propagate?

With the accumulation of H+ ions and metallic cations within an active crevice, Cl ions then migrate from the bulk electrolyte to the crevice electrolyte in order to maintain charge neutrality

Propagation stage of crevice corrosion involves formation of a highly corrosive internal electrolyte which is acidic and also concentrated in chloride ions and dissolved cations of the metal or alloy

Initiation stage of crevice corrosion can be quite prolonged (months to years), but propagation may proceed rapidly due to the highly corrosive crevice environment which is formed

How to test for crevice corrosion?

A crevice corrosion electrochemical cell

(remote crevice assembly)

A split electrode is used in which the metal in the crevice is physically separated (but electrically connected) to the open metal outside the crevice; studies mechanism of crevice corrosion

Crevice corrosion current can be measured as a function of time for various parameters, such as crevice dimensions, alloy composition, and electrolyte concentration


Effect of chromate concentration on the crevice corrosion of iron in 0.6 M NaCl; Crevice height = 0.25 mm, area of crevice iron = 7.9 cm2, area of

outer iron electrode = 50 cm2

For a fixed chloride concentration in the electrolyte, increasing concentrations of chromate decrease the crevice corrosion rate

Crevice corrosion can be inhibited by using an appropriate minimum amount of CrO42inhibitor


Composition of CrNiMo alloys and of crevice electrolytes formed in natural seawater after immersion for 160170 days; pH measured in separate experiments using synthetic crevice electrolytes

As a result of crevice corrosion, the crevice electrolyte contained dissolved cations of each of the major alloying elements for each of the alloys

What are area effects in crevice

corrosion?

Rate of crevice corrosion increases with an increase in the area of open metal outside the crevice

Effect of the area outside the crevice on the weight loss of an Fe-17% Cr steel inside a crevice in natural seawater.

What are area effects in crevice

corrosion?

Polarization curve for the limiting cathodic current density for oxygen reduction intersects the anodic polarization curve of the creviced metal (in the propagating electrolyte) at increasingly higher values of current

How to protect metals from crevice

corrosion? Use of corrosion inhibitors, such as chromates, dichromates,

nitrites, or phosphates

Cathodic protection (using anodes located outside the

crevice)

Design considerations to minimize the existence of crevices

Materials selection: titanium and its alloys and Mo-containing

alloys such as alloy 625 or alloy C-276 are more resistant to

crevice corrosion (at ambient temperatures) than

conventional stainless steels; Presence of nitrogen is also

beneficial to crevice corrosion resistance in alloys which

contain molybdenum


corrosion?

Cathodic polarization of type 304 stainless steel in 0.05 mm crevices in 0.6 M; Numbers indicate the distance (in cm) from the crevice opening

Cathodic protection (using anodes located outside the crevice)


corrosion?

Continuous welds in (b) are preferable to intermittent welds in (a) because intermittent

welds introduce additional crevices



corrosion?

Discharge valves can be re-designed as in (b) to prevent crevice corrosion under deposits


What is Pitting?

Cross-sectional view of a corrosion pit on Al 6061 formed by anodic polarization in 0.1 M NaCl

Form of localized corrosion in which the attack is confined to a small fixed area of the metal surfaceDue to localized breakdown of a passive film, usually by chloride ionsPits can serve as sites to initiate stress-corrosion crackingPits can be covered by corrosion products

What is the critical pitting potential?

Schematic anodic polarization curve showing the critical pitting potential (for a passive metal)

In the absence of chloride ions, the metal retains its passivity up to the electrode potential of oxygen evolutionIn the presence of chloride ions, the passive film suffers localized attack, and pitting initiates at the critical pitting potential


Experimental anodic polarization curves for type 304 stainless steel in 0.1 M NaCl or in 1 M Na2SO4

SS remains passive up to E = +1.2 V (potential for oxygen evolution)In 0.1 M NaCl, SS pitting occurs at E = +0.35 V


Critical pitting potentials, Epit, in 0.1 M NaCl for various metals and alloys

Epit is a characteristic property of a given metal; value depends on [Cl-]More positive the value of Epit, the more resistant the metal to pit initiationE < Epit (pitting does not occur); E > Epit (pitting initiates and propagates)

How do we determine experimentally

the pitting potential?Usually determined from steady-state anodic polarization curvesPotentiostatic technique; a constant potential is applied and the current is recorded as a function of timeWhen E < Epit, current decays to a constant valueWhen E > Epit, current increases with time

Schematic illustration of experimental currenttime curves obtained by the potential step method in the measurement of the critical pitting potential

What is the effect of chloride ions

on the pitting potential?

Effect of chloride concentration (activity) on the pitting potential of aluminum and type 304 stainless steel

Critical pitting potential decreases (is less positive) as the chloride concentration increases; Epit is a linear function of log [Cl-]

What is the effect of inhibitors on

the pitting potential?

Effect of sulfate additions on the pitting potential of type 304 stainless steel in 0.1 M NaCl solutions

Addition of a corrosion inhibitor to a solution increases the pitting potential

What is the penetration

mechanism of pit initiation?

The penetration mechanism of pitting showing the competing processes of film rupture and film formation

Aggressive anions are transported through the oxide film to the underlying metal surface where they participate in localized dissolution at the metal/oxide interfaceRecent evidence from X-ray photoelectron spectroscopy and X-ray absorption spectroscopy that Cl ions penetrate passive films on both stainless steel and aluminumCl migration through oxygen vacancies (Cl ion only slightly larger than oxide ion

What is the film thinning


Film thinning mechanism of pitting in which chloride adsorption (a) initiates the process of film thinning (b) leading to pitting (c)

Aggressive ions (Cl-, Br-, or I-) first adsorb on the oxide surface (perhaps in clusters) and then form surface complexes with the oxide film which lead to local dissolution and thinning of the passive film

What is the film rupture


The film rupture mechanism of pitting

Chloride ions penetrate the oxide through cracks or flaws in the filmFlaws may further develop by hydration/dehydration events in the oxide film and by the intrusion of Cl ions into the filmThe three mechanisms are not mutually exclusive

What is the mechanism of pit propagation?

Schematic representation of the propagation stage of pitting

Similar to propagation of crevice corrosionWhen corrosion pit has been initiated, local current density is very high because the current is confined to a small active geometrical areaVolume increases during pit growth; dissolved metal cations are confined within the pit and do not diffuse out into the bulk electrolyte due to the confinement of a restricted geometry or a cap of porous corrosion products

Accumulated metal cations undergo hydrolysis

Local acidity develops within the pit

Cl ions migrate from bulk electrolyte to the crevice electrolyte in order to maintain charge neutrality within the pit solution

What is the mechanism of pit propagation?

Variations in pH and electrode potential across a growing pit on iron in a dilute chloride solution.

Interior of a corrosion pit is acidified; pH adjacent to the pit becomes alkaline because the cathodic reaction (reduction of O2 to OH) occurs on the passive surface outside the pitElectrode potential above the pit is more active (more negative) than above the areas adjacent to the pitWhen the corrosive pit electrolyte has been formed, pitting is autocatalytic

What is protection potential?

Concept arose from cyclic anodic polarization curves in which the scan direction was changed at anodic potentials beyond the pitting potentialAt Epit, the growth of active pits is diminished or possibly stopped (because the passive current density has been regained).

Schematic illustration of the pitting potential Epit and the protection potential Eprot. The arrows show the direction of polarization

What is protection potential?Alloys immersed under crevices in natural seawater for 4.25 yearsCyclic anodic polarization curvess determined in 3.5% NaCl solutions for open samples cut from the same specimens previously immersed

Crevice wt loss versus difference bet. Epit and Eprot for open samples

Amount of hysteresis for the open samples (Epit Eprot) correlates with amount of crevice corrosion (first cousin to pitting corrosion)

Less hysteresis in cyclic polarization curve, less amount of crevice corrosion

Protection potential is useful as an indicator of corrosion behavior

What is protection potential?Measured protection potential depends on the experimental conditions used in its determinationDepends on the amount of propagation which has taken place within the pitShould not be regarded as a material property but can be used to rank alloys accdg to their pitting behavior

Effect of extent of propagation on the value of the protection potential

What are metastable pits and

repassivation?

Idealized curvesMetastable pitting current transients

observed for type 302 stainless steel in 0.1 M NaCl at +0.420 V vs. SCE.

At electrode potentials below the pitting potential, the currenttime curves often contain transient excursionsMetastable pits - pits that grow for a limited time but are repassivated and stop growing; have a limited lifetime because the concentrated acidic chloride solution which promotes pit propagation has not yet developed within the metastable pit

What are metastable pits and

repassivation?

Effect of the concentration of H+ and Cl on the corrosion of open iron in concentrated solutions

Corrosion rate of pure (open) iron in concentrated acidic chloride solutions depends on both the chloride ion and hydrogen ion concentrationsOpen-circuit corrosion rate increased as the chloride concentration increasedSynergistic effect of H+ and Cl ions in promoting corrosion

How to establish experimental

Pourbaix diagrams for pitting?

Experimental Pourbaix diagram for iron in 0.01 M Cl (right) determined from experimental anodic polarization curves, as on the left

general corrosion only

a pitting potential and a protection

potentialgeneral corrosion or

passivity but not pitting

Regions of immunity, general corrosion, perfect passivity, imperfect passivity, and pitting; perfect passivity = pits do not nucleate or growImperfect passivity = previously formed pits can grow before they are repassivated at potentials at or below Eprot

What is the effect of molybdenum

on the pitting of stainless steels?

Effect of molybdenum on pitting potentials of 13% Cr and 18% Cr stainless steels in 1 M NaCl at 25C

Addition of molybdenum to Fe-Cr alloys increases the pitting potentialReasons: 1) Active sites are covered with molybdenum oxyhydroxide or molybdate salts; 2) Dissolution of Mo in the alloy produces molybdate ions (corrosion inhibitor); 3) Mo interferes with the kinetics of active dissolution at the base of the developing pit; etc

What is the effect of sulfide inclusions


Top view scanning electron micrograph of a sulfide inclusion in type 304 stainless steel

EDAX (energy dispersive analysis by X-rays) showed that the sulfide inclusion

contained Mn, Fe, Cr, and S

Sulfide inclusions, especially manganese sulfide (MnS), are known to be pit initiation sites on stainless steels

What is the effect of sulfide inclusions


Cross-sectional illustration of a MnS particle as a pit initiation site

Pits initiate at the edge of the sulfide inclusion and that anodic zones exist around the inclusionAnodic zones to be due to an area around the MnS particle which is depleted in chromium, and thus susceptible to localized attackPitting resistance of type 304 stainless steel can be improved by laser surface melting

What is the effect of temperature

on pitting potential?

The effect of temperature on the pitting potential of type 304 and type 316 stainless steels in a dilute chloride solution

At all temperatures, the pitting potential of the Mo-containing 316 stainless steels is higher than that of type 304 stainless steelPitting potential of each alloy decreases with increasing temperature

What is the effect of temperature

on pitting potential?

Critical pitting temperature and critical crevice temperature as a function of molybdenum

content for several different stainless steels having the nominal composition Fe-18 Cr-20

Ni-x Mo in 10% FeCl3

First signs of pitting occurred at the critical pitting temperatureThe higher the critical pitting temperature, the more resistant the alloy to pittingPitting resistance increases with the Mo contentCritical crevice temperature is lower than the critical pitting temperature

Critical crevice temperature islower than the critical pitting temperature. Severe conditions already exist within the occluded crevice in the FeCl3 solution, but must develop within a corrosion pit on an open surface

What are some methods of

protection against pitting?

Effect of alloy selection on the pitting potential in solutions of fixed chloride concentration

1) Maintain the electrode potential below (more negative, i.e., less positive than) the critical pitting potential.

2) Add inhibitors to raise the critical pitting potential.3) Metals and alloys which are resistant to crevice corrosion are also usually

resistant to pitting.

What are occluded corrosion cells?Active crevices, corrosion pits, and stress-corrosion cracks each develop local internal acidities even when the bulk electrolyte is neutral or alkalineA special restrictive geometry seals off an active local corrosion cell by limiting the exchange of local and bulk electrolytesAll three forms of localized corrosion are different geometric manifestations of the same general phenomenon of OCC

Schematic illustration showing geometric similarities between pitting, crevice corrosion, and

stress corrosion cracking

What is occluded corrosion cell

(OCC) on iron?

Pourbaixs sketch of a corrosion pit or stress-corrosion crack on iron

M. Pourbaix suggested that when pitting, crevice corrosion, or stress-corrosion cracking occurs on iron in chloride solutions, the solution within the active cavities becomes saturated with respect to ferrous chloride (FeCl2) and also contains magnetite (Fe3O4)Three solid phases = FeCl24H2O, Fe3O4, and Fe in aq. solution

What is occluded corrosion cell

(OCC) on iron?

The Pourbaix diagram for iron in a localized corrosion cell in a chloride solution

Equilibrium E and pH within the OCC are given at the triple pointE=0.368 V vs. SHE (0.590 V vs. Ag/AgCl) and pH 4.8These results have been verified in various experimental studies

What are occluded corrosion cells

on copper and aluminum?

Sketch of a copper pit in cold tap water

Pit contains a layer of green malachite, CuCO3 Cu(OH)2, white crystals of cuprous chloride (CuCl), and a loose deposit of red cuprous oxide, Cu2OSolution at the bottom of the pit is in contact with Cu, Cu2O, and CuClE = +0.326 V vs. SHE and pH 2.45 in Pourbaix diagram

What are occluded corrosion cells

on copper and aluminum?

Electrode potential and pH within occluded cells on aluminum superimposed on a partial Pourbaix diagram for aluminum. CC refers to crevice corrosion and SCC to

stress-corrosion cracking

Hydrogen evolution is thermodynamically possible in each form oflocalized corrosion in aluminumHydrogen bubbles have been observed for aluminum in active crevices

What are differences between

pitting and crevice corrosion?

Comparison of the pitting potential of open iron with the internal potential of iron within active crevices for various chloride concentrations in 0.003 M/L chromate

Electrode potential for crevice corrosion is more negative than the pitting potentialCurrent density within a corrosion pit is much higher than that within a crevice

How do we detect corrosion pits?

Variations in pH and electrode potential across a growing pit on iron in a dilute chloride solution.

We can detect propagating pits on a metal surface by pH and electrode potential scanning over metal surfaces Detect the physical location of corrosion pits on a metal surface

How do we detect corrosion pits?

Distribution of current density over an iron surface showing the location and progressive growth of a pit in a solution of 1 mM NaCl plus 1 mM Na2SO4, as determined using a vibrating probe electrode

Distribution of current density from sampling of potential gradients normal to the surface

is the conductivity of the solution

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