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 65◦C 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

How does crevice corrosion initiate?


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

How does crevice corrosion initiate?

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

How does crevice corrosion initiate?


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

How does crevice corrosion initiate?


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

How does crevice corrosion initiate?

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

How does crevice corrosion initiate?
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

Cathodic polarization curves for open iron and iron in crevices in 0.06 M NaCl


When crevice height ~ thickness of oxygen diffusion layer, diffusion of oxygen into crevice is impeded; rate of oxygen reduction is decreased

How does crevice corrosion initiate?

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 μA/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

How to test for crevice corrosion?
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 CrO4−2 inhibitor

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

How to test for crevice corrosion?
Composition of Cr−Ni−Mo alloys and of crevice electrolytes formed in natural seawater after immersion for 160–170 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

How to protect metals from crevice corrosion?
Cathodic protection (using anodes located outside the crevice)

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

How to protect metals from crevice corrosion?
Design considerations to minimize the existence of crevices

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

How to protect metals from crevice corrosion?
Design considerations to minimize the existence of crevices

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

What is Pitting?
Form of localized corrosion in which the attack is confined to a small fixed area of the metal surface
Due to localized breakdown of a passive film, usually by chloride ions
Pits can serve as sites to initiate stress-corrosion cracking
Pits can be covered by corrosion products

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

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

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

What is the critical pitting potential?
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

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

What is the critical pitting potential? 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 initiation
E < 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 curves
Potentiostatic technique; a constant potential is applied and the current is recorded as a function of time
When E < Epit, current decays to a constant value
When E > Epit, current increases with time

Schematic illustration of experimental current–time 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?
Critical pitting potential decreases (is less positive) as the chloride concentration increases; Epit is a linear function of log [Cl-]

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

What is the effect of inhibitors on the pitting potential?
Addition of a corrosion inhibitor to a solution increases the pitting potential

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

What is the penetration mechanism of pit initiation?
Aggressive anions are transported through the oxide film to the underlying metal surface where they participate in localized dissolution at the metal/oxide interface
Recent evidence from X-ray photoelectron spectroscopy and X-ray absorption spectroscopy that Cl− ions penetrate passive films on both stainless steel and aluminum
Cl− migration through oxygen vacancies (Cl− ion only slightly larger than oxide ion

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

What is the film thinning mechanism of pit initiation?
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

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

What is the film rupture mechanism of pit initiation?
Chloride ions penetrate the oxide through cracks or flaws in the film
Flaws may further develop by hydration/dehydration events in the oxide film and by the intrusion of Cl− ions into the film
The three mechanisms are not mutually exclusive

The film rupture mechanism of pitting

What is the mechanism of pit propagation?
Similar to propagation of crevice corrosion
When corrosion pit has been initiated, local current density is very high because the current is confined to a small active geometrical area
Volume 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

Schematic representation of the propagation stage of pitting

What is the mechanism of pit propagation?
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 pit
Electrode potential above the pit is more active (more negative) than above the areas adjacent to the pit
When the corrosive pit electrolyte has been formed, pitting is autocatalytic

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

What is protection potential?
Concept arose from cyclic anodic polarization curves in which the scan direction was changed at anodic potentials beyond the pitting potential
At 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 years
Cyclic anodic polarization curvess determined in 3.5% NaCl solutions for open samples cut from the same specimens previously immersed


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

What is protection potential?
Measured protection potential depends on the experimental conditions used in its determination
Depends on the amount of propagation which has taken place within the pit
Should 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?
At electrode potentials below the pitting potential, the current–time curves often contain transient excursions
Metastable 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

Idealized curves

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

What are metastable pits and repassivation?
Corrosion rate of pure (open) iron in concentrated acidic chloride solutions depends on both the chloride ion and hydrogen ion concentrations
Open-circuit corrosion rate increased as the chloride concentration increased
Synergistic effect of H+ and Cl− ions in promoting corrosion

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

How to establish experimental Pourbaix diagrams for pitting?
Regions of immunity, general corrosion, perfect passivity, imperfect passivity, and pitting; perfect passivity = pits do not nucleate or grow
Imperfect passivity = previously formed pits can grow before they are repassivated at potentials at or below Eprot a pitting potential and a protection potential

general corrosion or passivity but not pitting

general corrosion only

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

What is the effect of molybdenum on the pitting of stainless steels?
Addition of molybdenum to Fe-Cr alloys increases the pitting potential
Reasons: 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

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

What is the effect of sulfide inclusions on the pitting of stainless steels?
Sulfide inclusions, especially manganese sulfide (MnS), are known to be pit initiation sites on stainless steels

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

EDAX (energy dispersive analysis by Xrays) showed that the sulfide inclusion contained Mn, Fe, Cr, and S

What is the effect of sulfide inclusions on the pitting of stainless steels?
Pits initiate at the edge of the sulfide inclusion and that anodic zones exist around the inclusion
Anodic zones to be due to an area around the MnS particle which is depleted in chromium, and thus susceptible to localized attack
Pitting resistance of type 304 stainless steel can be improved by laser surface melting

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

What is the effect of temperature on pitting potential?
At all temperatures, the pitting potential of the Mo-containing 316 stainless steels is higher than that of type 304 stainless steel
Pitting potential of each alloy decreases with increasing temperature

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

What is the effect of temperature on pitting potential?
First signs of pitting occurred at the “critical pitting temperature”
The higher the critical pitting temperature, the more resistant the alloy to pitting
Pitting resistance increases with the Mo content
Critical crevice temperature is lower than the critical pitting temperature 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


Critical crevice temperature is lower 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? 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.

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

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 alkaline
A special restrictive geometry “seals off” an active local corrosion cell by limiting the exchange of local and bulk electrolytes
All 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?
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 = FeCl2·4H2O, Fe3O4, and Fe in aq. solution

Pourbaix’s sketch of a corrosion pit or stresscorrosion crack on iron

What is occluded corrosion cell (OCC) on iron?
Equilibrium E and pH within the OCC are given at the “triple point”
E=−0.368 V vs. SHE (−0.590 V vs. Ag/AgCl) and pH 4.8
These results have been verified in various experimental studies

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

What are occluded corrosion cells on copper and aluminum?
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, Cu2O
Solution at the bottom of the pit is in contact with Cu, Cu2O, and CuCl
E = +0.326 V vs. SHE and pH 2.45 in Pourbaix diagram

Sketch of a copper pit in cold tap water

What are occluded corrosion cells on copper and aluminum?
Hydrogen evolution is thermodynamically possible in each form of localized corrosion in aluminum
Hydrogen bubbles have been observed for aluminum in active crevices

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

What are differences between pitting and crevice corrosion?
Electrode potential for crevice corrosion is more negative than the pitting potential
Current density within a corrosion pit is much higher than that within a crevice

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

How do we detect corrosion pits?
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

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

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

σ is the conductivity of the solution