Electrochemical studies of the pitting of austenitic stainless steel

Electrochemical impedance and noise measurements have been obtained both below and above the pitting potential on 316 stainless steel immersed in 3% NaCl solutions. The data are interpreted using an extended model of film rupture and repassivation which incorporates both stochastic processes and modified kinetics of film formation and metal dissolution. Pit initiation is considered as an electrocrystallization process with the chloride ions playing a major role by complex formation with the adsorbed intermediates produced on the metal surface. Pit growth and arrest is essentially seen as kinetic competition between the formation of non-passivating species (MOMOHCl)_ads, (MOMCl)_ads and passivating species [MOOH]_ads, [MOMOH]_ads. The pitting model proposed incorporated aspects from well-established theories. Passive film rupture is considered as a normal occurrence which increases in intensity with increase of potential and/or aggressive ion concentration. The electrostrictive model, involving chloride ion adsorption on the outer film, is therefore appropriate but the crack-heal process is stochastic. Progression from initial film rupture to pit propagation is also controlled by the potential, the chloride ion concentration and diffusion both within the initial crack and the growing pit. With increase of potential the influence of adsorbed intermediates on the metal dissolution kinetics becomes increasingly important.     

Sequence of steps in the pitting of aluminum by chloride ions

Corrosion pit initiation in chloride solutions is given by an electrode kinetic model which takes into account adsorption of chloride ions on the oxide surface, penetration of chloride ions through the oxide film, and localized dissolution of aluminum at the metal/oxide interface in consecutive one-electron transfer reactions. A previous model has been extended here to consider that penetration of chloride ions can occur by oxide film dissolution as well as by migration through oxygen vacancies. Pit initiation occurs by chloride-assisted localized dissolution at the oxide/metal interface. The electrode kinetic model leads to a mathematical expression which shows that the critical pitting potential is a linear function of the logarithm of the chloride concentration (at constant pH), in agreement with experiment. The model also predicts that the critical pitting potential is independent of pH (at constant chloride concentration), also in agreement with experiment. Corrosion pit propagation leads to formation of blisters beneath the oxide film due to localized reactions which produce an acidic localized environment. The blisters subsequently rupture due to the formation of hydrogen gas in the occluded corrosion cell. Calculation of the local pH within a blister from the calculated hydrogen pressure within the blister gives pH values in the range 0.85 to 2.3.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

The protection potential of aluminium

The corrosion behaviour of aluminium is investigated in chloride solutions at potentials below the critical pitting potential. Potentiodynamic and steady-state polarization data are presented and analyzed. Pit propagation is shown to continue as the applied potential drops below this critical value. The dissolution front shifts from macroscopic pits to more occluded, microscopic sites, which are known to branch out from the main pits in the form of crystallographic pits and tunnels. The protection potential is shown to be ca. 100 mV more active than the critical pitting potential and appears to be independent of the extent of pit propagation within the limits of experimental error. Polarization data are interpreted in terms of metal dissolution kinetics and diffusion. Results are compared to the available data for steel.     

The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more negative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more regative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

Kinetics of Pit Initiation at Passive Iron

Pit initiation at passive iron in borate and phthalate buffer solutions was investigated by separately measuring dissolution rates of Fe(III) and Fe(II) at rotating ring-dis electrodes. The rate of Fe(III) dissolution grow linearly with the chloride concentration. Fe(II) appears simultaneously with the first pits. At a constant electrode potential positive to the critical pitting potential a stage of pit initiation is observed ending at the time t_i of incubation after addition of chloride to the solution. The time t_i is independent of the time of prior passivation. The potential dependence of t_i described by a relation known from the theory of two-dimensional nucleation. The physical meaning of this formal analogy is discussed. According to the dependence of t_i on pH and chloride concentration the observed mechanism of pit initiation is possible only above a critical chloride concentration c=0.3 _mM and below a critical _pH 10.4. The absolute values of critical pitting potentials and the dependence of the critical pitting potentials on chloride concentration were found to be a function of the nature and the concentration of the supporting electrolyte. The influence of the supporting electrolyte on the time of incubation was also investigated.     

Real time pit initiation studies on stainless steels: The effect of sulphide inclusions

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.     

A Contribution to the Study of the Critical Pitting Potential of Oxide Covered Aluminum in aqueous chloride solutions

The critical pitting potential of oxide covered aluminum electrodes in aqueous chloride solutions has been investigated as a function of chloride ion concentration, temperature, solution pH and oxide film thickness. The steady state critical pitting potential decreases with chloride ion concentration and increasing solution temperature. Solution pH in the range 5 to 9 has no effect on the breakdown potential. The role of the oxide film thickness is to slightly increase the critical potential for passivity breakdown and intiation of pitting. It is postulated that at the critical pitting potential, passivity breakdown occurs by a process of field assisted Cl adsorption on the hydrated oxide surface and formation of a soluble basic chloride salt with a lattice cation which readily goes in solution. This process of localized dissolution of the hydrated oxide film via formation of a soluble basic aluminum chloride salt, once initiated, is likely to continue in an “autocatalytic” fashion until the oxide is locally “penetrated” and dissolution of the substrate metal begins.     

A micro-electrode study of the nitrate effect on pitting of stainless steels

Stainless steel micro-electrodes have been used to measure the effect of nitrate on pitting dissolution in sodium chloride solutions. No inhibiting effect of nitrate on active (film-free) dissolution is observed, even when the metal salt solution in the pit is supersaturated. However, nitrate causes abrupt passivation during diffusion-controlled dissolution across a salt film formed at higher potentials. The passivation potential of the salt-covered surface is highly reproducible and decreases with increasing ((NO₃⁻/(Cl⁻) ratio. This behaviour is probably related to redox reactions or electrochemical reduction of nitrate within the salt film, coupled with an increase in the pH of the salt environment with potential; a related observation in pure NaCl solutions is that local passivation-reactivation events occur under the salt film above a critical potential.     

The pit initiation mechanism on passive iron under both open-circuit and controlled potential conditions

The pit initiation mechanism on passive iron has been studied under open-circuit conditions and under anodic and cathodic polarization. In each case, an indispensable condition for pit initiation is that the passive film is thinned before pitting occurs, and at the most weak points on the passive film arises pitting. Under open-circuit conditions and under cathodic polarization, the pitting potential is determined by a coupling of the cathodic reduction of the passive film and the anodic dissolution of bare iron. The induction time for pit initiation under anodic polarization conditions is the period necessary for the passive film to reduce to a minimum thickness. The probable process for this is that the chloride ion detaches the iron ion from the passive film to form a chloride-iron complex ion.     

Corrosion relationships as a function of time and surface roughness on a structural AE44 magnesium alloy

Pit initiation, growth, and coalescence corrosion mechanisms of an AE44 magnesium alloy subjected to a salt-water environment were quantified. Stereological quantities were evaluated using optical microscopy, scanning electron microscopy, and laser beam profilometry. Three corrosion mechanisms clearly arose: pitting, intergranular, and general. Pitting began as the result of localized galvanic dissolution between the intermetallics and magnesium matrix. Intergranular corrosion arose as pits coalesced. General corrosion arose by dissolution and regeneration of a Mg(OH)₂ film at a continuous rate. Stereological quantification demonstrated that the corrosion pit number density and pit radius size distribution initially increased before decreasing due to pit coalescence.     

Lochkorrosion stickstofflegierter austenitischer CrNiMnMoN-Stähle in 3%iger NaCl-Lösung

Pitting corrosion of nitrogen alloyed austenitic CrNiMnMoN steels in 3% NaCl solution Nitrogen containing austenitic CrNiMnMoN steels investigated electrochemically in chloride containing aqueous solutions exhibit pitting corrosion susceptibility which may be attributed to the materials conditions after solution annealing and work hardening. The range of passivity of high chromium steels goes up to a potential of E ≈? 1300 mV_{H H}, but beyond the limiting potential E_L for stable pitting there may be pitting phenomena on the rolled surfaces of the specimens. At potentials between E ≈? 300 mV_H and E_N various current density peaks appear and indicate the range of repassivable pitting in terms of pit formation on the cutting edges of the specimens. After cold rolling of the sheet the current density is increased in the entire potential range, since the pit density cutting edges and rolled surfaces increases as deformation is increased. Such cold working, however, does not result in a shift of the limiting potential E_L for stable pitting. Investigations concerning the place of formation of the pits indicate that nuclei are preferentially formed at the sites of sulfide inclusions the different shapes of which produce pits of corresponding appearance on the different faces of the specimen. The growth of the pit is influenced by the depth of the pores resulting from the dissolution of the inclusion, and by lattice defects in the metal.     

Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate

The effects of dilute sulphate on metastable and stable pitting of 304 stainless steel in chloride solution have been studied. The presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, implying that pit nucleation is inhibited. Pit propagation, in both the metastable and stable states, is also inhibited by the sulphate ion. The reduced pit propagation current densities are described quantitatively with respect to the effect of sulphate on the solubility of the metal cation in the pit anolyte. The results are consistent with the observation that metastable and stable pits grow under diffusion control, at a rate which is independent of electrode potential. Pit nucleation and propagation in stainless steel are two distinct processes, of which only the former is directly affected by the potential.     

Neue Untersuchungsmethode zur Bestimmung der Lochwachstumskinetik – Resultate an Aluminium

New method for the determination of pit growth kinetics – Results on aluminium A new method to study the pit growth kinetics is proposed. On metal foils with an appropriate detecting system on the backside of the specimen the perforation time of a growing pit is easily measured. Using metal foils of different thicknesses the pit growth kinetics can be investigated for various metal/potential/environment-conditions. The results obtained on aluminium show that the pit growth rate is time dependent. It is also markedly influenced by the applied potential and the chloride concentration of the electrolyte. Furthermore pit growth limiting potentials have been determined by electrolyte exchange experiments for various chloride concentrations. Below them pit growth is not possible. Comparing these values with the potentiostatically determined pitting potentials it can be concluded that the pitting potentials of aluminium depend primarily on pit growth rather than on pit formation.     

Effects of applied potential and solution temperature on the pitting corrosion of pure aluminium in sulphate ion-containing chloride solution

Effects of applied potential and solution temperatureT _s on the pitting corrosion of pure aluminium (Al) were investigated in 0.01 M NaCl solutions containing various sulphate (SO₄ ^2-) ion concentrations using a potentiodynamic polarisation experiment, the potentiostatic current transient technique, ac impedance spectroscopy and atomic force microscopy (AFM). The potentiodynamic polarisation curves showed a rise in the pitting potentialE _pir values and a simultaneous increase in anodic current density at potentials much higher than theE _pit value as the SO42~ ion concentration increases. This implies that (SO₄ ^2-) ions impede pit initiation at potentials belowE _pit but enhance pit growth aboveE _pit. This was confirmed from the larger pit growth rate parameterb values of pure Al exposed to (SO₄ ^2-) ion-containing chloride solutions during the abrading action than those exposed to (SO₄ ^2-) ion-free chloride solution. Furthermore, at 7_s=25°C, the charge densityQ values for the Al metal dissolution in the presence of (SO₄ ^2-) ions were smaller than the value in its absence. By contrast, as validated by the capacitance values and the AFM images of the re-anodized specimens, an enhanced metal dissolution was observed in (SO₄ ^2-) ion-containing chloride solutions at 7_s=60° and 80°C. From the experimental findings, it is suggested that (SO₄ ^2-) ions act as inhibitors of pitting corrosion on pure Al belowE _pit and at 7_s=25°C, whereas they act as promoters at 7_s=60 ° and 80°C. This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive Cl ion attack, which occurs at potentials aboveE _pit     

On the Initiation process of pitting corrosion on austenitic stainless steel in chloride solutions

A literature survey has lead to the conclusion that a theory which postulates an increased anodic reactivity on a local site in the passive film is very probable. Experiments have been set up to confirm these suggestions. By means of the electron-microanalyser, it is shown that CI^?ions are preferentially adsorbed at singular points at the metal surface before the stage that pits can be observed. It has also been demonstrated that pH changes occur at local areas during the induction period. These two observations indicate that corrosion already occurs during the induction period. Induction time measurements have shown that the induction time is not very reproducible. The quantity of transferred charge per initiated pit before the breakdown of the film is redly a better re- producible figure. From this, the quantity of Cl^?ions necessary to create an active site is calculated. Experiments with the static potential band method reveal that pits can initiate at any potential higher than the pitting potential. Growing pits can repassivate at any potential. A model for the initiation is given. The pitting corrosion process starts with adsorption of chloride ion at singular points, mainly local stress points. The local anodic current density will be higher and in this way favourable conditions (low pH, high Cl^?concentration) are created for the formation of a local site in the metal surface free of a passivating film The creation of those conditions is an autocatalytic process. The time required to form those favourable electrochemical conditions corresponds with the observed induction period. The migration of activating ions and the occurring pH change at a singular point must exceed a critical rate, otherwise passive film stabilizing effects will dominate. This model for the pitting corrosion supports the acid theory and links this theory with the peptization theory.     

Deterioration in critical pitting temperature of 904L stainless steel by addition of sulfate ions

This paper deals with the effect of adding sulfate on the critical pitting temperature (CPT) of highly alloyed austenitic stainless steel. A large number of potentiodynamic CPT measurements and potentiostatic current-time curves were obtained in 1 M NaCl containing 0, 0.2, 0.5 and 0.75 M Na₂SO₄. Provided the CPT is defined as the first temperature where stable pitting occurs at intermediate potentials, such as 600 mV (Ag/AgCl), addition of sulfate is shown to have the unexpected effect of lowering the CPT. The growing pits formed in sulfate-containing solution passivate anodically as the potential is increased, perhaps via salt precipitation. The effect of sulfate on pitting kinetics was studied using 50 μm-dia. 302SS wire in 1 M NaCl and 1 M NaCl + 0.5 M Na₂SO₄ at 40 °C. Sulfate increases the critical concentration of metal salt in the pit, expressed as a fraction of the saturation concentration, that is required to sustain pit dissolution. Provided this fraction does not exceed 100% of saturation, passivation is enhanced just inside the pit rim, allowing earlier undercutting of the metal surface and a finer pore structure in the lacy metal cover over the pit. The pitting potential measured above the CPT is increased by sulfate addition, but the CPT itself is lowered. Related examples are cited where pitting shows an unusual dependence on some variable such as anion concentration or temperature.     

Recent Developments in Pitting Corrosion of Aluminium

Abstract

This paper draws attention to the practical importance of pitting. The environmental and metallurgical factors controlling pit initiation and propagation are reviewed in the light of recent researches. It is proposed that pits start because of the presence of flaws in the oxide film exposing anodic and cathodic areas and that these pits proceed because the conditions within the pit favour dissolution of the metal. The importance of copper and chloride ions in solution is stressed, as well as the lesser importance of sulphate.