Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

Eine charakterisierung der lochfraßkorrosion des nickels—I. Die bestimmung der lochfraßpotentiale in neutralen und alkalischen lösungen

Quasistationary values for the characteristic pitting potentials for nickel were determined by means of potentiokinetic polarization measurements and their dependence on chloride and hydroxide ion concentration was investigated. The pit nucleation potential U_np is a linear function of the logarithm of the anion concentration. The value being determined by the adsorption equilibrium of the anions on the passivated metal surface.The critical pitting potential U_cp depends on the pH of the solution only. Above pH 6-5 U_cp decreases with increasing pH. On the basis of the mechanism for crevice corrosion this behaviour may be explained by the influence of OH^?-ions being known to take part in the ionization of the metal atoms.     

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.     

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

The corrosion and protection of steel in saturated Ca(OH)2 contaminated with NaCl

The corrosion of steel in saturated Ca(OH)₂ containing different concentrations of NaCl has been investigated at different potentials, oxygen contents and temperatures. The initiation of pits takes place only above the pitting potential. Repassivation of developed pits is shown to follow the threshold concentration for the initial corrosion step. Above the threshold concentration, existing pits can continue to grow at potentials below the pitting potential, and partial cathodic protection with a potential change of 50 mV is needed to arrest the corrosion. An explanation of the corrosion process is proposed. The corrosion rate is shown to be under anodic control. The growth of pits below the pitting potential is caused by the change in pH of the pit solution, which reaches a steady state regulated by the hydrolysis of the solution within the pit, and by the diffusion rate of hydroxyl ions into the pit from the bulk solution.     

Mechanism of copper action on pitting phenomena observed on stainless steels in chloride media

The influence of the addition of copper on the resistance to pitting corrosion of stainless steels has been investigated using different experimental techniques--current transient analysis, polarization curves in acidic media, pitting and repassivation potential measurements, XPS and SEM observations--so that pit initiation, propagation and repassivation could be analysed separately. Copper addition is shown to act in three different ways on pitting corrosion. On the one hand, copper reduces steel dissolution rates in acidic chloride media and also pit propagation rates. On the other hand, copper addition in steel is shown to lower repassivation potentials in neutral chloride environments and also to delay pit repassivation. Lastly, when copper is injected into solution as CuCl₂ or when the steel is polarized at anodic potentials so that copper can dissolve from the steel into solution, pit initiation close to sulfide inclusions is prevented. A model is proposed for these three different actions of copper, showing that the role of this element is complex and that no relevant information can be drawn from only considering its effect on the pitting potential.     

Microbiologically influenced corrosion of stainless steels – What is required for pitting?

Pitting of stainless steels in environments normally regarded as completely harmless is often attributed to microbial activity. In this paper, attention is drawn on one hand to the basic requirements for pitting of stainless steels to be possible, and on the other hand to various ways how microbial activity could contribute to a fulfilment of these requirements. For pit growth to be possible, three basic requirements must be fulfilled: 1) the environment must contain anions that can form an aggressive solution into the pit, 2) there must be a potential difference between the interior of the pit and the open surface outside the pit, 3) the temperature must exceed a critical value. The main factors that normally influence the possibility of pitting are the chloride content and the oxidising power of the environment, presence of anions other than chloride, temperature, possible presence of deposits on the steel surface, and the composition of the steel. Anions other than chloride in the bulk solution, including sulphate, usually have an inhibiting effect. Thiosulphate, however, is known to promote pitting under certain conditions. The possible ways of microbial activity to enhance pitting could include deposit formation leading to crevice type of attack, local modification of the composition of the environment to a more concentrated one, raising the electrode potential of the steel surface (“ennoblement”), or formation of reaction products that permit active dissolution inside a pit at lower potentials. Special attention is drawn to the possible action of thiosulphate by enhancing the anodic reaction at low potentials of the dissolving surface inside the pits.     

Crevice corrosion of stainless steels in sodium chloride solution

Nucleation of crevice corrosion of five stainless steels in NaCl solution has been studied using potentiokinetic and galvanostatic methods. It is inferred that a well reproducible critical potential for crevice corrosion nucleation exists. This potential depends on the type of steel and is more negative than the critical potential for pit nucleation. The difference between the potential for crevice corrosion and that for pitting is higher for more resistant steels than for less resistant ones. A mechanism explaining the crevice corrosion in chloride solutions is proposed.     

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.     

Pitting corrosion of intermetallic compound Ni3(Si,Ti) in sodium chloride solutions

The pitting corrosion of intermetallic compound Ni₃(Si,Ti) was investigated as functions of test temperature and chloride concentration in sodium chloride solutions by using a potential step method. In addition, the pitting corrosion of solution-annealed austenitic stainless steel type 304 and pure nickel was also studied under the same experimental condition for comparison. The pitting potential obtained for the intermetallic compound decreased with increasing chloride concentration and test temperature. A critical chloride concentration below which no pitting corrosion took place was found to exist and to decrease with increasing test temperature. The specific pitting potential at the critical chloride concentration also decreased with increasing test temperature. In addition, the pitting potential at various constant chloride concentrations above the critical chloride concentration decreased with increasing test temperature. The pitting potential of Ni₃(Si,Ti) was higher than pure nickel, but lower than that of type 304.     

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.     

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.     

Analysis of the galvanostatic polarization method for determining reliable pitting potentials on stainless steels in crevice-free conditions

In this paper the potential of the galvanostatic polarization technique as accelerated method for determining the characteristic pit potentials on stainless steels in crevice-free conditions is examined. Measurement of the potential change as a function of time shows a maximum that agrees with the nucleation pit potential. Thereafter, a stationary potential is reached corresponding to the protection potential against pit. Possible limitations of this kind of measurements have been remedied by refinements in the test procedure and conditions. The state of the surface oxide film and the applied anodic current are two basic parameters that must be well defined because they govern the pitting susceptibility. It has been found that with applied anodic currents in the range 40–200 μA/cm² and with prior electrode exposures to solution between 30 and 60 min it is possible to obtain results in excellent agreement with the conventional potentiodynamic tests with the advantage of smaller data scattering and absence of crevice at electrode/holder interfaces. These effects are the result of the rapid pitting stimulated in the galvanostatic method. This implies a short duration of the experiment thus also favouring the elimination of the time-dependent crevice, which notoriously contributes to the poor reproducibility of pit potentiodynamic potentials. A detailed series of experiments have been conducted on several stainless steels and in different test conditions to validate the accuracy of the galvanostatic polarization method.     

Pit incubation at passive iron-nickel alloys as a stochastic process

The distribution functions of pit incubation times were measured with homogeneous iron-nickel alloys containing 40, 60, and 90 wt% nickel, respectively, at different electrode potentials in borate buffer, pH 7.5, after addition of 50 mM sodium chloride. The empirical moments of the distribution function are the same as for iron. Considering a general distribution function of the exponential type with a time-dependent frequency function, the logarithms of its parameters all are linearly related to the reciprocal potential difference towards the critical pitting potential. The same relationship is also observed for the mean incubation times. At constant electrode potential the mean incubation times grow longer with increasing concentration of nickel in the alloy. The critical pitting potentials become more positive with the nickel concentration. The independence of the kinetics of pit incubation from the nature of the metal indicates a universal mechanism of pit nucleation.     

On the Determination of Pitting Potentials by galvanostatic polarization techniques

Various characteristic pitting potentials are defined which may be observed in a system simultaneously. Following these definitions, galvanostatic polarization techniques are expected to result in potential oszillations whenever the pit nucleation potential, U_np, differs from the critical potential for pit repassivation, U_cp, of a system. With nickel polarized in neutral, chloride containing sulphate solution the electrode potential oscillates periodically. The lower limit responds to the critical potential, U_cp, determined by potentiostatic measurements.     

Effect of Sulphur and Manganese on Nucleation of Corrosion Pits in Iron

Abstract

A study has been made of the effect of sulphur and manganese concentrations in iron on its tendency to pitting in a buffered potassium chloride solution. As revealed by electron microprobe and microscopic examinations, (Mn, Fe)S_x inclusions are the main sources of pit nucleation. Corrosion most frequently starts within the boundary region between the inclusion and the passive metal. Electrochemical investigations have shown that the critical pitting potential of the alloys under investigation, irrespective of S content, is lower than the corresponding value for ultra-pure iron. Mn has a dual effect on fhe resistance of Fe to pitting: it slightly increases the critical pitting potential, but it forms the sulphide inclusions at which the pits nucleate.     

Die Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo‐Legierungen in Chloridlösung – Einflußfaktoren und Ausmaß

Limit of pitting corrosion at high‐alloyed special steels and NiCrMo alloys in chloride solution The phenomenon of the limit of pitting corrosion in direction to positive potentials is studied by potentiokinetic polarization after a jump in the transpassive range and by potentiostatic tests at technical wrought materials and at model alloys of the systems NiCrMo and NiMo in CaCl₂ solution in the concentration range 1 to 9 mol/l chloride at pH‐values of 1 to 9 at temperatures of 30 to 110°C. Surface‐analytical investigations gives in connection with knowledges from anodic polarization studies directions to the mechanism of the limit of pitting corrosion. Ranges of the limit of pitting corrosion are obtained at materials with a Mo content above 6.5% and contents of chloride of the media above 2 mol/l chloride. Increasing temperatures, increasing contents of chloride and sulfate shift the potential of the limit of pitting corrosion being always above 0.2 V (SCE) at potentiostatic determination to noble direction. There are indications that the mechanisms of limit of pitting corrosion is resulting from an inactivation of pitting nuclei by the formation of hardly soluble molybdenum chlorides in the potential range of limit of pitting corrosion.     

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.     

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.