Lochfraßkorrosion des Eisens in alkalischen Chloridlösungen

Pitting corrosion of iron in alkaline chlorine solutions The author has studied the corrosion of iron in chloride solutions (0.4 M KCl) with hydroxyl ions (0.1 M KOH), used as a pitting corrosion inhibitor. The rate of pitting corrosion in alkaline media can be represented in the form of a cubic parabola whilst the number of pits is a linear function. It was found pitting corrosion depends on the potential, on the concentration, and on the ratio of the concentration of activating and passivating ions, respectively. Another important factor is the adsorption of the anions on the electrode surface. The author has formulated equations for all these relations. The depassivation potential proves to be a suitable criterion for characterizing the pitting corrosion proneness under different conditions.     

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.     

Über den Mechanismus der Lochfraßkorrosion bei Aluminium

On the mechanism of pitting corrosion of aluminum In an extensive study the dependencies of the pit growth of aluminum on time, potential and electrolyte conductivity have been quantitatively analysed. From this analysis it could be concluded that the pit growth kinetics is governed by the ohmic potential drop inside the pit. This explains the square-root time law experimentally found for the pit growth and the low activation energy of 3.14 kcal/mole. Moreover this analysis led to the conclusion that pitting is caused by a primary change of the properties of the pit surface due to the adsorption of chloride ions, since high local composition changes are prevented by the gas bubbles generated in the pits increasing the mass transport enormously, and since the total ohmic potential drop inside and outside the pits can not be larger than the difference between the actual potential and the pit growth limiting potential. Additional statements refer to the reaction order of the chloride ions, to the minimum chloride concentration for pitting and to the influence of pH and motion of the electrolyte.     

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.     

On the electrochemical conditions within small pits

The electrochemical conditions within a corroding pit on an otherwise passive metal surface are discussed and recent observations of very large current densities in corroding pits are used to propose that, at least before local corrosion product precipitation occurs, the same potentials exist in the electrolyte adjacent to the passive surface and to the pit surface. Those theories of pitting which assume potential shifts to the active region of the polarization curve to be essential to the pitting process are considered inapplicable, at least in the early stages of pit growth.     

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.     

Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy

Extruded AZ61 magnesium coupons were exposed to immersion and cyclical salt spray environments over 60 h in order to characterize their corrosion rates. The characteristics of general corrosion, pitting corrosion, and intergranular corrosion were quantified at various intervals. General corrosion was more prevalent on the immersion surface. In addition, more pits formed on the immersion surface due the continuous exposure to water and chloride ions. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surface, due to the dried pit debris that trapped chloride ions within the pits.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

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.     

Beitrag zum Mechanismus der Lochfraßkorrosion des Nickels

A contribution to the mechanism of the pitting corrosion of nickel The pitting corrosion of nickel has been studied as a function of time, pH, potential and concentration in various salt solutions. It has been revealed that the number of pits as a function of time depends from the salt concentration and this dependence may be linear or parabolic. The potential dependence of the number of pits, on the other hand, follows an exponential law. From the salts included in the present study the chlorides in particular give rise to pitting; their effect is in part rather stimulated by nitrates. Sulfates, too, affect the action of chlorides, but their effect is different, depending on their concentration On the basis of the experimental results the author discusses possible mechanisms of pit formation and pit growth.     

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.     

Dissolution Anodique D'un Acier Inoxydable En Solution De Chlorure De Sodium

A stainless steel wire was anodically polarized in a sodium chloride solution in order to obtain, simultaneously, two active zones, where the components of the alloy became differently oxidized. Two areas, one passive and the other active—as in pitting corrosion—were obtained by reducing the anodic current. The anodic polarization diagrams for each zone made possible a separate investigation of the electrochemical behaviour of steel in pits and on passive areas.     

Lochkorrosion an Nichtrostenden Stählen

Pitting corrosion of stainless steels Stainless steels can get pitting corrosion in halide containing solution, which make them a big risk in industrial production. Many investigations were made in the past in order to understand processes involved in pitting corrosion, pit initiation and pit growth. Results about the influence of alloying elements, their contents, the state of the structure, the condition of the surface, the content of chloride, the temperatures, the pH-value, the velocity of flow and of the oxidizer on the chloride induced pitting corrosion of passive stainless steels are presented. Electrochemical measurements and the application of surface analytical methods (SEM, SAM, XPS) with high lateral resolution are carried out. A part of the samples received a diffusion annealing in order to obtain reproducible results. Pitting Resistance Equivalents (PRE) – Pitting Index – with different multipliers are given and discussed critical. An electrochemical method for selecting materials without susceptibility to pitting corrosion are also presented.     

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.     

Effects of tritiated hydrogen peroxide in tritiated water containing Cl and CO3 on behavior of welded type S32550 duplex stainless steel

We report an investigation of the corrosion susceptibility of welded S32550 Duplex stainless steel in the presence of tritiated hydrogen peroxide, chloride and carbonate, which are found in radioactive aqueous solutions. It is well known that a structure transformation occurs during welding, this could lead to localized corrosion of the welded zone. The electrochemical behavior of welded S32550 steel was studied using cyclic voltammetry and electrochemical impedance spectroscopy to provide an indication of mechanisms and oxide layer modifications. Increasing hydrogen peroxide concentration produces several effects. Although the corrosion potential does not change, the prepassive current is higher and, depending on passive potentials and hydrogen peroxide concentration, the passive oxide layer and its characteristics change showing the importance of the radiolytic species in passivity. Also, the breakdown potential shifts towards more positive values on increasing ³H₂O₂ concentration. Thicker passive oxide layers should limit localized corrosion. Examination of the impedance spectra indicates ionic diffusion in the outer oxide passive layer and a diffusion barrier effect for the inner oxide. As the hydrogen peroxide concentration is further increased, these effects appear more pronounced. Carbonate ions should keep the alkaline buffer pH constant giving protection from localized corrosion, and ³H₂O₂ should enhance the characteristics of the inner passive oxide layer. Due to the effects of these two parameters: alkaline pH kept constant at the electrode surface and enhancement of the characteristics of the inner oxide layer by ³H₂O₂, no pitting is observed in presence of chloride ions. Also, an equation, giving the pitting potential limit is derived. The ability of the nucleation sites to propagate as metastable pits is limited by the presence of the ³H₂O₂ and CO²⁻₃ buffers.     

Zur Frage des Mechanismus der Lochfraßkorrosion

The question of the pitting corrosion mechanism It is concluded from the time, concentration, temperature, PH and potential dependences of the corrosion current densities in the pits, that it is not justified always to suppose parallelity between the metal dissolution rates in the active pits and in the active region of the polarization curve. This is due to the fact, that in some cases diffusion is not the rate controlling factor of pitting corrosion, and that the metal dissolution in the pits and in the active states follows different mechanisms.     

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.     

Crevice corrosion on 316 stainless steel in 3% sodium chloride solution

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.     

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.     

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.