Electrochemical investigations of pitting corrosion

By using chronopotentiostatic and stepwise potential change experiments with potentiokinetic and galvanostatic testing, the following types of pitting corrosion of stainless steels in chloride-containing solutions have been investigated: sulphate inhibition of pitting; 35%Cr-Fe alloy; Cr-Ni-Mo stainless steels. The circuit resistance was found to be of fundamental importance. The pit passivation potential depends on the intensity of a corrosion attack before potential change only in the case of inhibited solutions. In uninhibited solutions passivation and formation potentials are nearly equal only in the case of potentiostatic circuit conditions. The Cr-Fe alloy and the Cr-Ni-Mo stainless steels show a potential range of repassivating pitting. Of practical interest is the critical potential of stable pitting which decreases with increasing circuit resistance. The beneficial effect of Mo is only valid for the pitting potential obtained potentiostatically and not at higher circuit resistances. Considering the practical meaning of the addition of Mo it may be concluded that this element is probably essentially connected with repassivation of pits and conditioning effects of the passive layer.     

Zu einem Phänomen der Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo-Legierungen in Chloridlösungen

On a phenomenon of the limitation of pitting corrosion at high alloyed special stainless steels and NiCrMo-alloys in chloride solutions Testing the pitting corrosion resistance of high alloyed special stainless steels and NiCrMo-alloys in chloride solutions there was observed a limitation of the pitting corrosion range toward more positive potentials. Above this limitation, the so-called pitting corrosion limitation potential, the pit initiation by all means is prevented, but the growth of pits which had been initiated before in the pitting corrosion range not necessarily comes to a stop. Therefore current density-potential curves which are obtained by downward polarization after an initial potential jump into the transpassive region and chronopotentiostatic tests are more suitable to investigate this phenomenon than cyclic polarization measurements and potentiostatic alteration tests. There is indicated a dependence of this phenomenon on temperature. The phenomenon of a limitation of the pitting corrosion range toward more positive potentials has been found until now at alloy 926, alloy 31, alloy 28, alloy 59 and alloy C-276.     

Elektrochemisches Verhalten und Deckschichtbildung hochlegierter Manganstähle

Electrochemical behaviour and scaling of high alloy manganese steels Passivating surface layers are considered to be one of the indispensable requirements for stress corrosion cracking of metallic materials. It is shown by potentiostatic and potentiokinetic current density-potential curves that the steel X 40 MnCrN 19 in neutral aqueous chloride solutions has a passive potential region. The passivation behaviour Of precipitation hardened samples is in agreement with the chromium depletion theory. The effect of alloying on the passivation behaviour of low carbon Mn steels is studied in 3 % NaCl solution at 20 and 100 °C Increasing proportions ε-martensite reduce the passivation of susceptibility. Increasing the Mn content has the same effect. The vital factor concerning passivation behaviour, however, is chromium content. Increasing the temperature of the corrodent results in an increased tendency to form scales of steels containing less than 8 % Cr. Long-term corrosion tests have shown, that increasing the Cr content produces a continuous transition from general localized and even pitting Corrosion. Tests made without applied current in aerated solutions have shown, that the variation in time of corrosion potentials depends from the tendency to be passivated of the materials and from the oxygen content of the solutions. In oxygen containing solutions passivable steels exhibit a pronounced corrosion in the pitting region, because with such alloys anodic dissolution current densities equal to those of the limiting diffusion current of oxygen reduction are obtained only at potentials above the pitting potential.     

Der Einfluß von Schwefeldioxid,Schwefelwasserstoff und Kohlenmonoxid auf die Lochkorrosion von austenitischen Chrom-Nickel-Stählen mit bis zu 4 Massen-% Molybdän in 1 M Natriumchlorid-Lösung

The Influence of SO₂, H₂S and CO on Pitting Corrosion of Austenitic Chromium-Nickel Stainless Steels with up to 4 wt. % Molybdenum in 1 M NaCl Active corrosion of chromium-nickel stainless steel X 5 CrNi 189 (AISI 304) in H₂SO₄ is stimulated by H₂S as well as by SO₂ (extension of the potential range of active corrosion, increase of the maximum corrosion rate in the active state and of the passivation current density), but is inhibited by CO (decrease of both maximum active corrosion rate and passivation current density). It is investigated whether likewise stimulating and inhibiting effects are valid also in case of pitting corrosion of austenitic stainless steels with molybdenum contents ranging from about zero (material no. 1.4301) to 4 wt. % (material no. 1.4449), tested in 1 M NaCl (ambient temperature) saturated with the gases mentioned above. The pitting corrosion behaviour of the materials investigated is judged by their pitting potentials measured by potentiostatically controlled experiments (testing time 24 hrs). The pitting potentials are compared with those measured in 1 M NaCl, N₂-bubbled. Pitting corrosion is stimulated by SO₂, CO and H₂S, with the stimulating efficacy increasing in the sequence given before. No stimulation is found only in 1 M NaCl, SO₂-bubbled,-with the highest Mocontent. In all other cases, stimulation of pitting corrosion increases with increasing Mo-content of the stainless steels. In coarse approximation, the critical limiting potentials of stable pitting in 1 M NaCl, bubbled with H₂S, SO₂ CO, correspond to the critical potentials of repassivating pitting corrosion found in N₂-bubbled 1 M NaCl. The chemical reactions and reaction products of SO₂ in aqueous solution are discussed. The nature of the stimulating component of the corrosive medium is not quite clear. The stimulating effects of SO₂ or one of its reaction products and of H₂S on pitting are in keeping with their stimulating effect on active corrosion of the steels investigated. The stimulating effect of CO, however, is in contradiction to the results expected and cannot be explained.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.     

Das potentialabhängige Repassivierungsverhalten der Titanlegierung Ti6A14V in neutralen Elektrolyten

The potential dependent repassivation behaviour of the titanium alloy Ti6A14 V in neutral electrolytes The potential dependent repassivation behaviour of Ti6A14V is very similar in aqueous sodium halogenide solutions (except NaF). Characteristic differences are found only at potentials in the vicinity of the particular pitting potentials. Comparative current-potential measurements have shown that it is possible from repassivation experiments to determine the pit passivation potential which is at 700 mV in NaCl and at 1200 mV in Nal. At potentials above–400 mV oxygen dissolved in the electrolyte has no effect on the repassivation processes; at lower potentials it has a bearing on the current; this effect is attributed to the oxygen reduction reaction. The method of investigation chosen for this particular case enables the measurement of current-potential curves on active Ti6A14V. The corrosion potential resulting therefrom is ?1100 mV (in the active state), the corrosion current density is about 10 mA/cm² (the corrosion current density for the passive material is about 10^?9 to 10^?10 A/cm²).     

Elektrochemische Untersuchung der Lochkorrosion austenitischer CrNi-Stähle mit rd. 0 bis 4 Gew.-% Mo unter besonderer Berücksichtigung der Meßverfahren und der Meßschaltung

Electrochemical study of the pitting corrosion of austenitic Cr Ni steels containing from about 0 to 4% Mo, with special regard to the measuring arrangement and method The authors have pointed out that the pitting potentials are no constants of the particular system, depending only on the material and the corrosion medium; it is always necessary to take account of the potential influences emanating from the measuring system and the particular experimental method. With respect to comparative tests the potentiostatic determination of U_b and the galvanostatic determination of U_p are particularly useful. In the case of short time tests the galvanostatically determined value of U_p yields a well defined limiting potential below which pitting occurs. This value, however, does not account for varying properties of Mo containing steels; in this case it is necessary to make potentiostatic experiments yielding inequivocal values of U_b which cannot be obtained reliably when potentiodynamic tests are carried out.     

Elektronenoptische und elektrochemische Untersuchungen Über das Passivverhalten von Feinblei in heißer konzentrierter Schwefelsäure

Electron-optical and electro-chemical investigation into the passive behaviour of fine lead in hot concentrated sulphuric acid The passivation of fine lead in hot concentrated sulphuric acid is exclusively governed by a lead sulphate layer. A stable passive condition can only be maintained within a narrow range of potentials. In the trans-passive zone (above + 600 mV), as with potentials below the passivation potential, a loose suphate layer is formed. In either case, pitting corrosion is liable to occur. Whether any compounds of tetravalent lead play a part in the passivation phenomena has not yet been fully clarified.     

Beitrag zur Methodik der Messung von Lochfraßpotentialen an austenitischen Chrom-Nickel-Stählen

Contribution to the methodology of pitting corrosion potential measurement on austenitic chromium nickel steels Reproducible values of the pitting potential and the incubation period are obtained only when definite techniques are applied to the specimen preparation. When the specimens are to be masked for the purpose of suspension they must be passivated without this mask because otherwise there would occur some type of crevice corrosion in the boundary zone between mask and open specimen surface. The corrosion potential in this is more negative (by 150 mV for a steel AISI 316) the the pitting potential as such. Preferential dissolution occurs in this zone when the pitting potential is attained. When, on the other hand, the passivation is effected without the mask, uniform pitting is achieved across the whole free surface and the pitting potential may be measured without difficulty. This potential turns out to be independent from the duration of passivation, while the incubation period distinctly increases with passivation time.     

Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.     

Untersuchungen über einen Lochfraßindikatortest an Chrom- und Chrom-Nickel-Stählen in chlorid- und bromidhaltigen Lösungen

Investigations into a pitting corrosion indicator test on chromium and chromenickel steels in solutions containing chloride or bromide An investigation has been carried out into the validity of the results of an indicator test for the examination of the pit corrosion proneness of stainless ferritic chromium steels and austenitic Cr-Ni and Cr-Ni-Mo steels in neutral solutions containing chloride, at room temperature. Suitable indicators are K₃(Fe(CN)₆) and equimolar mixtures of K₃(Fe(CN)₆) and K₄(Fe(CN)₆). The latter are preferable because their redox potential has a better constancy over time, because of the pH independence of this potential in neutral and alkaline solutions, and because of the position of the redox potential in the vicinity of the potential assumed by stainlèss steels in neutral aerated solutions. But the position of the redox potential depends on the concentration of the complex salts. The added indicatior inhibits pit corrosion at low chloride contents by shifting the pit corrosion potential. In diluted solutions, the indicator test is therefore only suited for comparative investigations on different steels, and not for the determination of the absolute value of the pit corrosion potential. Investigations carried out at higher temperatures show that, above 30° C, the age-resistance ranges of the steels examined are largely independent of steel composition and temperature. At room temperature, the well-known favourable influence of increasing molybdenum contents is apparent. In solutions containing bromide, pit corrosion was found to be generally less pronounced with solutions containing chloride.     

Zum Einfluß von Mangan auf die Korrosionseigenschaften von austenitischen 18.10-CrNi-Stählen

Influence of manganese on the corrosion properties of austenitic 18.10-CrNi stainless steels The influence of manganese in the range of 0.25 to 1.5 mass-% on the passivation and pitting corrosion behaviour of unstabilized and Tistabilized austenitic 18/10 CrNi stainless steels is examined by determination of useful characteristical electrochemical dates using potentiodynamical polarization measurements in H₂SO₄-acidic and neutral model electrolytes. In the case of Ti-stabilized steels, a trend to an improved ability to passivation and to an increased pitting corrosion resistance is signified with increased Mn-content. This is in agreement with the austenite stabilizing effect of manganese. In the case of unstabilized steels, a significant deterioration of the passivation and pitting corrosion behaviour is observed, if the Mn-content of the steel is increased from less than 0.7 to more than 1.0 mass-%. These observations are discussed in the viewpoint of segregation of Mn-rich sulfide inclusions in the steel, which are essentially influenced by the presence of titanium in the steel.     

Über den Einfluß der Austenitstabilität von 18/8-Chrom-Nickel-Stählen auf die Verformungseigenschaften und auf das Korrosionsverhalten dieser Stähle

Influence of austenite stability of 18-8 Cr-Ni-steels on the cold working and corrosion properties of these steels The martensite formed during cold working has a negative effect on uniform corrosion only when the specimen is active in the particular medium; if so, corrosion current density increases with the degree of cold working. The martensite generated by cold working reduces on the other hand the susceptibility to intercrystalline corrosion and – in amounts up tn 2% – increases the time to failure in stress corrosion cracking (15 and 45 kp/mm²). The pitting potential is not shifted by the martensite, but pit density increases with the martensite content. In the Kesternich test no negative effect of the martensite is found. It is therefore concluded that reducing the Ni content in 18-8 steels improves workability without having a pronounced deleterions bearing on corrosion behaviour.     

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.     

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.     

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.     

Die Bedeutung der metastabilen Lochkorrosion bei hochlegierten Stählen

The importance of metastable pitting corrosion in the case of high alloy steels A suitable computer-aided experimental method allows during potentiostatic tests to recognize and quantitatively treat current transients. The current transients result from metastable pitting phenomena below the pit propagation potential. It is possible under certain conditions to detect metastable pitting which in the SEM appears in the form of small (approximately 1 μm), in most cases hemispherical pits. A detailed study of metastable pitting has brought about fundamental knowledge about the mechanism of pit initiation, stable pit growth and repassivation in pitting and crevice corrosion processes.     

Die Inhibition der Aktivkorrosion nichtrostender Stähle in Säuren durch Kohlenmonoxid

The inhibition of the active corrosion of stainless steels in acids due to carbon monoxide The active corrosion of stainless austenitic chrome-nickel steels in H₂SO₄ is effectively inhibited by CO below the boiling temperature of the acid. The potential range of active corrosion is narrowed down, and the passivation current density as well as the maximum dissolution rate are reduced. In the boiling solution, pitting corrosion occurs in certain potential ranges, which can be reduced or completely prevented by increasing the Mo-content of the steels.     

Die Bestimmung von Lochfraßpotentialen unter besonderer Berücksichtigung der galvanokinetischen Meßmethodik

The evaluation of pitting potentials with particular consideration to the galvano-kinetic measuring method The galvano-kinetic method offers certain advantages with respect to the galvanostatic measurement. The last generally requires serveral potential-time curves to be determined at various current densities while the galvanokinetic measurement enables the pitting potentials to be determined by one curve only. The superiority of this method clearly appers in the case of stainless steels in chloride solutions containing nitrates as inhibitors: the inhibition which occurs in certain potential ranges may mask the pitting potential during potentio-kinetic method because of the slow potential feed rate enables these potentials to be determined without difficutly.     

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.