M.B. Ives

Synergism of alloying elements and pitting corrosion resistance of stainless steels

The anodic behaviour of the major pure metals of which stainless steels are composed has been studied in chloride-containing aqueous solutions at neutral and acidic pH values, both before and after a nitriding treatment. This suggests a mechanism by which molybdenum and nitrogen favourably influence the pitting resistance of austenitic stainless steels, and that the mechanisms are different at different stages of pit formation. It is shown that nitrogen is able to prevent the transpassive dissolution of molybdenum from the passive film of austenitic stainless steels. X-Ray photoelectron spectroscopy has shown also that the NH3 ligand is formed in the anodically formed films on nitrided molybdenum as well as in passive films formed on nitrogen-bearing stainless steels. This raises the local pH in the film/substrate interface which leads the electrode reactions to favour the forming of molybdate, a beneficial species which enhances the passivity of stainless steels, instead of forming MoO3, the loose transpassive product.

The improvement of the localized corrosion resistance of stainless steel by cerium

Abstract Rotating disc assemblies are employed to study the cathodic electrode process and its inhibition by cerium implantation into UNS S31603 stainless steel in a solution of 0.6 M NaCl + 0.1 M Na 2 SO 4 . The reduction of oxygen and protons on both gold and untreated steel are shown to be controlled by the mass transport processes in solution. Cerium implantation effectively inhibits the cathodic reduction, reducing the cathodic current by more than two orders of magnitude. The cathodic reduction of oxygen and protons on ion-implanted stainless steel is thus controlled primarily by charge transfer at the electrode. Thermodynamic data suggests that the highly stable cerium oxide may be responsible for blocking the active sites for both cathodic and anodic reactions.

Chemical treatment with cerium to improve the crevice corrosion resistance of austenitic stainless steels

Abstract The effect of cerium ion implantation in improving the localized corrosion resistance of stainless steels was demonstrated earlier. The work has now been extended to study the effect of cerium salt solution treatment. Rotating disc assemblies were employed to study the cathodic electrode process and its inhibition by cerium salt treatment on austenitic stainless steels in a simulated seawater solution. The reduction of oxygen and protons on both untreated steels is shown to be controlled by the mass transport processes in solution. Cerium treatment effectively inhibits the cathodic reduction of oxygen which is controlled primarily by charge transfer at the electrode. The overvoltage for cathodic reduction of protons increases after the cerium treatment and the electrode reaction is controlled by both the mass transport process in solution and the charge transfer on the electrode. As a result of inhibition of the electrode processes, cerium improves the localized corrosion resistance, and in particular the crevice corrosion resistance, of stainless steels. This is supported by results obtained from electrochemical measurements and crevice corrosion tests both in the laboratory and at a seawater test site.

A surface analytical and electrochemical study on the role of cerium in the chemical surface treatment of stainless steels

Abstract The mechanism of oxide layer formation and modification during chemical cerium nitrate treatment of stainless steel has been investigated. The aim of the work was to study the role of cerium in modifying the oxide layer properties, especially the kinetics of the cathodic reactions. For this, electrochemical and surface analytical studies were carried out. During exposure to hot (90 °C) cerium nitrate solution, oxide film formation by chromium passivation and an accompanying dissolution of iron oxide takes place, leading to an enrichment of chromium in the oxide layer. Further, insoluble cerium species are precipitated at the cathodic sites of the surface. The oxygen reduction reaction is inhibited on these films. The effect of the cerium treatment cannot be solely attributed to the formation of a chromium-rich oxide layer, since the cathodic reactions are more strongly inhibited on the cerium-treated stainless steel than on passivated pure chromium. Moreover, the cerium treatment is efficient in retarding the cathodic kinetics on pure chromium. Studies with a redox couple present in the electrolyte clearly show that the inhibition of the oxygen reduction reaction is not due to a lower electron conductivity of the oxide layer. The cathodic inhibition effect can be attributed to a high resistance against reductive dissolution. This is partially due to the chromium enrichment and in addition to the cerium precipitation at the weak sites of the oxide layer which otherwise under cathodic polarization would lead to reductive dissolution, thus providing current paths for electrons participating in the oxygen reduction reaction. Treatment parameters such as time, alloy composition, solution chemistry and potential during treatment were studied. Clearly, all factors leading to a maximum chromium enrichment and/or cerium precipitation increase the cathodic inhibition efficiency.

Cathodic reactions involved in metallic corrosion in chlorinated saline environments

The cathodic electrode processes which may be involved during the metallic corrosion in chlorine-treated and untreated seawaters have been studied. The reactions were produced at the surface of a rotating gold disc electrode using a base solution of 0.6 M NaCl + 0.1 M Na2SO4. The reductions of chlorine species are the major cathodic reactions in chlorinated water systems. They are pH dependent and particularly enhanced when the acidity of the solution increases. The overall reduction of chlorine is found to be mass-transport controlled. The influence of bromide in the seawater on the cathodic electrode processes has been studied. The free chlorine in the solution rapidly oxidizes the bromide to form free bromine. Bromine is hereby shown to slightly enhance the cathodic reduction processes in the potential range −200 to +600 mV(SCE), and may thereby enhance metallic corrosion. The reduction processes may be hindered when small quantities of ammonium ions are introduced to the solution. The beneficial effect on the pitting corrosion resistance of stainless steels in chlorinated seawater by alloying with nitrogen is discussed in this context.

On the electrochemical conditions within small pits

Abstract 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.

Pitting corrosion of Ni

Microscopic observations have been made in situ on the pitting corrosion of polycrystalline and single crystal Ni, at a passive potential, in 1N H2SO4 solution containing Cl−. Corrosion pits were observed to nucleate at scratches and grain boundaries. The susceptibility to pitting of polycrystal, mechanically polished {111} and electropolished {111}, {100} and {110} surfaces is compared with the anodic polarization currents for active dissolution. Under the same degree of passivation, the order of the susceptibilities to pitting is coincident with that of the reactivities for active dissolution. It is concluded that in Ni the active sites for pitting are identical with the active sites for active dissolution.

Anodic behaviour of stainless steel S43000 in concentrated solutions of sulphuric acid

Electrochemical, AES and XPS techniques were employed to characterize the anodic behaviour of S43000 stainless steel in concentrated sulphuric acid (90.0–96.4 wt.%). Electrochemical experiments showed that passivity is not spontaneous and requires anodic polarization in the acids studied. Rotating cylindrical electrode experiments showed that the corrosion rate is controlled by the mass transfer rate of FeSO4 from a saturated surface salt. AES and XPS analyses provided evidence that passivity involves the formation of a chromium-rich oxide–hydroxide film. The passivation mechanism and passive state stability are considered to relate to the manner in which undissociated H2SO4 molecules participate in the corrosion process. The findings have meaningful implications regarding the development of more corrosion resistant stainless steels for acid service.

Corrosion behaviour of molybdenum-implanted stainless steel

Abstract A low-molybdenum austenitic stainless steel (UNS S30100) has been surface implanted with molybdenum ions, using various doses of 50 keV and 140 keV ions at room temperature. It is found that in aqueous sulphate/chloride solutions similar to the constitution of sea-waters the implantation does not affect the potentiostatically-determined critical pitting potential, but does change the density and morphology of corrosion pits. Pitting initiation after the addition of chloride at a fixed potential indicates little change in the time for measurable current increase, but the rate of increase of the current is much lower for implanted material. Detailed examination using optical microscopy, scanning electron microscopy, transmission electron microscopy and selected area diffraction suggests that the pits produced in implanted material are hemispherical with smooth covers of unattacked alloy. The use of half-implanted samples demonstrates that molybdenum implantation causes the formation of smooth-covered pits rather than “lacy” attack characteristic of stainless steel attack in chloride solutions. The energy of implantation also affects the density of pit nucleation, suggesting that passive films formed after implantation at low energies are not able to completely protect the steel surfaces.

Microelectrodes for the study of localized corrosion

Microelectrodes have been developed which can measure the local pH, potential and chloride ion activity as a function of position within local corrosion sites. The electrodes have been characterized through studies of their stability, the interference of other ions involved in the corrosion of nickel- and iron-based alloys, and the spatial resolution of the microelectrodes. Techniques for eliminating the IR drop effects are discussed. Examples are given of the application of the microelectrodes to the study of localized corrosion.