Effect of tungsten alloying on passivity breakdown of nickel

Abstract

The addition of small amounts of tungsten (W) on the passivity breakdown of nickel (Ni) in borate buffer solutions as a function of chloride activity, pH, and voltage sweep rate has been evaluated and the data are interpreted in terms of the point defect model (PDM). The critical pitting potential of Ni–W alloys was found to become more positive (correspondingly a higher resistance to pitting) upon increasing the W content up to 3 at.% but decreases with further additions. The initial increase in the breakdown voltage with increasing [W] is attributed to changes in the diffusivity of the cation vacancy, the electric field strength, the annihilation rate of cation vacancies (Jm), changes in the standard Gibbs energy of adsorption of Cl− into surface oxygen vacancies, and in the standard Gibbs energy of generation of Schottky pairs, as predicted by the SVIM. At high [W] (4–5 at.%), it is postulated that all of the mobile cation vacancies are complexed with immobile WNi4⋅ [W(VI) cation substituted onto the Ni(II) vacancy of the cation sublattice of the barrier layer], so that further additions of W are predicted to have little positive effect in improving the pitting resistance of the alloy. Instead, W additions above 3% are postulated to increase the population density of breakdown sites on the surface via the formation of intermetallic precipitates, which has the effect of displacing the distribution in the breakdown voltage in the negative direction.

Statistical approach for electrochemical evaluation of the effect of heat treatments on the corrosion resistance of AlSi10Mg alloy by laser powder bed fusion

“…The stochastic nature of pitting initiation requires a statistical approach [20], it was done for stainless steels [21,22], copper [23], nickel [24] and aluminum alloys [25,26], but, on the author knowledge, it was never studied on additive manufactured alloys.…”

Effects of Ti addition on microstructure and the associated corrosion behavior of a 22Cr‐5Ni duplex stainless steel

“…Similar observations have been reported for the effect of alloying tungsten on the distribution in the pitting potential to a nickel system. [37] However, the excessive amount of Ti in duplex stainless steel would cause an increase in the ratio of ferrite to austenite phase. As a result, the main alloy elements of Cr, Ni, and Mo in the ferrite phase were diluted and pitting resistance was reduced.…”

“…Furthermore, the formation of large amounts of TiN caused a decrease of the N content in the matrix, whereas N is the strongest individual contributor to phase balance and corrosion resistance. [42] Moreover, the formation of TiN particles could increase the cation vacancy diffusivity by providing paths of high diffusivity through the passive layer at the intersection between the layer and the particles, [37] resulting in a higher current density. Therefore, excessive additional Ti deteriorated the corrosion resistance of the steel.…”

“…And the increase in the pitting potential with increasing Ti is attributed to a decrease in the diffusivity of the cation vacancy and an increase in the annihilation rate of cation vacancies. [37]…”