Quantifying Alloy 625 Crevice Corrosion using an Image Differencing Technique: Part II. A Diffusive Transport Model of Crevice Cation Concentration using Surface Current Density

Abstract

In this paper we present a diffusive transport model of alloy 625 crevice corrosion for simulated ocean water. The model uses the empirical electrochemical kinetics (local surface current density) and empirical damage profile to determine the source of cations from surface oxidation reactions and then calculates the total cation concentration in the crevice solution as a function of time. To gain insight into the solution speciation at the calculated cation concentrations, a thermodynamic software package was used. In those calculations, it was assumed that the anodic dissolution products of the individual alloy components form their corresponding metal salts in the crevice solution. The calculations, based on activity, predicted that precipitation occurred in the same time frame as the experimentally observed increase in active area, decrease in surface current density and transition to diffusion control. The salts that precipitated out of the model solution were NaCl, at 5.3 m, and NiCl2·6H2O, at 5.7 m. It is shown that the presence of the Cr3+, Fe2+, Mo3+ and Nb5+ in the model crevice solution increase the cation and chloride activities resulting in precipitation at the same Ksp values as in the pure salts, albeit at lower Na+ or Ni2+ concentrations. Implications of the speciation results as they relate to actual crevices are discussed.

A Peridynamic Model for Crevice Corrosion Damage

“…This stagnation of the active site leads to deep carving into the washer near the crevice mouth. This observation is consistent with other studies on crevice corrosion in Nickel Alloy 625 washers [61][62][63].…”

Recent Advances in Corrosion Science Applicable To Disposal of High-Level Nuclear Waste

“…Using a novel cryogenic specimen preparation approach, the structure and composition of the salt film formed on a SS304 1D electrode were characterized using FIB sectioning and SEM analysis. 409 The salt film was found to consist of salt crystallites and clusters, with ionic transport being possible through small connected pores and channels. The distributions of Ni and Fe ions were spatially similar, whereas Cr was concentrated in what appeared to be channels, suggesting that a coprecipitated salt containing Fe 2+ and Ni 2+ formed on the pit surface.…”

“…Experiments with SS316L one-dimensional (1D) artificial pit electrodes in 0.6 M NaCl validated the new framework for pit-growth stability by showing that the measured relationship between the critical pit surface potential for salt-film formation and the pit depth was in agreement with the prediction. Using a novel cryogenic specimen preparation approach, the structure and composition of the salt film formed on a SS304 1D electrode were characterized using FIB sectioning and SEM analysis . The salt film was found to consist of salt crystallites and clusters, with ionic transport being possible through small connected pores and channels.…”

“…The concepts developed in this new framework have been applied by Lillard et al to study and model crevice corrosion. − These authors first developed a novel optical method that allowed for detailed tracking of the crevice corrosion front for a crevice in Ni alloy 625 created by a transparent acrylic block crevice former . The differences in successive images could be used to determine the active area and, along with the current measured under potentiostatic conditions, the local current density.…”

Potential of a Thin-Film Cell for assessing the Sensitivity of Nickel-Base Alloys to Crevice Corrosion

“…Indeed, the most common set-up to study crevice corrosion susceptibility is multiple crevices assembly, which consists of two non-metallic segmented washers with a certain number of confined areas to form one or multiple crevices. [9][10][11][12][13][14] The torque applied to assemble these washers is a predominant factor affecting the crevice-corrosion initiation, as it will be more likely with higher torques. 9,14 To limit this bias, the crevice corrosion was initiated by confining an electrolyte above the working electrode with an insulating plate so no torque has to be applied.…”