In crevice corrosion, the development of a critical crevice solution (CCS) plays an important role in crevice corrosion propagation in nickel alloy 625. In this study CCSs were made from NiCl2, CrCl3, FeCl2, MoCl3, and NbCl5 metal salts following the same stoichiometric ratio as they appear in the alloy and ranging in concentration from 3.0 to 5.0 molal (m). From anodic cyclic potentiodynamic polarization results, it was found that minor elements such as Mo and Nb contributed to lower open circuit potentials (OCP) and increased critical peak current densities (icrit). Solutions that simulated an equivalent chloride content similar to that of the NiCrFeMoNb solutions, but without any Mo and Nb content, presented high OCP values and passive behavior. The active to passive transition in the polarization curves appeared to correlate with a critical concentration of Mo3+ in solution, i.e. at [Mo3+] > 0.09 m activation occurred. The properties of these CCSs were compared with simulations using OLI software to predict the solution pH and the solid species that precipitate at 25°C. It was found that for concentrations ranging from 3.0 to 5.0 m, all metal solutions presented pH values below 0. Additionally, HCl-based solutions within the range of 0.1 to 5.6 m were studied, and it was found that the icrit values in these solutions were typically lower than in the metal salt solutions at an equivalent pH. This study also presents an equation for preparing HCl solutions to obtain an equivalent icrit to that of the NiCrFeMoNb solutions.
A proposed mechanism for crevice corrosion damage evolution of remote crevice assemblies (RCAs) of nickel alloy 625 exposed to artificial ocean water under anodic potentiostatic control was also presented. Based upon the crevice corrosion morphology, the current vs time (I vs t) curves, and the analysis of corroded area over time, it was found that crevice corrosion occurred in three different stages:
Stage I – CCS development and initiation of crevice corrosion. During Stage I, two important subsequent events occur: (i) the deoxygenation within the crevice and (ii) the initiation of crevice corrosion. For an RCA polarized at 200 mV vs SCE, the deoxygenation event was calculated to occur in less than 10 min. Additionally, during this stage, visible damage was minimal, characterized by light etching beginning at the furthest point from the crevice mouth.
Stage II – Movement of the active front. During Stage II, the anodic dissolution rate (current) increased and was found to be dependent of the applied potential. The light etching damage moved from the deepest part of the crevice towards the mouth until it reached a critical distance near the crevice mouth (xcrit).
Stage III – Stable crevice corrosion propagation. Once a critical location from the mouth (xcrit) is reached, crevice corrosion propagation rates increase drastically and severe damage (with increased penetration depth) occurs. During this final stage, the current remains constant at an (Ilim) value, and brightened surfaces and deepening damage are observed.
A modified T.H.E. (Tsujikawa-Hisamatsu-Electrochemical) method was used for evaluating crevice corrosion repassivation potentials (Erp,crev) in the RCAs. It was found that there is a dependence of Erp,crev on the propagation time and location of xcrit. At early times, when the extent of crevice corrosion damage is small and xcrit is far from the crevice mouth, the Erp,crev values for 12 and 24 hours were found to be 80 and 150 mV vs SCE, respectively. Once xcrit was reached and crevice corrosion transitioned to stage III, Erp,crev stayed at a constant value: for both 36 and 72 hour immersion times, Erp,crev was 190 mV vs SCE.
Additionally, crevice corrosion products were examined at different immersion times during the crevice corrosion process. From Energy Dispersive Spectroscopy (EDS) and Inductively Coupled Plasma (ICP) analyses of the crevice corrosion products and bulk solution, it was found that, over time, the products within the crevice became enriched in O, Mo, and Nb, while the crevice corrosion products formed around the mouth of the crevice (outside of the crevice) became enriched in Ni, Cr, Fe, Mo, and O content. These results suggest that the metal cations diffusing out of the crevice form oxide/hydroxide precipitates due to the drastic pH change from the acidic conditions inside the crevice to the more neutral/alkaline bulk solution at the crevice mouth. The absence of Nb content in the corrosion products outside of the crevice suggests that the Nb exists as a stable precipitate inside the crevice, likely Nb2O5.
Finally, X-ray Computed Tomography (X-CT) was used to measure the crevice gap in the RCAs. It was found that the crevice gap varied within a range of 20 to 50 µm. The measurement of the crevice gap allowed the calculation of the IR-drop down the length of the crevice. It was found that the IR drop is small, and remains nearly constant during the crevice corrosion process.
This work is divided into 4 main chapters. Chapter I is a comprehensive review of the literature regarding crevice corrosion of stainless steels and nickel alloys, especially nickel alloy 625. In this literature review, the different theories for understanding crevice corrosion initiation and propagation mechanisms are described, along with models for predicting pH of highly concentrated solutions, such as critical crevice solutions found within crevices. Chapter II presents in detail the experimental set up and electrochemical techniques used for studying crevice corrosion initiation and propagation stages in this study. Chapter III displays the primary results of the study, which are then discussed in Chapter IV. Chapter V presents the conclusions drawn from this study, addressing the crevice corrosion propagation stage in RCAs, and the effect of metal cations in artificial crevice solutions.