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The development of a critical crevice solution plays an important role in crevice corrosion propagation in nickel alloy 625. In this investigation, the polarization response of alloy 625 in a set of simulated crevice chemistries using metal chloride salts has been studied. Specifically, solutions were made from NiCl 2 , CrCl 3 , FeCl 2 , MoCl 3, and NbCl 5 in the same stoichiometric ratio as they appear in the alloy and ranging in concentration from 3.0 to 5.0 molal (m). It was found that Mo and Nb contributed to lower open circuit potentials (OCP) and increased critical peak current densities (i.e. activation). Solutions that substituted NiCl 2 for MoCl 3 and NbCl 5 , such that the chloride content was equivalent to that of the NiCrFeMoNb solution, presented high OCP values and passive behavior. To better understand the polarization data, solution speciation and pH were calculated using a commercially available software package. It was found that for concentrations ranging from 3.0 to 5.0 m, the pH values were on the order of −1.0. In addition, the active to passive transition appeared to correlate with an increase of Mo 3+ concentration in solution. For comparison, the polarization response of alloy 625 in HCl-based solutions was investigated. Over the range of HCl concentrations studied, the critical peak current densities were typically lower than the metal salt solutions at equivalent calculated pH. In addition, an equation for preparing HCl solutions at the needed pH to obtain an equivalent critical peak current density as observed in the NiCrFeMoNb solutions is presented. Oldfield and Sutton's model 1,2 has been widely used for explaining crevice corrosion initiation and propagation based on the solution chemistry change within crevices. According to Oldfield's and Sutton's model, the initiation stage of crevice corrosion is divided into three processes occurring inside the crevice: 1) depletion of oxygen, 2) metal hydrolysis resulting in lower pH and chloride-migration, and ultimately 3) formation of a critical crevice solution (CCS). Once crevice corrosion initiates, the propagation stage immediately follows (stage 4). The passive current density (i pass ) and the crevice geometry (depth-to-gap ratio) are the key parameters that aid the development of a CCS. The model predicts that for a constant i pass , larger crevice gaps required deeper crevices to initiate crevice corrosion. Thus, occluded regions with large depth-to-gap ratios have increased susceptibility to crevice corrosion damage. The Oldfield-Sutton-model does not, however, consider the effect of the IR-drop within the crevice (i.e. the electrode-potential variation between the cavity and the outer surface), nor does it predict the location and rate of the crevice damage that occurs within the crevice.A model based solely on IR-drop as the cause for initiating and propagating crevice corrosion was proposed by Pickering.3,4 Early versions of the Pickering model did not separate the initiation and propagation stages. Instead, the critical requi...

Nickel alloy 625 is widely used in marine applications due to its excellent corrosion resistance when exposed to highly oxidizing and reducing environments. However, nickel alloy 625 has been found to be susceptible to localized corrosion when crevices with a large aspect ratio (depth/gap) are exposed to seawater. In this study, two different electrochemical techniques were used to evaluate the mechanisms of crevice corrosion in nickel alloy 625: potentiostatic polarization of remote crevice assemblies (RCAs) exposed to ASTM artificial ocean water and potentiodynamic polarization curves in simulated critical crevice solutions (CCSs). From the electrochemical response of the RCAs, i.e. current vs. time curves, it was found that crevice corrosion damage under potentiostatic conditions occurred in three stages: Stage (I) CCS development, stage (II) crevice corrosion under IR control, and stage (III) crevice corrosion under diffusion control. It was also found that the CCS formed near the crevice tip and moved toward the crevice mouth. Once the CCS reached a critical distance from the mouth, presumably IR*, the corrosion rate drastically increased and severe damage occurred. During the RCA experiments, light green deposits around the outside edge of the RCA were found. When the crevices were opened after the test, additional corrosion products were found. For short exposure times, dark green and brown corrosion products were found spread out over the etching damage while at longer exposure times, accumulation of dark brown corrosion products were found closer to the crevice mouth. Energy Dispersive Spectroscopy (EDS) analysis showed that the crevice corrosion products formed inside the crevice were rich in Mo, Nb, and O, suggesting the possible formation of Mo and Nb oxides. The crevice corrosion products outside of the crevice were rich in Ni, Cr, Fe, Mo and O. The behavior of 625 in the CCS was characterized using potentiodynamic polarization in concentrated metal salt solutions prepared by dissolving NiCl2∙6H2O, CrCl3∙6H2O, FeCl2∙4H2O, MoCl3, and NbCl5 salts in deionized water at concentrations ranging from 3 to 5 molal. It was found that Mo and Nb content increased the current density of the active peak. The properties of these simulated CCSs were determined using thermodynamic calculations via the OLI Stream Analyzer software. Specifically, OLI software was used to predict the precipitation products and solution pH at 25°C. The predictions were used to compare the polarization data generated with the traditional simulated CSS using HCl base solutions.