Metal Salt Solutions as a Simulant for the Chemistry in Crevices of Nickel Alloy 625

Abstract

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...

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

“…In addition, the models show that early propagation occurs in an alloy 625 solution concentration less than 1 M, far below the solution concentration necessary for an active to passive transition in alloy 625. We have previously shown that an active to passive transition in alloy 625 does not develop in the solution of cations below 3 m. 7 In comparison, here, after 5.76 ks of propagation the solution composition was approximately 0.9 M alloy 625 (Fig. 5, g = 40 × 10 −6 m).…”

“…This model solution consisted of 0.6 M NaCl, the initial OW crevice environment (converted to molality using a solution density of 1.022 g cm −3 , OLI) and, to represent the anodic dissolution products of Ni, Cr, Fe, Mo and Nb from alloy 625 we used the metal salts CrCl 3 , NiCl 2 , FeCl 2 , MoCl 3 , and [Nb 2 O 5 + 10HCl]. 7 The relative mass% of metal cations in the simulation was that of the alloy in the experimental study and equal to Ni-22.36 wt% Cr, 8.76% Mo, 3.74% Fe and 1% Nb (sub stochiometric w.r.t. Nb).…”

“…In addition, the Mo transport from the crevice appeared to lag the other cations, possibly forming an insoluble oxide. 4,5 Despite the findings of non-stochiometric cationic solutions, in general, metal salt solutions that are congruent with the alloy are often used to simulate the solution that develops in occluded cells, 6,7 however, Cl − base solutions acidified with HCl 8-10 are often used as well. While both HCl base and metal chloride solutions produce potentiodynamic polarization curves with an active to passive transition, which has been considered to be the first step in alloy 625 crevice corrosion, 8,9 HCl base solutions typically have a transpassive potential (E trans ) that is much higher in comparison to metal chloride base solutions as shown by Salgado for alloy 625.…”

“…While both HCl base and metal chloride solutions produce potentiodynamic polarization curves with an active to passive transition, which has been considered to be the first step in alloy 625 crevice corrosion, 8,9 HCl base solutions typically have a transpassive potential (E trans ) that is much higher in comparison to metal chloride base solutions as shown by Salgado for alloy 625. 7 The change in E trans as a function of solution composition is often overlooked as the active to passive transition is typically the focus. Further, some authors have proposed that crevice initiation due to transpassive dissolution is an entirely separate mechanism.…”

Quantifying Alloy 625 Crevice Corrosion Using an Image Differencing Technique: Part I. Initiation and Propagation

“…Recall, the Flade potentials for alloy 625 in simulated CCS are on the order of −0.2 V to 0.0 V vs SCE. 31 As such, for the case of alloy 625, breakdown most likely owes to transpassivity. There is a wealth of literature on the transpassivity of nickel alloys and these investigations typically focus Cr(III) oxidation to soluble Cr +6 in the transpassive region and little attention is given to the role of other alloying elements.…”