Studies on accumulation of chloride ions in pits growing during anodic polarization

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...

supporting

confidence: 88%

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

Salgado

Lillard

2017

“…Variants of this procedure to measure i L have been employed in other pitting studies as well. 27,28 This dissolution current density was converted to a flux value J as shown in Equation 1.…”

Section: Methodsmentioning

confidence: 99%

Geometric Evolution of Flux from a Corroding One-Dimensional Pit and Its Implications on the Evaluation of Kinetic Parameters for Pit Stability

Srinivasan

Liu

Kelly

2016

The flux from a one-dimensional (1-D) artificial pit electrode corroding under a salt film was examined using experimental and modeling techniques. Finite element simulations showed that the flux at shallow depths was consistently lower than analytically determined calculations for 1-D Fickian diffusion. This deviation was due to a substantial contribution of the external hemispherical boundary layer to the overall diffusion length. Increasing the pit diameter resulted in a larger boundary layer, which in turn affected the flux characteristics to greater depths. Data from experiments and simulation converged with the theoretical 1-D calculations only when pit depths approached nearly ten times the pit diameter. The experimental data from this artificial pit study as well as data from related published work were observed to span the range bounded by the numerically simulated and the analytically determined flux predictions. Comparison with published pit stability phenomenology showed that only deep pits provided kinetic data based on the cation concentration gradient unadulterated by bulk chloride effects. Finally, this work also provided insight into the origin of the dependence of the measured repassivation potential with pit depth, contributing towards a quantitative framework relating the various critical factors governing pitting. The stable growth of corrosion pits requires the presence of an aggressive chemistry at the corroding surface as characterized by a high metal chloride concentration and low pH.1,2 The conditions leading to the maintenance of such chemistry were mathematically considered by Galvele 3 through an investigation of the steady state relationship between metal dissolution and mass transport 4,5 out of a one-dimensional (1-D) pit. For the case of a 1-D pit, it was theoretically demonstrated that a minimum critical value of cation flux -expressed as the product of the current density and the pit depth, (i·x) -was necessary for the pit to maintain a critical chemistry and thus stably corrode. As such, should the product of the current density and the pit depth fall below this critical value, repassivation would set in due to the loss of the aggressive chemistry. Subsequent studies on stainless steel pitting referred to this parameter and equivalent relationships for other pit geometries as the pit stability product. 6,7 This critical pit stability product (denoted (i·x) crit ) has been employed as the anodic stability parameter to determine the maximum pit size that can be attained on a particular metal surface in a given corrosive environment. 8-11Experimental results obtained using the artificial pit or lead-inpencil electrode 12-17 can be directly used to quantify 1-D dissolution kinetics because the construction of this electrode results in inert walls surrounding a single active surface 13,14,16 corroding under a precipitated salt film 18-24 upon the application of high anodic potential in corrosive solution. The presence of the salt film results in diffusionlimited dissolution conditi...

“…4 An undercutting mechanism was suggested for pit growth at high electrochemical potentials, leading to the development of dish shaped pits covered by metal lacy covers. [5][6][7][8][9][10] Pit propagation is controlled by diffusion of species through the salt film at the inner surface of pits. A high anodic dissolution rate, which facilitate critical ion concentrations inside a pit, is the criterion for pits to propagate.…”

mentioning

confidence: 99%

Application of a Quasi In Situ Experimental Approach to Estimate 3-D Pitting Corrosion Kinetics in Stainless Steel

Al-Muaili

McDonald

Withers

et al. 2016

Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ X-ray computed tomography (X-ray CT) approach. A miniature electro-chemical cell was constructed to allow imaging during potentio-dynamic polarization of a wire specimen in chloride solution. The formation of three discrete pits was observed, allowing comparison between real pit geometry and different geometrical assumptions to estimate pit growth kinetics. The pit volumes obtained by X-ray CT showed a good fit with the volume of metal dissolution calculated using Faraday's law. Large fluctuations of the mean current density were observed during the pit nucleation stage, followed by pit growth with mean current densities of 1-3 A.cm −2 . Stability products associated with these pits were on the order of 0.3-0.6 A.m −1 , with a diffusivity parameter (D . C) of 1. Pitting or crevice corrosion can occur on stainless steels exposed to halide containing environments. The breakdown of the passive surface film results in the nucleation of local attack, which is followed by pit growth, with a number of mechanisms proposed for each stage. 1,2Electrochemical polarization above the critical pitting potential leads to formation of pits, in the form of localized metal dissolution, with corrosive electrolyte developing inside the nucleated pit due to hydrolysis of dissolved metal ions, and chloride ions being attracted from bulk solution to maintain charge neutrality.3 At high applied electrochemical potentials, pits grow with a polished inner surface morphology, associated with salt film formation, whereas pits grown at lower applied potentials display more irregular etch morphologies of the internal pit surface. 4 An undercutting mechanism was suggested for pit growth at high electrochemical potentials, leading to the development of dish shaped pits covered by metal lacy covers. 5-10Pit propagation is controlled by diffusion of species through the salt film at the inner surface of pits. A high anodic dissolution rate, which facilitate critical ion concentrations inside a pit, is the criterion for pits to propagate. 11 The anolyte concentration inside a pit influences pit growth kinetics and the pit shape. For example, a 3 M concentration of dissolved metal ions was proposed as the minimum concentration to turn an open hemispherical meta-stable pit into a stable pit. However, pits developed with lacy metal covers grow at far lower metal ion concentrations, since these covers are believed to provide an effective diffusion barrier 12 and an increased ohmic resistance between pit interior and the bulk electrolyte.1 Based on these observations for the transition between meta-stable and stable pits, the "pit stability product" (0.3 A. m −1 ≤ i.r ≤ 0.6 A. m −1 ) was proposed, with the latter based on the evolution of pit current density (i) and pit depth (r). 12To sustain stable pit growth under anodic polarization, a minimum current density is required, with typical values for stainless steel in the region of 1-5 A.cm −2 . 13,14 For ...