Xi Shan

Characterization of the Corrosion Products of Crevice Corroded Alloy 22

The crevice corrosion performance of Alloy 22 was tested under potentiostatic polarization more positive than the repassivation potential in 4 M NaCl solution at 100°C. Under this aggressive condition, crevice corrosion initiated under the crevice contacts and four stages of corrosion behavior were observed: initiation, propagation, stifling (corrosion slowed), and arrest (corrosion stopped). During the exposure, dark green deposits were found on the uncorroded metal surface around the crevice contacts, light green precipitations were found in the test solution, and the solution color changed from clear to light green. After exposure, loose black corrosion products were found under the crevice contacts. Surface analyses at the sites of varying corrosion penetrations showed similar composition depth profiles, and the results indicated that a 3-5 nm thick, chromium-rich oxide film was formed on the alloy surface. The composition depth profiles indicated that the crevice corrosion occurred by a uniform, nonselective dissolution. Surface analysis results showed that the corrosion products in the crevice were rich in O, enriched with Mo and W, and depleted of Ni and Cr relative to the bulk alloy. A solution analysis showed that Ni was the main element dissolved into the solution during the exposure.

Modeling the Effects of Crevice Former, Particulates, and the Evolving Surface Profile in Crevice Corrosion

Crevice corrosion may initiate in confined regions due to transport limitations, followed by an accumulation of a highly corrosive chemistry, capable of dissolving the metal. The metal and the crevice former surface roughness, the presence of particulates under the crevice former and the accumulation of solid corrosion products at the corroding site would significantly affect the current and potential distribution at the anode by increasing the ohmic potential drop. Most crevice corrosion models focus on a smooth walled crevice of uniform gap and do not account for the changing profile after crevice corrosion has been initiated. In this work we analyze the crevice (anodic) region and apply current and potential distribution models to examine the effects of the perturbed surface topography. The analysis focuses on three related issues: (1) the effects of surface roughness of the metal and the crevice former, (2) the effects of particulates under the crevice former, and (3) the evolution of the crevice profile with corrosion product accumulation at the active, anodic region.

Examination of Corrosion Products and the Alloy Surface after Crevice Corrosion of a Ni-Cr-Mo Alloy

The objective of this study is to investigate the composition of corrosion products and the metal surface within a crevice after localized corrosion. The analysis provides insight into the propagation, stifling and arrest processes for crevice corrosion and is part of a program to analyze the evolution of localized corrosion damage over long periods of time, i.e. 10,000 years and longer. The approach is to force the initiation of crevice corrosion by applying anodic polarization to a multiple crevice assembly (MCA). Results are reported here for alloy C-22, a Ni-Cr-Mo alloy, exposed to a high temperature, concentrated chloride solution. Controlled crevice corrosion tests were performed on C-22 under highly aggressive, accelerated condition, i.e. 4M NaCl, 100 C and anodic polarization to -0.15V-SCE. The crevice contacts were by either a polymer tape (PTFE) compressed by a ceramic former or by a polymer (PTFE) crevice former. Figure 1 shows the polarization current during a crevice corrosion test. After an incubation period, several initiation-stifle-arrest events were indicated. The low current at the end of the test indicated that the metal surface had repassivated.