A Framework for Localized Corrosion Prediction of Stainless Steel
Abstract
Approved for Public Release-The US Navy operates in a corrosive environment and so makes extensive use of corrosion resistant alloys(CRAs) like stainless steels. CRAs are protected from corrosion by the spontaneous formation of a Cr-rich surface oxide layer, but this layer is susceptible to breakdown in chloride-containing environments via one of the forms of localized corrosion. The most fundamental form of localized corrosion is pitting corrosion and it can lead to failure by more aggressive forms such as environmental cracking or crevice corrosion, so it is critical to have a thorough understanding of pitting corrosion if Navy assets are to be effectively managed. Accurate prediction of corrosion performance can enhance Navy readiness and decrease maintenance costs. Similarly, the understanding needed to enhance corrosion performance prediction can also be used for promoting the design of more corrosion resistant materials. A new framework for describing the pitting of stainless steel has been developed over the past 5 years. One of the gaps of that framework relates to recent work showing that pit passivation under a salt film is critical to pit stabilization and the corrosion performance prediction of CRAs. The objectives of the proposed work are to elucidate the mechanism of pit repassivation under a salt film, particularly during downward potential scanning at a fixed temperature, and to develop the parameters needed to transform the understanding of pitting corrosion provided by the new framework into a practical predictive tool. The proposed research is divided into tasks involving repassivation under a salt film, metastable pitting analysis, cryogenic studies of salt film structure and composition, and CPT prediction.1-D pit electrode experiments done previously on deep pits have shown unusual dependence of repassivation temperature on potential and unexplained repassivation under a salt film during downward E scanning. These experiments must be extended for smaller pits, with depth as small as practical, but probably on the order of 25 um. Both E and T scanning experiments will be performed to determine the conditions for repassivation. It is expected that the results on less deep pits will provide the information needed to validate the proposed relationships for the boundaries between pit stability and repassivation. These types of repassivation experiments will be performed on CRAs with a range of pitting resistance, from SS430 to SS904. We will use an approach described by Pistorius and Burstein to determine the condition for repassivation of very small pits, on the order of a few um. We will initiate pits at a potential of 0.7 V SCE and rapidly scan the potential both upwards and downwards. This will be done for different temperatures, pit depths and alloys. We previously used cryo-SEM after flash freezing in LN2 and sample preparation by cryo-FIB to view the salt film/metal interface at high resolution. We will extend this approach to higher resolution using cryo-TEM and APT. The goal is to better understand the structure and composition and to look for the proposed inner barrier salt film layer. - Our newly developed framework describes an expression for the prediction of CPT as well as the distribution of these critical values. This task will use both 1-D pit electrodes and metastable pitting analysis to develop the parameters in the expression for potentiostatic-CPT, including critical pit concentration, activation energy for dissolution, and pit size distribution parameters.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 06, 2023
- Source ID
- N000142312202
Entities
People
- Gerald S. Frankel
Organizations
- Office of Naval Research
- Ohio State University
- United States Navy