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ASME BPVC Sec. VIII Div II fatigue curves in corrosive environment?

ASME BPVC Sec. VIII Div II fatigue curves in corrosive environment?

(OP)
I am attempting to analyze oil and gas well components for failure from fatigue using ASME BPVC Sec. VIII Div II 5.5.2.4 (screening) and 5.5.3 (if screening fails). The material is Inconel 718 (per API 6A) in a sour corrosive environment using the curve 3-F.3. The environment is corrosive (H2S, CO2, high chlorides, low pH). My question is in this environment, is the ASME fatigue analysis still valid, is the ASME S-N curve still valid? Is there an environmental limit to where the ASME curves are or are not valid?

We have standard smooth bar/uniaxial S-N fatigue test data for 718 in a slightly harsher environment (H2S PP, CO2 PP, chloride concentration, and pH) than the application. The failure points fall to the upper right of curve 3-F.3 showing that 3-F.3 would still conservatively predict fatigue failure, although not nearly as conservative as compared to a relatively inert environment. We have evaluated the material for failure from corrosion, stress corrosion cracking (SCC), and hydrogen embrittlement/sulfide stress corrosion (SSC) and the material passes in our application (according to appropriate NACE and API standards). However, obviously the environment degrades the material's fatigue properties but is it enough to invalidate the ASME analysis given how conservative their S-N (3-F.3) curves are?

If the curves/analysis is not valid, do you have a recommendation of how to do the analysis "to code" using ASME or another design/analysis standard?

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