Alloy UNS(1) N08825 is a titanium-stabilized fully austenitic nickel-iron-chromium alloy with addition of copper and molybdenum. The alloy was designed for applications in the chemical process industry and was later applied in the oil and gas industry.

Because of its high nickel content, UNS N08825 shows an outstanding resistance to stress corrosion cracking in aqueous and acidic chloride-containing solutions. However, the molybdenum content of 2.5 to 3.5 wt.% limits its resistance to pitting and crevice corrosion in highly concentrated chloride-containing environments.

To address this weak point, a new advanced alloy with increased molybdenum content, which will be called Alloy 825 CTP, was developed. Previous corrosion tests showed an increased critical pitting temperature measured on the new alloy1 and based on these data further corrosion tests were performed.

Alloy 825 CTP has been tested according to ASTM G48 in solution-annealed and in PWHT condition to study the pitting and crevice corrosion resistance. In addition, corrosion tests were performed to study its corrosion resistance considering different mechanisms of environmental cracking. Sulfide stress cracking (SSC) and galvanically induced hydrogen stress cracking (GHSC) tests were performed according to NACE(2) TM0177-2016 Method A. Stress corrosion cracking (SCC) resistance was investigated using C-ring tests according to NACE TM0177 Method C at Level VI for 3 months and Level VII for 1 month. Slow strain rate (SSR) tests under cathodic polarization were performed to study the resistance to hydrogen embrittlement (HE).

In terms of resistance to (localized) corrosion and hydrogen embrittlement, the newly developed Alloy 825 CTP was found to perform equally well or even better than the conventional material.

Subject
Test methods, Hydrogen embrittlement, Materials, Corrosion resistant alloys, Molybdenum alloys, Corrosion testing, Cracking resistance, Heat, Pitting, Corrosion resistance, Sulfide stress cracking, Alloys, Stress corrosion cracking
Keywords:
Alloy UNS N08825, Alloy 825 CTP, PWHT, Ni39Fe28Cr22Mo6, corrosion behavior, ASTM G48-C, ASTM G48-D, SCC, SSC, GHSC
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2019
Association for Materials Protection and Performance (AMPP)
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