The international standard NACE(1) MR0175/ISO(2)15156-3 is essentially used for the material selection in H2S-containing environments when choosing corrosion resistant alloys (CRAs). Temperature and H2S partial pressure are mainly considered. Earlier experiences indicated that UNS N08028 and UNS N08029 could withstand more severe environmental testing conditions compared to the standard required for type 4c alloys.

A recent laboratory testing program has been carried out on the OCTG tubing of UNS N08028 and UNS N08029 in 91000 mg/l chloride, 1000 psi CO2, and 2200 psi H2S at temperature from 150°C and up to 220°C, using constant load and slow strain rate testing (SSRT). The materials could in essence withstand the testing conditions. Pitting was observed on these materials when chloride concentration was increased to 250000 mg/l using constant load testing. Failures have been observed after SSRT when chloride was 254000 mg/l. Critical pitting temperature (CPT) determination in 3-12 wt% chloride solutions showed that UNS N08029 has significantly higher CPT compared to UNS N08028 because of its higher Mo content. However the environmental limits have not been defined for these materials in the tested conditions. The effects of chloride and other possible parameters on the pitting and failures are discussed.


Sandvik Sanicro 28(3) (UNS N08028) and Sanicro 29(4) (UNS N08029) belong to solid-solution nickel-based alloys. Tube products in these material grades have been successfully used in oil country tubular goods (OCTG) applications. Earlier studies have shown that these materials have high resistance to stress corrosion cracking (SCC) in testing environments with high concentrations of hydrogen sulphide (H2S), carbon dioxide (CO2) and chlorides (Cl-), at elevated temperatures.1, 2, 3

Cold worked UNS N08028 and N08029 tube products with specified minimum yield strength 125 ksi were tested per NACE(5) TM01774 method A, method C using tensile, C-ring; and per NACE TM01985 using slow strain rate testing (SSRT), with good results. SCC resistance was proven in the sour environments outside the environmental limits according to NACE MR0175/ISO(6) 15156-3:20156 for the 4c type materials.7, 8, 9, 10 UNS N08029 showed better corrosion resistance compared to UNS N08028.11, 12 The ISO limits might be too conservative for these materials.

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