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The NACE MR0175 standard for sulfide, ( sulphide ), stress corrosion cracking resistant metallic materials for oilfield equipment is now also available as an ISO standard 15156. This article summarises the requirements for all types of stainless steel austenitic , ferritic, duplex, martensitic and precipitation hardening stainless steels which are covered by the standard. Mill softened material will normally be found to comply with the requirements.
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Salt spray testing is an accepted method for assessing the suitability of stainless steel parts and fabrications that are likely to encounter chloride environments in service. The test outcome is sensitive to the shape of the parts, (designed-in crevices), surface finish and the test conditions and so specific results for ‘hours to failure’ for steel grades alone is not appropriate. Specified test methods are shown.
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The 316 types are used widely in marine applications, but their corrosion resistance in contact with seawater is limited. They cannot be considered ‘corrosion proof’ under all situations. These grades are susceptible to crevice and pitting corrosion, which limits there use in seawater applications. The affects of water chloride levels, flow rates, temperature and oxygen levels are noted and cathodic protection that can be derived from contact with less noble metals such as carbon steels and aluminium. The 304, and more especially the free machining 303 types, should not be considered for seawater service.
