Metals for Sour Oilfield Environments
What is NACE MR0175?25th June 2019
NACE formally stands for the National Association of Corrosion Engineers, but is commonly used as short-hand for the internationally recognised standard NACE MR0175. A ‘NACE compliant’ product, really meaning one that is compliant to NACE MR0175, is one that meets the requirements of NACE MR0175 and therefore can be considered for sour service environments within the set operational limits.
The National Association of Corrosion Engineers started life as a small group of corrosion engineers working on a regional study, but subsequently developed into an international association focused on the challenges on corrosion in industrial processes. It was originally a standard for the USA market, intended to assess the suitability of materials for oilfield equipment where sulphide stress corrosion cracking may be a risk in hydrogen sulphide ‘sour’ environments. However, given the importance of this standard to the global Oil and Gas industry, it was ultimately co-opted by ISO, the world standards body.
To give its full name, NACE MR 0175 (‘Metals for Sulphide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments’), specifies the types of corrosion-resistant materials that can be used in specific oilfield environments. One particular feature of this standard is the use of limits for material hardness, as it is the only practical material measurement that can be conducted in the field as a validation of material specification. Despite this restriction, hardness has a reasonable correlation with overall mechanical properties and is applied to both the parent metal and any weld features.
Given that the standard is focused purely on sulphide stress corrosion cracking, it does not address the issues of general corrosion or pitting corrosion. Similarly, it does not stipulate minimum mechanical properties, the required manufacturing route nor general testing standards, so does not provide a complete selection tool.
The hydrogen sulphide threshold limits have been established based upon a combination of real-world experience and laboratory testing. The standard has evolved over time, such that both the operating limits and the material requirements will vary quite considerably. The standard also provides guidance for the selection and specification of materials when the hydrogen sulphide thresholds are exceeded. This allows for ‘fit-for-purpose’ testing to be undertaken in order to qualify grades for a particular application.
As previously mentioned, NACE MR0175 is now published and maintained as ISO 15156-3.
As a default requirement, Langley Alloys stock will be produced and expect to meet the ‘NACE’ standard if applicable.
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