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.
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.
NACE MR0175 was issued as a recommendation of threshold limits of H2S above which precautions against environmental cracking are necessary. In 2003 the NACE ‘MR0103 Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments’ document was issued to cover the refining industry as well as MR0175 being adopted by ISO and designated as ISO 15156.
NACE MR0175 consists of three parts –
H2S threshold limits have been established based on a combination of laboratory evaluation and operator experience. The document continues to evolve with additions of alloys and refinement of the threshold limits. The threshold limits between alloys and alloy families can vary dramatically. For a number of alloys, the H2S threshold limits consider chlorides, pH, temperature, hardness, yield strength and CO2 whereas for other alloys limits are not as strictly defined.
MR0175 also gives guidance for the selection and specification of materials when the H2S thresholds have been exceeded, which allows for fit-for-purpose testing to qualify a material for a specific application.