Question:

We have some 316 stainless steel housings with a large through bore machined. Inadvertently this bore was machined oversize. We would like to flame spray build up the surface with 316 or 316L stainless material and remachine to size. As we understand the standard, 316 and 316L stainless are both included in a lengthy list of materials accepted for direct exposure to sour gas. As we intend to apply stainless to stainless for the purpose of remachining to dimension and not as a corrosion-inhibiting coating, would this process be acceptable and compliant with the NACE Standard MR0175/ISO 15156?

Answer:

1.0 Flame spraying as a coating for corrosion resistance over a base material that is resistant to sulfide stress cracking is acceptable within the requirements of NACE MR0175/ISO15156 Part 2 Paragraph A.2.1.5 when applied over carbon steels and of Part 3 Paragraph A.1.5.1. In the case of your inquiry, the 316 or 316L base materials are acceptable coating substrates if they conform to the metallurgical requirements of Part 3 Table A.2 and are used within the environmental restrictions of this table for any equipment.
2.0 If this application of flame spray is for the replacement of material that will be load bearing of tensile stresses, then the inquiry is not currently addressed by NACE MR0175/ISO15156. NACE/ISO have not been balloted with data to demonstrate that the 316 SS or 316L SS deposited flame spray coating has the same cracking resistance as the materials referenced in Part 3 Table A.2, which are assumed to be in the cast or wrought conditions.

This question is in relation to NACE MR0175/ISO 15156-3 Annex A.1.5.1

Reference: ISO 15156 Maintenance Panel Inquiry #2005-01