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What are the NACE MR0175 electrode requirements for 625 overlay welding? what about dilution?

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Reference: NACE MR0175/ISO 15156-3, Clause A.13.1 (Corrosion-resistant claddings, linings and overlays) The first sentence of Clause A.13.1 states: “The materials listed and defined in Clauses A.2 to A.11 may be used as corrosion-resistant claddings, linings or as weld overlay materials.” The fifth paragraph of Clause A.13.1, as stated below, recognizes that dilution of the weld metal with the substrate occurs during welding. “Dilution of an overlay during application that can impact on its corrosion resistance or mechanical properties should be considered.” Discussion: While the composition of a starting filler metal electrode may meet the composition requirements of an applicable UNS alloy listed in Clause A.2 to A.11, the as-deposited filler metal may be diluted (as noted in the fifth paragraph of A.13.1) to the point where it no longer falls within the applicable UNS alloy’s composition range. For example, a starting electrode for a 625 weld overlay may meet the composition requirements of UNS N00625, but API 6A, 625 overlay made to class FE10 allows an iron content in the as-deposited filler metal of up to 10.0% which exceeds the 5.0% maximum iron limit in UNS 06625. QUESTIONS:

Q1: Does the as-deposited filler metal have to comply with the UNS composition of the starting electrode?

ANSWER:

ISO15156-3 does not state anything specific about the composition of the as-deposited filler metal. It does state that dilution can affect the corrosion resistance. The purpose of an overlay has historically been for corrosion resistance and not cracking resistance. If the enquirer wishes to consider the overlay as a barrier for cracking resistance, a ballot is required to define cracking limits for a specific as deposited composition for a define location within the weld.

Q2: If the answer to Q1 is yes, then what locations within a weld must meet the UNS composition allowing for some dilution?

ANSWER:

This issue is not addressed by this standard. See answer to Q1.

Q3: Does a 625 overlay made to the prescribed API 6A requirements for FE10 meet NACE MR0175/ISO 15156?

ANSWER:

The standard does not define the cracking limits for as-deposited filler material beyond the UNS compositions. See answer to Q1

This is in relation to NACE MR0175/ISO 15156-3 A.13.1

Reference: ISO 15156 Maintenance Panel Inquiry #2013-02

UPDATE 2018-06-04: A recent Technical Circular brings relevant updates on NACE MR0175 and 625 weld overlays: https://oilandgascorrosion.com/nace-mr0175-iso-15156-technical-circulars-for-part-2-and-3-published-2018-06-01/

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