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NACE MR0175 and the requirements for HSC, GHSC and SSC

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Question:

HSC describes cracking in metals that are not sensitive to SSC but which can be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is corroding actively as an anode. The term “galvanically induced HSC” has been used for this mechanism of cracking. It means HSC is the same as “galvanically induced HSC” and different from SSC. But in NACE MR0175-1 clause 3.23 SSC is a form of hydrogen stress cracking (HSC) and involves the embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. It means SSC is a type of HSC. The words from two paragraphs are contradictory. I think the words should be understood as below. There are two types of HSC. One is “galvanically induced HSC”, it is abbreviated as GHSC, because in NACE MR0175-3, clause 3.7,the definition galvanically induced hydrogen stress cracking cracking that results due to the presence of hydrogen in a metal, induced in the cathode of a galvanic couple, and tensile stress (residual and/or applied) The other one is “Sulfide induced HSC” .e.g “Sulfide Stress Crack”(SSC) because in NACE MR0175-1 clause 3.23, the definition is as below. SSC is a form of hydrogen stress cracking (HSC) and involves the embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulfides. The atomic hydrogen can diffuse into the metal, reduce ductility and increase susceptibility to cracking. High strength metallic materials and hard weld zones are prone to SSC. Please help me if my understanding about HSC, GHSC, and SSC right or not?

Answer:

We thank you for your inquiry and agree that there is some confusion in the notes to these definitions. The HSC includes cracking in metals that are not sensitive to SSC but which can be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is corroding actively as an anode. The term “galvanically induced HSC” has been used for this mechanism of cracking. We believe that this can be clarified by a ballot on our definitions; we are initiating the ballot process.

This question is in relation to NACE MR0175/ISO 15156-1 3.1.3

Reference: ISO 15156 Maintenance Panel Inquiry #2016-02

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