Role of pH in sulfuric process fluid - NACE MR0175 (ISO 15156-2)

[aegis4048]

I've been reading through ISO 15156-2. There's two options for choosing corrosion-resistant materials in ISO:

1) Selecting from Annex A.2's Table A.2, which includes A106
2) More detailed selection guide based on the pH vs. H2S partial pressure plot

Since I'm interested in A106 already, it means I'm simply going for option 1. For H2S partial P exceeding 0.05 psi, requirements in A.2 must be met. Some of the requirements I read is that the metals need post-heat treatment (if hardness test not met), hardness test, base material percentage, blah blah blah. In section 8, I read that to avoid HIC/SWC, flat-rolled (cold-rolled) products must have sulfur less than 0.003% and seamless products less than 0.01%.

I don't see any other guidelines or requirements other than these post-heat treatment, hardness test, some pressure limits, and sulfur contents. What if the H2S ppm is like above 20,000 ppm or higher? What if the pH of the stream is very acidic? When do I switch from carbon steel to stainless steel?

It just seems that ISO 15156-2 is simply saying use A106 pipe with post-heat treatment, and I will be good. It doesn't make sense to me. Please give me feedback on if I'm understanding this document correctly.

 

[EdStainless]

Look at the chart with categories of service.
You need to respect the H2S pressure, pH, temperature, and Cl content when making these choices.

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P.E. Metallurgy, consulting work welcomed

 

[SJones]

You switch from carbon steel to a CRA when you deem the risk of failure with the carbon steel to be unacceptable. ISO 15156 deals only with H2S cracking, there are many other damage mechanisms that could be in play. If the hardness criterion is met without heat treatment, it is not mandatory to apply it. There is no limit of H2S concentration for carbon steel. You don’t appear to be fully understanding the standards, and it may prove beneficial to consult someone who does.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.