4130 for wet CO2 and H2S service

[stonerabroad]

just looking for material suggestions for a gaseous environment operating at 300PSI.

it's currently about 50% CO2 and 50% H2 were processing, with only around 50PPM H2S. the gas stream is also humid, 100%RH. temperature is around 60C

considering we're making pressure vessels out of the material, it must be manufacturable. thinking instead of the mild steel, maybe 4130 might be a better option.

we can't have any rust form on the inside of these vessels. it messes with our valves.

 

[Hush]

Have you considered a polymer coating on the wetted surfaces?

 

[stonerabroad]

hush,

by polymer coating i'm assuming you mean a powder coat, or a polymer lining.

i'm uncertain of how you'd line these as vessels. powdercoating the inside could be uneven and would require QA.

 

[TVP]

SAE 4130 with small additions (~0.5%) of molybdenum are often used in H₂S environments. Hardness should be kept on the low side (26 HRC maxmimum) in order to minimize susceptibility to sulfide stress cracking.

Regarding your question about coatings, you are correct, iron phosphate really doesn't offer much corrosion protection. Zinc phosphate and zinc chromate are both good pretreatments when used in conjunction with a polymer coating. Epoxies and polyurethanes are typically applied over these pretreatments, by any number of methods-- electrostatic spray, cathodic electrocoating, etc. I am not familiar enough with H₂S to know if it seriously degrades either of these polymers. I would talk to a good paint supplier (PPG, BASF, etc.) about compatibility and potential application methods.

 

[stonerabroad]

the compatibility info i've got suggests typical polyester-epoxy powdercoating blends would be adequate for H2S environments.

i'll chat with our vendors to determine if coating the inside is a possibility.