H2S is a great passifying agent as it converts the inside pipe surfaces made of Carbon steel to iron sulfide which then protects the pipe (at low process velocities).  Your H2S content is too low to achieve this passivation effect. Also the CO2 in water forms carbonic acid which will corrode carbon steel.  You need 304 Stainless Steel or better.  I am not sure what the effect of the hydrogen is.

My experience is from amine units stripping H2S and CO2 out of natural gas and just stripping CO2 out of natural gas.

Regards,

Rocco8@flash.net

Not having dealt with a gas of such composition (steam reforming for ammonia production or water gas shift reaction ?) and based on general corrosion concepts I'd venture that Rocco8 is probably right in proposing austenitic St St or better to deal with gases of the given composition (H2O + CO2 + H2 + H2S) at ambient temperatures.

Austenitic stainless steels, as mentioned by Rocco8, are apparently immune to hydrogen attack, but allow atomic hydrogen diffusion and its probable attack to the underlying carbon steel when protected by a St St thin loose lining or integral cladding. Atomic hydrogen forms by dissociation of molecular hydrogen at temperatures above 500F and partial pressures above 100 psia.

Besides, when planning the construction of vessels it would be in line to consider the thermal expansion of the St St steel usually much larger than for carbon steel.

I don't know if it will help, but here is a reference:

API 941, "Steels for Hydrogen Service at Elevated Temperatures and Pressures in the Petroleum and Petrochemical Industries"

I think we have used Duplex 2205 in a similar type service, though with much less pressure.