mixed reactions and corrosive environments? 1

[stonerabroad]

could anyone tell me of mixed reactions between CO2, H2, H2S and water that could create a corrosive environment in contact with mild steel?

CO2 readily dissolves in H2O to make carbonic acid, at room temperature.

does H2S dissolve to form sulfuric acid? at temperatures of around 60C and concentrations of only 50PPM? 300PSI?

is there anything i should be watching for here? if so, at what concentrations should i limit the H2S to?

 

[DeltaCascade]

Do you have a vapour or a liquid system?

H2S reacts with iron to form iron sulfide. Iron sulfide is typified by "black water" seen in many oilfields. Acid gases will reduce water pH, depending upon the water's buffering capability, and increase corrosiveness of the water.

Maye check NACE's web site (National Association of Corrosion Engineers).

Good luck.

 

[grumpus]

Stoner,

My knowledge here is limited to aqueous H2S in carbon and stainless steels.

When H2S is present in aqueous acidic solutions heavy corrosion can occur. The hydrogen sulfide in solution will react with the iron in the carbon steel forming two products: molecular hydrogen and iron sulfide. The molecular hydrogen blisters (actually diffuses into) the steel and to make matters worse the iron sulfide flakes off the piping/vessel and gets carrried along. This iron sulfide can BOTH scour surfaces causing "erosion-corrosion" and can pocket causing "under deposit corrosion". However, I offer some good news the iron sulfide can form a protective passive layer on the carbon steel and prevent further corrosion (provided it is not continuously disturbed). Iron sulfide is easily detected in samples by using a magnet. Stainless steel is a good metallurgical upgrade here. Piping velocities should be kept low and sharp elbows should be avoided as well (scouring prevention). Filtration is often used to remove iron sulfides.

Grumpus

 

[stonerabroad]

thanks grumpus.

at concentrations of only 50PPM though, would these reactions occur?

 

[25362]

If conditions are stagnant, and/ot at low velocities, and the temperature level mentioned, depending on the partial pressures, I believe that a tenacious film of FeS could form protecting the underlying carbon seel from corrosion.
Hydrogen penetration in presence of H2S, would probably also be limited. In the "reducing" atmosphere present no sulfuric acid would form.

Since the quality of the water isn't defined, at the end, actual experience may tell you more.