H2S, CO2 and Chloride Ion corrosion are having their independent attributes and also can influence each other. Chloride Ion in vapour (HCl gas) form is noncorrosive itself and if H2S & CO2 creates any dew point then carbon steel corrosion due to HCl liquid will be significant. More the S , higher will be the dew point and that facilitates formation of aquous phase . Further discussion from other expert end of Refinery side is reqd.

hi I agree with them but I have some addtion. Principle is very clear no water no corrosion you know the HCL gas is crried in the process and corrosion will not occcurs until the temp. is lowerd to near the dew point of water so, in your gas line you have to predict where the location that may be you have lower temp. dew point and do some corrosion protection on it for example coating . H2s play a game with HCL on the carbon steel as : EXAMPLE .........THE HCL IN PRESENCE OF H2S BECAOME CYCLIC REACTION AND THE CORROSION BECOME MORE AND MORE SERIOUS AND THE CORROSION ENGINEER SHOULD DO SOMETHING ON IT. Fe + 2 HCL==== FeCl2 +H2 FeCl2 + H2S=== 2 HCL+Fes H2S ACTIVATE THE HCL REACTION TO DESTROY THE CARBON STEEL. H2S CAUSE THE HIC CARCKING BECAUSE WET H2S WILL PUSH THE HYDROGEN ATOMS TO THE CARBON STEEL AND TWO HYDROGEN ATOMS WILL CARETS HYDRON Molecule THAT EXPAND BECAUSE ITS SIDE IS TOO BIG AND CRABON STEEL Boundaries WILL EXPAND MORE AND MORE UNTIL IT CRACK. CO2 IF IT PRESENT WILL CAUSE PITTING CORROSION ON THE GAS LINE SO, H2S OR HCL OR CO2 CAUSE CORROSION ITSELF AT DEW POINT ONLY BUT IN THE DRY GAS NO CORROSION EVEN WE HAVE HCL 10000000000 PPM (joking ) Thanks and good luck Mark Corrosion Engineer