Corrosion at Heat Exchanger( condenser )

[Reformator]

Dear all

At plant we have corrosion issue at heat exchanger. Could you please provide me some information about how to solve this issue and in case of choosing right alloy, which alloy will be best for my case.

Fluid at tubeside is: Treated water
{ 1.Cl = 114mg/l
2.pH = 7.90
3.SO = 25.43 mg/l
4.Sulfur = 347 mg/l
5.CaCO3 = 396 mg/l
6.temp (at tube bundle):max 70 C, min: 15 C. Pressure: 0,75 MPa
Material of tube bundle is: OLT35K

Fluid at shell: Hydrocarbon + H2S
{1. temp = max: 90 C min:15
2. Pressure = 0,75 MPa
Material of shell: OLT35K

Problems are mainly at tube bundle, they are all black and corroded, I believe it is due to FeS.

Please, help me to choose right alloy

Thank you!