Copper alloys are widely used in seawater cooled heat exchangers because of their good thermal conductivity and resistance to corrosion. It is well documented that the presence of small amounts of sulphide, as a pollutant, can cause rapid failure of aluminium brass and copper-nickel alloys. Concentrations as low as 0.01mg/L have been observed to cause corrosion.

Chlorine or hypochlorite is often added to seawater to reduce fouling, particularly of carbon steel, stainless steels and non-metallic piping. Dosing levels are usually small and heat exchanger inlet values may vary from 0.25 to 1.0mg/L free chlorine. What is less well known is that when chlorine is also present, the corrosion can be greatly increased. When both chlorine and sulphide are present together, the 66/30/2/2 Cu-Ni-Fe-Mn alloy is the most susceptible to this type of attack, followed by aluminium brass and 90/10 copper nickel.

The photograph below shows pitting of a 66/30/2/2 Cu-Ni-Fe-Mn heat exchanger tube that perforated just a few months after start-up. Chlorine concentrations were high initially (~1mg/L at the heat exchanger). The sulphide concentration is not known, but sulphide was clearly detected in the corrosion products.

                                  Pitting in a 66/30/2/2 Cu-Ni-Fe-Mn tube due to sulphide and chlorine.