Galvanic corrosion of copper-base alloys in contact with molybdenum-containing stainless steels in Arabian Gulf water

Abstract

The condenser tubes of MSF distillers of the Water and Electricity Department of Abu Dhabi are made of either 90 10 or 70 30 CuNi alloys or of Al-Brass and are mounted on 90 10 or 70 30 CuNi tube plates. The tubes suffer pitting-, SCC and erosion corrosion from the water side and general attack from the vapour side. The resulting copper and nickel ions are potential health hazards and harmful pollutants to the environment. In the event of retubing, it is recommended to use more corrosion-resistant, environmentally more friendly tube material. High strength Mo-containing stainless steels qualify both requirements. As these are going to be mounted on the already present copper-base tube plates, galvanic corrosion of the plates is liable to occur. The present study was undertaken to establish the extent of attack that would evolve and its dependence on operation variables. The study was carried out on six Mo-containing stainless steels with Mo contents varying between 0.3 and 6.0 wt% and three copper-base alloys ( 90 10 and 70 30 CuNi alloys and Al-bronze) in sterile Arabian Gulf Water. The study involved the measurement of the open circuit potentials (OCPs) of single metals and the couple potentials (CPs) till constant values. The corrosion current produced upon coupling was followed simultaneously. The effects of varying cathode/anode surface area, of temperature and of stirring on these parameters were examined in detail. The results obtained indicated that the stainless steel cathodes were more polarizable than the copper-alloy anodes. The (CPs) were very near to the (OCPs) of the anodes. The cathode polarizability decreased slightly with increase of cathode/anode are a ratio in the case of couples involving the 90 10 and 70 30 alloys, but was not affected in the case of Al-Bronze. The rates of galvanic attack of the copper alloys increased linearly with cathode/anode area ratios in the range 0.36–6. These rates were the same for couples incorporating the 90 10 and Al-Bronze anodes, and these were higher than those measured with the 70 30 material. In all cases the corrosion rates increased with temperature to reach maximum at 50°C. Above this temperature the rates dropped sharply as result of expulsion of oxygen from solution. Stirring increased the galvanic corrosion rates as result of increased mass transfer of oxygen to the cathodes and of removal of corrosion products from anodes surfaces. Under otherwise the same conditions the corrosion of the tested copper alloys decreased in the order: Al-Bronze 90 10 CuNi alloy> 70 30 CuNi alloy