GALVANIC CORROSION OF COPPER-BASE ALLOYS IN CONTACT WITH MOLYBDENUM-CONTAINING STAINLESS-STEELS IN ARABIAN GULF WATER

The condenser tubes of MSF distillers of the Water and Electricity Dep artment 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 a nd general attack from the vapour side. The resulting copper and nicke l ions are potential health hazards and harmful pollutants to the envi ronment. In the event of retubing, it is recommended to use more corro sion-resistant, environmentally more friendly tube material. High stre ngth Mo-containing stainless steels qualify both requirements. As thes e are going to be mounted on the already present copper-base tube plat es, galvanic corrosion of the plates is liable to occur. The present s tudy was undertaken to establish the extent of attack that would evolv e and its dependence on operation variables. The study was carried out on six Mo-containing stainless steels with Mo contents varying betwee n 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 involve d the measurement of the open circuit potentials (OCPs) of single meta ls and the couple potentials (CPs) till constant values. The corrosion current produced upon coupling was followed simultaneously. The effec ts of varying cathode/anode surface area, of temperature and of stirri ng on these parameters were examined in detail. The results obtained i ndicated 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 o f 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 sa me 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 degrees 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. U nder otherwise the same conditions the corrosion of the tested copper alloys decreased in the order: Al-Bronze > 90/10 Cu-Ni alloy > 70/30 C u-Ni alloy.