Localized corrosion at welds in structural steel under desalination plant conditions Part I: Effect of surface roughness and type of welding electrode

Abstract

The present research was conducted into the causes of and remedies for seawater corrosion in desalination plants. Several localized corrosion along structural steels and its weldments has occurred in fabricated structures, such as the shells of desalinators, which are exposed to corrosive environments. This is to be expected if localized parts of steel or the weld metal are anodic with respect to other parts of the parent metal surrounding it because the anode-cathode size ratio in large structures then facilitates rapid corrosion. However, high corrosion rates at welds has also been promoted by various other factors such as surface finish, variations in the metallographic structure between the weld metal, HAZ, and parent metal, or by residual thermal stresses. An experimental program has been undertaken to identify the pertinent factors which determine the corrosion of structural steels and welds under desalination plant conditions and to optimize the welding process variables. The work has involved: 1. The correlation of such variables as type of welding process and procedure (number of passes, welding rate, etc.), electrode composition, heat treatment processes, with the corrosion properties of weld metal and welded test pieces in high temperature seawater. 2. The determination of any galvanic effects — in this respect there may be value in producing a weld which is cathodic relative to the surrounding steel since there is then a favorable area ratio in large structures. 3. The comparison of the separate corrosion rates of weld metal, HAZ and parent metal and then correlation with the microscopical structure of the various regions.