The settling of critical levels of soluble salts for painting

The presence of hydrosoluble species, mainly chlorides and sulphates, at the metal/paint interface promotes osmotic blistering of the coating and underfilm metallic corrosion when the concentration of the soluble salts exceeds a critical level. Both processes can lead to the deterioration of the paint system in a very short period of time. The International Standards Organization (ISO) has for some time been trying to develop a standard about guidance safe levels for water-soluble salt contamination before the application of paints and related products. However, it is difficult to set acceptable unique levels since each type of coating and thickness varies in susceptibility to soluble salt degradation and also the exposure conditions vary. In this study, by a variety of accelerated and natural weathering trials, realistic series of limits of soluble salt have been provided depending on the coating systems and exposure conditions. The results show that the generalised levels suggested in the draft version of the ISO standard are far too low compared with the critical levels obtained here. It is suggested that higher levels could be defined, which would save costs and time.