Soluble Salt Contamination on Blast Cleaned Surfaces and the Effect on the Durability of Subsequently Applied Coatings

摘要

It is well known that residual water-soluble contaminants on steel surfaces, prior to protective coating application are detrimental to subsequent coating performance. Levels as low as 0.3-0.6 μgcm~(-2) have been suggested by some organisations as allowable soluble salt contamination on steel surfaces. if levels this low are incorporated into standards documents, then this could result in expensive and unnecessary cleaning operations prior to the coating of steel surfaces. Corus R,D&T has undertaken an RFCS project (in collaboration with ProfilArbed -Luxemburg, MPIE -^sGermany and CENIM, Spain) in order to assess the impact of water-soluble salt contaminants on in-service performance of subsequently applied coatings and to establish realistic working limits. Typical industrial coating systems incorporating elements such as zinc silicate primers, fusion bonded epoxies, glass flake epoxies, micaceous iron oxide (MIO) primers, glass flake epoxy polyamides and zinc rich systems with different exposure conditions (immersion, splash or atmospheric environments) were evaluated. Acceptable contamination levels of 10 -40 μgcm~(-2) were typically observed. Results also show that different coating systems can tolerate different levels of residual salt and the maximum tolerable salt value, for a specific coating system, depends on the exposure conditions. In general the safe limits suggested by this work [2] are 1 - 2 orders of magnitude higher than the more conservative assessment [1]. These results are reinforced by the observation that the artificial doping of surfaces generates a greater "aggressiveness" compared to natural contamination. Thus the limits highlighted by this study are probably conservative. Basic mechanistic studies on model systems performed by MPIE support the idea of a critical contamination level below which little effect on durability is seen. One point of concern, however, is the impact on cathodic protection (CP), where the effect of sulphate, when either present alone or in combination with chlori de, could be seen. CP is widely used (particular offshore) and this observation requires further investigation to fully understand the corrosion mechanism.