Abstract
Thermally sprayed aluminium (TSA) has successfully been used for corrosion protection for several decades. Coating lifetimes of more than 30 years in corrosive marine atmosphere has been documented. However, it has also been found that coating TSA with thick, protective organic coatings may result in rapid corrosion of the TSA and very short lifetimes. A corrosion mechanism resembling crevice corrosion is causing the degradation. The problem is avoided by not covering TSA with thick organic coatings. However, in many situations it is difficult to avoid coating the TSA. E.g. when repairing damages in the TSA in the field, the repair coating often overlaps with the surrounding TSA. Hence, a repair coating that can protect steel, but not trigger the crevice corrosion mechanism when applied on the TSA is needed. In this investigation different repair coatings have been studied with respect to performance when applied on both TSA and bare steel. Coatings have been selected based on four different hypotheses: (i) coatings that can buffer the low pH developed in the corrosion of TSA, (ii) open coatings that can let the acidic environment be washed out and (iii) conductive coatings that can make electrochemical reactions take place outside the TSA/coating crevice and (iv) sacrificial coatings. A number of coatings have been selected for testing with this in mind and tested in the ISO 20340 cyclic ageing test.
