Thermally sprayed aluminium (TSA) has successfully been used for corrosion protection for several decades. Coating lifetimes of 30 years in corrosive marine atmosphere has been documented. However it has also been found that coating TSA with thick protective organic coatings will result in rapid corrosion of the TSA and very short lifetimes. The TSA under a protective coating corrodes in a crevice corrosion like mechanism. Corrosion of the TSA under the paint film results in formation of aluminium chloride which hydrolyze and acidify the electrolyte. The acidification destabilizes the protective aluminium oxide and corrosion of the TSA accelerates.The problem is avoided by not covering TSA with thick organic coatings. However in some 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 TSA around. 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 three 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. A number of coatings have been selected for testing with this in mind and tested in the ISO 20340 cyclic ageing test. The repair coatings were zinc rich primers of various types and silicate coatings with aluminium particles magnesium particles and magnesium oxide.
Key words: TSA, repair coating, durability