Improved Splash and Tidal Zone Coatings for Extended Design Life of Offshore Structures

Offshore operators are currently looking to extend the design life of offshore facilities, structures and components to improve the affordability, and to increase their availability in later years of operation. Duplex stainless steels, often used in offshore structures, are susceptible to localised corrosion and environmentally assisted cracking when subjected to loads and temperatures approaching their material design limits in marine environment. Conventional organic coatings (paints) provide limited mitigation due to their rapid degradation at elevated temperature, particularly in hot risers. Whilst the use of thermally sprayed aluminum (TSA) to mitigate the corrosion of C-steels and austenitic stainless steels is well-documented, there are little or no published data relating to its behaviour on duplex stainless steels (DSS) or other high strength alloys.

This paper summarises the results of a TWI Joint Industry Programme which comprised a detailed evaluation of the performance of TSA-coated, 22Cr DSS welds subjected to pitting corrosion, external chloride stress corrosion cracking (SCC) and hydrogen embrittlement (HE). The effect of plastic strain (at values simulating pipe reeling) on TSA coating integrity and behaviour was investigated. TSA corrosion rate data were produced at ambient and elevated temperature using two electrochemical techniques. The effects of sealant selection on TSA coating behaviour were characterised. Recommendations for the application of TSA coatings on 22Cr DSS are provided.