Thermal-sprayed aluminum (TSA) sacrificial coatings have been used in offshore oil and gas platforms for decades. The low corrosion rate of aluminum in seawater, coupled with its ability to act as an anode with respect to steel, makes TSA an ideal candidate for offshore applications. It provides a barrier layer when intact and cathodic protection (CP) when damaged.
The damage tolerance of TSA coatings is known from experience, but the level of damage these coatings can endure and still provide sacrificial protection is unexplored. Understanding the effect of large defects or damage on the performance of TSA is important. While TSA would polarize steel even if damaged, the polarization may be insufficient to protect the steel or the dissolution rate of TSA may be high, and the protection offered may only be short lived.
To study the damage tolerance of TSA, NACE International member Shiladitya Paul with TWI (Cambridge, United Kingdom) and the University of Leicester (Leicester, United Kingdom) exposed coated carbon steel (CS) bar with a high degree of damage to synthetic seawater and monitored the potential. Paul observed that TSA polarized the CS bar to below –800 mV vs. a silver/silver chloride (Ag/AgCl) reference electrode, even when damage exposed 90% of the steel surface.
The potential, however, became less negative with time and reached values close to the corrosion potential of steel in 35 days. Even after values close to the steel corrosion potential were reached, rust was not seen on the exposed CS surface due to the deposition of a fine layer of calcareous matter, which implies that TSA can provide corrosion protection to offshore CS structures even when extreme damage occurs. Paul notes, however, that rust spots began to emerge after 100 days, leading to the conclusion that extreme damage may lead to accelerated corrosion of TSA with subsequent reduction of its service life.
Further, in cases where TSA coating with extreme damage is exposed to splash and tidal zones with only intermittent contact with seawater, the efficacy of TSA may be limited as continuous electrolytic contact is essential for CP.
Details of this study can be found in CORROSION 2018 paper no. 10949, “Cathodic Protection of Offshore Structures by Extreme Damage Tolerant Sacrificial Coatings,” by S. Paul.