Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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38 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM Continued f rom page 37 COATINGS & LININGS ESSENTIALS Damage-Tolerant Sacrificial Coatings Provide Offshore Protection ermal-sprayed aluminum (TSA) sacrificial coatings have been used in offshore oil and gas platforms for decades. e low corrosion rate of aluminum in seawater, coupled with its abil- ity to act as an anode with respect to steel, makes TSA ideal for offshore applications. It provides a barrier layer when intact and ca- thodic protection (CP) when damaged. e damage tolerance of TSA coatings is known from experience, but the level of damage these coatings can endure and still provide sacrifi- cial protection is unexplored. Understanding the effect of large defects or damage on TSA's performance is important. While TSA would polarize steel even if damaged, the polariza- tion may be insufficient to protect the steel or the dissolution rate of TSA may be high. 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. e potential, however, became less nega- tive with time and reached values close to the corrosion potential of steel in 35 days. Even after values close to the steel corrosion poten- tial 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 sub- sequent reduction of its service life. Further, in cases where TSA coating with extreme dam- age 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.

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