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18 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIAL MATTERS Continued f rom page 17 Photographs of the TSA-coated sample (a) before and (b) after testing in synthetic seawater. White corrosion product, Al(OH) 3 , is evident on the surface of the intact TSA coating. Images courtesy of Shiladitya Paul. The calcareous deposit formation mechanism in the sample's holiday region. Image courtesy of Shiladitya Paul. seawater reference electrode. The polar- ization process also promotes a calcare- ous deposit on exposed steel that reduces the cathode area and, as a result, decreases the corrosion rate; however, dissolution of the aluminum in the coat- ing will occur during this activity. Subsea gas pipelines account for 45% of natural gas exports to Europe, and in some areas the seabed can be several kilo- meters deep. 3 In deep sea environments, a TSA coating can sustain damage from movement of the seabed that can cause wear, or from the impact of surrounding geological features, such as rocks. Paul notes the behavior of TSA-coated CS in natural seawater—with and without external CP—has been studied. The mechanism responsible for calcareous deposit formation on CS in seawater due to CP, and the inf luence of parameters such as temperature, pH at the steel/sea- water interface, and the various ions pres- ent in seawater, have been examined as well, although the research has ty pically focused on calcareous deposit formation under ambient pressure and ambient or low temperatures with the polarization stemming from galvanic CP or impressed current CP rather than a TSA coating. Not much has been reported on the corrosion performance of TSA when it is damaged, and data on the performance of damaged TSA under the level of pressure experi- enced in deep seawater are virtually nonexistent. "The solubility of the constituents that form these calcareous compounds is dependent on pressure," Paul explains. "When using a thermally sprayed alumi- num in very high-pressure environments, we don't know if the deposits that form will be protective or not if the coating is damaged." To address these knowledge gaps, Paul conducted a research project to study the protective performance of TSA in deep seawater. In CORROSION 2017 paper no. 9009, "Protection of Deep Sea Steel Structures Using Thermally Sprayed A luminum," he reports the results. For the study, a f lat 42- by 40- by 6-mm coupon made of carbon-manganese steel (BS EN 10025 Grade S355J2+N), which is often used in offshore environments, was thermally sprayed on one side with com- mercially pure aluminum (99.5% A l) using a twin-wire arc spray to deposit a TSA coating with a nominal thickness of 300 µm. A defect (holiday) amounting to 3% of the sample area was created by drilling through the entire thickness of the TSA coating and exposing the underlying CS substrate. In an autoclave, the sample was exposed to synthetic seawater (per ASTM D1141 4 ) at 5 °C for 90 days at 50 MPa to simulate water pressure at a depth of 5,000 m. Since water pressure increases ~1 atm (the atmospheric pressure at sea level) for every 10 m of water depth, the pressure at a depth of 5,000 m is ~500 atm or 500 times greater than the pressure at