Evaluation of Protective Coatings for High-Corrosivity Category Atmospheres in Offshore Applications

The interest in renewable energies obtained from the resources availed in the ocean has increased during the last few years. However, the harsh atmospheric conditions in marine environments is a major drawback in the design of offshore structures. The protective systems that are employed to preserve offshore steel structures are regulated by several standards (ISO 12944, NORSOK M-501), which classify the corrosivity category of offshore installations as C5-M and Im2. In this work, three coatings employed in offshore components protection have been evaluated according to these standards by performing weathering aging tests in different climatic cabinets. The coatings studied were a thermally sprayed carbide coating with an organic sealant (C1), a thermally sprayed aluminum (TSA) coating with an organic topcoat (C2), and an epoxydic organic coating reinforced with ceramic platelets (C3). The only coating that reached the higher categories in all the tests was the C2 coating. The C1 coating presented ferric corrosion products coming from the substrate in some of the tests, and blistering was detected in the C3 coating.

Evaluation of Aluminum-Coated Flexible Pipe Armour Wires Exposed to a Sour Environment

In general, tensile armour wires of flexible pipes that are designed for sour applications have their strength limited to 850 MPa due to the possibility of embrittlement phenomena to occur. A Thermally Sprayed Aluminum (TSA) coating 250 μm thick was applied to high strength steels with UTS of 1470 MPa and YS of 1280 MPa. Three specimens conditions were evaluated: full coating, no coating and coating with a designed defect. The load was applied using a four point bending fixture, maintaining a constant stress of 90% of material’s yield strength. All tests were performed in accordance with recommendations of NACE TM 0177 method B. The test solution was distilled water with NaCl 5.0% saturated with a gas mixture of 10,000 ppm of H2S in balance with CO2 during 720 hours. It was observed that samples without coating were more susceptible to the effect of the environment presenting higher degradation and failure. The fractures presented typical characteristic of the Sulfide Stress Corrosion Cracking (SSCC). Furthermore, it was detected parallel cracks to the surface of the wires indicating the embrittlement phenomenon of Hydrogen-Induced Cracking (HIC). On the other hand, coated samples with and without defects did not fail during the 720 hours of testing. A posterior non-destructive testing and a metallographic analysis did not identify the presence of cracks. These results were attributed to the physical barrier of the aluminum coating and the cathodic protection generated by the preferential aluminum corrosion. This preliminary study shows that TSA coatings can be a good alternative to increase the corrosion resistance of armour wires in sour environments allowing the application of higher strength steels.