Rapid Evaluation of Metallic Coatings on Large Cylinders Exposed to Marine Environments

Hydraulic riser tensioners for offshore drilling platforms and piston rods for hydraulic systems are used extensively in offshore oil and gas drilling and production environments. These materials are exposed to a wide variety of severe operating conditions that can range from marine atmospheres to splash zone exposures in brackish and salt water. Typically, these devices are all large cylinders that are difficult to test because of the curvature of the external surface. The present work describes a novel testing methodology, based on the zero-resistance amperometry (ZRA), which allows the determination of the critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) in small curved samples sectioned from tensioner risers and piston rods. The samples were made of carbon steel cylinders with a thick metallic coating (i.e., weld overlay) of a corrosion-resistant alloy (CRA). Moreover, the determination of the CPT and the CCT is a rapid way of prescreening candidate materials and alloys for applications that are not necessarily limited to the oil and gas industry. Several solid materials and CRA-coated carbon steel samples have been prescreened successfully using this testing methodology. The relative ranking of the CPT values of these CRA materials have been used to determine the quality of the metallic coating and the relative corrosion resistance of the material. Interestingly, two critical temperatures were found when the materials were tested with crevice formers. It is hypothesized that while part of the sample underwent crevice corrosion at the lower CCT, the rest of the surface remained unaffected. At higher temperatures, pitting corrosion took place in the areas that remained passive, leading to a second sharp current increase associated with their CPT.