High Corrosion Resistant Metallic Coatings for Oilfield Applications Exposed to Aggressive Environments

Highly corrosion resistance alloys (HCRAs) are currently being employed for offshore riser tensioners and piston rods for hydraulic systems in marine environments. These materials are often exposed to a wide variety of severe operating conditions that can range from marine to splash zone atmospheres. The fabrication of these HCRAs metallic coatings typically involves coating low grade steels with a layer(s) of the HCRAs using novel approaches (i.e., thermal spray coating, chemical vapor deposition, weld overlay, electrodeposition, etc.).  

Previous work by the authors has described a novel testing methodology, based on the Zero Resistance Ammeter, which allows the determination of the Critical Pitting Temperature (CPT) as well as the Critical Crevice Temperature (CCT) in small angular sectioned samples taken from actual production cylinders. In this paper, the CPT and CCT measurements that were successfully employed for screening UNS N06625 weld overlay materials will be compared with those of other potential HCRAs (e.g., UNS R31233, UNS N10276, UNS W73021, etc.) to determine a relative ranking for offshore applications.  In addition, galvanic coupling of various HCRAs will also be assessed to determine corrosion susceptibility that might occur due to weld repairs (typically UNS N06625).  Different suitable materials were characterized using electrochemical techniques as well optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).

References:

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