INTRODUCTION
Highly corrosion resistance alloys (HCRAs) are becoming more popular for oilfield applications, where not only corrosion resistance is important, but also mechanical integrity and long term reliability. These materials are often exposed to a wide variety of aggressive environments that can range from marine splash zone to down-hole sour environments. The fabrication of these HCRAs metallic coatings typically involves coating low grade carbon steels with one or different layers of the HCRAs using techniques such as 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 amperometry (ZRA), 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 hydraulic cylinders. In this paper, the CPT and CCT measurements that were successfully employed for evaluating UNS N06625 weld overlay materials compared with two potential HCRAs coated steels (UNS R31233 and UNS W73021). The main objective was to develop a rapid approach for the relative ranking of these materials for aggressive offshore applications. From the results obtained in this work, it is expected that the UNS W73021 weld overlay should have relatively better performance than the UNS N06625 weld overlays, which is expected to have relatively better performance than the R31233 weld overlays.
Highly corrosion resistance alloys (HCRAs) have been employed in offshore riser tensioners, hydraulic cylinders, pipeline coatings and in general, where the environments are extremely corrosive in the processing industry. These materials are often exposed to a wide variety of aggressive environments and the ultimate end user requires increasingly higher lifetimes and reliability.
The fabrication of these metallic HCRA coatings typically involves coating a low grade carbon steels with one or more layers of the HCRA in order to provide similar HCRA properties (corrosion, wear, mechanical, etc.) to the surface of steel. The majority of the currently employed coatings are either a mixture of corrosion and wear resistant coatings, for example cobalt alloy, nickel alloys and tungsten carbide). In order to obtain improvements in metallic coatings, producers have been “fine tuning” the selected technique (i.e., thermal spray coating, chemical vapor deposition, weld overlay, electrodeposition, etc.) while striving for the best combination of the desired properties (corrosion, wear, mechanical, etc.).
Of particular interest, laser welding, a weld overlay method, focuses the energy of a solid-state or gas laser onto the surface of a substrate producing a concentrated heat source that generates a well controlled and narrow melt pool1. The HRCA powder is injected into the melt pool using a co-axial, lateral or any combination of nozzles. Rotation of the substrate combined with the lateral movement of the laser nozzle allows the melt pool to solidify producing a uniform and fused HRCA cladding overlay on the substrate with excellent adhesion properties. Currently, weld overlay HCRAs formed using laser welding/cladding techniques are being actively researched in industry and have shown promising properties for offshore applications2.
