Hydraulic riser tensioners (for offshore drilling platforms) and piston rods (for hydraulic systems) have been extensively used in marine environments. They are exposed to a wide variety of severe operating conditions that range from marine atmospheres to splash zone in brackish and saltwater. The present work describes a novel testing methodology, based on the Zero Resistance Ammeter that allows the determination of the Critical Pitting Temperature (CPT) as well as the Critical Crevice Temperature (CCT) in small angular sectioned samples taken from cylinders (tensioner risers and piston rods). The determination of the CPT and CCT allows the pre-screening of various materials and alloys for their use as piston rods. Several materials and alloys have been successfully pre-screened using this testing methodology combined with metallurgical analysis. Of particular interest, a part of the sample may undergo crevice corrosion while the sample is being tested, whereas the rest of the sample will remain intact. As the temperature is further increased, pitting corrosion can take place in those passive areas that have not undergoing crevice corrosion.
Hydraulic riser-tensioners for offshore drilling platforms and piston rods for hydraulic systems have been extensively used in marine and saltwater environments (see Figure 1). Additional applications can be found in different industries that range from the Energy Sector to the maritime and construction. Consequently, these materials are exposed to a wide variety of severe operating conditions that range from marine atmospheres to splash zones in brackish and seawater. Currently, different options are being considered for the production of piston rods (see Table 2). The most common and economic material being the carbon steel piston rods (made out of UNS K02501, UNS G10450, UNS G41400, etc.) coated either with a hard chromium or a ceramic coatings with a bond coat in between these materials, which serves as an adhesive layer (NiCr for example). Survival of these materials depends fundamentally on the properties of the outer coating (porosity, corrosion resistance, coating adhesion, wear resistant, etc.). Unfortunately, both hard chromium and ceramic coatings are know to exhibit corrosion failures within 3 years of service in marine and saltwater environments1. Another alternative being considered currently is the use of highly alloyed stainless steels or highly corrosion resistance Ni alloys, both of them being corrosion resistant for offshore applications in aggressive conditions. Currently, hydraulic riser-tensioner and piston rods composed of solid UNS N06625 and 25% Cr duplex stainless steels (DSS) exhibit no failures to date. However, in this case, price, availability and machinability may be a problem, when it comes to deploy hundreds of these devices (see Reference 1). Currently, a new generation of riser-tensioner and piston rods using highly corrosion resistant Ni-alloy claddings are being evaluated for implementation in marine and saltwater applications. These materials combine the strength and availability of low cost steels with the corrosion resistance and excellent adhesion of metallic coatings. Ideally, the new metallic coatings are selected because of their relatively low porosity, high corrosion-resistance, good mechanical properties, and extreme adhesion to the substrate.
