Reducing Hydrogen Embrittlement Risk of Steel Bolting in Seawater Environment with Nickel-Cobalt Electroplating

Abstract

AbstractAmongst all the several threats derived from corrosion in bolting in marine environments, hydrogen embrittlement is perhaps one of the most dreaded. Hydrogen embrittlement is a form of environmentally assisted cracking (EAC) where hydrogen generated from different sources penetrates inside the basematerial and makes it more fragile. This paper addresses the severity of the environment and the influence of different metallic coatings on the amount of hydrogen present in the base substrate steel bolting. From laboratory generated data, environmental severity and risk of hydrogen embrittlement are estimated and correlated to actual field performance from the literature. This investigation relates the interplay of environment, stress, and materials.The metallic coatings evaluated in this investigation are: Zinc, Zinc-Nickel, and Nickel-Cobalt ASTM B994, the two formers are typical coatings currently used in bolting in Oil and Gas, the latter is a novel material with offshore applications including subsea environments. Cracking resistance of the AISI 4130/4140 and 4340 also specified as ASTM A193 B7/B7M, L7/L7M and L43 studs or bolts and ASTM A194 nuts, all referred to as bolting, is characterized in terms of the variation in environmental severity. The environmental severity is measured by both, Devanathan–Stachurski cell technique to measure the amount of hydrogen absorbed into the steels substrate across the coatings, and the use of Tafel scans of the coatings and a Pourbaix diagram to measure Ecorr and amount of hydrogen generated. From these results, the metallic plated coatings are ranked as to environmental severity relative to each other.The results showed an outstanding performance of the Nickel-Cobalt coating compared to the sacrificial coatings. Sacrificial coatings produced large amounts of hydrogen in the cathodic reaction and allowed its penetration to the base-material; Nickel-Cobalt did not produce hydrogen at OCP and the permeation was much lower than in the other materials under cathodic charging conditions. The results of mechanical testing showed that Nickel-Cobalt is not affected by the cathodic charging whilst the tested sacrificial coatings reduce their mechanical resistance in a 50%; these results mean that hydrogen permeation into Nickel-Cobalt is negligible, even after being subject to high cathodic charging. Nickel-Cobalt alloy ASTM B994 is proposed as the emerging technology for carbon and high strength steel bolts to be used in marine environment and subsea where failure is not an option.The results from this effort have an outstanding relevance in offshore and subsea deep-water drilling and production equipment that depends on high strength carbon and low allow steels. This work addresses failures caused by hydrogen embrittlement that have been documented by the American Petroleum Institute (API) and the Bureau of Safety and Environmental Enforcement (BSEE).