In the present work, the corrosion resistance of carbon steel studs coated with Ni-Co, Cd, Zn, and Zn-Ni was tested and measured by electrochemical techniques and environmental exposure tests.
Omar Rosas / Jose Hernandez / Oscar Garcia / Carlos Girault
One of the most common problems when dealing with bolted joints in the oil and gas industry is corrosion; corrosion damage can affect the stability of the clamping force and it accelerates mechanical failures such as thread stripping and fatigue. Coatings are the most popular solution to protect bolts from corrosion, and electroplating has been used to overcome problems such as oversizing. Cadmium and zinc have been the two most popular selections for electroplating due to their high corrosion protection properties however, some problems associated to environmental risks and hydrogen embrittlement lead to keep searching for other materials; nickel-cobalt electroplating is a proven solution providing high corrosion resistance and preventing oversizing in bolts and nuts.
In the present work, the corrosion resistance of carbon steel studs coated with Ni-Co, Cd, Zn, and Zn-Ni was tested and measured by electrochemical techniques and environmental exposure tests. The preliminary results showed an outstanding performance of Ni-Co plating compared to the other materials after 10,000 h of salt spray chamber tests; electrochemical techniques showed higher values of polarization resistance for Ni-Co plating compared to the other materials. Galling tests on Cd, Zn, and Ni-Co showed the ability of the latter to dismantle the nut from the stud after applying torque and environmental exposition. Finally, mechanical tests on cathodically protected Zn, Zn-Ni, and Ni-Co were performed, the results showed that the mechanical resistance of Zn coatings was reduced after cathodic protection, Ni-Co was unaltered. Thus, Ni-Co electroplating is proposed as an emerging technology in carbon and high-strength steel bolts to be used in the harsh conditions found in oil and gas and subsea.
Key words: Fasteners, Bolts, corrosion resistant, Ni-Co, electroplating, hydrogen embrittlement Oil and Gas