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19 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 Information on corrosion control and prevention C onnector bolt failures due to stress corrosion cracking (SCC) caused by hydrogen embrittlement (HE) are a major problem for structures in many indus- tries, including safety-critical equipment currently deployed in oil and gas opera- tions. Electroplating fasteners with a Ni-Co alloy has been found to decrease hydrogen production on the coating's surface and prevent hydrogen from penetrating the underlying metal bolt. According to the Bureau of Safety and Environmental Enforcement (BSEE) (Washington, DC), many failures of bolts used to connect blowout preventers, risers, and other subsea equipment have been reported. Leaks detected during an oil and gas drilling operation in the Gulf of Mexico pointed to failures from severe SCC fracture of bolts on the lower marine riser package. The BSEE formed a Quality Control Failure Incident Team to investi- gate known failures and research possible causes. The f indings concluded that the connector bolt failures were primarily caused by hydrogen-induced SCC due to HE. 1 NACE International members Oscar Garcia and Omar Rosas, senior corrosion specialists with Doxsteel Fasteners (The Woodlands, Texas), note that SCC of con- nector bolts is caused by a combination of three factors: material susceptibility; tensile stress; and a corrosive environ- ment, which can include the presence of hydrogen. HE is caused by the ingress of hydrogen into metals such as steel and can seriously reduce a component's duc- tility and load-bearing capacity, cause cracking, and lead to catastrophic brittle failures (e.g., a bolt becomes brittle and can break off) at stresses below the yield stress of susceptible materials. Garcia and Rosas explain that the presence of hydrogen is often a result of the unintentional introduction of hydro- gen while forming and f inishing the material (known as internal HE), but can also be linked to corrosion as well as cor- Nickel-Cobalt Alloy Coating Protects Steel Fasteners from Hydrogen Embrittlement Bolts with an electrodeposited Ni-Co alloy coating have been used in offshore installations (left) and refinery locations (right). Photos courtesy of Doxsteel Fasteners. rosion-control processes (known as envi- ronmental HE). Hydrogen can be pro- duced by electroplating; corrosion reactions such as rusting; and the chemi- cal reactions of cathodic protection (CP) systems and sacrif icial coatings, which are metal coatings designed to corrode in place of the steel they coat. According to Doxsteel, hydrogen is produced by sacrif i- cial coatings as they oxidize, which can permeate the bolt and cause HE. 2 Addi- tionally, the company notes that any plat- ing process can produce hydrogen; and in parts with hardness higher than 31 Rock- well C hardness (HRC), such as industrial fasteners, an uncontrolled plating pro- cess can lead to HE. 3 Ni-Co alloy has been used in applica- tions by both the military and NASA in environments where withstanding high temperatures and resisting corrosion are critical. Nickel and cobalt are adjacent to each other in the periodic table of ele- ments and are similar, highly noble met- als—more noble than steel, says Rosas. He notes the two elements form a stable, solid solution that can form a very hard, conductive coating that is able withstand temperatures up to 1,500 °C, with proper- ties that include abrasion resistance to guard against erosion, and kinetics that make it impervious to atomic hydrogen penetration and ingress that can cause steel to become brittle and crack. "Hydrogen is every where: in the water, the environment, the air, and vapor phases. It's a natural reaction. The more hydrogen we have, the more likely we are to have hydrogen embrittlement," says Rosas. "The nickel-cobalt alloy has a more positive potential than steel, and hydro- gen doesn't affect it. That's why it is corro- sion resistant," he explains. Electrodeposited Ni-Co alloy coating is one of the materials promoted by NASA's Technolog y Transfer Program. Developed by NASA for its space shuttle program, the electrodeposited Ni-Co coating method enhances the mechanical properties of electrodeposited nickel by adding small amounts of cobalt. Com- pared to conventional electrodeposited nickel, the resulting Ni-Co alloy is strong, Continued on page 20