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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.

Connector bolt failures due to stress corrosion cracking (SCC) caused by hydrogen embrittlement (HE) are a major problem for structures in many industries, including safety-critical equipment currently deployed in oil and gas operations. 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 investigate known failures and research possible causes. The findings 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 connector bolts is caused by a combination of three factors: material susceptibility; tensile stress; and a corrosive environment, 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 ductility 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 hydrogen while forming and finishing the material (known as internal HE), but can also be linked to corrosion as well as corrosion-control processes (known as environmental HE). Hydrogen can be produced by electroplating; corrosion reactions such as rusting; and the chemical reactions of cathodic protection (CP) systems and sacrificial coatings, which are metal coatings designed to corrode in place of the steel they coat. According to Doxsteel, hydrogen is produced by sacrificial coatings as they oxidize, which can permeate the bolt and cause HE.2 Additionally, the company notes that any plating process can produce hydrogen; and in parts with hardness higher than 31 Rockwell C hardness (HRC), such as industrial fasteners, an uncontrolled plating process can lead to HE.3

Ni-Co alloy has been used in applications 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 elements and are similar, highly noble metals—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 properties 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 everywhere: 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 hydrogen doesn’t affect it. That’s why it is corrosion resistant,” he explains.

A Ni-Co alloy-coated fastener (left) and a cadmium-coated fastener (right) after eight months of installation in a refinery environment. Photo courtesy of Doxsteel Fasteners.

Electrodeposited Ni-Co alloy coating is one of the materials promoted by NASA’s Technology 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. Compared to conventional electrodeposited nickel, the resulting Ni-Co alloy is strong, has significantly enhanced tensile yield strength—the increase in tensile strength is directly proportional to the concentration of cobalt—and also retains weldability, corrosion resistance, and ductility.4

Garcia and Rosas stress that electrodeposited Ni-Co alloy is not a sacrificial coating. The electrodeposition process creates a smooth, hard, dense, abrasion-resistant protective coating, with a very low coefficient of friction. The Ni-Co alloy coating resists chlorides, ultraviolet light, and high humidity, as well as hydrogen. Due to its high melting point, the coating can also be used in more extreme temperatures, such as manholes in heat exchangers, without the risk of liquid metal embrittlement.

The coating does not change the mechanical or chemical properties of any steel it coats. It elongates and contracts in similar ways to steel, so it doesn’t chip or break as the substrate is put under stress such as torquing. Additionally, the Ni-Co alloy doesn’t generate hydrogen as part of its reduction process, Rosas adds. This means it will not introduce hydrogen or cause HE no matter how long the coating is in service and how much it oxidizes.

Since it is an excellent electrical conductor, note Garcia and Rosas, the Ni-Co alloy coating maintains the electrical continuity of a CP system that may be applied to the base material. The Ni-Co alloy also has a low kinetic rate, so it is slow to react to its surrounding environment, which enhances its corrosion resistance when exposed to corrosive environments or coupled with dissimilar metals. Recent laboratory experiments by Doxsteel have indicated the Ni-Co coating is also resistant to hydrogen permeation in seawater at the high pressures (300 to 600 psi [2 to 4 MPa]) found in subsea operations.

The requirements for corrosion-resistant electrodeposited Ni-Co coatings are described in ASTM B994,5 which also provides processing steps for the coating to reduce the risk of HE from hydrogen introduced during substrate fabrication and electroplating. The standard specifies a range of 43 to 61% for nickel content and a range of 39 to 57% for cobalt. While the Ni-Co alloy is mainly electrodeposited as a coating on steel products such as machined parts, springs, latches, threaded parts, and fasteners, it also can be deposited on iron, stainless steel, aluminum, titanium, or any other metal substrate. The electrodeposition process applies the coating evenly across the entire part; and thicknesses can range from 5 µm for slightly corrosive service environments to >25 µm for highly corrosive environments, depending on the shape of the substrate. For threaded pieces, the deposited alloy should be thick enough to provide coating protection without interfering with the fit and function of the threads. For bolts in marine and industrial environments, Rosas recommends using the ASTM B994 service condition SC18 Class 1, which signifies that the application is corrosive and a minimum coating thickness of 18 µm is sufficient to protect against corrosion.

Applying an electrodeposited coating involves several steps. To reduce the risk of HE, ASTM B994 calls for a heat treatment as a pretreatment of steel parts with an ultimate tensile strength of >1,000 MPa (31 HRC) that have been machined, ground, cold formed, or cold straightened. The metallic substrate is then typically cleaned to remove oil or grease, rinsed, and put through a pickling process to remove impurities such as rust, scale, or inorganic contaminants.

After cleaning, the metal substrate is placed in an electrolyte bath as the cathode, with nickel and cobalt anodes. The bath is electrically charged so the nickel and cobalt ions are attracted to the metal substrate, and the process is monitored for time, temperature, and chemical composition so the substrate is ultimately covered with a consistent thickness of the protective alloy coating. After the electrodeposited alloy coating is applied, ASTM B994 recommends baking the coated steel parts with a tensile strength >1,000 MPa to allow the diffusion of any internal hydrogen and reduce the risk of HE.

In addition to acceptance tests for appearance, adhesion, and thickness of the coating, ASTM B994 also lists several qualification tests. These include a HE test using ASTM Test Method F519,6 which detects possible HE of steel parts due to the plating and coating processes and also due to chemical contact during service life, and an electrochemical corrosion rate test using ASTM Test Method G59.7 Garcia and Rosas note that results of the HE test run by Doxsteel showed that bolts coated with the electrodeposited Ni-Co alloy coating did not fail after 200 h of applied tensile stress at 75% of their notched tensile strength. An electrochemical corrosion test performed by Doxsteel showed a corrosion rate of 0.1603 µm/y for Ni-Co alloy coated bolts in a 5% sodium chloride (NaCl) solution vs. 29.7572 µm/y for bare steel. In environmental tests using ASTM B117,8 the Ni-Co alloy reached 3,000 h with no significant presence (<1%) of red rust.

Source: Doxsteel Fasteners, www.doxsteel.com. Contact Omar Rosas, Doxsteel Fasteners—email: omar.rosas@doxsteel.com.

References

1 “Bolt and Connector Failures,” Bureau of Safety and Environmental Enforcement, https://www.bsee.gov/what-we-do/offshore-regulatory-programs/emerging-technologies/bolt-and-connector-failures (October 6, 2017).

2 “Layers of Protection,” Doxsteel Fasteners, http://www.doxsteel.com/wp-content/uploads/2017/08/DSF_BSEEPresentation_082617.pdf (October 6, 2017).

3 “Doxsteel Coating—Hydrogen Embrittlement,” Doxsteel Fasteners, http://www.doxsteel.com/doxsteel-coating/ (October 6, 2017).

4 “Method Increases Tensile Strength for Electrodeposited Nickel-Cobalt Alloy,” NASA Technology Opportunity, Case No. MFS-32777-1, https://quicklaunch.ndc.nasa.gov/file/resource/MSFC-QL-0046/Mini%20Listing%20MFS-32777-1.docx (October 8, 2017).

5 ASTM B994/B994M-15, “Standard Specification for Nickel-Cobalt Alloy Coating” (West Conshohocken, PA: ASTM, 2015).

6 ASTM F519-17, “Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments” (West Conshohocken, PA: ASTM, 2017).

7 ASTM G59-97 (2014), “Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements” (West Conshohocken, PA: ASTM, 2014).

8 ASTM B117-16, “Standard Practice for Operating Salt Spray (Fog) Apparatus” (West Conshohocken, PA: ASTM, 2016).

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