Materials Performance

NOV 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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20 NOVEMBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIAL MATTERS 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. Continued f rom page 19 has signif icantly enhanced tensile yield strength—the increase in tensile strength is directly proportional to the concentra- tion of cobalt—and also retains weldabil- ity, corrosion resistance, and ductility. 4 Garcia and Rosas stress that electro- deposited Ni-Co alloy is not a sacrif icial coating. The electrodeposition process creates a smooth, hard, dense, abrasion- resistant protective coating, with a very low coeff icient 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 temper- atures, such as manholes in heat exchang- ers, 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 con- ductor, note Garcia and Rosas, the Ni-Co alloy coating maintains the electrical con- tinuity 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 resis- tant 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-resis- tant electrodeposited Ni-Co coatings are described in ASTM B994, 5 which also pro- vides processing steps for the coating to reduce the risk of HE from hydrogen introduced during substrate fabrication and electroplating. The standard speci- f ies 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, alu- minum, titanium, or any other metal sub- strate. 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 corro- sive 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 f it and func- tion of the threads. For bolts in marine and industrial environments, Rosas rec- ommends using the ASTM B994 service condition SC18 Class 1, which signif ies that the application is corrosive and a minimum coating thickness of 18 µm is suff icient 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 electroly te bath as the cath- ode, 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 composi- tion so the substrate is ultimately covered with a consistent thickness of the protec- tive alloy coating. After the electrodepos- ited 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 hydro- gen and reduce the risk of HE. In addition to acceptance tests for appearance, adhesion, and thickness of the coating, ASTM B994 also lists several qual- ification 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 cor- rosion rate of 0.1603 µm/y for Ni-Co alloy

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