OTC EXTRA: Review Of Different Wear-Resistant Coatings

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In this comparison, both hard chrome and MaxShield coatings are smooth and exhibit a silvery metallic color. (Source: Maxterial)

New system is an environmentally friendly replacement for a regulated hard chrome coating used in the oil and gas industry.

Hard chrome has been used extensively for coating the surfaces of different parts (e.g., hydraulic shafts and cylinders, pump internals and housings, piston rods, compressor rods, mandrels, mud motors, valves, bushing, head pulley, nipples, plunger, spindles and rotating equipment). However, hard chrome is heavily regulated by environmental and safety agencies, such as the U.S. Environmental Protection Agency and Occupational Safety and Health Agency, as well as the EU regulation act called REACH. The reason behind this concern is due to a carcinogenic chemical known as hexavalent chromium, which is used in the process of making hard chrome. This has resulted in the potential banning of hard chrome in the near future, upon emergence of a reasonable replacement.

The industry has been searching for an alternative to hard chrome in wear-resistant coating applications for many years. A high-velocity oxygen fuel (HVOF) coating is the existing replacement for hard chrome in some applications. Other than high cost, an HVOF thermal spray coating exhibits some technical issues, such as poor adhesion (debonding) and poor corrosion performance due to porosity. Both hard chrome and HVOF lack ductility (less than 1%), which causes coatings to de-bond when the parts rotate or twist during drilling or oil exploration. There is still a huge demand for an economic replacement for hard chrome that can provide better corrosion resistance and bonding. Therefore, Maxterial has introduced a novel coating system called MaxShield.

Corrosion performance of MaxShield was evaluated by an accredited third-party laboratory per an ASTM B117 salt spray corrosion test. During this test, the coated steel sample is exposed to 5% sodium chloride mist, which simulates marine environments. In this test, the corrosion performance of hard chrome coating was compared with that of MaxShield’s coating through 5,000 hours (about seven months) of exposure to salt fog. The hard chrome coating failed around 400 hours with more than 10% of the area covered with rust, while MaxShield passed the 5,000-hour salt spray test with less than 0.03 % of the area covered with rust. Both hard chrome and MaxShield coatings tested were 75-µm thick. In fact, the corrosion performance of Maxshield is found to be similar or better than expensive special alloys, such as Hastelloy and Monel, used in the oil and gas industry.

As shown in the figure, both hard chrome and MaxShield coatings are smooth and exhibit a silvery metallic color. However, under an optical microscope, hard chrome deposits show many microcracks (500 to 1,500 microcracks/cm) while MaxShield’s coating is more compact, which is desirable for corrosion protection applications. HVOF thermal spray coatings have porosities in their structure (0.5% to 2%), which also reduces their corrosion resistance.

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