Materials Performance

MAY 2015

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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14 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 A fastener assembly coated with a zinc-based nanolaminated coating. Photos courtesy of Modumetal. MATERIAL MATTERS Nanolayers in metallic coating enhance its corrosion resistance O ffshore oil and gas assets are continuously exposed to cor- rosive atmospheric conditions such as moisture and chlo- rides, and often at temperatures above ambient. To protect metal components in these environments against corrosion, a protective zinc coating is often applied. A zinc coating protects the underlying sub- strate in two ways—by creating a barrier that prevents oxygen and moisture from reaching the surface of the steel, and by providing localized cathodic protection of the steel when the zinc coating is dam- aged. To improve the corrosion protection provided by a zinc coating, Modumetal, Inc. (Seattle, Washington) developed an electrochemical process that creates a nanolaminated zinc coating, and this emerging coating technology is being eval- uated by the industry on its ability to en- hance corrosion resistance and extend the service life of offshore equipment. According to Christina Lomasney, president, CEO, and co-founder of Modu- metal, nanolaminated metallic coatings enable more efficient use of the coating material because of the architecture of the coating. "It's a concept that is very similar to ply wood in terms of the ability to archi- tect the material, but much more profound in terms of the implications for the mate- rial's properties," she says. "There is a dramatic shift in material properties that we can affect through the nanolayering process." Nanolaminated coatings are com- prised of nanometer-scale metal particles (ions) that are electrochemically deposited onto a substrate in nanolayers that vary in composition and material microstructure. Coating attributes achieved by nanolami- nation include enhanced corrosion resis- tance as well as improvements in strength, toughness, wear characteristics, and em- brittlement resistance. The nanolamination process is similar to electroplating in that the component to be coated (the cathode) is immersed in an electrolyte that contains the zinc and other raw metal materials (the anodes). An electric field is used to reduce metal ions from the metals in the electrolyte into a metallic form that bonds to the surface of the component. The process is unique in that the elec- tric field is modulated to create different conditions in the electrolyte for very short time periods during the coating process, which results in the controlled deposition of a distinct zinc alloy nanolayer during each of these time periods. All parameters of the electric field—voltage, current, and frequency—are carefully controlled to achieve a very specific chemistry and microstructure for each snapshot of time. These laminated layers cannot be seen optically because they are smaller than the wavelength of visible light. The resulting microns-thick zinc alloy coating comprises several 10- to 100-nm thick layers that form the coating's nano- laminated architecture, with each nano- layer possessing a unique microstructure. The process can be used to apply nanolam- inated coatings to complex parts and sur- faces such as fastener threads. For thicker protective zinc coatings, the coating depo- sition process is repeated—each time applying one coat of the nanolaminated coating with its specific nanolayer archi- tecture—until the required coating thick- ness is attained. Nickel-based nanolami- nated coatings that isolate downhole components from hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) also can be ap- plied using this technology.

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