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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15 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 Shown are black oxide bolts (left) and bolts with a zinc-based nanolaminated coating (right) after 24 months of service during an independent analysis and feld trial of a zinc-based nanolaminated coating system conducted by the United States Coast Guard. Photos courtesy of Modumetal. Continued on page 16 The coating's properties not only in- corporate the ability to resist particular corrosive environments due to the alloy's inherent composition and microstructure, but also include enhanced performance resulting from the nanolamination archi- tecture, says Lomasney. Corrosion pro- ceeds laterally along the interface of the nanolayers rather than toward the sub- strate, and fracture propagation also fol- lows the lamination planes. Because the discontinuities tend to move along a layer boundary rather than down into the layers themselves, corrosion and fatigue resis- tance are improved. Manipulating the architecture of the nanolayers also can enhance the coating's corrosion- and fatigue-resistant proper- ties, and yield a coating that is very differ- ent from a single-layer coating made of the same base material. "It's the interface be- tween layers in the nanolaminated archi- tecture that creates the properties of the coating and provides the mechanism to take the coating's performance to the next level," Lomasney notes. When building the architecture of the coating, the nanolayers can be changed and arranged to achieve the desired coating properties. "We're not changing the average alloy composition or mixing the zinc with something else to change how it corrodes; we are creating an interface that enables a dramatic change in performance," she says. Lomasney cites galvanic coupling in the zinc-based alloy as an example. "We have found that if we create a potential dif- ference between the nanolaminated layers that comprise the coating, we can enhance the corrosion protection behavior of the zinc. In the process we use, we can actually control that galvanic couple, which creates the improvement in performance. That is an architectural parameter that doesn't exist in a homogeneous material," she explains. By creating small galvanic cells within the zinc coating's nanolayers instead of be- tween the zinc coating and the substrate, Lomasney continues, oxidation occurs within the coating itself if the coating is scratched or damaged, which delays oxida- tion of the underlying steel surface and makes it less susceptible to pitting corro- sion. At the same time, the zinc's consump- tion rate decreases, which provides a lon- ger coating life. This more efficient use of Information on corrosion control and prevention

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