NANOSTRUCTURED COATINGS AND THEIR POTENTIAL BENEFITS FOR CORROSION APPLICATIONS

Nanostructured metals have been shown to possess superior resistance against localized corrosion, i.e., pitting [1-3].  Localized corrosion is often regarded as one of the most challenging degradation issues due to difficulties in predicting the remaining component life and the extent of structural compromise.  Thermal spray coatings have been used to provide sacrificial and barrier coatings to protect components against corrosion.  In addition, coatings such as WC-CoCr have been used to provide wear and corrosion resistance.   

There are questions as to whether having a nanostructured sacrificial metal coating, for cathodic protection, will provide any advantage over conventional thermal spray deposits of the same composition.  This will likely be determined in the near future from an on-going collaborative work between Perpetual Technologies, Army Research Lab (ARL), Naval Surface Warfare Center and US Naval Academy.

To evaluate the merits of having a nanostructured coating for corrosion protection, it is important to transfer the nanostructure characteristics found in bulk materials to a coating in a reasonable, cost-effective manner.  A dense, oxide-free, and well bonded deposit with fully retained nanostructure has been successfully attained using the cold spray process.  The other key element towards eventually taking this effort from R&D to commercialization is the access to quality, cost-effective powder.  Recently, n-WERKZ, Inc., has developed a potentially cost-effective means of processing conventional metal-base powder into nanostructure form.  Even at the present, smaller-scale, n-WERKZ is providing nanostructured metal-base powder at lower prices.  The figure below is a cross-sectional micrograph of a nanostructured AA5083 aluminum alloy deposited by ARL using n-WERKZ’s powder.  The deposit shows no signs of oxidation or porosity (a few pull-outs) and retained its nanostructure with no signs of grain growth when characterized by high resolution TEM.

Amongst the numerous potential applications for nanostructured corrosion-resistant coatings, there are two applications that are being considered by the author and his collaborators.  One is the protection of and possibly rebuilding of worn sections of landing craft air cushion (LCAC) buoyancy boxes.  These parts are exposed to pitting corrosion, as well as to erosive and abrasive wear.  Having a protective layer of a nanostructured aluminum alloy of the same composition as the part may provide superior resistance to both chemical and physical degradations.  The second application of interest is in seeking for an alternative to ALCLAD for the corrosion protection of Legacy aircraft skin.  This approach may not only improve on the corrosion protection of the skin; preliminary results indicate the likelihood of increased fatigue and erosive wear resistance of the skin.  The other added advantage of the cold spray approach is in its ability to repair localized damage which is not a capability of the ALCLAD approach.

As mentioned earlier, there are materials other than monolithic metals that are used in corrosive environments.  It would be interesting to see if having nanostructured cermet coatings such as WC-CoCr, where nanoparticles of WC are incorporated into a nanostructured matrix of CoCr, will be beneficial or not.

REFERENCES

1      E. Sikora, X.J. Wei, and B.A. Shaw, Corrosion, 2004, vol. 60, no4, pp. 387-398

2      Mala M. Sharma and Constance W. Ziemian, JMEPEG (2008) 17:870–878

3      Li Liu, Ying Li, Fuhui Wang, Electrochimica Acta 54 (2008) 768–780 

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca

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