Nanocrystalline coatings were the scope of numerous studies due to their specific corrosion resistance and their particular mechanical, electrical and magnetic properties. The improvement of the corrosion resistance due to the reduction of the grain size, particularly at the nanometer scale, was not yet clearly established [1]. As grain size decreases, other metallurgical parameters that can affect the corrosion resistance like surface roughness, crystallographic orientation, chemical contamination, vacancy concentration, internal stresses at different scales … are also changed. The objective of our study is to better understand the influence of the microstructural parameters, mainly grain size and tungsten content, on the corrosion resistance in acidic solution of nanocrystalline coatings. It was previously observed that the deposition process strongly influences the defect density inside the coatings, so we have investigated two different deposition processes to obtain Ni-W alloys [2].

Electrodeposition by applying direct and pulse currents in an additive free citrate-ammonium bath and PVD magnetron sputtering were used for the elaboration of Ni-W coatings with different amounts of W. A multi-scale characterization of the metallurgical states of the nanocrystalline coatings using a combination of several microstructural techniques has permitted the characterization of the structural morphology, the grain size and the crystallographic texture [3,4].

Polarization curves were used to study the anodic behavior and the kinetics of the Hydrogen Evolution Reaction in acidic (H₂SO₄ 1M, pH 3) deaerated solutions.

According to our results, the grain size refinement strongly depends on the elaboration parameters which also influence the light elements contamination (O, H, N, C) and/or the alloying addition. The analyses of the polarization curves in acidic media revealed that the tungsten addition considerably affects the stability of the protective passive film. It was suggested that the tungsten addition plays an important role on the electrochemical reactivity in both anodic and cathodic domains and that the other metallurgical parameters such as grain size only intervene at second order.

[1] K.D. Ralston, N. Birbilis and C.H.J. Davies, Scripta Materialia 63 (2010) 1201-1204.

[2] M. Lagarde, N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall. Eurocorr Graz 2015, 6-10 sept.2015.

[3] C. Savall, A. Godon, J. Creus, X. Feaugas, Surface and Coatings Technology 206 (2012) 4394-4402.

[4] N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall, Journal of Alloys and Compounds 609 (2014) 296-301.