Electrochemical Aspects of Laser Clad Nickel Base Coatings

Abstract

A comparative assessment is presented, by cyclic voltammetry, of the electrochemical performances of experimental samples of nickel base superalloy layer samples L1, with a maximum thickness of 0.5 mm, and L2 and L3 successively thinned, produced by laser cladding on a support of carbon steel OLC45 as compared with the reference materials, commercial nickel base superalloys with a similar chemical composition, as well as an analysis of the variation of these properties in the bulk of the layer. The electrolyte used was 1N sulphuric acid; the polarisation rate, between -400 and 1200 mV(SCE), was 200 mV s-1 at 18°C. The superalloy samples present complete electrochemical passivation in two stages during the first polarisation cycle, with passivation currents (ip) of order only 20–60 m A cm-2. The spontaneous passivation tendency shown by all the analysed superalloy samples is not seen on the L1, L2 and L3 samples; they suffer net electrochemical passivation processes during all 10 polarisation cycles, and the passivation current does not fall below 800 m A cm-2; the passivation critical potentials (icp) are also situated in another, more noble field. The existence of two passivation critical potentials is specific for nickel and hardening γ and γ phases. The nickel behaves characteristically only for the L1 sample, the two current maxima (icp1 and icp2 ) being attributed to it. The other L samples, with the successively thinned layers, are electrochemically passivated in a single stage, marked by high current consumption. Nevertheless, the laser clad layers have typical passivation properties in reducing acids, such as the sulphuric acid in the present study. The electrochemical performance of these layers decreases as the investigation depth increases, approaching the steel base which has an active behaviour, lacking passivation properties. It can be supposed that iron diffusion from the steel base took place, as higher as the section is approaching the steel/layer interface.