Influence of the interlamellar oxide on the electrochemical behavior of flame sprayed alloy 718 coatings.

Alloy 718 thermally sprayed coatings are candidates for wide industrial applications. However their morphological defects such as porosity and oxides can hinder their corrosion resistance. High Velocity Oxy-Fuel (HVOF) process was used to decrease the porosity level but interlamellar oxidation remained. The aim of this work is to highlight the relationship between the thermal spraying parameters and the corrosion resistance of the coatings. The oxidation state of coatings have been identified and quantified by XRD and EDS spectroscopic techniques. Their electrochemical behaviour have been examined in an electrolyte supporting a passive state. Experimental results show a selective oxidation during the particles flight according to the flame temperature and the energy of the enthalpic source. Interlamellar spinel oxide built up in flight control the electrochemical activation of coatings. Moreover, this oxidation leads to a subsurface chromium depletion in the alloy 718 which weaken the passive state. For economical reasons, other solutions than post-treatments are proposed. These solutions have been tested in order both to limit chromium depletion and to improve coating electrochemical properties.