Cavitation behavior of active thermal-sprayed coatings

As a first stage in the development of active coatings to improve both localized corrosion resistance and cavitation resistance in sea water, we studied flame-sprayed coatings of aluminum, zinc and Zn-15 wt.% Al applied to steel substrates. Cavitation was carried out in distilled water and in 3.5 wt.% NaCl solution, and was characterized by weight loss and by scanning electron microscopy. Open-circuit potentials in 3.5 wt.% NaCl solution were measured in an attempt to monitor internal fracture, which would allow seepage of solution through the coating to the more noble steel substrate. Cavitation damage rates were found to be significantly higher in the saline solution for the alloy and the aluminum coatings, whereas little sustained change in damage rate was found for the zinc coatings. The data indicate that the strength of the particle matrix essentially governs the overall strenght of the coating. However, in saline solution the effects of alloy strengthening are slightly reduced by the tendency of localized corrosion attack to initiate coating flaws and eventual failure.