Features of Cavitation Wear of Chrome Electrolytic Coatings

Abstract—Cavitation wear is a pervasive phenomenon on water transport. The parts of ship propulsion system, as well as the water-cooled surface of cylinder liners of high-speed ship diesels, are subjected to cavitation attack. Bright chromium deposits are used to protect the water-cooled surface of liners. Nevertheless, the effect of cracks appearing in the electrolytic chrome coating upon deposition on the cavitation resistance of coating is still an open relevant problem. The aim of the study is to analyze the kinetics and mechanism of destruction of the bright chrome electrolytic deposit under cavitation attack. The cavitation resistance of chrome electrolytic coatings after grinding and polishing was studied. The chrome electrolytic coatings were obtained on plates (90 × 30 × 7 mm in size) made of SCh21 cast iron. The chrome was deposited on one side (90 × 30 mm) of the plate, placed vertically by flow anodic jet chrome plating in a standard electrolyte (kg/m³): chrome anhydride 250, sulfuric acid 2.5, trivalent chromium no greater than 5, and trivalent iron ions no greater than 10. The electrolyte was prepared using distilled water of a single distillation. The parameters of depositing: electrolyte temperature is 50°C; the current density amounts to 60 A/dm². Four samples cut from the same plate were used. The coatings on the first two samples were ground using the 320 grit abrasive paper. The coatings on the second pair of samples were first ground successively on the abrasive paper with different grits: 320, 500, 800, 1000, and 1200, following by cloth polishing with GOI paste. The cavitation wear tests were carried out on a magnetostrictive vibratory rig in fresh water; the frequency and amplitude of the rig horn vibration were 22 kHz and 28 μm, respectively. The spacing between the chrome-plated sample surface and the horn butt was 0.5 mm. The wear of the samples was evaluated by periodical weighing during testing. The wear of the ground samples was significantly higher than that of polished samples. Photographs of the surface after different durations of the cavitation attack show that the initial cracks that formed on the coating during chrome plating are centers of cavitation destruction. To reduce the negative impact of initial cracks on the cavitation resistance of the bright chrome electrolytic coatings deposited on the water-cooled surface of the cylinder liners of diesels, it is shown that it is expedient to include polishing in the technological process of depositing chrome coatings.