Influence of the structure of chrome electrolytic coatings on their resistance against cavitation wear

Bright chromium electrolytic deposits are widely used on water transport to provide the wear resistance of the crucial equipment parts in particular to protect the water-cooled surface of ship diesel liners against the cavitation attack. There were seven bright chromium coatings tested. The coatings were deposited in different regimes and the surface of the coatings was subjected to different treatment. The coatings were distinguished from each other by thickness, microhardness and the character of cracks distribution. The chromium plating was carried out in standard electrolyte of the following composition (kg/m³): chrome anhydride — 250; sulphuric acid — 2,5; trivalent chrome — no more than 5; trivalent iron ions — no more than 10. The electrolyte was prepared on the distilled water of a single distillation. The chrome electrolytic coatings were obtained on the plates cut out of the used ship diesel liners made out of cast iron SCh21. The cavitation wear tests were carried out on a magnetostrictive vibratory rig in fresh water, the frequency and amplitude of vibration of the rig horn being equal to 22 kHz and 28 μm accordingly. The distance between the sample chrome surface and the horn butt amounted to 0,5 mm. The wear of the samples was evaluated by periodical weighing during the testing on an analytical balance with step-type indication equal to 0,1 mg. Along with the samples weighing, the roughness of their surface was measured before and during testing, the roughness was evaluated by the value of arithmetical mean deviation of the assessed profile. The coatings structure was investigated on their cross microsections using a metallurgical microscope before and after the test. Cavitation wear resistance of a chromium coating is determined by two factors: the character of the cracks in the coating and the coating thickness. The coating thickness affects the coating longevity under cavitation attack, and the cracks character affects both the coating longevity and the wear rate of the chromium coating. The density of the cracks emerging on a coating surface and the cracks breadth define the surface roughness, and the cavitation wear begins from the coatings spots adjacent to the cracks edges: there occurs the brittle breaking off of the particles from the coating by its destruction along the cracks already present in the original coating. So there has to be a dependence of the wear rate on the height parameter of the roughness: the dependence was established by results of the experiments. The existence of the dependence of the wear rate on the value of the arithmetical mean deviation of the assessed profile of initial coating surface allows one to draw a conclusion, that after chromium plating, liners surface is necessary to expose to grinding with a fine-grained tool and, if possible, to polishing. The chrome coating has to be dense, that is the brоad extended cracks have to be absent in the coatings.

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