Evaluation of the incidence of micro-cracks and their influence on corrosion resistance of steel coated with different thicknesses of chromium

Hard chromium coatings are often used for applications where corrosion resistance and wear degradation protection are need. The aim of this study was to evaluate the incidence of micro cracks networks and corrosion resistance of different thickness of chromium coatings deposited on C45K steel. The coatings were applied through the electrolysis process. Five samples were prepared with different chromium thickness in a range of 0 to 34 µm. The coatings were characterized by optical (OM) and scanning electron microscope (SEM) and Vickers micro-hardness. The corrosion resistance was evaluated by anodic polarization and potential versus time in a solution of 3,5% NaCl. Tests on salt-spray chamber were performed as well. The results showed that the progressive increase of chromium thickness drastically improves the corrosion barrier resistance of the sample surface. By the magnifications on SEM was clear to see that the micro-cracks networks has developed after the polarization tests. However, tests on salt spray chamber showed that all samples presented localized corrosion after 120 hours. The incidence of micro-cracking is speculated as the determining factor for surface degradation, once chloride ions could easily penetrate into the cracks, causing localized corrosion on the bulk material.