This study investigated the effects of surface conditions on phosphatability for Si- and Mn-containing high-strength cold-rolled steel sheets. Si-, Mn-, and Fe-containing oxides of approximately 3 μm form on the steel surface after annealing. During 3 s pickling using nitric acid, these oxides are partially removed, exposing the steel surface. Additional pickling (6 s pickling time) mostly removes Si-, Mn-, and Fe-containing oxides, but Fe-based oxides of 50 nm maximum thickness are formed on the steel surface. These observations suggest that Si-, Mn- and Fe-containing oxides (about 3 μm thickness) and Fe-based oxides (about 50 nm thickness) interfere with the steel dissolution in the phosphate solution. Results demonstrate the importance of controlling the Fe-based oxide formation as well as that of Si- and Mn-containing oxides to obtain good phosphatability.
CrN, Cr-B-N，Cr-C-N and Cr-O-N coatings were prepared by arc ion plating, and their corrosion resistances and microstructures were investigated. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) in hydrochloric acid and acetic acid solutions were used to evaluate the corrosion resistances of the coatings. Results obtained from polarization curves revealed that all coatings exhibited much better corrosion resistance than the WC-Co substrate. The Cr-O-N coating offered the best corrosion resistance, showing the lowest passivation current densities in both acid solutions. The Cr-B-N coating had a higher current density than the CrN coating. EIS measurement results showed that all the coatings in both acid solutions exhibited capacitive loops with different diameters of semicircles in Nyquist plots. Equivalent circuit analyses revealed that the resistance to corrosion was highest for the Cr-O-N coating, and the corrosion resistances of the CrN and the Cr-C-N coatings were almost the same. The reason that the Cr-O-N coating showed the best corrosion resistance could be its characteristic periodic microstructure, as confirmed by TEM observations, and it showed repeating layers of chromium oxynitride and thermodynamically stable amorphous chromium oxide.