PREPARATION AND CHARACTERIZATION OF A COLORED ANODIC COATING ON HOT-DIP GALVANIZED STEEL

This study reports a method by which a colored anodized layer can adhe re to hot dip galvanized steel by utilizing an alternating current pow er supply that automatically controls and maintains proper current and voltage values. To be effective, this anodizing process was divided i nto two stages. A solution containing 0.75 M sodium hydroxide, 0.2 M s odium borate, 0.75 M sodium silicate and 0.03 M cobalt salt was used a s an electrolyte. An alternating current of 60 Hz at a constant 60 V w as utilized in the first stage for 10 min. A milky white film was form ed on the structure as a barrier layer to the passage of zinc species from the electrode to the solution, which further provides a sufficien t barrier layer for the subsequent coating formation. In the second st age, the current density was kept at 10 A/dm2 and the temperature was maintained at 10-degrees-C for 10 min. The anodic coating appeared fai rly smooth and the colors were attractive. Multi-color variations were obtained, such as blue, green, brown, white, gray, etc. The hardness of the anodic coating is about 20 percent higher than that of pure zin c layers. Microscopically, two distinct structures were observed-one i sland-like, the other like fritted glass. Anodic coating of the hot di p galvanized steel offers excellent resistance to saline environment a nd is significantly better than the untreated galvanizing steel, based on the results of the salt fog test.