Hard Anodic Oxide Coating on the Piston-Ring Groove of a Cu-Rich Aluminum Piston with Mixed Acid Electrolytes

Hard anodic oxide coating (hard anodizing) technology giving hardness values above HV300 was developed for a piston alloy containing a high Cu concentration (Al-12%Si-4Cu-0.5Mg). This technology was developed to improve the result that the anodic oxide coating in a sulfuric acid bath on the alloy can give hardness values as low as HV200. The combination of mixed acid electrolytes (40gL^-1 oxalic acid and sulfuric acid less than 150gL^-1) and periodic reverse electrolyzing enables the piston-ring groove to form a hard anodic oxide coating film having hardness values above HV300, coating thickness of 20 μm, and surface roughness of Ra 2.0μm. This mixed acid electrolyzing was found to prevent the electrochemical dissolution of Cu. The periodic reverse waveform cools the piston-ring groove to prevent burning.