Layer-by-layer self-assembled polyurea layers onto MAO surface for enhancing corrosion protection to aluminum alloy 6063

Structural defects such as micro-pores and cracks generated during the micro-arc oxidation process can greatly reduce the corrosion resistance of the coating. This work proposed a method to improve the anti-corrosion property of the micro-arc oxidation (MAO) coating by layer-by-layer (LBL) assembling a polyurea layer on the surface of it. Organic silane (KH) and isophorone diisocyanate (IPDI) were self-assembled alternately on the MAO coating surface and formed a polyurea layer. The assembled layers were investigated using XPS, FTIR, EDS-SEM, EIS and polarization curves measurements. Three organic layers obtained couldn't be seen from SEM morphology. Whereas the XPS results showed the existence of polyurea layers with the disappearance of Al 2p signals and appearance of N 1s signals. The MAO coating surface was hydrophilic, and the contact angles of samples with odd-numbered layer (KH550 surface) were up to 95°, while the samples with even-numbered layer (IPDI surface) were about 87°, meaning that the silane and IPDI were successfully assembled on the MAO surface, improving the hydrophobicity of coating. The KH550/IPDI coating exhibited remarkably corrosion protection, and the corrosion resistance of the coating was maintained above 109 Ω·cm2 after exposure in NaCl solution for 288 h. In addition, the self-corrosion current density was under 1.46 × 10−9 mA·cm−2 after 288 h immersion, indicating that the KH550/IPDI coating would keep the anti-corrosion ability for longer time.