Investigation of Corrosion Behavior of Ti/TiN Multilayers on Al7075 Deposited by High-Vacuum Magnetron Sputtering in 3.5% NaCl Solution

Abstract

Although Al 7075 has many favorable mechanical properties such as the large strength-to-weight ratio, the relatively poor corrosion resistance has restricted industrial applications. In this work, Ti/TiN as hard multilayered and nanostructured coatings are deposited on the relatively soft Al 7075 structure by high-vacuum radio-frequency magnetron sputtering and the phase, structure, and morphology are investigated in details. The corrosion behavior is evaluated by electrochemical impedance spectroscopy in 3.5% NaCl at a pH of 7.5 for 1, 6, 12, 24, 36, 48, 60, and 72 h. At time points of 1, 6, 12, and 24 h, primary oxide layers and double layers are formed, but the corrosive medium penetrates the primary titanium nitride columnar structure. At longer time points of 24, 36, 48, 60, and 72 h, formation of stronger oxide and double layers leads to better corrosion resistance which is 14.8 times better than that observed from the uncoated substrate after immersion for 36 h. According to Rct, the corrosion resistances of the short and long immersion groups are 808.5-1984 and 808.5-1248 kΩ cm2, respectively, thereby confirming the effectiveness of the Ti/TiN coating against corrosion in comparison with the corrosion resistance of 84.3 kΩ cm2 observed from the uncoated Al 7075. The smallest corrosion resistance of 808.5 kΩ cm2 observed at the time point of 24 h is 9.6 times that of the uncoated substrate. A 1.4-µm-thick Ti/TiN hard nanostructured coating comprising six layers is deposited on the relatively soft Al 7075 substrate by high-vacuum radio-frequency magnetron sputtering at 100 °C. The first layer of the intermediate Ti layer cannot improve the corrosion resistance of the TiN super hard coating with a columnar structure. The second and third intermediate Ti layers play an important role in improving the corrosion resistance of Al 7075 by obstructing defects and coating damage from aggressive Cl− ions and penetration of water. The mechanism involves self-healing of defects by oxide formation and Warburg resistance by diffusion control.