Magnesium is the lightest metal structural material, with good thermal conductivity, electromagnetic shielding, shock resistance and high specific strength, high specific stiffness, non-toxic, recyclable and easy processing characteristics, has broad application prospects. However, the electrode potential of magnesium alloy is far below zero, which is the lowest among industrial alloys, and the oxide film of magnesium is loose and porous, which makes it have extremely high chemical and electrochemical activity, so the corrosion resistance is poor. In addition, magnesium alloy has low hardness and poor wear resistance. These disadvantages greatly restrict the application of magnesium alloy in the engineering field. How to effectively improve the wear resistance and corrosion resistance of magnesium alloys has become a key technical problem to be solved in the industrial application of magnesium alloys, and the surface treatment of magnesium alloys is the most promising research direction.
Preparation of Al coating on the surface of magnesium alloy can improve the corrosion resistance of magnesium alloy, because :(1) a dense and hard Al2O3 film is easily formed on the surface of Al coating, which has self-repair in the atmosphere and can protect the base material; The corrosion resistance of magnesium alloy can be multiplied or even tens of times by increasing the Al content in the alloy. At the same time, during the forming process of aluminum coating on the surface of magnesium alloy, Al element in the coating will diffuse to the direction of the matrix magnesium alloy, increase the Al content in magnesium alloy, and improve the corrosion resistance of magnesium alloy. (2)Al is a common alloying element in magnesium alloy, and its addition will not affect the recycling of magnesium alloy, and Al is an environmentally friendly material, pollution-free, conducive to recycling; (3)Al and Mg can form intermetallic compounds with good corrosion and wear resistance; (4) Among many elements, the chemical potential of Al and Mg is the closest, and the destruction of the two to form corrosion galvanic cells is the least.
Some scholars use aluminum and magnesium thermal expansion coefficient is similar (Al thermal expansion coefficient is 23μm/℃; The thermal expansion coefficient of Mg is 26μm/℃. The aluminum based amorphous nanocrystalline composite coating was prepared on the surface of AZ91 magnesium alloy by high speed arc spraying technology to achieve surface protection. The results show that there are amorphous, nanocrystalline and crystalline phases in the Al-Ni-Y-Co coating prepared by high-speed arc spraying technology. The coating microstructure is dense, the bonding strength with magnesium alloy matrix is greater than 25MPa, the porosity is less than 2.0%, the average micro-Vickers hardness value is greater than 300HV0.1, and the coating exhibits in 5% NaCl solution The corrosion resistance is better than that of pure Al coating.
However, many reports have pointed out that Al coatings obtained on the surface of magnesium alloys often have problems of large porosity and low bonding strength, and it is necessary to adopt sealing treatment, hot pressing treatment or anodic oxidation technology to improve the corrosion resistance of the coating, so as to achieve surface protection of magnesium alloy substrates. For example, arc spraying technology is used to form Al protective layer on the surface of AZ91 magnesium alloy. The results show that the corrosion of the unsealed coating sample is more serious than that of the original magnesium alloy, and the corrosion resistance of Al coating is greatly improved after hole sealing treatment. The experimental results show that after hot pressing and anodic oxidation, the corrosion resistance of the coating can be improved significantly, and the corrosion current density can be reduced by 4 orders of magnitude. It was found that the treated coating surface formed a dense Al2O3 protective film.
