Evaluation of Porosity and Corrosion Protection Ability of Sol-Gel Deposited Oxide Thin Films on Magnesium

Some valuable properties of magnesium like high strength-to-weight ratio, high thermal conductivity, good electromagnetic shielding and machinability have made it an excellent material for various applications in automobile and aerospace components, sporting goods and household equipments. Despite of these valuable properties, poor corrosion and wear resistance restrict its application. The deposition of a protective film acting as a barrier between the material and its environment is an effective solution to preserve the metal from a corrosive attack. Good adhesion, uniformity and freedom from defects are necessary properties to ensure a good protection performance. Sol–gel deposition can provide ceramic-like films with high density and hardness at very low temperatures in a fast and inexpensive way. The low reaction temperature and the mixture of organic and inorganic materials lead to the formation of coatings containing both inorganic and organic moieties. The inorganic component enhances hardness, durability and adhesion to the metal substrate while the organic component leads to an increased flexibility and density. The major challenge of sol–gel deposited films is the prevention of pinholes and open porosities arising from the deposition process. Local porosities can produce direct paths between the corrosive environment and the substrate and increase the susceptibility of the materials to corrode. Despite of huge effort on resolving of this drawback, there is still no systematical manner to overcome this problem. In this work, several thin films obtained from sol-gel solutions containing silicon and zirconium were deposited on magnesium samples. Effect of sol solution parameters such as the amount of catalyst, stabilizing agent, water, co-solvent and heat treatment temperature on the porosity of the final film was investigated. Electrochemical methods such as potentiodynamic scans and electrochemical impedance spectroscopy were applied. It was shown that along with other parameters, the structure of precursor also plays an important role on the porous nature of the resulting film. It was also shown that these films are able to increase the corrosion resistance of magnesium in several environments. Film characterization methods proved the formation of an inter-diffusion layer between substrate and the coating which could be responsible for corrosion resistance enhancement