Steel is one of widely used materials in various industries because of its excellent mechanical properties and low manufacturing costs. However it is highly susceptible to aggressive corrosion environments such as seawater, thus surface protection as coatings is necessary when using it properly.
Aluminium coatings are used for the corrosion protection of ferrous metal for two main reasons. Firstly, aluminium, with its air-formed passive oxide film, forms an effective barrier layer against attacking materials.
Secondly, if the underlying substrate is exposed at, for example, the base of pre-existing defects or corrosion pits, the aluminium coating will afford a degree of sacrificial protection to the substrate.
A drawback of pure aluminium is, nevertheless, that since its corrosion potential is not far removed from that of many steels, it is unable to provide adequate levels of sacrificial protection, particularly under natural exposure conditions. If aluminium is alloyed with small amounts of active metals such as magnesium, its corrosion potential can be reduced, leading to a significant improvement in sacrificial properties. Therefore, Al-Mg alloy coating was studied and Al-Mg films using the physical vapor deposition(PVD) method that is drawing great attention as an alternative to the conventional coating process were also examined. In addition, cutting section is inevitable when using the surface-treated steel sheet, thus galvanic corrosion properties of Al-Mg films were also studied.
In this experiment, Al-Mg films were deposited onto steel substrates using the eco-friendly PVD method under 10 ~ 31 wt.% Mg contents. The total thickness of every specimen was equal to 5㎛. And then heat treatment was conducted at temperature of 400℃ for 10 minutes. The material properties of Al-Mg films were investigated by using SEM, GDS and XRD. The corrosion behaviour of the Al-Mg films determined using neutral salt fog test and electrochemical techniques. After the salt spray test, corrosion products were analyzed by SEM, EDS and XRD at the beginning, middle, and end of the test. Non heat treated Al-Mg specimens were analyzed after 48h, 96h, 168h, and heat treated Al-Mg specimens were analyzed after 72h, 168h, 240h.
Based on the results of the immersion and salt spray tests, heat treated Al-Mg films obviously showed good corrosion resistance compared to non-heat treated Al-Mg films. In the salt spray test in a 5% NaCl environment, Al-Mg films containing 31 wt.% Mg had the most excellent corrosion resistance. In case of the immersion test in a 3% NaCl solution, Al-Mg films ranging from 18 to 22 wt.% Mg had the most excellent corrosion resistance. Also the mechanism of anti-corrosion can be obtained by the analysis of Al-Mg film's corrosion products at each stage.
Consequently, the results of these experiments obtained were as follows -. The difference in corrosion properties between the salt spray test and immersion test is considered the density of corrosion products by the difference of oxygen concentration. Therefore, it is needed to apply the method of estimating the corrosion resistance considering the differences according to environmental conditions.
-. It is thought therefore that the property of Al-Mg films can be improved by heat treatment and uniform distribution of intermetallic compounds.