Study porosity oxide coatings on aluminum alloys
Аuthors*, **, ***, ****
Siberian State University of Science and Technology named after academician M.F. Reshetnev, 31, Krasnoyarsky Rabochy av., Krasnoyarsk, 660014, Russia
AbstractThe corrosion resistance of the metal depending on operation conditions significantly effects to the operability of aluminum alloy structural elements. One of the most promising methods of protective coatings application is a microarc oxidation (MAO) due to the high wear and corrosion resistance, insulating and decorative properties of coatings. The effect of the application process’ parameters on the structure and properties of oxide coatings is not investigated adequately. The insufficient study results together with the absence of technologies and standards limits the use of MAO, so the study of the properties of MAO coatings is an important problem.
Corrosion resistance, electrical and other properties of the coatings depend on the porosity. Therefore estimation of the protective properties of the coating must consider its porosity.
Several methods for determining the porosity of the protective or protective and decorative coatings are known. The simplest and most common measurement methods are corrosive electroplating methods based on treatment with a special solution or applying the paste.
To identify the effect of process’ parameters on the porosity of the MAO coating the aluminum alloy AD33 and AMg6 specimens of thickness of 1 mm and 2.5 mm and in-plane dimensions 50x50 mm were used. Coating formation was carried out in two electrolytes, calcium silicate and aluminate, for the following modes of MAO: current density - 15 ... 60 A/dm2, the duration of oxidation - 60 min., the ratio of the cathode and anode component of the current Ik / Ia of 0.6 to 1, 2, bath temperature - 15 ... 60C.
After coating application the phase and structural composition of MAO coatings were investigated. The maximum content of solid oxide Al2O3 on the surface of the has been observed when the ratio of the anodic and cathodic current component Ia / Ik was equal to 0.6.
Since the porosity effects on the corrosion resistance of coatings the in rapid testing for corrosion resistance have been made.
To estimate the rate of corrosion both qualitative and quantitative properties were used. The change the look of the metal surface and the microstructure variation were the used to estimate the corrosion rate qualitatively.
Investigation of the corrosion resistance in laboratory was based on the accelerated corrosion tests by creating conditions of increasing environment aggressivity allowing rapid corrosion attack. Signs of corrosion were observed after 7 days of test. On the uncoated specimens the corrosion attack was observed on about 70% of the surface. On the conventional anodized specimens slight pockets of corrosion were observe and the specimens with micro-arc oxidation coatings survived the tests without any surface damage.
Thus, the investigation revealed that the protective properties of the coating are significantly depending on the technology. It was found that the coatings obtained by microplasma processing have good barrier properties (corrosion resistance) in the marine environment compared as compared with coatings of the classical anodizing.
Keywords:microarc oxdation, corrosion resistance, porosity, protective coating
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