Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 15-21 doi:10.4236/jsemat.2011.11003 Published Online April 2011 (http://www.SciRP.org/journal/jsemat) Copyright © 2011 SciRes. JSEMAT 15 Formation Process and Properties of Phytic Acid Conversion Coatings on Magnesium Jian-Rui Liu, Yi-Na Guo, Wei-Dong Huang State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, P. R. China Email: ljr@nwpu.edu.cn Received March 7 th , 2011; revised March 10 th , 2011; accepted March 16 th , 2011. ABSTRACT A chromium-free conversion coating treatment for magnesium by phytic acid solution was studied. The formation proc- ess of phytic acid conversion coating was studied through measuring the open circuit potential (OCP) and weight change of the pure magnesium in the different conversion treatment time. The morphologies and compositions of the coatings were determined by SEM and EDS respectively. The conversion coating has the multideck structure with net- like morphology which is similar to the chromate conversion coating, and is mainly composed of Mg, P, O and C. The contents of C and P and the size of the cracks in different layers decrease from the external layer to the inner layer. The hydroxyl groups and phosphate carboxyl groups in the coating which have the similar properties to organic paintcoat are beneficial to the combination of substrate and organic paintcoat. The formation mechanism and thickness variation of the conversion coatings are also discussed. Keywords: Magnesium, Chemical Conversion Coatings, Phytic Acid, Formation Mechanism 1. Introduction As the lightest structural alloys, magnesium alloys are attractive for automotive and aerospace applications to achieve significant reduction in energy conservation and green-house gas emission because of its good combina- tion of mechanical properties and castability. However, the poor corrosion resistance of magnesium alloys which results from the properties of metal magnesium is one of the critical factors limiting its wide applications, espe- cially in an environment containing corrosive ingredients [1-3]. Surface treatments can improve the corrosion re- sistance of magnesium and its alloys, which have been extensive researched in recently years [4]. In the surface treatment methods, Chemical conversion treatment is an effective and simple method, such as chromate [5], per- manganate [6], phosphate [7,8], phosphate/permanganate [9,10], etc. Chromate conversion treatment as a conven- tional chemical conversion method, it has been widely used in industry for long time. But the conversion solu- tion containing toxic hexavalent chromium carcinogen is harmful to the environment, which has been restricted and forbidden in many countries. The pollution level of the treatment solutions containing phosphate or perman- ganate is less than that of chromate, although the metal ions and existing in the solutions have certain harmfulness to environment. Therefore it is urgently needed to develop new environment-friendly surface treatments for magnesium and its alloys. 3 4 PO  Phytic acid (C 6 H 18 O 24 P 6 , inositol hexaphosphate ester), an innocuity macromolecule natural compound with 24 oxygen atoms, 12 hydroxyl groups and 6 phosphate car- boxyl groups, is extensively applied in the area of surface protection of metals due to its particular physical and chemical properties which has powerful capability of chelating with many metals ions [11,12]. The researches [13-16] indicate that the chemical conversion coatings based on phytic acid solution have better corrosion resis- tance for magnesium alloys, which can substitute for those harmful methods. It can be seen from these research results that the properties of conversion coatings are closely related to their microstructure and compositions, which greatly depend on the conversion treating paramet- ers such as treating time, temperature and concentration of the solution. In order to further know the formation mechanism of phytic acid conversion coating and the influence of processing parameters on the microstructure and corro- sion resistance of the chemical conversion coating, in this study, the formation process of the phytic acid conver- sion coating was investigated by electrochemical and