Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy

Magnesium and its alloys are susceptible to corrosion, which has limited their use in the automotive and aerospace industries, where low weight is of significant concern. In this study, AlN films were coated on the magnesium alloys (AZ31, AZ61, AZ63, and AZ91) using physical vapour deposition (PVD) technique of DC magnetron sputtering, and the influence of the coating on the corrosion behaviour of the magnesium alloys was examined. A physical vapour deposition (PVD) system for coating processes, a potentiostat for electrochemical corrosion tests, X-ray difractometer (XRD) for compositional analysis of coating, and scanning electron microscopy (SEM) for surface examinations were used. It was found that DC magnetron sputtering AlN coatings deposited on magnesium alloys increased the corrosion resistance of these alloys. However, the basis factor determining the ability of protection from corrosion was that small structural defects in the coating, e.g., pores, pinholes, cracks that could arise from the coating process or substrate.

The effect of DC magnetron sputtering AlN coatings on the corrosion behaviour of magnesium alloys

Zinc phosphate coating was formed on AZ91D magnesium alloy through a phosphating bath where H3PO4, ZnO, Zn(NO3)2 and NaF were applied. Chlorate (NaClO3) was used as an accelerator of phosphatization to replace nitrite. The coating compositions were hopeite (Zn3 (PO4)2·4H2O) and Zn. The growth process of the phosphate film on the magnesium alloy substrate was investigated by SEM observation and XRD analysis. In the early stage (0–60 s) of phosphatization, both hopeite and metallic zinc nucleated and grew together on the substrate to form flower structure. A great number and uniform nucleation of hopeite and metallic zinc implied that they formed on the β and α phases of the AZ91D magnesium alloy simultaneously. Afterwards, because magnesium alloy substrate was almost fully covered, only hopeite deposited continuously to form slab-like structure. The formed zinc phosphate coatings exhibited high corrosion resistance in 5% NaCl solution as shown by polarization measurement.

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Growth of zinc phosphate coatings on AZ91D magnesium alloy

Chemical conversion treatment is a basic method to improve the corrosion resistance of magnesium and its alloys. In the method of conventional conversion, however, the solution contains hexavalent chromium compounds which are harmful to the environment. The replacement surface treatment has been strongly emphasized in recent years. A chemical conversion treatment solution containing phytic was developed in this study for magnesium alloys. The surface conversion coating was characterized by SEM and EDS. The factors of influence to the corrosion resistance of conversion coating were discussed through hydrogen evolution method and polarization potential. The corrosion resistance of phytic acid conversion coating was comparable with the chromate containing. The experimental results indicate that the phytic acid conversion coating had better corrosion resistance when magnesium alloy was treated with a solution containing 0.5% to 1% phytic acid at temperature 25 to 60 °C and pH value at the range of 3∼5 for 30 to 60 min.

Study on the corrosion resistance of phytic acid conversion coating for magnesium alloys

A Review of Magnesium/Magnesium Alloys Corrosion and its Protection~!2009-10-01~!2009-11-25~!2010-02-11~!

Magnesium-based light-metal alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. However, the limitations according to mechanical strength and the low corrosion resistance restrict their practical application. To improve the surface hardness and the corrosion resistance, PVD-coating techniques offer possibilities to overcome these drawbacks. The paper presented reveals relevant mechanical and chemical properties of various PVD-coatings on high purity (hp) AZ31 magnesium alloy specimens. The industrially very important layer systems TiN, CrN, TiAlN, NbN-(TiAl)N, CrN-TiCN and the multi-layer composite AlNyTiN are discussed in detail. Furthermore, the protective effect of CrN yNbN superlattices has been studied. All of these coatings were deposited by d.c. magnetron sputtering. The sputtering processes have been performed by two different coating devices-PLS 500 (laboratory scale) and HTC 1000y4 ABS (industrial scale). Both the mechanical behaviour and the � corrosion resistance of the specimens have been studied after coating. The chemical composition of the thin films was analysed by GDOES. A special emphasis of the investigation was laid on searching for appropriate stripping procedures due to the fact that the substrate material is very reactive. � 2002 Elsevier Science B.V. All rights reserved.

Jana Scholz

Papers

Characteristics of PVD-coatings on AZ31hp magnesium alloys