In the present study, coating of some magnesium alloys including AZ31 and AZ91 were studied using different techniques namely anodizing, electroless and hot press cladding. AZ91 alloy was coated using anodizing process using three types of environmental friendly electrolytes; the first based on sodium silicate, the second based on sodium hydroxide-boric acid-borax and the third on sodium silicate-potassium hydroxide-sodium carbonate-sodium tetra borate. Characterization of the anodizing layer was achieved by determination of surface morphology, microstructure, phase analysis, coat thickness, adhesion and corrosion resistance. It was found that the anodic film thickness increases with increasing the current density, anodizing voltage and deposition time until the deposition stops due to the formation of a thick anodic film. The range of the anodic film thickness is 28 42 µm.Optimization of the anodizing conditions - current density and deposition time was determined for each electrolyte. A corrosion efficiency ranging from 94% to 97% was reached; the highest value corresponding to the third electrolyte. Another study is the electroless Ni plating technique with zinc pre-treatment applied on several magnesium alloys and the effect of pre-treatment and post heat treatment on the coat characteristics. The surface morphology, surface roughness, thickness of the layer, EDX analysis, adhesion, hardness and corrosion resistance are covered in this part. The electroless layer thickness is about 6 µm. The results showed good bond quality of the coat maintaining good corrosion behaviour of electroless Ni-P based on potentiodynamic polarization tests in chloride solution where it was improved after heat treatment process. On the other hand, AZ31 was covered by a commercial pure aluminum sheet by hot pressing. The influence of the applied pressure, holding time and temperature on the bond characteristics was studied. The experimental investigation has revealed a good bond quality due to the effective mutual diffusion of Mg and Al. The phase analysis resulted in the formation of two equilibrium phases namely; Mg17Al12and Mg2Al3. The corrosion resistance of AZ31 is enhanced as a results of this process by 98.6%. Further points will be covered.Keywords: Magnesium alloys; Coating; Anodizing; Electroless; Hot press cladding; environmental friendly electrolytes; corrosion
Environmental Friendly Corrosion Inhibitors for Magnesium Alloys
