Anodisation of Magnesium and Magnesium Alloys in a Phosphate Electrolyte

by Clapp, Christina

Abstract (Summary)
Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. An anodic phosphate coating was formed by anodisation of pure magnesium, and two magnesium alloys, (AM60 and, AZ91), in an aqueous electrolyte containing 1.5 M NH3, 0.3 M NH4+ and 0.15 M HPO4 2-. Under, constant current conditions (200 A m-2) the cell voltage rose steadily to 430 V as the coating grew. Potential transients and scanning electron microscopy revealed three main stages during anodic coating growth, defined by a change in potential gradient. Glowing, sparking and dissolution processes occurred in addition to the growth of the anodic coating, which was highly porous. Further kinetic studies showed that increasing the current density increased the passivation rate. While electrolyte agitation was found to be advantageous, the type of magnesium substrate had a negligible effect. The following techniques were used to characterise the anodic coating: x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Rutherford backscattering, Raman and infrared spectroscopy, x-ray diffraction and differential scanning calorimetry. The anodic coating was amorphous, anhydrous Mg3(PO4)2 containing a small amount of residual water and ammonia extraneous to its lattice. Electrochemical impedance spectroscopy and cyclic voltammetry experiments led to an equivalent circuit of the coating, which had a bilayer structure. The experimental data is consistent with a duplex film consisting of a barrier layer (0.4 µm) and a porous layer (with the mean porosity 8 % and mean pore size 2 µm) up to 20 µm in thickness. Simulation of the heat transfer at the growing anode showed that the surface reached a temperature greater than 100 °C. This heat was generated in the growing anodic coating and resulted in an increase in temperature of the aqueous electrolyte in the pores. This heated electrolyte then vaporised causing electric discharge through the vapour producing a microplasma. The rise in temperature also caused sintering effects in the anodic coating. This study of the anodisation of magnesium and magnesium alloys resulted in a novel model for the mechanism of anodic phosphate coating growth. The insulating nature of the coating meant that high electric fields were required to drive coating growth via ionic migration in the solid state barrier layer in which the rate-determining step was postulated as cation injection from the metal substrate into the coating.
Bibliographical Information:


School:The University of Auckland / Te Whare Wananga o Tamaki Makaurau

School Location:New Zealand

Source Type:Master's Thesis



Date of Publication:01/01/2002

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