PLASMA ELECTROLYTIC OXIDATION OF MAGNESIUM ALLOYS FOR AUTOMOTIVE APPLICATIONS

Abstract

The present work concerns the plasma electrolytic oxidation (PEO) ofmagnesium alloys. The first part focuses on the optimisation of the PEO processparameters on Elektron 21 magnesium alloy in order to produce a protective coating onthe metal surface. Two electrolytes were employed, the first one contains 11 g/LNa2SiO4, 10 g/L Na4P2O7*10H2O and 2.5 g/L KOH, while the second one additionallyconsists of 8 g/L KF. The influence of the solution composition, current density andtime during the PEO process on the coating morphology and thickness are discussed. An addition of KF in the electrolyte, an increase of a current density and prolongationof the time of the process resulted in an increase of the thickness of the PEO coatings.Longer times of the process led to a transition to the so-called "soft-spark" regime.However, their occurrence caused the generation of a non-uniform coating in thesolution without KF. On the other hand, the prolongation of the time in the KFcontaining electrolyte led to a thicker and more uniform coating. XRD analysis revealedthe presence of amorphous and crystalline phases, with the latter including MgO,Mg3(PO4)2 and MgSiO4 for treatment in the solution without KF, and MgO and MgSiO4for the PEO process in the electrolyte with the addition of KF. The PEO coatings haveprotective properties and reduce the corrosion current density by two orders ofmagnitude. Moreover, an increase of noise resistance and impedance at low frequencyby up to two orders of magnitude were recorded for the PEO coated material. Thesecond part of the work was focused on application of a post-treatment to fill theporosity and defects present in the PEO coatings. A sol-gel technique was chosen. Itwas found that a sol can easily penetrate through the PEO layer and fill the pores. Thesol-gel coating was found to act as a barrier layer for the penetration of the corrosivesolution and decreases the corroson current density by one order of magnitude inpotentiodynamic polarization tests. In the last part of the work the PEO process wascarried out on AZ31 magnesium alloy. The PEO coating produced at a current densityof 420 mA/cm2 in the solution without addition of KF has a uniform and relativelycompact structure. The potentiodynamic polarization test showed that the coating hasprotective properties and results in a reduction of the corrosion current density by twoorders of magnitude.

Date of Award	1 Aug 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Skeldon (Supervisor) & George Thompson (Supervisor)