Micro Arc Oxidation of AZ91 Magnesium Alloy – Effect of Organic Compounds in the Electrolyte

Abstract

AZ91 Mg alloy was micro arc oxidized under constant electrical parameters in silicate based and phosphate based electrolytes with and without addition of organic chemicals, namely Hexamethylenetetramine (HMTA), TRIS (hydroxymethyl) aminomethane (THAM) and Glycerol in two different concentrations. Following oxidation, samples were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), coating thickness measurements, hardness measurements and electrochemical corrosion tests. Results showed that coating layers mainly consisted of MgO, Mg2SiO4 and MgF2 for silicate based electrolytes, and MgO for phosphate based electrolytes. Incorporation of organic chemicals into electrolyte composition did not change the type of the phases in the coating. However, when they are added in silicate based electrolytes, pore density and coating thickness are reduced and pore size is increased. On the other hand, there is no significant change in surface morphology when organic chemicals are added in phosphate based electrolyte. In the view point of corrosion resistance, organic chemicals did not enhance corrosion resistance of the samples oxidized in silicate based electrolytes, but exhibited some increment in corrosion resistance of the samples oxidized in phosphate based electrolytes.

The Effects of Hydroxyapatite on the Corrosion Behaviour of AZ Series Mg Alloys

“…1). Magnesium and its alloys have very low corrosion resistance in high Clenvironments [44]. Because unlike the protective passive layer formed in metals such as aluminum or stainless steel, an unstable semi-passive magnesium hydroxide layer is formed in magnesium and its alloys.…”

Optimization of Surface Properties of Plasma Electrolytic Oxidation Coating by Organic Additives: A Review

“…Hexamethylenetetramine (HMT), which is a safe soluble amine, was extensively employed during the PEO treatment of Mg [98][99][100]. For example, Bai and Chen [99] In the work of Wang et al [77], a complex electrolyte system composed of 13-17 g/L Na 2 SiO 3 , 9-15 g/L NaAlO 2 , 2-4 g/L Na 2 B 4 O 4 , 1-3 g/L NaOH, 3-7 mL/L GLY, and 3-7 g/L SC i was developed to improve the corrosion resistance of AZ91D Mg alloy in a 3.5 wt.% NaCl solution.…”

“…Hexamethylenetetramine (HMT), which is a safe soluble amine, was extensively employed during the PEO treatment of Mg [98][99][100]. For example, Bai and Chen [99] examined the influence of sodium borate (0.1 M) and HMT (0.1 M) as additives on the morphology and corrosion behavior of PEO coatings made on AZ91D Mg alloy utilizing an electrolyte composed of 1.5 M KOH, 0.04 M sodium citrate, 0.1 M H 3 PO 4 , and 0.08 M Na 2 SiO 3 .…”

“…As a result, PDP tests in 5 wt.% NaCl solution indicated that both additives helped to improve the protective properties of PEO coatings. Gozuacik et al [100] reported an increase in the pore size after the inclusion of HMT (2 g/L) into an alkaline silicate-fluoride solution (2 g/L KOH+ 1g/L KF+ 1.85 g/L Na 2 SiO 3 ) during the PEO treatment of AZ91 Mg-alloy, accompanied by a reduction in the thickness of the coatings obtained in the solution without HMT. Echeverry-Rendon and co-workers [101] compared the corrosion behavior of pure magnesium coated via PEO utilizing alkalinesilicate electrolytes (0.07 M KOH + 0.1 M Na 2 SiO 3 ) without and with separate addition of 0.2 M NaF, 0.05 M mannitol (MAN), and 0.07 M HMT.…”