Magnesium alloys oxide film

The Stability of the natural oxide film reinforced by the chromate ion determines the conditions of pH, ratio of activating anion to chromate, and temperature at which the oxide is broken down and a chromate film deposited. Thus magnesium alloys can be chromate-treated in nearly neutral solutions, whereas aluminium alloys can be treated only in solutions of appreciable acidity or alkalinity. 

Magnesium alloys are very lightweight, and are being used in the aerospace industry. Because they are very reactive, these alloys need to be protected from corrosion. Dr. Birss holds a patent on a new approach to the electrochemical formation of protective oxide films on magnesium alloys. Dr. Birss also works on developing new catalysts for fuel cells, and studies the factors that lead to the breakdown of fuel cells. 

The metallic elements are all potentially very reactive towards air, water and most elements, but Be and Mg form passivating oxide films. Elemental magnesium is manufactured in large quantities either by electrolysis of molten MgCl2 or by reduction of MgO, and is used in lightweight alloys and... 

Duan, H.P., Yan, C.W., and Wang, F.H. (2007) Effect of electrolyte additions on performance of plasma electrolytic oxidation films formed on magnesium alloy AZ91D. Electrochim. Acta, 52 (11), 3785—3793. 

C. Chemical modification of the glued surfaces by the formation of passivating layers. The modification technique depends on the nature of the metal. The parts are most often subjected to acid pickling, e.g. aluminum alloys are anodized in sulfuric and chromic acids. It is preferable to anodize aluminum parts in sulfuric acid followed by treatment of the anodic film in a bichromate. There are several methods of pickling carbon and stainless steels, chemical oxidation of magnesium alloys as well as copper and titanium alloys before gluing [4]. 

Magnesium metal is industrially important as a major component in lightweight alloys (with aluminum and zinc). The metal surfaces develop an impervious oxide film which protects them from progressive deterioration. 

The effect of plasma electrolytic oxidation (PEO) treatment on the SCC of surface-modified magnesium alloys was studied [167]. PEO coating offered improved corrosion resistance. However, the barrier film did not improve the SCC resistance in ASTM D 1384 test solution. The SCC of PEO-coated specimens was attributed to the development of micro cracks in the coating, leading to substrate cracking under SSRT test conditions [167]. 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

When strength-to-weight ratio is an important consideration, magnesium alloys compete with aluminum alloys. Magnesium has a density of 1.74 g/cm, which is 36% less than that of aluminum. However, aluminum is less expensive and has a greater corrosion resistance. The oxide film formed on magnesium provides only limited protection, unlike the adherent protective oxide film on aluminum. The primary application of magnesium alloys are for die-cast products. 

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