Magnesium surface preparation

Active metals (Li, Mg, Ca, etc.) react spontaneously with the main atmospheric gases (N2, O2, H2O, CO2) and with most relevant polar aprotic solvents and salt anions. All active metals are covered initially by native surface films formed during their production by their reaction with atmospheric gases. It should be noted that even a usual glove box atmosphere that officially contains less than 1 ppm of H2O and O2 (but may contain hundreds of ppm of CO2 and N2) should be considered as reactive towards lithium or magnesium surfaces prepared freshly in the glove box. Active metals are usually covered by bilayer surface films. The inner layer is comprised of metal oxide, while the outer layer contains mostly carbonates and hydroxides. When an active metal is introduced into a polar aprotic electrolyte solution, several processes take place in parallel. These include dissolution of part of the initial surface species, nucleophilic reactions between metal oxide and hydroxide and electrophilic solvents such as esters and alkyl carbonates, and diffusion of solvent molecules towards the active metal-native film interference and their reduction by the active metal. 

High-humidity and salt spray environments have been found to cause the greatest decrease in bond strength of magnesium adhesive joints. Of the several surface treatments that have been evaluated, the Dow 17 surface preparation (ASTM D 2651 Method C) provides the best overall performance with all adhesives under these types of environmental... 

While it is possible to bond to a freshly abraded or cleaned metal surface, chemical treatments are preferred for rendering the metal surface inactive to corrosion over time. For low carbon steel, phosphatising is the recommended pre-bond surface preparation treatment. Stainless steel should be passivated or acid etched, while titanium is usually treated with a hydrofluoric acid pickle. Almninium or magnesium are best treated with a chromate conversion coating. Zinc and cadmium are generally prepared mechanically but a phosphate or chromic acid treatment may be used. Brass and copper may be treated with an ammonium persulphate etch or an acid-ferric chloride etch. 

Titanium aiioys. Because of the usual use of titanium at high temperatures, most surface preparations are directed at improving the thermal resistance of titanium joints. Like magnesium, titanium can also react with the adhesive during cure and create a weak boundary layer. 

The required surface preparation depends on which substrate will be bonded and which kind of adhesive is applied. Some metals, for example, have oxide layers like aluminium, magnesium, or titanium. In this case, the surface preparation focuses on stabilizing the oxide layer. 

The surface preparation methods for magnesium alloys are closely associated with corrosion prevention. Because magnesium is highly reactive, corrosion-preventive coatings must be applied for most service applications. The major problem is to apply a sufficient thickness of coating to prevent corrosion, but not so thick that the bond fails cohesively in the coating. ... 

Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

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