Magnesium Substrates

A problem unique to magnesium substrates is the formation of loose particles (termed smut) on the surface of the metal. The source of the smut can be varied but is usually related to the environment during the cleaning process. Such loose, gray, black, or brown smut particles are believed responsible, in some cases, for erratic adhesive strength results as well as lower bond strengths. These loose particles generally can be rubbed off with a solvent-dampened pad. 

Almost any epoxy adhesive can be used on magnesium provided that proper surface protection is maintained. In view of the corrosion potential, water-based adhesives or adhesives that allow water permeation may be expected to cause problems with magnesium substrates. 

Blair, J. (1969). Magnesium and the aconitase equilibrium Determination of apparent stability constants of magnesium substrate complexes from equilibrium data. Eur.. Biochem. 8, 287-291. 

Kaabi Falahieh Asl, S., Nemeth, S., and Tan, M.J. (2014) Electrophoretic deposition of hydroxyapatite coatings on AZ31 magnesium substrate for biodegradable applications. Progr. Cryst. Growth Character Mater, 60 (3-4), 74—79. 

Pure magnesium substrates were chemically pretreated at 70 °C with a Ca/Na phosphate solution to obtain a porous brushite layer and subsequently PEO at a voltage of 450 V, a frequency of 1000 Hz and a duty cycle of 40% to deposit strontium-containing hydroxyapatite (Lu et al., 2014). Figure 7.59 shows that the Sr-hydroxyapatite coating provided improved corrosion resistance in HBSS to the pure Mg substrate. The characteristic corrosion parameters are presented in Table 7.6. 

Fig. 2. SEM micrographs of ANOF generated oxidic films on (a) aluminum, (b) titanium and (c) magnesium substrates.

Yue TM, Li T, (2008), Laser cladding of Ni/Cu/Al functionally graded coating on magnesium substrate . Surface and Coatings Technology, 202, 3043-3049. 

The oxide layer produced by micro-arc oxidation or anodizing has pores that can form interlocks with the paint deposited by subsequent electrocoating, which is believed to have excellent bonding with the magnesium substrate [49]. 

A kinetic study on the activation of deoxyribonuclease I by magnesium has shown that the activation curve is biphasic (the substrate being the Mg salt of bovine spleen DNA). This indicates that activation occurs at two sites on the protein. Free Mg + was required for enzyme activity, confirming that a metallo-enzyme as well as a metallo-substrate is necessary for deoxyribonuclease I activity. In contrast, it has been shown that the pyrophosphatase activity of bovine brain alkaline phosphatase depends on Mg + bound to the enzyme but not on the formation of a magnesium-substrate complex. 

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