Magnesium alloy in contact with coated components

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Magnesium alloy in contact with coated components

Boese, E. / Goellner, J. / Heyn, A. / Eisenberg, S.

Investigations were carried out to study the galvanic corrosion of a magnesium alloy in contact with various coated components (e.g., fastening devices, threaded bushings, plugs, and screws). Such coated components are needed, for example, to assemble steering gear made out of pressure die-casted -,magnesium alloy. The quality of the coatings as well as their protective properties are of decisive importance in preventing galvanic corrosion. The determination of the coupling current density was the focal point of this research to evaluate galvanic corrosion. The difference between the rate of weight loss per unit area (v_tot calculated from the weight loss of the magnesium alloy MgAl9Zn1 (UNS M11910, AZ91) coupled to a coated component, and the rate of weight loss per unit area (v_I calculated from the coupling current density of a galvanic element was compared with natural corrosion data of MgAl9Zn1 without contact to a coated component under the same conditions. At high-contact corrosion rates, this difference had a much higher value than the natural corrosion data of MgAl9Zn1. Consequently, the quota of the natural dissolution rises also with increasing galvanic corrosion. In addition to the investigations of galvanic corrosion via determination of the coupling current density, measurements of polarization resistance, electrochemical noise, and polarization curves were examined to characterize the properties of the coatings before and after contact with MgAl9Zn1. This allowed us to further substantiate the conclusions with regard to the risk of galvanic corrosion.