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An unusual feature known as the negative difference effect (NDE) can be observed in magnesium when recording corrosion current density–potential curves. More hydrogen is evolved at a more positive potential which does not occur in conventional metals. Several models have been proposed in the literature in order to explain the phenomenon of NDE. They succeed in explaining some effects, and fail to deal with others. A new model, which explains the NDE by two electron consuming processes, is presented in this paper. By potentiostatic investigations of magnesium in a chloride electrolyte, measurements of hydrogen evolution and chemical analysis of the electrolyte the new model was experimentally verified.
While the possibilities of improving corrosion resistance (especially galvanic corrosion) by alloying are limited, surface finishing of magnesium alloys is the alternative for improving corrosion resistance. Due to the low corrosion potential of Mg and the danger of galvanic corrosion in the case of a coating defect, the choice of coatings is limited. This chapter will summarize the corrosion behavior of magnesium alloys and the metallurgical possibilities to improve the corrosion resistance of the alloys and reviewing critically the most commonly used surface treatments and coatings for magnesium.
Corrosion Education 2.0
(2013)
In the presentation different zinc coatings are presented and investigated. Their naturally formed protective layers are electrochemically characterized and corrosion relevant values are determined by using a gel pad based on polysaccharide. The corrosion relevant values allow the differentiation of various zinc coating systems and can describe the current protective effect provided by the coating. It is shown that gel-type electrolytes influence protective layers and coatings considerably less than corresponding bulk electrolytes. From the results it is evident that gel-type electrolytes represent a viable and promising field in corrosion research.
A new approach to the assessment of the corrosion behaviour of zinc coatings under atmospheric conditions can be achieved using gel-type electrolytes as an alternative to standard aqueous test electrolytes. Gel-type electrolytes allow minimally invasive corrosion investigations on zinc coatings. They also provide the possibility to carry out corrosion investigations onsite. By using gel-type electrolytes corrosion-relevant characteristic values such as corrosion currents and polarization resistances can be determined. These allow to describe the formation and the stability of the formed corrosion product layers under various climatic conditions. With this knowledge, a prediction of the zinc coating life cycle is possible. In this contribution, the value of gel-type electrolytes for corrosion research on zinc coatings is presented.