A hybrid test method to evaluate the adhesion characteristics of soft coatings on steel substrates - Application to Zn-Mg coated steel

Coatings have been widely used to improve the surface properties of materials. For long-term applications and the complex forming of coated components, the adhesion strength between the coating and the substrate must be sufficiently high. However, it is difficult to evaluate adhesion strength quantitatively in the case of soft coatings. Thus, appropriate methods of evaluating adhesion characteristics are required. In this work, a punch stretching test with a potentio-dynamic polarization test is proposed as a hybrid adhesion test method that simultaneously allows quantitative studies on the adhesion characteristics and the corrosion properties of soft coatings. With this test, the adhesion characteristics are able to be expressed quantitatively with a corrosion potential variation after the deformation of the coated steel. This hybrid adhesion test was applied to Zn-Mg coated steel, and the results showed that the adhesion between the Zn-Mg coating and steel substrates decreased with increasing Mg content in the coatings, while the corrosion resistance of the coatings increased with the Mg content. This could be attributed to the microstructural change of the Zn-Mg coatings from a crystalline porous structure to a mixed structure of amorphous and nano-crystalline featureless structure with a wt.% Mg content above 11.2 in the coatings.