Understanding corrosion mechanisms and hydrogen permeation in Zn and ZnAlMg flake coatings

Abstract

The properties of hot-dip galvanised and electroplated zinc coatings on steel have been widely studied, but the corrosion mechanisms of zinc flake coatings have not yet been investigated in similar detail. This work contributes to expanding the knowledge in the field of corrosion protection with zinc flake coatings. For this purpose, industrially used accelerated corrosion tests were carried out on the coatings and additionally examined using analytical in-situ and ex-situ methods. The behavior of inorganic Zn and ZnAlMg flake coatings after salt spraytest, humidity test, cyclical corrosion testing and outdoor exposure inrural climate on intact and defined pre-damaged coatings is investigated and related to the metal dissolution rate measured by ICP-MS.Furthermore, SKP measurements were conducted to better understand the sacrificial corrosion protection. In order to draw conclusions about the corrosion products and the mechanism of passive layer formation,a further combination of SEM EDX, GID, and XPS investigations was performed on zinc flake coated sheets. In addition to focusing on the anodic partial reactions and the associated metal dissolution, the barrier properties regarding hydrogen formation and permeation associatedwith corrosion in steel sheets coated with Zn and ZnAlMgflakes were finally carried out using hydrogen permeation measurements according to Devanathan and Stachurski. Different corrosion mechanisms were identified for the Zn and ZnAlMg coatings, which prove that ZnAlMg exhibits significantly more passive behavior and a 2 - 3 times lower zinc dissolution rate, with Mg playing a key role for the passivation process. Both zinc flake systems tested show a diffusion coefficient in the hydrogen permeation tests that is lower than that of the underlying steel. The ZnAlMg system has a diffusion coefficient that is 2 to 3 times higher and a hydrogen activity that is 2 times higher than the pure Zn flake system. Based on this, studies of coating solutions tailored to different corrosive environments using zinc flakecoatings can be developed.