Abstract
This paper describes a systematic study into the initiation and propagation of filiform corrosion (FFC) on industrially important Zinc-Aluminium-Magnesium (ZAM) alloy coatings for steels. An artificial scribe defect is created in model organic coatings applied to ZAM. Corrosion is initiated using HCl, NaCl, FeCl2 and acetic acid. Only acetic acid is able to reproducibly initiate FFC. The FFC (area) rate is shown to be proportional to the amount of acetic acid introduced. The FFC mechanism is explained in terms of the relative susceptibility of alloy phases to anodic dissolution, determined using scanning electron microscopy and scanning Kelvin probe force microscopy.
Highlights
In recent years there has been a great increase in the use of magnesium as an alloying element in galvanized coatings applied to steel surfaces to provide cathodic protection
The apparent Volta potential difference existing between the MgZn2 lamellae and primary zinc is approximately 0.24 V. It must always be borne in mind that factors such as electrostatic charge spreading, tip dihedral capacitance and cantileversample capacitance all contribute towards the finite lateral resolution of the scanning Kelvin probe force microscopy (SKPFM) and tend to reduce potential contrast [68,69,70]
Given the approximately linear relationship typically observed between Volta potential and electrochemical potential [32,71], the Volta potential distributions in Fig. 2 suggest that the MgZn2 lamellae will form the anode in any galvanic couple with primary zinc
Summary
In recent years there has been a great increase in the use of magnesium as an alloying element in galvanized coatings applied to steel surfaces to provide cathodic protection. Both hot dip [1,2,3,4,5,6,7,8,9,10,11] and physical vapour deposition (PVD) [12,13] processes have been used to apply ZAM and zinc-magnesium coatings to steel used for building and automotive purposes. MgZn2 and MgZn11 intermetallics (IMs) have been found to undergo preferential anodic dissolution [1,7,18,22,23]
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