Corrosion Resistance of Chromium-Free Conversion Layers on Hot Dip Galvanized Steel

The corrosion protection properties of a phosphate based conversion coating on hot dip galvanised zinc coated steel (Z) and hot dip galvanized zinc magnesium coated steel (ZM) was characterized in a standardized salt spray test. Attention was turned first to the appearance of white rust and second to the first appearance of red rust. On treated Z material the corrosion was faster than on treated ZM coated steel, like on samples without conversion coating. Cross-section samples were made analysed by scanning electron microscopy. By means of this SEM pictures it was concluded that soluble corrosion products must have been leached out through pores and defects of the conversion coating until precipitation over the conversion coating. This first visible corrosion products were characterized by Infrared spectroscopy which demonstrated that the corrosion products, built up on ZM coatings, contains more carbonates than on Z coatings.

Because of this, larger amount of carbonates, the pH on a ZM surface must be lower than on a Z surface. This is visualized on samples painted with a polymer/colour-indicator mixture. These experiments reveal that on ZM the building of regions with increased pH needs much more time. This corresponds to a different occurrence of anodes and cathodes on the surface which are visualized via the scanning vibrating electrode technique.

The resistance to alkaline solutions is a very important factor for the barrier property of a conversion coating. The formation of alkaline corrosion products of treated Z and ZM, was tested with dissolution experiments. After each exposure for a certain time a cyclic voltammogram in a borate puffer solution and an X -ray fluorescence analysis was made. From this results it is obvious that phosphates were washed out during exposure. This causes a conversion of the manganese component which slows down corrosion indicated by an decrease of anodic peak currents in the cyclic voltammograms.