The effects of manganese (Mn) cations on the anticorrosive, morphological and adhesion properties of a room temperature phosphate coating on steel substrate

The corrosion protection, morphological and adhesion properties of a room temperature phosphate coating containing manganese ions (Mn) were studied on a steel substrate. For this purpose, phosphate coating solutions containing different concentrations of Mn were prepared. The steel samples were treated by the phosphate coatings at room temperature, pH of 3.1 and immersion time of 30 min. Then, an epoxy / polyamide coating was applied on the phosphated samples. The corrosion protection properties of the samples (in 3.5% NaCl solution) were studied by a potentiostatic polarization technique. The microstructure and morphology of the surface treated sample were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Adhesion strength values of the epoxy coatings were measured by a pull-off test before and after exposure to 3.5 wt% NaCl solution. Results showed that addition of Mn (especially at 7 gr/lit) to the phosphate coating caused an improvement of its corrosion protection properties compared to the sample without additive. A less porous and more compact phosphate coating formed in the presence of Mn than the sample without additive. The lowest adhesion loss of the epoxy coating was observed on the sample treated by a phosphate coating containing Mn. Generally, results showed that Mn as an additive reduced adhesion loss and improved corrosion protection properties of a room temperature phosphate coating on steel.