A Comprehensive Structural and Electrochemical Study on the Performance of Mn-phosphate Layers

Several manganese phosphating routes, including the influence of immersion time (15 min. and 30 min.), addition of accelerators (Ni(NO3)2, 2 g/l), modification in the activation stage (increased concentration in the activator), and ultrasonic stirring, were used to obtain films with a variety of characteristics. This research uses the following common analytical methods for the phosphate characterisation: scanning electron microscopy (SEM) and electrochemical techniques (potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). Thus, modifications in the coating mass, covering efficiency, surface appearance, and chemical composition/structure could be determined. This study showed that accelerators induced the formation of thicker layers (up to 125 % bigger), whereas ultrasonic stirring assisted with reducing the crystal sizes and developing denser films (up to 50 % reduction in the icorr values). The dry frictional tests showed that the wear behaviour depended on the phosphate thickness, crystal density, and film tortuosity. A fine-grained structure or a thick film contributed to an improved wear performance (near 80 % reduction in the coefficient of friction), but grains that are too fine along with an intricate structure worsened the behaviour under frictional forces.