Analysis of the Physical-Mechanical Properties of the Zinc Phosphate Layer Deposited on a Nodular Cast Iron Substrate

Abstract

The rotors of submersible wastewater pumps, generally made of nodular graphite cast iron, are subjected to complex processes of corrosive erosion during operation. To improve the characteristics of erosion resistance by impact with solid particles in the corrosive environment of wastewater, cast iron was subjected to a chemical phosphating treatment. In the paper, the scratch test behaviour of nodular cast iron and phosphate nodular cast iron is analysed comparatively, studying the behaviour of the deposited layer and its adhesion to the substrate. The nanoindentation characteristics of nodular cast iron and phosphate nodular cast iron were also studied. It was observed that the deposited layer is not compact, but when pressed, it does not crack and does not detach from the substrate; it is impregnated in the substrate in the metal matrix, but not on the area with carbon nodules. The SEM micrographs show that the deposited phosphate layer is relatively porous and can change the behaviour of the liquid flow moving on the surface of the rotor due to its hydrophilic behaviour; this also allows the formation of a boundary layer that adheres to the surface of the rotor and protects it from the impacts of microparticles driven by the liquid stream.

The Effect of Different Accelerators on the Corrosion Protection of a Surface Coating on Spheroidal Graphite Cast Iron (Ductile Iron)

“…Nejrenu et al [21] showed in their work that the zinc phosphate coating created on the surface of the nodular cast iron substrate deposited by immersion contains acicular dendritic crystallites uniformly distributed over the entire surface with small gaps between them. No needle-like crystals were visible on the surface during our experiments and the coating was free of gaps.…”

“…The size, form and morphology of carbon-based particles as well as the ferrite and pearlite content are usually investigated by light microscopy [17]- [20]. Since adhesion of the conversion layer during the zinc phosphating process carried out on ductile iron may be weaker in the case of larger graphite nodules on the surface than on the adjacent surfaces, it is also important to examine its microstructure [21]. Although sufficient dispersion of fine carbon particles in the phosphating bath can improve the degree of corrosion protection [22], this effect cannot be achieved by including graphite particles in the matrix.…”

The influence of surface imperfections on phosphate coating performance of nodular cast iron substrates

“…Hence, automotive components are usually coated to enhance their corrosion resistance. Several coating technologies have been applied to automobile parts to improve their service life [5,[11][12][13] . When the functionality of parts and threaded fasteners depends on certain fixed functions, such as the torque-tension relationship, phosphate coating is a prescribed process.…”