Ni-Co-Fe-P quaternary alloy coatings were electrodeposited and the dependence of microstructure and erosion– corrosion resistance with varying P content was investigated. With the addition of H3PO3 from 0 to 40 g L−1 into the electrolyte bath, the P content in Ni-Co-Fe-P quaternary alloy coating increased to 12.92 wt%. Along with the increase of P content, the microstructure evolved from a composite structure of crystalline and amorphous to a pure amorphous and then to another composite structure, while the grain size refined gradually and the microhardness increased obviously from about 510 to 1150 HV. The corrosion rate and the synergy between erosion and corrosion first increased and then decreased with the increase of P content. The polarization curves detected in the slurry flow at impact velocity of 4.19 m s−1 shows a tendency to passivity for all of as-deposited coatings. Ni-Co-Fe-12.92P coating deposited from H3PO3 concentration of 40 g L−1 possesses the best erosion–corrosion resistance due to the highest hardness and the lowest corrosion current density. Large amounts of cracking lips along the micro-plowing grooves can be observed on the worn surface morphologies of as-deposited coatings and the predominant wear mechanism is mechanical micro-cutting in Ni-Co-Fe-P coatings when subjected to erosion–corrosion in slurry flow.
