Microstructural characterization and surface properties of zinc phosphated medium carbon low alloy steel

The present study concerns an in-depth investigation of the influence of process parameters on zinc phosphating of medium carbon steel to have an understanding of nucleation and growth mechanism and the microstructures developed thereafter. Furthermore, the wear and corrosion behavior of phosphated steel were evaluated in details. Phosphating was carried out in zinc phosphate bath (6–8% ZnPO4) using 0.43 wt.% Fe as catalyst at 90 °C with variable time ranging from 60 s to 1800 s followed by water rinsing and oil sealing. The microstructural evolution with time was recorded to understand the nucleation and growth phenomena. The morphology of coating was both plate and needle shaped. The microhardness of the coated layer is significantly lower than that of as-received steel. Fretting wear behavior against hardened steel ball showed that phosphating reduced the wear resistance which was possibly attributed to the presence of phosphate needles causing an increased coefficient of friction and the reduced hardness of the phosphate layer. Corrosion potential of the phosphated steel shifted towards noble direction as compared to the as-received one. The study of the corrosion rate also shows that the rate is considerably reduced by phosphating. Pitting corrosion resistance was also found to be increased (in terms of increased critical potential for pit formation) due to phosphating as compared to as-received steel.

► Detailed microstructural study of zinc phosphate coating on medium carbon steel.
► Residual stress developed on the surface is mainly compressive (− 88 MPa to − 575 MPa).
► Increased fretting wear rate of phosphate steel against hardened steel ball.
► Detailed corrosion behavior of the coated steel is evaluated in 3.56% NaCl solution.