Iron phosphate coatings on cold-rolled steel: Morphology formation and intrinsic adhesion mechanisms

Abstract

The surface morphology, composition and performance of iron phosphate (P) coatings obtained in an industrial phosphating process on cold-rolled steel (CRS) base material (S4) at selected service lifetime (SLT) intervals ((1−330) × 10 3 m 2) and under operating conditions of the industrial plant were studied. Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) were used to image directly and characterize the surface morphology and elemental composition of iron phosphate thin coatings as well as the corresponding bare CRS panels used for phosphating. The performance of the resulting iron phosphate was evaluated by assessing the interface bonding (using a mechanical shear test) and extent of bonding deterioration (using an accelerated electrochemical test) of a fast-cured electrocoated thermosetting powder coating. SEM results indicated that the iron phosphate layers formed on CRS substrate have similar morphology and thickness throughout most of its cycle of service, indicating that the system operates under optimum conditions. Pretreatment of iron phosphate increased the shear strength from 38 N mm -2 for untreated CRS to 45–48 N mm -2, thus enhancing the adhesion bond by about 25% at all S4PL interfaces studied over the control S4L interface. Accelerated corrosion test results show that the percentage of delaminated areas for most SLT stages ((60−280) × 10 3 m 2) was about 27%; for the SLT stages of 10 3 and 330 × 10 3 m 2 the percentages of delaminated areas were about 85% and 75% respectively. A model of the mechanism of formation of iron phosphate on CRS is proposed which accounts for the dependence of the microstructure on the operating conditions. On the basis of the results, a model for the mechanism of the intrinsic adhesion link at the organic-coated iron-phosphated CRS interfaces is presented suggesting that it cannot be totally attributed to a mechanical interlocking bond but rather to the presence of hydration water or crystallization water molecules in the Fe 3(PO 4) 2 · 4H 2O molecular structure.