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

Abstract

The service performance of the nodular cast iron wheel hub was modelled by investigating the effect of surface morphology and characteristics on the phosphate coating size, morphology, and corrosion resistance properties of the zinc phosphate conversion coated cast iron substrates. The machined and unmachined surfaces as well as the coatings were examined by a scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (SEM-EXD). The influence of the substrates’ surface imperfections on the phosphate coating and the subsequent corrosion resistance was assessed and rated according to the standard procedures. Surface analysis of the machined and unmachined cast iron hubs reveals the existence of graphite and foreign material inclusions on the substrate surface that impact the phosphate coating properties and resistance to corrosion. The average phosphate coating crystal size is 1.74 µm and 2.58 µm for the unmachined and machined cast iron substrates, respectively. The corrosion resistance of the coated unmachined wheel hub surfaces was rated poor and disapproved based on the application requirements. The poor corrosion resistance was ascribed to the influence of the substrates’ surface characteristics on the coating adhesion to the substrate. However, the cast surfaces should be properly shot-blasted to remove any adhere foreign materials on the as-cast skin to enhance the coating adhesion.