Inspection the corrosion prevention performance and dry/wet interfacial adhesion qualities of the melamine-cured polyester coating applied on the treated mild steel surface with a nanostructured composite cerium-neodymium film

The aim of this study is to investigate the influence of a cerium-neodymium (Ce-Nd) conversion coating on the corrosion protection performance and surface characteristics of a polyester/melamine coating (PMC) applied on the mild steel (MS) surface. The energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM) methods were exploited to examine the surface morphology and roughness of the deposited layers. Also, an X-ray photoelectron spectroscopy (XPS) analysis was carried out for the characterization of the deposited Ce-Nd film chemical composition. The influence of MS surface chemical treatment via Ce-Nd film on the PMC corrosion protection activity was surveyed through accelerated salt spray (SS) and electrochemical impedance spectroscopy (EIS) tests. The impact of Ce, Nd, and Ce-Nd conversion coatings on the adhesion properties of PMC was studied by the pull-off (PO) test in dry and wet conditions. Through the steel surface decoration with Ce-Nd oxide film the highest improvement in the work of adhesion (about 47.5 %) and surface free energy (about 42.7 %), compared to the untreated sample, were obtained. The results of the cathodic delamination (CD) test revealed a 69 % improvement of the PMC resistance against disbondment by employing the Ce-Nd conversion coating. Pull-off test results evidenced that the steel substrate modified by Ce-Nd film resulted in the maximum improvement (about 57 %) in dry adhesion strength and the lowest adhesion loss of about 20 % compared to the bare (untreated) sample. FE-SEM and AFM observations showed the uniform growth of the Ce and Nd layers and Ce-Nd complexes (with particle sizes almost less than 100 nm) on the substrate, coincidentally. EIS results evidenced 67 %, 43 %, and 77 % improvement in total resistance (R-t) of the epoxy coating with artificial defect on the steel surface with Ce, Nd and Ce-Nd treatments, respectively, confirming that the redox reactions have been limited in the Ce-Nd/PMC interface more than each of Ce and Nd treatments. It should be noted that the Ce-Nd is a novel non-toxic and environmentally friendly chemical treatment method for replacement of the conventional metals' surface pretreatment methods like phosphating and chromating with numerous advantages like excellent corrosion resistance as well as good adhesion to the metal surface and subsequent organic film. One major practical application of the developed coating can be in the automotive and gas/oil industries for the car body and pipe surface chemical treatment prior to the application of the organic coating.