Influence of deposition parameters on the behavior of nitro-cobalt-based and Ti-hexafluoride-based pretreatments

Two different chemical pretreatments for hot-dip galvanized steel, typically used in the coil coating field, were considered comparing their electrochemical behavior (electrochemical impedance spectroscopy (EIS), potentiodynamic curves) after a morphological characterization of the deposited films. The wet and dry adhesion of the following deposited paint is tested employing neutral salt spray chamber test (NSST) and pull-off test. Despite these technologies being widely adopted in industrial plants, the literature lacks some relevant research about the actual improvement in durability, especially in the case of nitro-cobalt based conversion. This paper analyzes the factors affecting the protection provided by the deposited film and varying the principal production parameter, the immersion time in the chemical conversion solution, an optimization study was carried out. The better behavior of the titanium hexafluoride-based artificial conversions, with respect to the nitro-cobalt one, is highlighted. Such treatment analyzed as bare surface gives higher impedance modulus and lower corrosion current density compared to the second one. For both the technologies, the uselessness of longer treatment periods in terms of performance improvement is found. The results obtained from the electrochemical characterization are in accordance with the technological test on the painted samples, such as NSST, where the delamination rate is significantly higher for the substrate treated employing a nitro-cobalt-based bath.