Molybdate-based passivation treatments for tin, tinplate and zinc
The level of toxicity of chromate conversion coating solutions has led to a search for alternatives. Molybdate has been considered, being a relatively non-toxic analogue from group (VIA) of the periodic table. The ability of molybdate to perform as an inhibitor in aerated conditions is well-reported, although its application in conversion coatings is less widely documented. The experimental work reported in this thesis was to a large extent based on potentiodynamic polarisation which is considered a powerful tool for evaluating potential coating solutions. The cathodic polarisation characteristics of zinc in group (VIA) oxy-anion solutions has been studied quantitatively. The effect of temperature, pH and aeration has been examined. The film-forming ability of molybdate-based solutions such as molybdate-orthophosphoric acid (MP), molybdate-orthophosphoric acid-nitrate and molybdate-orthophosphate has been examined with respect to a tin, tinplate or zinc substrate. Films were formed electrochemically (anodically and cathodically) or by a simple immersion technique. The performance of such coatings has been studied using classical corrosion tests such as sulphide stain and salt spray. The cathodic reduction characteristics of molybdate solutions have also been examined in detail with respect to possible lower valency films being produced (possibly as a result of molybdate's complex electrochemistry) on tinplate, zinc and platinum metal surfaces. Experimental results have indicated that simple, relatively dilute, molybdate solutions at pH 3 are capable of imparting good staining resistance to tinplate in sulphide stain tests. However, other mixed systems have proved less successful, only molybdate-orthophosphoric acid providing acceptable protection on tin foil. Simple immersion techniques in dilute, pH 5 molybdate solutions have provided moderate protection on zinc in 24-hour salt spray tests. Inflections exhibited on cathodic polarisation curves of tinplate in simple molybdate solutions have been attributed to a physical degradation of the coating as opposed to a reduction step.