Wire drawing is a cold deformation process in which a steel wire is pulled through several dies of decreasing size, in order to reduce its diameter. To ensure smooth operations and to maximize the lifetime of the expensive dies, lubrication of the wire is mandatory prior to the drawing step. More importantly, the lubricating layer is required to remain adherent to the steel surface for multiple deformation steps, while also maintaining its performance throughout the entire process. For this application, zinc phosphate conversion coatings are widely utilized in combination with an aqueous lubricant based on stearate salts. In fact, phosphate coatings can easily deform under the compressive load of the drawing step, while their porous nature maximizes the amount of lubricant that can be applied to the wire surface. [1]Despite the good performance offered by conversion phosphating, there is an active interest in replacing this technology as it generates a significant amount of waste byproduct, the so called “phosphate sludge”. At the same time, finding an alternative conversion formulation that ensures the same performance is not a trivial task, especially when price is also factored in. In this context, electrolytic phosphating could offer an elegant, yet effective solution: the process allows the formation of coatings with the same chemical composition and morphology as conversion phosphating but prevents the formation of the sludge byproduct. With the application of an external voltage, it is in fact possible to trigger the local increase in pH responsible for phosphate formation without having to rely on the dissolution of the substrate. [2]In this talk we will present the implementation of electrolytic phosphating in what is arguably its most relevant industrial application. In particular, by designing the phosphating tank around the symmetry of the steel wire, it is possible to ensure uniform distribution of the current density on the substrate, effectively avoiding the main drawback of the electrolytic process. Furthermore, the higher deposition rate obtained with the electrolytic setup allows the implementation of a continuous operation process, with a higher throughput than traditional batch conversion phosphating. The presentation will showcase results coming from laboratory-scale experiments as well as data resulting from real case-studies at the industrial level.
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