Industrial R&D on Innovative Surface Treatments for an Ecological Descaling of “Acciaierie Valbruna” Stainless Steel Wire Rods in Replacement of the Traditional Acid Etching – Vlada Pastushenko PhD Thesis

Author: Vlada Pastushenko; Type of thesis: PhD Thesis
Abstract: This work deals with an industrial research on ecological innovative descaling treatments for stainless steels, in substitution of the acid etching process: from the study and the research on samples, the most efficient techniques and their application to industrial purpose are explained. The research has basically covered the study of two pre-finishing treatments (high pressure water blasting and “dry ice”-blasting) and two etching treatments (electropolishing with ionic liquids and in aqueous media). In the field of stainless steels, the surface oxide of iron Fe (III) is combined with the multiple elements added to alloys in order to increase their characteristics (carbon, chromium, nickel and other). Moreover, the surface oxide is presented as a layer very tenacious and compact. In addition, heat treatment leads to the formation of a layer without chromium more readily attacked by oxygen from the atmosphere. Surface treatments are required to remove the surface oxide and recover the
chromium layer. The chemical pickling, that is one of the most common etching processes, depends on many factors, such as the size of the pieces, the type of plant, the type of alloy et al. In general, the traditional solutions contain from 10% to 20% by weight of nitric acid, and 1% to 5% by weight of hydrofluoric acid. The oxidizing environment is provided by nitric acid, which effectively removes the oxide surface, and is subsequently used without the hydrofluoric acid to restore the passive layer (passivation). From the environmental point of view, however, the use of this reagent is very costly:
• Air pollution: the formation of nitrogen oxides (NOx) during the process causes fumes and vapours. These gases are harmful to health, highly polluting (production of acid rain) and extremely aggressive towards metals.
• Water pollution: the high concentration of nitrates and nitrites is one of the causes of eutrophication. In particular, nitrites may form carcinogenic compounds such as nitrosamines, which can enter the food chain through fish.
• Health and safety: hydrofluoric acid is highly corrosive and a poison. It should be handled with extreme attention, using protective equipment and safety precautions. Once absorbed into the blood through the skin, it reacts with blood calcium and may cause cardiac arrest. In addition, it combines with calcium and magnesium of the bones. Since its action can be delayed for many hours, it can distribute throughout the body, causing the erosion of bones. These features have shown how the study of alternative “green” treatments is crucial.