Study On Removement Of The Oxide Layers From The Surface Of Stainless Steel By Low Pollution Chemical Pickling Method

Because of its unique corrosion resistance and good mechanical processing performance, stainless steel is widely used in many fields such as civil and industrial etc. But in the production process of stainless steel, it is easy to produce continuous, compact and adherent black oxide layers on the surface of stainless steel. If oxidized layers is not completely removed, it will not only affect the appearance and properties of stainless steel, but also it can accelerate the corrosion of stainless steel, as well it can affect the subsequent processes of stainless steel.This paper first analyzed the composition and structure of the oxide layers of hot-rolled316stainless steel, hot-rolled304stainless steel and cold-rolled321stainless steel. The results showed that the outside oxide layers of hot-rolled316stainless steel are composed of loose iron oxides layers, inner layers contain NiO·Cr2O3and Fe3O4, and the layer depletion in chromium; The thickness of oxide layers is about50μm. The oxide layers of hot-rolled304stainless steel are mainly composed of iron oxides, chromium oxides and little NiO·Cr2O3.The oxide layers of cold-rolled321stainless are mainly composed of iron oxides, chromium oxides, manganese oxides and nickel oxides.The experiments investigated the pickling solution containing hydrochloric acid, ferric ion and hydrogen peroxide. The result showed that the descaling rate increase with an increase of the concentration of Fe3+, HCl and H2O2; but because of the corrosion and economic, we chose HCl200ml/L, FeCl3·6H2O150g/L, H2O2150ml/L. With further study, it is found that the oxidant can increase the descaling rate, the H2O2is the best one in all of the oxidants.The pickling process was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The study found that the descaling process is controlled by charge transfer process, increasing redox potential will help remove the oxide layers. In descaling process, the oxide layers is etched, dissolved, and the oxide layers are removed evenly because the porous solid corrosion product is produced, it has large active surface area. But the pickling solution can leed to pitting corrosion of the stainless steel substrate.In this pickling solution, the outer and simple oxides are first dissolved. With the pickling furthering, chemically and electrochemically, the oxide layers intereact with the pickling solution, and the evolved gases make the oxide layers loose and peel-off. Thus, it cleaned faster and more complete. The metal surface is uniformly corroded.It also has better removal efficiency for hot-rolled304and cold-rolled321stainless steel. Especially for cold-rolled321stainless steel, it can remove the oxide layers completely only for30s.