The surface of hot-rolled stainless steel plate usually has a thick oxide layer. If chemical oxidation is used to remove oxide layer, it will not only increase pickling time, reduce pickling efficiency, but also increase the pickling cost too much. Therefore, to use other methods to treat the stainless steel plate as an auxiliary process, there are three main pretreatment processes for pickling.
The first is the shot peening process, this is a kind of mechanical removal way at the present time most frequently used, it is the principle of using shot peening equipment ejected fine particles of the shot to hit the surface of stainless steel plate, stainless steel plate in order to get rid of the thick oxide layer. After shot peening, some oxide layers will be removed, and the remaining oxide structure on the stainless steel plate will become loose and discontinuous, which is very beneficial for later pickling.
The second is alkali leaching process, which includes two kinds of alkaline leaching with oxidation type and reduction type alkali leaching. The oxidized alkali leaching is also called the “salt law”, its mechanism is after alkali treatment for stainless steel oxide refractory iron chromium spinel (FeCr2O4) modified into alkali soluble CrO3, and because of changes in the volume and structure of oxide layer, the oxide layer falling off. Reduced alkali leaching is the strong reducing agent NaH, the oxide of iron, nickel, chromium and other refractory metal oxide or metal oxide reduction becomes low, and let the oxide layer break off, shortening pickling time and enhance efficiency.
In the process of oxidizing alkali leaching, a certain degree of Cr6+ pollution will be formed. The reduction type alkali leaching can remove the problem of Cr6+ pollution, and the key material is NaH. Domestic use Potassium Permanganate oxidized alkaline leaching process, while the foreign countries mostly use the leaching of alkali reduction.
The third one is neutral salt electrolysis, which takes Na2SiO4 aqueous solution as electrolyte. The stainless steel plate passes through the electric field between cathode and anode, changing cathode and anode continuously, and removing the surface oxide layer after current action. The mechanism of electrolysis of neutral salt, which is oxidized to higher soluble ions by chromium, manganese and iron oxide in the insoluble oxide based, so that the oxide layer is dissolved; second is based on the stainless steel substrate is oxidized to metal ions, so that the spallation of the oxide adhered on the surface of the stainless steel plate.
In general, the most common use now is the neutral salt electrolysis process. These pretreatment processes can remove the oxidation layer on the surface of the hot rolled stainless steel plate and reduce the load produced during the pickling.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)