During the heat treatment process of the stainless steel pipe (SS pipe), due to the action of high temperature heating, the inner and outer surfaces are oxidized, thus forming a surface oxide scale. In order to improve the corrosion resistance and surface cleanliness of stainless steel pipes, and provide a high-quality surface for future use and reprocessing, the surface oxide skin of stainless steel pipes after heat treatment must be removed, and passivation treatment is required after removal. These are essential for heat-treated stainless steel pipes. After the heat treatment of the stainless steel pipe, the process of removing the oxide skin on the surface and forming a passivation film on the surface is commonly referred to as surface pickling.
1. Composition of stainless steel pipe scale
Stainless steel contains chromium, nickel, iron and a small amount of carbon and silicon, and there may also be elements such as manganese, titanium, molybdenum, tungsten, etc. During heat treatment and welding, due to high temperature heating, oxide skin will inevitably be formed on the surface . The structure of stainless steel oxide scale formed by elements such as chromium, nickel, iron and a small amount of carbon and silicon during heat treatment is very complicated. There are a variety of oxide structures in stainless steel oxide scales, which often form oxide solid solutions and are densely bonded to the surface of stainless steel in the form of pure chemical compounds. Chromium is the main element that forms scale that is difficult to remove. During the heating process of stainless steel, the oxide layer formed by chromium makes the diffusion of oxygen very difficult. Because of the protective layer formed by chromium, the amount of FeO generated is very small, and FeO rarely exists alone, and exists in the oxide scale in the form of chromium spinel (FeO Cr2O3) and nickel spinel (Fe2O3 Ni).
The thickness, characteristics and composition of the scale on the surface of the stainless steel tube after heat treatment vary with the chromium content in the steel, the atmosphere in the furnace during heat treatment, the temperature and time of heat treatment. Such as: Cr-Ni austenitic stainless steel scale, from the surface to the inside, in order: Fe2O3, Fe3O4, FeO·Cr2O3. The scale structure formed by heat treatment in oxidizing atmosphere is looser and thinner; while heat treatment in reducing atmosphere is denser and thicker.
The atmosphere in the furnace during heat treatment has a great influence on the formation and composition of the oxide layer of stainless steel tubes. When Gr-Ni austenitic stainless steel is heated in an oxidizing atmosphere, the thickness of the formed oxide layer is thicker than that in a reducing atmosphere. The oxide layer formed is much thinner. Therefore, if the atmosphere in the furnace is adjustable, it is required to be adjusted to an oxidizing atmosphere, so that the pickling of the surface after heat treatment will be easier.
Under normal circumstances, the metal loss of stainless steel pipes during pickling is 0.3%. In order to reduce the loss of metal and the consumption of pickling solution during pickling of stainless steel pipes, it is very necessary and reasonable to require Cr-Ni austenitic stainless steel pipes to be heat treated in an oxidizing atmosphere.
2. The method of stainless steel pipe scale removal
The oxide scale of stainless steel pipe after heat treatment is dense and firm, and it is more difficult to remove than the oxide layer of ordinary steel. There are three commonly used methods for removing stainless steel scale:
Mechanical removal: Generally, bending and shot blasting are used for removal.
Chemical removal: use salt bath, pickling to remove.
Electrochemical removal: removal by electrolytic pickling.
1) Shot peening
For stainless steel pipes processed by hot-rolled strip steel or steel plate, after heat treatment, they need to be shot blasted and descaled before pickling. Because shot peening effectively loosens and removes dense scale, subsequent pickling will be easier.
Shot peening of stainless steel pipes is to use centrifugal force to rapidly throw steel shots to the surface of stainless steel workpieces to achieve the purpose of removing scale and rust on the surface. Shot peening is the most effective method to quickly remove thick and tight iron oxide scales before pickling, but for stainless steel, shot peening itself is not a complete and thorough final cleaning process, and the final process for removing oxide scales is pickling. It is very economical and effective to use shot peening and pickling together to remove scale on the surface of stainless steel. Shot peening usually uses carbon steel pellets or sheared steel wire segments. During the shot peening process, it is likely to cause iron filings to be embedded in the stainless steel surface, which will seriously damage the corrosion resistance of the stainless steel surface. Unless the pickling method is used after shot peening, these impurities can be removed. This is why pickling must be carried out after shot peening.
The steel shot used for shot peening is a round spherical cast steel shot with a particle size of 0.3mm~0.5mm. The steel shot has high crack resistance and wear resistance, and the carbon content should be low to ensure that it is hard but not brittle, and it is easy to reuse. The steel shot can also be a sheared steel wire segment.
2) Salt bath
The purpose of salt bath treatment is to transform the insoluble chromium oxide Cr2O3 into easily soluble lower oxides. The salt used in the stainless steel salt bath is NaNO3, and the reaction formula is:
2FeO·Cr2O3+7NaNO3→Fe2O3+4CrO3+7NaNO2
The salt bath temperature is around 450°C.
Salt bath removal is generally used for oxide scale removal before pickling of stainless steel pipes processed by cold-rolled strip steel or steel plate. After the stainless steel pipe comes out of the salt bath furnace, it must be brushed by superheated steam to force the residual molten salt and oxide layer to fall off. In this way, it is much easier to pickle again and the consumption of acid solution will also be reduced.
3) Pickling
Pickling is a common method for removing scale. Pipe pickling after heat treatment of stainless steel pipes is generally pickled with mixed acid solution (nitric acid + hydrofluoric acid).
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