Stainless steel fasteners have been widely used in various fields of equipment and mechanical manufacturing, and their usage is increasing day by day.
In practical applications, some stainless steel fasteners, after a series of treatments such as welding, annealing, and cold heading, will form an oxide layer on their surface, which will affect the quality of the fasteners. Therefore, the oxide layer must be removed.
The oxide layer on the surface of stainless steel fasteners is a layer of iron-rich oxides formed on the surface of the metal substrate. This layer of iron-rich oxides is mainly composed of Fe2O2, Fe2O2, and FeO. Some factory tests have shown that it also contains small amounts of other oxides such as Cr2O3, FeO, Cr2O3, NiO, and MnO.
During the high-temperature short-time annealing process, the oxygen in the combustion atmosphere and the high-temperature water vapor will cause further oxidation of the metal matrix, increasing the thickness of the oxide scale on the workpiece surface and significantly increasing the inner layer of oxides.
The reason is that high temperature greatly reduces the ability of the oxide scale to impede ion diffusion, resulting in a rapid increase in the oxidation rate. A large number of holes and cracks will also form in the oxide scale on the surface of stainless steel during the welding and annealing processes, which poses a quality hazard to the product.
Generally, there are three methods to remove the oxide layer, namely, immersion method, salt bath method and salt film method.
① Soaking method.
a. Sulfuric acid pickling method. During sulfuric acid pickling, the acid penetrates into the oxide scale and the surface of the metal substrate through the cracks and pores inside the oxide scale, reacting with Fe and Mn in the metal substrate to generate a large amount of hydrogen gas (bubbling effect).
The dissolution of the metal substrate and the bubbling effect of hydrogen gas cause the oxide scale to quickly separate from the metal substrate. In actual production, when the sulfuric acid pickling solution is changed, there are often a large amount of undissolved oxide scales at the bottom of the pickling tank, which proves that during sulfuric acid pickling, the oxide scale is mainly removed by mechanical stripping.
b. Mixed acid pickling method. The nitric acid-hydrochloric acid pickling solution has a strong dissolving ability for the oxide scale of stainless steel fasteners, and the surface is relatively clean after erosion, but it is prone to over-corrosion if not properly controlled.
During mixed acid pickling, the acid solution penetrates into the oxide scale and the surface of the metal substrate through the cracks and pores inside the oxide scale, dissolving the chromium-poor layer and causing the oxide scale to quickly separate and fall off from the metal substrate.
c. Nitric acid-hydrofluoric acid pickling method. The nitric acid-hydrofluoric acid pickling solution has a strong dissolving ability for the oxide scale of stainless steel fasteners and a weak dissolving ability for the metal substrate, resulting in good surface erosion quality.
However, hydrofluoric acid is highly toxic and requires good protection and wastewater treatment conditions. Hydrofluoric acid penetrates into the oxide scale and the surface of the metal substrate through the cracks and pores inside the oxide scale, quickly reacting with Fe, Cr, Mn, and Si in the chromium-poor layer to generate a large amount of hydrogen gas, dissolving the chromium-poor layer, which exerts a mechanical stripping effect on the entire oxide scale.
② Salt bath method.
The salt bath treatment is essentially also a pretreatment process. It takes advantage of the different thermal expansion coefficients of the base metal and the oxide scale. Through rapid cooling, cracks are generated in the oxide scale.
Meanwhile, the low-valent oxides existing in the outer layer of the oxide scale are transformed into acid-soluble high-valent oxides under the action of strong oxidants. The descaling effect is better than that of the alkaline sodium nitrate solution, but it requires a dedicated salt bath treatment tank and the operating environment is also poor.
The treatment effect of the salt bath method is relatively good, but it also has some drawbacks. The salt bath is solid at room temperature, and the preheating process consumes too much energy. When stainless steel fasteners are removed from the tank, a large amount of dissolved salt is carried out, causing waste and pollution. The working environment for workers is also harsh.
③ Salt film method.
The salt film method has the same treatment principle as the salt bath method. The operation methods are divided into two types: heating and cooling.
Heating method: The immersed alkali solution is kept in the heating furnace at 400-500℃ for 2-3 minutes → water washing → acid washing.
Cooling method: The stainless steel fasteners are taken out of the furnace and cooled naturally to about 600℃ → spray alkali salt solution (10% sodium hydroxide; 3% sodium nitrate; 87% water) → water washing → acid washing.
After the treatment by salt bath or salt film method, the bond between the oxide scale and the metal substrate becomes relatively loose. At this time, a short-term treatment with a low-concentration mixed acid can obtain a clean surface. Post-treatment after acid washing:
After the stainless steel fasteners’ oxide scale is treated by the acid washing process, the oxide layer can be completely eliminated, but there are often residues of ash and acid washing residues on the surface of the stainless steel fasteners, which should be removed.
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