How does chromium oxide improve the properties of stainless steel?

Although stainless steel itself has good corrosion resistance and oxidation resistance, in practical applications, different degrees of corrosion damage will still occur in different corrosive medium environments. Therefore, it is necessary to properly treat the surface to make it The surface layer of the alloy is modified to achieve the purpose of improving the oxidation resistance of the alloy. 

Among the ingredients of stainless steel, chromium is the most important element. Chromium is a very active element that can form a layer of dense chromium oxide film (Cr2O3) on the surface, which has a very good protective effect especially in extremely bad/acidic environments, and can well prevent further damage to the substrate. Corroded. By injecting a certain amount of chromium oxide into the surface, the chromium oxide film makes the surface metal with larger pores denser, and also enhances the oxidation resistance inside the metal, making the overall performance of the metal more superior.

In addition to chromium, zirconium, tin, etc. are also important elements commonly used in stainless steel. Zirconium can provide stronger corrosion resistance, and is often used in production sites involving strong corrosive media, and its addition is usually between 0.07% and 0.12%. Tin can enhance the heat resistance, corrosion resistance and machinability of stainless steel, and is widely used in the manufacture of food containers such as stainless steel food barrels, beverage barrels, and wine barrels. 

When people increase the chromium oxide content in the oxide layer on the surface of stainless steel and optimize the structure of the oxide layer, the corrosion resistance of the stainless steel surface can be greatly improved. Improving the corrosion resistance of stainless steel surfaces can be achieved in two steps by chemical treatment. Even stainless steel welds can be improved in this way to resist corrosion. 

The first step is to surface treat the stainless steel surface with a water-based organic solution. This surface treatment process will last for 3 to 5 hours. During this period of time, some iron elements are removed from the oxide layer on the surface. With the reduction of iron elements, the ratio of chromium to iron increases from 6% to 8%, which has a positive effect on improving the corrosion resistance. 

The second step is the optimization of the surface structure of the oxide layer. This further improves the corrosion resistance of the surface oxide layer. What affects the surface structure of the oxide layer is the heat treatment process: purposeful heat treatment in air at 140 ° C to 200 ° C according to the type of alloy and the lattice.

Chromium heat treatment in air

The chromium oxide layer is a typical representative of the corrosion resistance of stainless steel materials. This oxide layer is oxidized by the chromium element in the alloy and the oxygen element in the air. The chemical reaction of the chromium oxide layer formed by oxidation is very slow. Thus, as an oxide layer, the stainless steel metal material is protected from further oxidation and corrosion. Other types of alloy materials can be used if higher corrosion resistance is required. Generally speaking, stainless steel alloy materials contain higher nickel, molybdenum and chromium. Grinding, ray irradiation, corrosion or electrolytic polishing on the surface of stainless steel materials can all create conditions for the formation of a highly oxidized, defect-free, corrosion-resistant oxide layer. 

The principle of stainless steel passivation

The principle of stainless steel passivation is to use the chromium element naturally existing on the surface of stainless steel to oxidize it into a stable chromium oxide film, which can effectively prevent the surface of stainless steel from being affected by oxidation and corrosion.

After the surface of stainless steel is passivated, a layer of chromium oxide film will be formed. Compared with the surface of steel, this oxide film is denser and can prevent surface corrosion from more aspects. This film-like substance can fill the pores on the stainless steel surface, thereby inhibiting the possibility of external factors corroding the stainless steel surface. The thickness, color, hardness and other parameters of the chromium oxide film are determined by factors such as process conditions, treatment dosage and temperature. 

Process flow of stainless steel passivation

The process flow of stainless steel passivation mainly includes: surface treatment, cleaning, passivation, rinsing, drying and other links. 

1. Surface treatment

Remove steel slag, dirt, scale or welding slag from the stainless steel surface, so that the passivation solution can fully contact the stainless steel surface, thereby forming a dense oxide film.

2. Cleaning

Remove the pollutants on the surface of the stainless steel by pickling, alkali washing, etc., to meet the requirements of surface cleanliness, and avoid residual impurities and rust substances on the surface from interfering with the normal reaction of the passivation solution. 

3. Passivation

Commonly used passivating agents include chromium sulfate, chromium nitrate, chromium fluoride, etc., by soaking the surface of stainless steel to form a stable chromium oxide film. Under different treatment doses and temperature conditions, chromium oxide films of different colors, thicknesses and hardnesses can be formed, which can be selected according to requirements. 

4. Rinse

The waste liquid and impurities produced by the reaction of the passivation solution on the surface of the stainless steel should be thoroughly cleaned to ensure that the surface of the stainless steel is clean without any impurities that affect the performance of the passivation film.

5. Dry

Make the surface of the stainless steel meet the dry requirements by drying, drying, etc., so as to prevent the residual moisture on the surface of the stainless steel and the liquid produced by the reaction of the passivating agent from continuing to affect the surface of the stainless steel. Stainless steel passivation plays an important role in the production process of stainless steel products. Through the gradual optimization and improvement of the surface treatment process, the corrosion resistance, aesthetics and service life of stainless steel products have been greatly improved and guaranteed. According to the requirements of different use environments on the surface performance of stainless steel, different passivation processes and links can be selected to obtain the best effect and performance guarantee.