1. Introduction to stainless steel varieties

      1). The main components of stainless steel: generally contain chromium (Cr), nickel (Ni), molybdenum (Mo), titanium (Ti) and other high-quality metal elements.

      2) Common stainless steel: chromium stainless steel, containing Cr ≥ 12%; nickel-chromium stainless steel, containing Cr ≥ 18% and Ni ≥ 12%.

      3) Classification from the metallographic structure of stainless steel: there are austenitic stainless steels, such as: 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5. Martensitic stainless steel, such as: Cr17, Cr28, etc. Commonly referred to as non-magnetic stainless steel and magnetic stainless steel.

2. Common stainless steel surface treatment methods

      1) Surface whitening treatment: During the processing of stainless steel, black oxide skin is produced by rolling, edging, welding or artificial surface fire baking and heating. This hard gray-black oxide scale is mainly composed of two EO4 components, NiCr2O4 and NiF. In the past, hydrofluoric acid and nitric acid were generally used for strong corrosion removal. However, this method has high cost, pollutes the environment, is harmful to human body, and is highly corrosive, and is gradually eliminated. At present, there are two main methods of oxide skin treatment:

      ①Sand blasting (pellet) method: mainly use the method of spraying micro glass beads to remove the black oxide scale on the surface.

      ②Chemical method: Use a non-polluting pickling passivation paste and a non-toxic cleaning solution with inorganic additives at room temperature for dipping. So as to achieve the purpose of whitening the natural color of stainless steel. After processing, it basically looks like a dull color. This method is more suitable for large, complex products.

      2) Mirror bright treatment method of stainless steel surface: According to the complexity of stainless steel products and different user requirements, mechanical polishing, chemical polishing, electrochemical polishing and other methods can be used to achieve mirror gloss. The advantages and disadvantages of these three methods are as follows:

      3) Surface coloring treatment: Stainless steel coloring not only imparts various colors to stainless steel products, increases the variety of product colors, but also improves product wear resistance and corrosion resistance.

Stainless steel coloring methods are as follows:

     (1) Chemical oxidation coloring method: It is the color of the film formed by chemical oxidation in a specific solution, including dichromate method, mixed sodium salt method, vulcanization method, acid oxidation method and alkaline oxidation method. Generally, "INCO" (INCO) is used more, but if you want to ensure the same color and luster of a batch of products, you must use a reference electrode to control.

     (2) Electrochemical coloring method: It is the color of the film formed by electrochemical oxidation in a specific solution.

     (3) Ion deposition oxide coloring method chemical method: the stainless steel workpiece is placed in a vacuum coating machine for vacuum evaporation plating. For example: titanium-plated watch cases and watch straps are generally golden yellow. This method is suitable for high-volume product processing. Because of the large investment and high cost, small batch products are not cost-effective.

     ⑷ High temperature oxidation coloring method: In a specific molten salt, the workpiece is immersed in a certain process parameter, so that the workpiece forms a certain thickness of oxide film, and presents various colors.

     ⑸ Gas-phase cracking coloring method: It is more complicated and less used in industry.

Common causes of corrosion of stainless steel parts

1. Chemical corrosion

     1) Surface pollution: The oil, dust, acid, alkali, salt, etc. attached to the surface of the workpiece are converted into corrosive media under certain conditions, and chemically react with some components in the stainless steel parts, resulting in chemical corrosion and rust.

     2) Surface scratches: The damage to the passivation film by various scratches reduces the protective ability of stainless steel, and it is easy to react with chemical media, resulting in chemical corrosion and rust.

     3) Cleaning: After pickling and passivation, the cleaning is not clean, resulting in residual liquid remaining, which directly corrodes stainless steel parts (chemical corrosion).

2. Electrochemical corrosion

     1) Carbon steel pollution: The scratches and corrosive medium caused by contact with carbon steel parts form a primary battery and cause electrochemical corrosion.

     2) Cutting: The adhesion of rust-prone substances such as cutting slag and splashing and the corrosive medium form a primary battery to produce electrochemical corrosion.

     3) Baking school: The composition and metallographic structure of the flame heating area change unevenly, and form a galvanic cell with the corrosive medium to produce electrochemical corrosion.

     4) Welding: Physical defects (undercut, pores, cracks, lack of fusion, lack of penetration, etc.) and chemical defects (coarse grains, poor chromium at grain boundaries, segregation, etc.) in the welding area and corrosive media form primary cells to generate electricity. chemical corrosion.

     5) Material: Chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (porosity, blisters, cracks, etc.) of stainless steel are conducive to the formation of galvanic cells with corrosive media to produce electrochemical corrosion.

     6) Passivation: The poor passivation effect of pickling will cause the passive film on the surface of stainless steel to be uneven or thin, and it is easy to form electrochemical corrosion.

     7) Cleaning: The remaining pickling passivation residue and the products of chemical corrosion of stainless steel form electrochemical corrosion with stainless steel parts.

surface treatment

     1. Cleaning and grinding: If there is any damage, it should be polished, especially the scratches and splashes caused by contact with carbon steel parts, and the damage caused by cutting slag must be carefully cleaned and polished.

     2. Mechanical polishing: Use appropriate polishing tools for polishing, requiring uniform treatment, and avoiding over-polishing and re-scratching.

     3. Degreasing and dust removal: Before pickling and passivation of stainless steel parts, oil, scale, dust and other sundries must be removed according to the process.

     4. Water blasting treatment: According to different treatment requirements, different micro glass beads and different process parameters should be selected, and overspray should be avoided.

     5. Pickling passivation: The pickling passivation of stainless steel parts must be passivated in strict accordance with the process requirements.

     6. Cleaning and drying: After pickling and passivation, neutralization, rinsing and drying should be carried out in strict accordance with the process to completely remove the residual acid solution.

     7. Protection: After the surface treatment of stainless steel parts is completed, they should be well protected to avoid secondary pollution from personal touches and sundries such as oil and dust.

     8. Avoid reprocessing: After the surface treatment of stainless steel parts is completed, reprocessing of the parts or products should be avoided.