1.Common surface treatment methods for stainless steel
1.1 Introduction to Stainless Steel Varieties
Main components of stainless steel: generally contain chromium [Cr], nickel [Ni], molybdenum [Mo], titanium [Ti] and other high-quality metal elements.
Common stainless steel: chromium stainless steel, containing more than 12% Cr; nickel-chromium stainless steel, containing Cr≥18%, containing Ni≥12%.
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.
1.2 Common surface treatment methods of stainless steel
Commonly used stainless steel surface treatment techniques include the following treatment methods: surface natural color whitening treatment, surface mirror bright treatment, and surface coloring treatment.
Surface whitening treatment: During the processing of stainless steel, black oxide skin is produced after coiling, edge binding, welding or artificial surface heating. This hard gray-black scale is mainly composed of two kinds of EO4, NiCr2O4 and NiF. In the past, hydrofluoric acid and nitric acid were generally used for strong corrosion removal. However, this method costs a lot, pollutes the environment, is harmful to the human body, and is highly corrosive, so it is gradually eliminated. At present, there are two main methods of oxide scale treatment:
⑴ Sandblasting (shot) method: mainly uses the method of spraying glass beads to remove the black oxide skin on the surface.
⑵ Chemical method: Use a non-polluting pickling passivation paste and a non-toxic cleaning solution with inorganic additives at room temperature for immersion. 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 and complex products.
(3)Surface mirror bright treatment method: According to the complexity of stainless steel products and user requirements, methods such as mechanical polishing, chemical polishing, and electrochemical polishing can be used to achieve mirror gloss. The advantages and disadvantages of these three methods are as follows:
Mechanical polishing: good leveling, high labor intensity, serious pollution, complex parts are difficult to process, and gloss declines.
Chemical polishing: It can polish complex parts with high efficiency, fast speed and insufficient brightness. Gas escaping during polishing requires ventilation.
Electrochemical polishing: the mirror gloss can be maintained for a long time, the process is stable, less pollution, and low cost. Complex parts require auxiliary motor equipment, and cooling is required for mass production. Can be widely used in various production lines.
Surface coloring treatment: stainless steel coloring not only endows stainless steel products with various colors, increases the variety of designs and colors of products, but also improves the wear resistance and corrosion resistance of products. Stainless steel coloring methods are as follows:
(1) Chemical oxidation coloring method;
⑵ Electrochemical oxidation coloring method;
(3) Lon deposition oxide coloring method;
⑷ High temperature oxidation coloring method;
⑸ Vapor phase pyrolysis coloring method.
A brief overview of the various methods is as follows:
(1) Chemical oxidation coloring method: In a specific solution, the color of the film is formed by chemical oxidation, including dichromate method, mixed sodium salt method, vulcanization method, acid oxidation method and alkaline oxidation method. Generally, “Inco method” [INCO] is used more, but if you want to ensure the same color of a batch of products, you must use the reference electrode to control.
(2) Electrochemical coloring method: the color of the film is formed by electrochemical oxidation in a specific solution.
(3) Chemical method of ion deposition oxide coloring 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. Due to the large investment and high cost, small batch products are not cost-effective.
⑷ High-temperature oxidation coloring method: immerse the workpiece in a specific molten salt to maintain a certain process parameter, so that the workpiece forms a certain thickness of oxide film and presents various colors.
⑸ Vapor-phase pyrolysis coloring method: it is more complicated and less used in industry.
1.3 Treatment method selection
Which method to choose for the surface treatment of stainless steel depends on the product structure, material, and different requirements for the appearance, and choose an appropriate method for treatment.
Sandblasting (shot) method
Surface mirror bright treatment method
2.Common causes of corrosion of stainless steel parts
2.1 Chemical corrosion
Surface pollution: oil, dust, acid, alkali, salt, etc. attached to the surface of the workpiece are transformed into corrosive media under certain conditions, and chemically react with certain components in the stainless steel parts, resulting in chemical corrosion and rust.
Surface scratches: All kinds of scratches damage the passivation film, reduce the protective ability of stainless steel, and easily react with chemical media, resulting in chemical corrosion and rust.
Cleaning: After pickling and passivation, the cleaning is not clean, resulting in residual liquid, which directly corrodes stainless steel parts (chemical corrosion).
2.2 Electrochemical corrosion
Carbon steel pollution: scratches caused by contact with carbon steel parts and corrosive media form primary batteries to produce electrochemical corrosion.
Cutting: The adhesion of rust-prone substances such as cutting slag and splashes and corrosive media form primary batteries to produce electrochemical corrosion.
Baking school: The composition and metallographic structure of the flame heating area change unevenly, and form a primary battery with the corrosive medium to cause electrochemical corrosion.
Welding: Physical defects (undercut, pores, cracks, incomplete fusion, incomplete penetration, etc.) and chemical defects (coarse grains, chromium-poor grain boundaries, segregation, etc.) in the welding area form primary batteries with corrosive media to produce electrochemical corrosion.
Material: Chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (porosity, trachoma, cracks, etc.) of stainless steel are conducive to the formation of primary batteries with corrosive media and electrochemical corrosion.
Passivation: Poor pickling passivation effect causes uneven or thin passivation film on the surface of stainless steel, which is prone to electrochemical corrosion.
Cleaning: The remaining pickling passivation residue and the products of chemical corrosion of stainless steel form electrochemical corrosion with stainless steel parts.
2.3 Stress concentration is easy to cause stress corrosion
In short, due to its special metallographic structure and surface passivation film, stainless steel is difficult to be corroded due to chemical reaction with the medium under normal circumstances, but it is not impossible to be corroded under any conditions. In the presence of corrosive media and incentives (such as scratches, splashes, cutting slag, etc.), stainless steel can also undergo slow chemical and electrochemical reactions with corrosive media and be corroded, and the corrosion rate is quite fast under certain conditions, resulting in corrosion, especially pitting and crevice corrosion. The corrosion mechanism of stainless steel parts is mainly electrochemical corrosion.
Therefore, all effective measures should be taken during the processing of stainless steel products to prevent the occurrence of corrosion conditions and causes as much as possible. In fact, many corrosion conditions and causes (such as scratches, splashes, cutting slag, etc.) also have significant adverse effects on the appearance quality of the product, and should and must be overcome.
Brightness grade of stainless steel after polishing
Using the visual method, the brightness of the surface of the polished part is divided into 5 levels:
Level 1: There is a white oxide film on the surface, without brightness;
Level 2: Slightly bright, the outline cannot be seen clearly;
Level 3: The brightness is good, and the outline can be seen;
Level 4: The surface is bright, and the outline can be seen more clearly (equivalent to the surface quality of electrochemical polishing);
Level 5: Bright as a mirror.