Common problems and preventive measures in stainless steel surface treatment process

Stainless steel has excellent characteristics such as unique strength, high wear resistance, superior corrosion resistance and resistance to rust. Therefore, it is widely used in chemical industry, food machinery, electromechanical industry, environmental protection industry, household appliance industry and home decoration, finishing industry, giving people a feeling of magnificence and nobility.
The application and development prospect of stainless steel will become wider and wider, but the application and development of stainless steel largely determines the development level of its surface treatment technology.

1 Common surface treatment methods for stainless steel

1.1 Introduction to stainless steel varieties

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

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

1.1.3 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. Generally called non-magnetic stainless steel and magnetic stainless steel.

1.2 Common stainless steel surface treatment methods

Commonly used stainless steel surface treatment techniques have the following treatment methods: ① surface natural whitening treatment; ② surface mirror bright treatment; ③ surface coloring treatment.

1.2.1 Whitening treatment of the natural surface color: During the processing of stainless steel, black oxide scale is produced after coiling, binding, welding or artificial surface fire-bake heating treatment. This hard gray-black oxide scale is mainly composed of NiCr2O4 and NiF, two EO4 components. In the past, hydrofluoric acid and nitric acid were generally used for strong corrosion to remove. However, this method is costly, pollutes the environment, harmful to the human body, and corrosive, and is gradually eliminated. At present, there are two main methods for treating oxide scale:

⑴ Sandblasting (pill) method: mainly adopts the method of spraying micro glass beads to remove the black oxide scale 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.

1.2.2 Mirror bright treatment method of stainless steel surface: According to the complexity of stainless steel products and the different requirements of users, 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:

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

There are several coloring methods for stainless steel:

⑴Chemical oxidation coloring method;

⑵ Electrochemical oxidation coloring method;

⑶ ion deposition oxide coloring method;

⑷High temperature oxidation coloring method;

⑸ Gas phase cracking coloring method.

A brief overview of the various methods is as follows:

⑴Chemical oxidation coloring method: It is the color of the film formed by chemical oxidation in a specific solution. There are dichromate method, mixed sodium salt method, vulcanization method, acid oxidation method and alkaline oxidation method. In general, INCO is used more, but if you want to ensure the color of a batch of products are consistent, you must use a reference electrode to control.

⑵Electrochemical coloring method: the color of the film formed by electrochemical oxidation in a specific solution.

⑶ Ion deposition oxide coloring 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 bands are generally golden yellow. This method is suitable for mass product processing. Because of the large investment and high cost, small batch products are not economical.

⑷High temperature oxidation coloring method: It is to 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 a variety of different colors.

⑸ Gas phase pyrolysis coloring method: It is more complicated and less applied in industry.

1.3 Selection of treatment methods

Which method to choose for the surface treatment of stainless steel should be based on the product structure, material, and different requirements on the surface, and choose an appropriate method for treatment.

2 Common causes of corrosion of stainless steel parts

2.1 Chemical corrosion

2.1.1 Surface contamination: 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.

2.1.2 Surface scratches: All kinds of scratches damage the passivation film, which reduces the protective ability of stainless steel. It is easy to react with chemical media and produce chemical corrosion and rust.

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

2.2 Electrochemical corrosion

2.2.1 Carbon steel pollution: scratches and corrosive media caused by contact with carbon steel parts form a galvanic cell and cause electrochemical corrosion.

2.2.2 Cutting: The adhesion of rust-prone substances such as cutting slag, splash and corrosive medium forms a galvanic cell and produces electrochemical corrosion.

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

2.2.4 Welding: physical defects (undercuts, pores, cracks, lack of fusion, incomplete penetration, etc.) and chemical defects (large grains, chromium depletion at grain boundaries, segregation, etc.) in the welding area and corrosive media form galvanic cells. Electrochemical corrosion.

2.2.5 Material: The chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (loose, blisters, cracks, etc.) of the stainless steel material are conducive to the formation of galvanic cells with corrosive media and electrochemical corrosion.

2.2.6 Passivation: poor pickling and passivation results in uneven or thin passivation film on the surface of stainless steel, which is prone to electrochemical corrosion.

2.2.7 Cleaning: The remaining pickling passivation residue and the chemical corrosion products of stainless steel will 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 generally difficult to be corroded by chemical reaction with the medium, but it is not impossible to be corroded under any conditions. In the presence of corrosive media and inducements (such as scratches, splashes, cutting slag, etc.), stainless steel can also be corroded by slow chemical and electrochemical reactions with corrosive media, and the corrosion rate is quite fast under certain conditions. Rust phenomenon, 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 try to avoid rust conditions and inducements. In fact, many rust conditions and inducements (such as scratches, splashes, cutting slag, etc.) also have significant adverse effects on the appearance of products, and they should and must be overcome.

3 There are problems in the processing of stainless steel products

3.1 Welding seam defects: Welding seam defects are more serious, and manual mechanical grinding treatment methods are used to make up. The grinding marks produced will cause uneven surface and affect appearance.

3.2 Inconsistent surface: only pickling and passivation of welds will also cause uneven surface and affect appearance.

3.3 Scratches are difficult to remove: the overall pickling passivation can not remove all kinds of scratches generated during processing, and can not remove carbon steel, splashes and other impurities that adhere to the stainless steel surface due to scratches and welding spatter. Causes chemical corrosion or electrochemical corrosion to rust in the presence of corrosive media.

3.4 Uneven grinding and polishing passivation: pickling passivation after manual grinding and polishing, it is difficult to achieve a uniform treatment effect for a workpiece with a large area, and an ideal uniform surface cannot be obtained. And the cost of working hours and auxiliary materials are also higher.

3.5 Limited pickling capacity: pickling passivation paste is not a panacea. It is difficult to remove black oxide scale produced by plasma cutting and flame cutting.

3.6 Scratches caused by human factors are more serious: in the process of hoisting, transportation and structural processing, the scratches caused by human factors such as bumping, dragging and hammering are more serious, making the surface treatment more difficult, and it is also rusty after treatment. main reason.

3.7 Equipment factors: Scratches and creases caused during the bending and bending of profiles and plates are also the main reasons for corrosion after treatment.

3.8 Other factors: During the procurement and storage of stainless steel raw materials, bumps and scratches caused by hoisting and transportation are also more serious, which is also one of the causes of corrosion.

4 Precautions should be taken

4.1 Storage, hoisting and transportation

4.1.1 Storage of stainless steel parts: special storage racks should be provided. The storage racks should be wooden or painted carbon steel brackets or pads with rubber pads to isolate them from carbon steel and other metal materials. When storing, the storage location should be convenient for lifting, relatively isolated from other material storage areas, and protective measures should be taken to avoid contamination of stainless steel by dust, oil, and rust.

4.1.2 Stainless steel parts hoisting: when hoisting, special hoisting tools, such as hoisting belts, special chucks, etc., should be used. Wire ropes are strictly prohibited to avoid scratching the surface; and when hoisting and placing, avoid impacts and scratches caused by scratches.

4.1.3 Transportation of stainless steel parts: during transportation, transportation tools (such as trolleys, battery carts, etc.) should be used, and isolation and protection measures should be taken to prevent dust, oil, and rust from polluting the stainless steel. Procrastination is strictly prohibited to avoid bumps and scratches.

4.2 Processing

4.2.1 Processing area: The processing area of ​​stainless steel parts should be relatively fixed. The platform in the processing area of ​​stainless steel parts should take isolation measures, such as laying rubber mats. The fixed management and civilized production of stainless steel parts processing areas should be strengthened to avoid damage and pollution to stainless steel parts.

4.2.2 Cutting: The cutting of stainless steel parts adopts shearing, plasma cutting, sawing, etc.

⑴ Shearing: When shearing, it should be isolated from the feeding bracket, and the falling hopper should also be covered with rubber pads to avoid scratches.

⑵ Plasma cutting: After plasma cutting, the cutting residue should be cleaned up. When batch cutting, the completed parts should be cleaned up in time to avoid contamination of the workpiece by cutting slag.

⑶ Sawing and cutting: When sawing and cutting, the clamping should be protected by rubber. After sawing, the oil and residue on the workpiece should be cleaned.

4.2.3 Mechanical processing: The stainless steel parts should also be protected during machining such as turning and milling. After the operation, the oil stains, iron filings and other sundries on the surface of the workpiece should be cleaned up.

4.2.4 Forming processing: Effective measures should be taken to avoid scratches and creases on the surface of stainless steel parts during the rolling and bending process.

4.2.5 Riveting welding: When assembling stainless steel parts, compulsory assembly should be avoided, especially flame baking assembly should be avoided. If plasma cutting is used temporarily during the assembly or production process, isolation measures should be taken to avoid contamination of other stainless steel parts by cutting slag. After cutting, the cutting residue on the workpiece should be cleaned up.

4.2.6 Welding: Before welding stainless steel parts, oil, rust, dust and other debris must be carefully removed. When welding, use argon arc welding as much as possible. When using manual arc welding, use small current and fast welding to avoid swinging. It is strictly forbidden to ignite the arc in the non-welded area, and the ground wire should be in proper position and firmly connected to avoid arc scratches. Splash-proof measures (such as white ash, etc.) should be taken during welding. After welding, use stainless steel (no carbon steel) flat shovel to thoroughly clean slag and spatter.

4.2.7 Multi-layer welding: In multi-layer welding, the slag between layers must be removed. When multi-layer welding, the temperature between layers should be controlled, generally not exceeding 60℃.

4.2.8 Welding seam: Welding seam joints should be ground. The surface of the welding seam shall not have defects such as slag, pores, undercuts, splashes, cracks, lack of fusion, incomplete penetration, etc. The welding seam and the base metal shall be smoothly transitioned and shall not be low. In the base material.

4.2.9 Orthopedics: For the orthopedics of stainless steel parts, flame heating methods should be avoided, especially repeated heating of the same area is not allowed. When orthopedic, use mechanical devices as much as possible, or hammer with a wooden hammer (rubber hammer) or a rubber pad. Hammering with an iron hammer is forbidden to avoid damage to stainless steel parts.

4.2.10 Handling: When handling stainless steel parts during processing, transportation tools (such as trolleys, battery carts or cranes, etc.) should be used, and isolation and protection measures should be taken to prevent dust, oil and rust from polluting the stainless steel. It is strictly forbidden to drag directly on the platform or the ground, and it is strictly forbidden to bump and scratch.

4.3 Surface treatment

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

4.3.2 Mechanical polishing: Use appropriate polishing tools for polishing, requiring uniform and consistent treatment, and avoid over-polishing and re-scratching.

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

4.3.4 Water blasting treatment: According to different treatment requirements, choose different micro glass beads, different process parameters, and avoid overspray.

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

4.3.6 Washing and drying: After pickling passivation, neutralization, washing, and drying should be carried out strictly according to the process to thoroughly remove the residual acid.

4.3.7 Protection: After the surface treatment of stainless steel parts is completed, protection should be done to avoid the secondary pollution of personnel touching and oil, dust and other sundries.

4.3.8 Avoid reprocessing: After finishing the surface treatment of stainless steel parts, avoid reprocessing the parts or products.