Common Problems and Precautionary Measures of Stainless Steel Surface Treatment of Fasteners

Stainless steel has unique strength, high wear resistance, superior anti-corrosion properties and is not easy to rust and other excellent characteristics. Therefore, it is widely used in the chemical industry, food machinery, mechanical and electrical industry, environmental protection industry, household appliances industry and home decoration, finishing industry, giving people a feeling of noble and noble.

The prospects for the application of stainless steel will be more and more wide, but the application development of stainless steel largely determines the degree of development of its surface treatment technology.

1 stainless steel commonly used surface treatment method

1.1 Introduction of Stainless Steel Varieties

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

1.1.2 common stainless steel: chromium stainless steel, containing Cr ≥ 12% or more; nickel-chromium stainless steel, containing Cr ≥ 18%, with Ni ≥ 12%.

1.1.3 Classification from stainless steel metallurgical structure: There are austenitic stainless steels, for example: 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5. Martensitic stainless steel, such as: Cr17, Cr28 and so on. Commonly referred to as non-magnetic stainless steel and magnetic stainless steel.

1.2 Common Stainless Steel Surface Treatment Methods

Commonly used stainless steel surface treatment technology has the following treatment methods: 1 surface color whitening treatment; 2 surface mirror light treatment; 3 surface coloring treatment.

1.2.1 Surface color whitening treatment: stainless steel in the process of processing, after coiling, binding, welding or artificial surface heating and heat treatment, resulting in black oxide scale. This hard gray-black scale is mainly NiCr2O4 and NiF two EO4 components, previously used hydrofluoric acid and nitric acid for strong corrosion removal. However, this method is costly, pollutes the environment, is harmful to the human body and is highly corrosive, and is gradually being eliminated. Currently there are two main methods for treating oxide scales:

(1) Sandblasting (Pills) Method: The method of spraying micro glass beads is mainly used to remove the black oxide scale on the surface.

(2) Chemical method: Use a non-polluting pickling passivation paste and a non-toxic, non-toxic cleaning solution with inorganic additives. In order to achieve the purpose of the white 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 Stainless steel mirror surface light treatment method: According to the complexity of stainless steel products and user requirements different conditions can be used mechanical polishing, chemical polishing, electrochemical polishing and other methods to achieve a mirror gloss. The advantages and disadvantages of these three methods are as follows:

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

Stainless steel coloring methods are as follows:

(1) chemical oxidation coloring;

(2) electrochemical oxidation coloring method;

(3) ion deposition oxide coloring method;

(4) High temperature oxidation coloring method;

(5) Gas phase cracking coloring method.

The various methods are as follows:

(1) Chemical Oxidation Coloring: It is the color of the film formed by chemical oxidation in a specific solution, such as dichromate method, mixed sodium salt method, sulfurization method, acidic oxidation method, and alkaline oxidation method. In general, INCO is used more often. However, in order to ensure that the color of a batch of products is consistent, the reference electrode must be used for control.

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

(3) Ion-deposition oxide coloring chemical method: The stainless steel workpiece is placed on a vacuum coating machine for vacuum evaporation deposition. For example: titanium-plated watch cases, watch bands, are generally golden yellow. This method is suitable for processing large quantities of products. Because of the large investment and high cost, small-volume products are uneconomical.

(4) high-temperature oxidation coloring method: in a specific molten salt, immersed in the workpiece to maintain a certain process parameters, the workpiece to form a certain thickness of oxide film, and showing a variety of different colors.

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

1.3 Processing Method Selection

Which method is used for the surface treatment of stainless steel depends on the product structure, material, and different requirements on the surface.

2 common causes of corrosion of stainless steel parts

2.1 Chemical corrosion

2.1.1 Surface contamination: Oil, dust, acids, alkalis, salts, etc. attached to the surface of the workpiece are converted into corrosive medium under certain conditions, chemical reactions occur with certain components in the stainless steel, and chemical corrosion occurs and rust occurs.

2.1.2 Surface scratches: Various scratches damage the passivation film, which reduces the protective ability of the stainless steel, easily reacts with chemical media, and causes chemical corrosion and rust.

2.1.3 Cleaning: Cleaning after acid washing is not clean, causing residual liquid to remain and directly corroding stainless steel (chemical corrosion).

2.2 Electrochemical corrosion

2.2.1 Carbon Steel Pollution: Scratches and corrosives caused by contact with carbon steel elements form galvanic cells and cause electrochemical corrosion.

2.2.2 Cutting: Adsorption and corrosion of rust-prone materials such as slag, splashing, etc. form a primary battery to generate electrochemical rot.

2.2.3 Roasting school: The composition and metallographic structure of the flame-heating zone change and become non-uniform, forming a primary battery with the corrosive medium and causing electrochemical corrosion.

2.2.4 Welding: physical defects in the welding area (undercuts, pores, cracks, unmelted, incomplete penetration, etc.) and chemical defects (greater grain size, chromium-depleted grain boundaries, segregation, etc.) and the formation of primary batteries by corrosive media. Electrochemical corrosion.

2.2.5 Material: Stainless steel chemical defects (inhomogeneous composition, S, P impurities, etc.) and surface physical defects (loose, trachoma, cracks, etc.) are conducive to the formation of primary batteries with corrosion media and electrochemical corrosion.

2.2.6 Passivation: The passivation effect of pickling is not good. The passivation film on the surface of stainless steel is uneven or thin, and it is easy to form electrochemical corrosion.

2.2.7 Cleaning: Electrochemical corrosion is formed between the retained acid-washing passivation residual solution and the stainless steel and the stainless steel.

2.3 Stress concentration is prone to stress corrosion

In short, because of its special metallographic structure and surface passivation film, stainless steel makes it difficult to corrode with chemical reaction with the medium under normal circumstances, but it cannot be corroded under any conditions. In the presence of corrosive media and inducing agents (such as scratches, splashes, slags, etc.), the stainless steel can also corrode slowly with the corrosive media, and the corrosion rate under certain conditions is very fast. Corrosion phenomena, 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 avoid rust conditions and incentives. In fact, many rust conditions and incentives (such as scratches, splashes, slags, etc.) also have a significant adverse effect on the appearance of the product and should and must be overcome.

3 stainless steel product processing problems

3.1 Weld Defects: Weld defects are more severe and are repaired by manual mechanical sanding. The resulting wear marks cause uneven surfaces and affect the appearance.

3.2 Inconsistent surface: only the passivation of the weld passivation, also caused uneven surface, affecting the appearance.

3.3 Scratch hard to remove: The whole pickling passivation, can not remove the various scratches produced during processing, and can not remove the carbon steel, spatter and other impurities adhered to the stainless steel surface due to scratches, welding spatter, Causes chemical corrosion or electrochemical corrosion and rust in the presence of corrosive media.

3.4 Non-uniformity of polishing and passivation: After pickling and polishing, the pickling and passivation treatment is performed. It is difficult to achieve a uniform and uniform treatment effect for a workpiece with a large area, and an ideal uniform surface cannot be obtained. And working hours, accessories costs are also high.

3.5 limited pickling ability: pickling passivation paste is not a panacea, plasma cutting, flame cutting and black oxide production, it is more difficult to remove.

3.6 Scratching caused by human factors is more serious: In the process of lifting, transportation and structural processing, scratches, dragging, hammering and other human factors cause serious scratches, making the surface treatment more difficult, and it also causes corrosion after treatment. main reason.

3.7 Equipment factors: The scratches and creases caused during the bending and bending of profiles, plates, etc. are also the main causes of corrosion after treatment.

3.8 Other factors: During the procurement and storage of stainless steel raw materials, bumps and scratches due to lifting and transportation are also serious and one of the causes of corrosion.

4 should take preventive measures

4.1 Storage, Hoisting, Transportation

4.1.1 Storage of stainless steel parts: There should be special storage racks. The storage racks should be wood or painted carbon steel brackets or pads with rubber mats to isolate them from other metal materials such as carbon steel. When storing, the storage location should be easy to lift, isolated from other material storage areas, and there should be protective measures to avoid contamination of stainless steel by dust, oil, and rust.

4.1.2 Stainless steel lifting: When lifting, special lifting devices such as lifting belts and special chucks should be used. It is forbidden to use steel ropes to avoid scratching the surface. When lifting and placing, collisions should be avoided to cause scratches.

4.1.3 Transportation of stainless steel parts: During transportation, transportation tools (such as trolleys, car batteries, etc.) shall be used, and shall be cleaned and protected against dust, oil, and rust to contaminate the stainless steel. Do not drag, avoid bumps, scratches.

4.2 Processing

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

4.2.2 Cutting: Cutting of stainless steel parts is carried out by cutting or plasma cutting, sawing, etc.

(1) Shear: When shearing, it should be isolated from the feeding bracket, and the falling hopper should be covered with a rubber pad to avoid scratching.

(2) Plasma cutting: After plasma cutting, the cutting slag should be cleaned. When batch cutting, the field should be cleared in time for the completed parts to avoid the slag slag on the workpiece.

(3) Sawing cutting: When sawing and cutting, the clamp should be protected by rubber cover. After sawing, the oil and residue on the workpiece should be cleaned.

4.2.3 Machining: The stainless steel parts should also be protected during the machining of vehicles, milling, etc., and the surface of the workpiece should be cleaned of oil, iron and other debris.

4.2.4 Molding: In the process of bending and bending, effective measures shall be taken to avoid scratches and creases on the surface of the stainless steel parts.

4.2.5 Rivet welding: When the stainless steel parts are paired, forced assembly should be avoided, especially the flame roasting assembly. If there is temporary plasma cutting in the pair or production process, isolation measures shall be taken to avoid contamination of other stainless steel parts by slag cutting. After cutting, the slag on the workpiece should be cleaned.

4.2.6 Welding: The stainless steel parts must be carefully cleaned of oil, rust, dust and other debris before welding. When welding, try to use argon arc welding. When using manual arc welding, use small current and fast welding to avoid swing. It is strictly forbidden to start the arc in the non-welding area. The grounding position is appropriate and the connection is firm to avoid arcing scratches. Anti-splash measures should be taken when welding (such as brush white ash and other methods). After welding, stainless steel (not carbon steel) spatula is used to thoroughly clean slag and splash.

4.2.7 Multi-layer welding: When multi-layer welding, the inter-layer slag must be removed. When multi-layer welding, the temperature between layers should be controlled, and generally should not exceed 60 °C.

4.2.8 Welds: Weld joints shall be ground. There shall be no defects such as slag, air holes, undercuts, splashes, cracks, incomplete fusion, incomplete penetration, etc., and the welds and parent metal shall be smoothly transitioned. They shall not be low. In the parent material.

4.2.9 Orthopedic: For the reshaping of stainless steel parts, flame heating methods should be avoided. In particular, repeated heating of the same area is not allowed. When correcting, use mechanical devices as far as possible, or hammer them with wooden hammers (rubber hammers) or pad rubber pads, and do not hammer them with hammers to avoid damage to the stainless steel parts.

4.2.10 Handling: When transporting stainless steel parts during processing, use transportation tools (such as trolleys, battery carts, or overhead cranes), and clean and isolate protective measures to prevent dust, oil, and rust from contaminating the stainless steel. It is forbidden to drag directly on the platform or the ground. Bumping and scratching are strictly prohibited.

4.3 Surface treatment

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

4.3.2 Mechanical Polishing: The use of a suitable polishing tool for polishing requires uniform handling and avoids overswing and re-scratching.

4.3.3 Degreasing and dust removal: Before the stainless steel parts are pickled and passivated, they must be cleaned according to the process to remove oil, oxide scale, dust and other debris.

4.3.4 Water blasting: According to different processing 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 Cleaning and Drying: After pickling is passivated, it should be neutralized, rinsed and dried strictly according to the process to completely remove residual acid.

4.3.7 Protection: After the surface treatment of stainless steel parts is completed, protection shall be provided to prevent secondary pollution of personnel touching and impurities such as oil, dust and the like.

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