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Stainless steel surface treatment

Stainless steel surface treatment

Time:2022-04-07 Source:Wuxi Jianuo Special Steel Co.

Stainless steel has unique strength, high wear resistance, superior anti-corrosion properties and not easy to rust and other excellent characteristics. Therefore, it is widely used in the industry, food machinery, electromechanical industry, household appliances industry and home decoration, finishing industry. The use of stainless steel development prospects will become more and more extensive, but the use of stainless steel development is largely determined by its surface treatment technology development degree.

Basic information

Stainless steel has unique strength, high wear resistance, superior anti-corrosion properties and not easy to rust and other excellent characteristics. Therefore, it is widely used in the chemical industry, food machinery, electromechanical industry, environmental protection industry, household appliances industry and home decoration, finishing industry, giving people a gorgeous and noble feeling. [1] The application of stainless steel development prospects will become more and more extensive, but the development of stainless steel applications largely determine its degree of development of surface treatment technology. [1]

Variety Introduction

1.1.1 Stainless steel main components: generally contains chromium (Cr), nickel (Ni), molybdenum (Mo), titanium (Ti) and other high-quality metal elements.

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

1.1.3 Classification from stainless steel metallographic organization: there are austenitic stainless steel, for example: 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5. martensitic stainless steel, for example: Cr17, Cr28, etc.. Generally known as non-magnetic stainless steel and stainless steel with magnetism.

Causes of rust and corrosion

Chemical corrosion

2.1.1 Surface contamination: attached to the surface of the workpiece oil, dust and acid, alkali, salt, etc. in certain conditions into a corrosive medium, and some components of the stainless steel parts in a chemical reaction, resulting in chemical corrosion and rust.

2.1.2 Surface scratches: various scratches on the destruction of the passivation film, so that the stainless steel protection capacity is reduced, easy to react with chemical media, chemical corrosion and rust.

2.1.3 Cleaning: pickling passivation after cleaning is not clean resulting in residual liquid retention, direct corrosion of stainless steel parts (chemical corrosion).

Electrochemical corrosion

2.2.1 carbon steel pollution: contact with carbon steel parts caused by scratching and corrosion media to form primary cells and electrochemical corrosion.

2.2.2 cutting: cutting slag, splash and other rust-prone material adherence and corrosion media to form primary cells and electrochemical corrosion.

2.2.3 Baking school: flame heating area composition and metallurgical organization changes and uneven, and corrosion media to form a primary cell and electrochemical corrosion.

2.2.4 welding: physical defects in the welding area (edge biting, porosity, cracks, not fused, not welded through, etc.) and chemical defects (coarse grains, grain boundary poor chromium, segregation, etc.) and corrosion media to form a primary cell and electrochemical corrosion.

2.2.5 material: stainless steel material chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (loose, sand eyes, cracks, etc.) is conducive to the formation of primary cells with corrosive media and electrochemical corrosion.

2.2.6 passivation: pickling passivation effect is not good cause stainless steel surface passivation film is not uniform or thin, easy to form electrochemical corrosion.

2.2.7 Cleaning: the residue of the surviving pickling passivation and stainless steel chemical corrosion occurs with the formation of electrochemical corrosion of stainless steel parts.

Stress corrosion

In short, stainless steel due to its special metallurgical organization and surface passivation film, making it more difficult in general to be corroded by chemical reaction with the medium, but not under any conditions can not be corroded. In the corrosive medium and causative factors (such as scratches, splashes, cutting slag, etc.) under the conditions of the presence of stainless steel can also be corroded with the corrosive medium of slow chemical and electrochemical reactions, and in certain conditions of the corrosion rate is quite fast and rust corrosion phenomenon, especially pitting and crevice corrosion. The corrosion mechanism of stainless steel parts is mainly electrochemical corrosion.

Therefore, in stainless steel products in the process of processing operations should take all effective measures to try to avoid rust conditions and causes of corrosion. In fact, many rust conditions and causative factors (such as scratches, splashes, cutting slag, etc.) for the appearance of the product quality also has a significant adverse impact, but also should and must be overcome.

Preventive Measures Editorial Podcast

Storage lifting

4.1.1 Stainless steel parts storage: there should be a special storage rack, storage rack should be wooden or surface painted carbon steel bracket or pad with rubber mat, in order to isolate from other metal materials such as carbon steel. Storage, storage location should be easy to lift, and other materials storage area relative isolation, there should be protective measures to avoid dust, oil, rust pollution of stainless steel.

4.1.2 Stainless steel lifting: lifting, should use special lifting tools, such as lifting straps, special chuck, etc., strictly prohibit the use of steel wire rope to avoid scratching the surface; and in lifting and placing, should avoid impact knocking caused by scratching.

4.1.3 Stainless steel transport: transportation, the application of transport tools (such as carts, battery car, etc.), and should be clean with isolation protection measures to prevent dust, oil, rust pollution of stainless steel. Dragging is strictly prohibited to avoid bumping and scratching.

Processing

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

4.2.2 undercutting: stainless steel parts undercutting using shear or plasma cutting, sawing, etc.

(1) shear: shear, should be isolated from the feeding bracket, the drop bucket should also be laid with a rubber mat to avoid scratching.

(2) Plasma cutting: After plasma cutting, the cutting slag should be cleaned up. When batch cutting, for the completed parts should be cleaned out of the site in time to avoid the tarnishing of the workpiece by cutting slag.

(3) sawing off: sawing off, clamping should be protected by rubber, and the workpiece should be cleaned of oil, residue, etc. after sawing.

4.2.3 Mechanical processing: stainless steel parts in turning, milling and other mechanical processing should also pay attention to protection, the work should be cleaned up the surface of the workpiece oil, iron filings and other debris.

4.2.4 Forming process: in the rolled plate, bending process, effective measures should be taken to avoid causing surface scratches and folds on the stainless steel parts.

4.2.5 riveting: stainless steel parts in the grouping, should avoid forced assembly, especially to avoid flame baking school assembly. Grouping or production process such as the temporary use of plasma cutting, isolation measures should be taken to avoid cutting slag on other stainless steel pieces of pollution. After cutting, the cut slag on the workpiece should be cleaned up.

4.2.6 Welding: stainless steel parts must be carefully removed before welding oil, rust, dust and other debris. Welding try to use argon arc welding, manual arc welding should be used when using a small current, fast welding, to avoid swinging. It is strictly forbidden to draw an arc in the non-welding area, and the ground wire should be properly positioned and firmly connected to avoid arc abrasion. Welding should take anti-spatter measures (such as brushing white ash and other methods). Apply stainless steel (not carbon steel) flat shovel to thoroughly clean up slag and spatter after welding.

4.2.7 multi-layer welding: multi-layer welding, inter-layer slag must be removed clean. Multi-layer welding, should control the interlayer temperature, generally shall not exceed 60 ℃.

4.2.8 welding seam: welded joints should be repaired, the weld surface shall not have slag, porosity, biting, spatter, cracks, not fused, not welded through and other defects, the weld and the base material should be a smooth transition, not lower than the base material.

4.2.9 orthopedic: stainless steel parts of the orthopedic, should avoid the use of flame heating methods, especially do not allow repeated heating of the same area. When orthopedic, try to use mechanical devices, or with a wooden hammer (rubber hammer) or pad rubber pad hammering, prohibit the use of iron hammering to avoid damage to stainless steel parts.

4.2.10 Handling: stainless steel parts in the processing of handling, the application of transport tools (such as trolleys, battery carts or overhead cranes, etc.), and should be clean with isolation protection measures to prevent dust, oil, rust pollution of stainless steel. It is strictly forbidden to drag directly on the platform or ground, bump and scratch.

Surface treatment

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

4.3.2 Mechanical polishing: appropriate polishing tools should be used for polishing, requiring uniform and consistent treatment and avoiding over-polishing and re-scratching.

4.3.3 Oil and dust removal: stainless steel parts in the pickling passivation before, must be removed according to the process of oil, oxide, dust and other debris.

4.3.4 Water blasting treatment: according to different treatment requirements, choose different micro-glass beads, different process parameters, and avoid overblasting, etc.

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

4.3.6 Cleaning and drying: After pickling and passivation, neutralization, rinsing 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, should be well protected to avoid secondary pollution by personnel touching and debris such as oil and dust.

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

Including: surface treatment of stainless steel / mild steel products such as chemical vessels, storage tanks, pipeline boilers, such as grinding / polishing, pickling / passivation, inspection / supervision, technical advice, etc.

The main service targets are industries such as ships, petrochemicals and food equipment.

Problems

5.1 Weld defects: Weld defects are more serious, and manual mechanical polishing treatment methods are used to make up for them, resulting in polishing traces that cause uneven surfaces and affect aesthetics.

5.2 Surface inconsistency: Only pickling passivation is applied to the weld seam, which also results in an uneven surface and affects the aesthetics.

5.3 Scratches difficult to remove: overall pickling passivation, but also can not remove the various scratches generated during processing, and also can not remove the carbon steel, spatter and other impurities adhered to the surface of stainless steel due to scratches, welding spatter, resulting in the presence of corrosive media conditions of chemical or electrochemical corrosion and rust.

5.4 grinding and polishing passivation is not uniform: manual grinding and polishing after pickling passivation treatment, the area of a larger workpiece, it is difficult to achieve uniform and consistent treatment effect, can not get the ideal uniform surface. And the cost of man-hours and auxiliary materials is also higher.

5.5 Limited pickling ability: pickling passivation paste is not a panacea, and it is difficult to remove the black oxide skin produced by plasma cutting and flame cutting.

5.6 Serious scratches caused by human factors: In the process of lifting, transportation and structural processing, scratches caused by human factors such as bumping, dragging and hammering are more serious, which makes surface treatment more difficult and is also the main cause of rust and corrosion after treatment.

5.7 Equipment factors: in the profile, plate roll bending, bending process, resulting in scratches and creases is also the main reason for rust and corrosion after treatment.

5.8 Other factors: stainless steel raw materials in the procurement and storage process, due to lifting, transport process generated by the bump and scratch is also more serious, is also one of the reasons for the generation of rust and corrosion.

Basic treatment

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

1.2.1 Surface natural whitening treatment: stainless steel in the processing process, after the rolled plate, tied edge, welding or after artificial surface fire baking heating treatment, resulting in black oxide skin. This hard gray-black oxide skin is mainly NiCr2O4 and NiF two EO4 components, the previous general use of hydrofluoric acid and nitric acid for strong corrosion methods to remove. However, this method is costly, polluted environment, harmful to human body and corrosive, gradually eliminated. There are two main methods of treatment for oxide skin.

(1) sandblasting (pill) method: mainly using the method of spraying micro glass beads to remove the black oxide skin on the surface.

(2) chemical method: the use of a non-polluting pickling passivation paste and room temperature non-toxic cleaning solution with inorganic additives for dipping. Thus achieving the purpose of whitening treatment of stainless steel natural color. After the treatment basically looks like nothing light color. This method is more suitable for large, complex products.

1.2.2 Stainless steel surface mirror bright treatment method: according to the complexity of stainless steel products and user requirements can be used in different situations mechanical polishing, chemical polishing, electrochemical polishing and other methods to achieve a mirror luster. 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, increasing the variety of products, but also to improve product wear resistance and corrosion resistance.

Stainless steel coloring methods are as follows.

(1) chemical oxidation coloring method.

(2) electrochemical oxidation coloring method.

(3) ion deposition oxide coloring method.

(4) high-temperature oxidation coloring method.

(5) gas phase cracking coloring method.

A brief overview of the various methods is as follows.

(1) Chemical oxidation coloring method: It is the color of the film formed by chemical oxidation in a specific solution, and there are dichromate method, mixed sodium salt method, sulfide method, acidic oxidation method and alkaline oxidation method. Generally “INCO method” (INCO) is used more often, but to ensure a batch of products with the same color, you must use reference electrodes to control.

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

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

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

(5) gas phase cracking coloring method: more complex, less used in industry.