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Scope of Application
Stainless steel possesses exceptional properties such as unique strength, outstanding wear resistance, superior corrosion resistance, and minimal rusting. As a result, it finds extensive use across industries such as chemical engineering, food machinery, electromechanical sectors, environmental protection, home appliances, home decoration, and fine finishing, providing users with an elegant and luxurious aesthetic experience. The future of stainless steel applications holds immense potential, yet the extent of its development will largely hinge on advancements in its surface treatment technologies.
Approaches to Surface Treatment
1. Common Surface Treatment Techniques for Stainless Steel
1.1 Overview of Stainless Steel Varieties
1.1.1 Key Components: Stainless steel typically contains high-grade metallic elements such as chromium (Cr), nickel (Ni), molybdenum (Mo), and titanium (Ti).
1.1.2 Common Types: Chrome stainless steel, with Cr ≥ 12%; nickel-chrome stainless steel, with Cr ≥ 18% and Ni ≥ 12%.
1.1.3 Crystal Structures: Austenitic stainless steel (e.g., 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5); martensitic stainless steel (e.g., Cr17, Cr28). These are commonly referred to as non-magnetic and magnetic stainless steel respectively.
1.2 Common Surface Treatment Methods: Three primary approaches include: â‘ Natural color whitening treatment; â‘¡ Mirror finish treatment; â‘¢ Coloring treatment.
1.2.1 Whitening Treatment: During processing, stainless steel often develops black oxide scales due to rolling, edging, welding, or heat exposure. Previously, hydrofluoric acid and nitric acid were used to remove these scales, but this method was costly, environmentally harmful, and hazardous. Today, two main methods are employed:
â‘´ Sandblasting (with glass beads) removes the black oxide layer.
⑵ Chemical treatment uses non-toxic, organic additive-based solutions at room temperature to achieve a dull finish, ideal for large, complex products.
1.2.2 Mirror Finish Techniques: Depending on product complexity and user requirements, mechanical polishing, chemical polishing, and electrochemical polishing can be used. Each method has its pros and cons.
1.2.3 Coloring Treatment: Coloring enhances the product’s wear resistance and corrosion resistance while adding visual appeal. Methods include:
â‘´ Chemical oxidation coloring
⑵ Electrochemical oxidation coloring
⑶ Ion deposition oxide coloring
â‘· High-temperature oxidation coloring
⑸ Gas phase cracking coloring
Choosing the Right Method: Selecting the appropriate treatment method depends on the product's structure, material, and specific surface requirements.
Common Causes of Rust
2.1 Chemical Corrosion
2.1.1 Surface Contamination: Substances like oil, dust, acids, alkalis, and salts can chemically react with stainless steel components under certain conditions, leading to rust.
2.1.2 Surface Scratches: Damage to the passive film leads to reduced protection and increased susceptibility to chemical corrosion.
2.1.3 Improper Cleaning: Residual liquids post-pickling and passivation can cause chemical corrosion.
2.2 Electrochemical Corrosion
2.2.1 Carbon Steel Contamination: Contact with carbon steel can create galvanic cells, causing electrochemical corrosion.
2.2.2 Cutting: Adhered rust-prone substances and corrosive media can generate electrochemical corrosion.
2.2.3 Heating and Calibration: Uneven changes in structure and composition can form galvanic cells.
2.2.4 Welding: Physical and chemical defects in the weld zone can lead to corrosion.
2.2.5 Material Defects: Intrinsic issues like uneven composition or surface defects can facilitate corrosion.
2.2.6 Poor Passivation: An uneven or thin passivation layer can exacerbate electrochemical corrosion.
Stress concentration can lead to stress corrosion. While stainless steel is resistant to corrosion under normal conditions, scratches, splashes, and slag can trigger corrosion under specific conditions, particularly pitting and crevice corrosion. The primary mechanism of corrosion in stainless steel parts is electrochemical corrosion.
Challenges in Stainless Steel Processing
3.1 Weld Defects: Serious welding flaws require manual grinding, which introduces visible grinding marks affecting aesthetics.
3.2 Surface Inconsistency: Post-weld pickling and passivation lead to uneven surfaces.
3.3 Difficult to Remove Scratches: Overall pickling passivation fails to eliminate scratches or remove impurities like carbon steel splatter, causing chemical or electrochemical corrosion.
3.4 Uneven Polishing: Manual grinding followed by pickling passivation results in inconsistent outcomes, especially for large pieces. Labor and material costs are high.
3.5 Limited Pickling Capacity: Certain black oxides from plasma or flame cutting resist standard pickling treatments.
3.6 Human Factors: Bumps, drags, and hammers during handling cause severe scratches, complicating surface treatment and leading to post-treatment rust.
3.7 Equipment Issues: Bends and folds during profile bending introduce rust risks.
3.8 Raw Material Handling: Transportation-related bumps and scratches during procurement and storage also contribute to rust.
Preventive Measures
4.1 Storage, Handling, and Transportation
4.1.1 Proper Storage: Stainless steel parts should be stored on dedicated racks with wooden or rubber-lined supports to avoid contact with carbon steel. The storage area should be isolated and kept clean.
4.1.2 Safe Handling: Special lifting devices like slings and chucks should be used to avoid scratches.
4.1.3 Clean Transport: Use clean tools to avoid contamination and damage.
4.2 Processing
4.2.1 Fixed Processing Areas: Isolate platforms with rubber mats. Maintain cleanliness and avoid contamination.
4.2.2 Cutting: Use shears, plasma cutters, or saws. Ensure cleanliness of cutting tools and remove slag promptly.
4.2.3 Machining: Protect surfaces from debris and clean after operations.
4.2.4 Forming: Avoid scratches during rolling and bending.
4.2.5 Riveting and Welding: Avoid forced assembly and use isolation measures. Clean slag after plasma cutting.
4.2.6 Welding: Remove debris before welding. Prefer argon arc welding. Use anti-splash measures and clean thoroughly afterward.
4.2.7 Multi-Layer Welding: Clean interlayers and maintain temperatures below 60°C.
4.2.8 Final Weld Inspection: Ensure smooth transitions and no defects.
4.2.9矫形: Avoid flame heating; prefer mechanical devices.
4.2.10 Handling: Use clean tools and avoid direct dragging or collisions.
4.3 Surface Treatment
4.3.1 Cleaning and Grinding: Repair scratches and splatter meticulously.
4.3.2 Polishing: Achieve uniform finishes.
4.3.3 Oil Removal: Clean thoroughly before passivation.
4.3.4 Blasting: Adjust bead size and parameters.
4.3.5 Passivation: Follow strict protocols.
4.3.6 Neutralizing and Drying: Rinse and dry thoroughly.
4.3.7 Protection: Cover treated parts to prevent contamination.
4.3.8 Avoid Reprocessing: Once treated, do not reintroduce parts to unclean environments.
By addressing these challenges and implementing preventive measures, stainless steel applications can achieve enhanced durability and aesthetic appeal.
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