Fastener surface treatment process is a key process to enhance corrosion resistance, wear resistance, aesthetics and other specific properties by forming a cover layer on the surface of the workpiece or changing the surface properties. There are many methods and processing techniques for surface treatment of fasteners. For example, physical, chemical, electrical processing, mechanical, metallurgical.
Fastener Surface Treatment Process
| Hot-dip Galvanizing | Chemical Plating | Vacuum Coating | Chemical Vapor Deposition (CVD) | Carburizing | Blue-white Zinc Plating |
| Black Oxide | Spray coating | Anodizing | Surface quenching | Teflon | Dacromet |
| Yellow Zinc | Powder Coating | Stainless Steel Passivation | Copper Plating | Ruspert | Zinc Plating |
| Brass Plating | Chrome Plating | Silver Plating | Gold Plating | Nickel Plating | Cadmium Plating |
| Mechanical Plating | Electroplating | Magni Coating | Geomet Coating | Tin Plating | |
| Color Zinc Plating | Phosphating | Natural Finish | Black Zinc Plating | Rilsan |
Electroplating
Electroplating is the process of depositing metal plating on the surface of fasteners through electrolysis, in which fasteners are immersed in an electrolytic solution containing the target metal ions, and the metal precipitates out to form a uniform, dense plating layer after energization. Commonly used coatings include zinc, nickel, cadmium, chromium and copper-nickel alloys.
Electro zinc plating is the most economical fastener treatment option with a nice appearance, available in white zinc, blue zinc, colored zinc, black zinc and other colors.
Process: degreasing→Water washing→Acid washing→Water washing→Electroplating→Hot water washing→Baking (if necessary)→Purification treatment (or closure)→or closure or additional lubrication→Water washing→Drying→Delivery inspection.
Phosphating
Phosphating is a treatment in which fasteners are immersed in an acidic solution containing phosphates to form a phosphate film on the metal surface through a chemical reaction.
Phosphated fasteners have good lubricity and wear resistance, and are suitable for high temperature and high pressure working environment. Phosphating cost is low, but the anti-rust performance is weaker than electro-galvanized, need to be used with anti-rust oil (salt spray resistance depends on the oil: ordinary oil 10~20 hours, high-grade oil 72~96 hours).
Fastener phosphating two commonly used, zinc phosphating and manganese phosphating. Zinc phosphating lubricating properties than manganese phosphating, manganese phosphating corrosion resistance, wear resistance is better than galvanized. It can be used up to 225 degrees Fahrenheit to 400 degrees Fahrenheit (107 ~ 204 ℃). High-strength bolts are phosphated without hydrogen embrittlement problem, which is suitable for high-strength bolts of grade 10.9 and above.
Dacromet
Dacromet is a flaky zinc-based chromium salt protective coating, also called zinc-aluminum coating. The basic process is: degreasing → coating → preheating → sintering → cooling. The process will generally have 2~4 times from coating to cooling process, because the screws with dip coating such as to achieve a certain thickness needs to be done a few more times.
Black Oxide
Black oxide is a chemical treatment process by generating a dense Fe₃O₄ oxide film on the surface of steel, which can be completed in 130-150°C alkaline solution or 15-40°C room temperature solution, forming a blue-black protective layer of 0.5-1.5μm. The process has the advantages of low cost, simple operation, no change in the size of the parts, etc. It can improve the appearance and eliminate reflections, but the anti-rust performance is weak (neutral salt spray test is only 4-24 hours), and it needs to be used in conjunction with the application of oil closure.
Boiling black process: decontamination and degreasing → cleaning → pickling and descaling → cleaning → blackening → cleaning → saponification → oil immersion.
Hot Dip Galvanizing
Fasteners hot dip galvanizing is the pre-treatment of fasteners immersed in the lower melting point of the liquid metal zinc, hair infiltration, dissolution, chemical reaction, diffusion, the formation of zinc on the surface of the steel parts and/or zinc – iron alloy plating, in the centrifugal separation machine to shake off the excess zinc, or brush off the threaded portion of the excess zinc, so as to obtain the metal cover layer of the process and method.
Process flow: degreasing → washing → pickling → washing → melt treatment → drying → hot dip galvanizing → shaking dry → post-treatment (purification treatment when needed) → delivery inspection
Anodizing
Aluminum is able to form a light gray dense oxide film in humid air under atmospheric corrosion to prevent further corrosion of the metal, and these corrosive pollutants will not contaminate the surface of aluminum. Industry uses anodizing to further strengthen the surface of aluminum in order to increase its surface hardness and enhance its wear resistance. The anodizing process not only improves corrosion resistance, but also enhances protection against wear and scratches. Anodized coatings are available in a wide range of colors according to decorative aesthetics.
51fasteners aluminum alloy fasteners anodic oxidation color options, strong texture, the production of products with high aesthetic, glittering, extremely stylish. And anodized aluminum alloy fasteners, scratch resistance, corrosion resistance, high surface hardness. Anti-fouling ability is strong, touch does not leave handprints, does not fade. Excellent fire resistance, anti-static and does not absorb dust, anti-ultraviolet and many other advantages.
Spray Finishing
Spray coating is a convenient way to improve the surface treatment of fasteners by applying a special coating to the surface of the fastener. Common spray materials include anti-corrosion paints, anti-wear coatings, and powder coatings. Anti-corrosion paints form a protective film on the surface of the fastener to prevent corrosion and oxidation. Anti-friction coatings can improve the wear resistance of fasteners and extend their service life. Powder coatings smooth the surface of fasteners and improve their appearance. Spray coated fasteners are available in a variety of colors for a better decorative effect.
Heat Treatment
Heat treatment is the process of changing the internal structure of a material and optimizing the surface properties (strength, hardness, toughness) of a fastener by heating, holding and cooling. Heat treatment methods include annealing, normalizing, quenching, tempering and carburizing.
- Annealing: heated to a critical temperature above the slow cooling (such as furnace cooling), for the elimination of internal stresses, grain refinement, improve machinability, for subsequent quenching to prepare.
- Normalizing: heated to the appropriate temperature and then cooled in air, the effect of normalizing is similar to annealing, the organization is finer than annealing, which can improve cutting performance.
- Quenching is the fastener is heated and kept warm, in water, oil or other inorganic salt solution, organic aqueous solution and other quenching medium rapid cooling. Substantially increases hardness and strength, but increases brittleness.
- Tempering: after quenching in low temperature (150 ~ 650 ℃) heating and insulation, reduce brittleness, improve toughness and wear resistance.
- Carburizing: through carbon penetration into the surface layer of mild steel and quenching, forming a high hardness wear-resistant carbonized layer (such as 12.9 grade bolts), while maintaining the core toughness, suitable for high load, high wear environment.
- Tempering: quenching + high temperature tempering (500 ~ 650 ℃) is called tempering, can make the fasteners both high strength and good toughness, applicable to 8.8 and above high-strength bolts.