Color And Corrosion Resistance Surface Treatment Processes For Fastener

Fastener corrosion resistance

Fasteners play a very important role in the industrial system, but they are easily overlooked due to their small size. In addition to specifications, the appearance, color and corrosion resistance are mainly considered when selecting fasteners. This article summarizes common surface treatment processes and the differences in corrosion resistance of materials.

Material selection
There are many materials to choose from for fasteners. Among them, stainless steel has good anti-corrosion ability. Commonly used materials are 0Cr18Ni9 (Japanese standard SUS304, American standard 304). Stainless steel such as SUS202 or SUS201 is also used in the industry. It has certain anti-corrosion ability, but Lower cost. SUS304 is resistant to neutral salt spray for 48-96 hours. SUS202 can resist neutral salt spray for more than 48 hours after surface cleaning and passivation. The stainless steel screws we call SUS410 are resistant to neutral salt for about 20 hours. They are made of stainless steel. The cost is higher than all ferrite plus surface treatment.

surface treatment
blackened
Blackening is a common surface treatment method. It is completed through steps such as oil removal, rust removal, neutralization, drying, surface sealing, and blackening treatment to form an oxide film on the surface. Its anti-rust ability is poor, and anti-rust oil needs to be added during storage and use. The main component of the oxide film is iron tetraoxide. The film thickness is only 0.6-1.5um. In the absence of oil or closed, the neutral salt spray is only 1 -About 2 hours, it can be delayed to 12-120 hours with anti-rust oil.

Phosphate
The fasteners are immersed in a phosphate solution to form a dense phosphate film on the surface, which is completed through steps such as oil removal, rust removal, surface sealing, phosphating, water washing, and drying to improve the hardness and wear resistance of the metal surface. and corrosion resistance. Because the phosphating film has poor salt spray resistance, it can generally withstand neutral salt spray for 2-3 hours, but with phosphating anti-rust oil, the corrosion resistance time can be delayed to 12-120 hours.

plating
Use electrochemical methods to form different metal coatings on the surface of fasteners, such as zinc, zinc-nickel, nickel, copper, silver, chromium + nickel, etc. The appearance can be black, white, silver, colored, etc., with a more beautiful appearance. The anti-corrosion performance of electroplated layers depends largely on the type and thickness of the metal layer. Conversion coatings, sealants or surface coatings on zinc, zinc-iron, zinc-nickel, cadmium coatings provide additional protection to the metal coating. Neutral salt spray can be used for more than 48 hours, and the zinc-nickel coating can be used for more than 720 hours through black passivation and sealing processes.

hot dip galvanizing
The zinc layer is thicker and has strong anti-corrosion properties, but the appearance is relatively unsightly, and the thickness of the zinc layer should not be controlled, resulting in poor accuracy. Generally, the rust-removed fasteners are immersed in a plating bath with high-temperature molten zinc liquid at about 455℃-560℃. Generally, the minimum coating thickness is 40um, and neutral salt spray is required for more than 48 hours.

Dacromet
Also called zinc-aluminum coating, the basic process is: degreasing - degreasing - coating - preheating - sintering - cooling. On the metal surface, a layer of Dacromet solution (that is, a highly dispersed mixed aqueous solution containing scaly zinc, aluminum and special organic matter) is coated. After baking at about 300°C for a certain period of time, the six Dacromet solutions in the Dacromet solution Valent chromium is reduced to trivalent chromium to form an amorphous complex chromate compound. The corrosion resistance is very good, neutral salt spray is generally 500-1000 hours. The anti-corrosion layer has no cracks or breaks after being subjected to tension and stress.

Electrophoretic paint
A technology that utilizes the different moving speeds of charged ions in an electric field to achieve separation. It is a deposition process of polymer resin, and the film-forming material is resin. The cost is lower than that of the electroplating process, it has good construction performance, causes less pollution and harm to the environment, and its neutral salt spray resistance is 300 hours or more.

Jumeite
The Jiumeite process is close to the Dacromet process, with a matte silver-gray appearance and a slightly darker luster than the Dacromet coating. It is a chromium-free coating that combines ultra-fine zinc flakes and aluminum flakes and wraps them in a special binder. Inorganic coating. For threaded parts, the 8-10 micron JUMET coating can withstand 720 hours of salt spray testing without red rust.

Powder zinc infiltration
The powder zinc infiltration process is a surface protection process that obtains a zinc-iron alloy layer on the steel surface through thermal diffusion. The corrosion resistance is about 10%-30% higher than that of hot-dip zinc. It has good wear resistance and scratch resistance. The surface hardness of zinc-infiltrated zinc can reach HV250-400, which does not affect the mechanical properties of steel and does not cause hydrogen embrittlement. risks of.

Summarize

About salt spray test
Salt spray test is an environmental test that mainly uses artificial simulated salt spray environmental conditions created by salt spray testing equipment to evaluate the corrosion resistance of products or metal materials.

Salt spray test

The artificial simulated salt spray environment test is to use a test equipment with a certain volume space - a salt spray test chamber, to use artificial methods in its volume space to create a salt spray environment to assess the salt spray corrosion resistance performance and quality of the product. . Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several or dozens of times that of the general natural environment, which greatly increases the corrosion rate. Conduct a salt spray test on the product and obtain the results. The time is also greatly shortened.