Fasteners play a very important role in industry, yet because of their small size they are easily overlooked. When selecting them, besides the specification, the main considerations are the colour of their appearance and their corrosion resistance. This article summarises the differences in corrosion resistance between common surface treatment processes and base materials.

Material selection

Fasteners can be made from many materials. Stainless steels have good inherent corrosion resistance, and 0Cr18Ni9 (SUS304 in the Japanese standard, 304 in the US standard) is commonly used. Some manufacturers also use SUS202 or SUS201, which offer a certain corrosion resistance at lower cost. SUS304 resists neutral salt spray for 48-96 hours; SUS202, with proper surface cleaning and passivation, can also resist neutral salt spray for more than 48 hours; while so-called stainless iron screws, SUS410, resist neutral salt spray for about 20 hours. Stainless steel costs more than all ferritic materials with surface treatment.

Surface treatment

Black oxide

Black oxide is a fairly common surface treatment. It is completed through degreasing, derusting, neutralising, drying, surface sealing and blackening, forming an oxide film on the surface. Its rust prevention is poor, and anti-rust oil is needed during storage and use. The oxide film consists mainly of ferroferric oxide and is only 0.6-1.5 um thick. Without oil or sealing it resists neutral salt spray for only about 1-2 hours, which can be extended to 12-120 hours with anti-rust oil.

Phosphating

The fastener is immersed in a phosphate solution to form a dense phosphate film on the surface, through degreasing, derusting, surface sealing, phosphating, rinsing and drying. This improves the hardness, wear resistance and corrosion resistance of the metal surface. Because the phosphate film has poor salt spray resistance, it generally withstands neutral salt spray for 2-3 hours, but with phosphating anti-rust oil the corrosion resistance can be extended to 12-120 hours.

Electroplating

Electrochemical methods are used to form different metal coatings on the fastener surface, such as zinc, zinc-nickel, nickel, copper, silver, and chromium plus nickel. The appearance can be black, white, silver or colour, giving a relatively attractive look. The corrosion protection of an electroplated layer depends largely on the type and thickness of the metal layer. Conversion coatings, sealing layers or surface coatings on zinc, zinc-iron, zinc-nickel and cadmium coatings provide additional protection for the metal coating. Neutral salt spray resistance exceeds 48 hours, and zinc-nickel coatings with black passivation and sealing can reach more than 720 hours.

Hot-dip galvanizing

The zinc layer is thick and gives strong corrosion protection, but the appearance is less attractive and the zinc layer thickness is difficult to control, resulting in poor precision. Generally, derusted fasteners are immersed in a plating bath of molten zinc at about 455-560 degrees C; the minimum coating thickness is usually 40 um and neutral salt spray resistance exceeds 48 hours.

Dacromet

Also called zinc-aluminium coating. The basic process is: degreasing, oil removal, coating, preheating, sintering and cooling. A Dacromet solution (a highly dispersed mixed aqueous solution containing flake zinc, aluminium and special organics) is applied to the metal surface and baked at about 300 degrees C for a period, during which the hexavalent chromium in the solution is reduced to trivalent chromium, forming an amorphous complex chromate compound. Corrosion resistance is very good, with neutral salt spray generally reaching 500-1000 hours. The coating does not crack or break under tension and stress.

Electrophoretic coating

This technology separates charged ions according to their different migration speeds in an electric field; it is a deposition process of polymer resin, and the film-forming substance is resin. It costs less than electroplating, has good workability and causes less environmental pollution and harm, with neutral salt spray resistance of 300 hours or more.

Geomet

The Geomet process is similar to Dacromet. The appearance is matte silver-grey, slightly darker in gloss than a Dacromet coating. It is a chromium-free inorganic coating in which ultrafine zinc flakes and aluminium flakes are laminated and wrapped in a special binder. For threaded parts, an 8-10 um Geomet coating can withstand 720 hours of salt spray testing without red rust.

Powder sherardizing

Powder sherardizing is a surface protection process that produces a zinc-iron alloy layer on steel surfaces by thermal diffusion. Corrosion resistance is about 10%-30% higher than that of hot-dip galvanizing, wear and scratch resistance are good, and the surface hardness of the sherardized layer can reach HV250-400. It does not affect the mechanical properties of the steel and carries no risk of hydrogen embrittlement.

Summary

About the salt spray test

The salt spray test is an environmental test that mainly uses the artificial salt spray conditions created by salt spray test equipment to assess the corrosion resistance of products or metal materials.

An artificial salt spray environmental test uses a salt spray test chamber, a piece of equipment with a certain volume, in which a salt spray environment is created artificially to assess the salt spray corrosion resistance of products. Compared with the natural environment, the chloride concentration of the salt spray can be several times or several tens of times that of a normal natural environment, greatly accelerating corrosion, so the time needed to obtain results from salt spray testing is greatly shortened.