FAQs on Stainless Steel Fasteners – Magnetism/Rusting

FAQs on Stainless Steel Fasteners – Magnetism

“Stainless steel can be completely non-magnetic only under vacuum conditions. All austenitic stainless steel fasteners are normally non-magnetic in the solution-treated state; some will exhibit obvious magnetism after cold deformation processing.” During work hardening, part of the austenitic structure transforms into martensite, resulting in increased magnetism.

The magnetic permeability of materials measured in a magnetic field is referenced to the magnetic permeability of the material in vacuum. If the magnetic permeability is close to 1.0, the material features low magnetic permeability.

Examples: A2 (304): Magnetic permeability ≈1.8 A4 (316): Magnetic permeability ≈1.015 A4L (316L): Magnetic permeability ≈1.005

For applications with magnetism requirements, magnetism can be controlled from three aspects:

1. Control chemical composition to lower the MD30 value. MD30=551−462(C+N)−9.2(Si)−8.1(Mn)−29(Ni+Cu)−13.7(Cr)−18.5(Mo). Lower MD30 value means weaker magnetism.
2. Mitigate the impact of work hardening on magnetic permeability during processing by minimizing deformation.
3. Implement solution treatment for demagnetization (this will reduce product strength and hardness grade).

Magnetism on stainless steel fasteners is a normal phenomenon and will not affect the mechanical properties of products.

FAQs on Stainless Steel Fasteners – Rusting

Stainless steel resists atmospheric oxidation (stain resistance), and also withstands corrosion in media containing acids, alkalis and salts (corrosion resistance). However, its corrosion resistance varies depending on the chemical composition of the steel, surface condition, service conditions and type of ambient medium. Therefore, not all stainless steel grades can remain corrosion-free in all environments.

Stainless steel is not rust-proof, only rust-resistant. It may rust under certain conditions, yet it will not corrode completely like carbon steel.

A thin, dense, firm and stable chromium-rich oxide film forms on the stainless steel surface. This film prevents oxygen atoms from penetrating and triggering oxidation reactions, which protects stainless steel from rusting. Rust easily occurs once the oxide film is damaged.

1. In humid environments with water and oxygen, organic acids form and erode the stainless steel surface.
2. Mechanical damage caused by installation tools scratches the surface protective film. For instance, when installing stainless steel bolts for exterior curtain walls, wrench contact creates mechanical scratches on bolt heads. Light surface rust will appear on bolt heads after rain exposure.
3. Dust, impurities or metallic particles adhering to the surface trigger electrochemical reactions with stainless steel in humid air and accelerate corrosion.
4. Contact with acids, alkalis, salts and other substances leads to chemical corrosion. For example, grade 316 stainless steel fasteners are generally selected for curtain wall projects in coastal cities (more corrosion-resistant than 304), as high salinity in coastal air causes corrosion to stainless steel.

To keep stainless steel products bright and corrosion-free, selecting the appropriate stainless steel grade is essential. Regular cleaning and maintenance are also required to remove surface contaminants and avoid corrosive reactions.

If surface rust arises from adhered sand, dust, iron powder generated during construction or vehicle traffic, or contamination by harmful exhaust substances: Wipe the surface with a sponge or cloth using neutral detergent or soapy water, then rinse thoroughly with clean water. For heavy rust stains, use commercial stainless steel cleaners or 15% dilute nitric acid for removal. If sandpaper or stainless steel brushes are used for scrubbing, rinse thoroughly afterwards.

It is recommended to clean stainless steel fasteners 1 to 2 times per year, followed by complete clean water rinsing after each cleaning.