What Is a Stainless Steel Surface Treatment?
A stainless steel surface treatment is a surface treatment that takes advantage of stainless steel’s property of forming a film called passive film.
Stainless steel is an alloy steel that is composed mainly of iron and contains 11% or more chromium. The passive film of stainless steel does not oxidize, so corrosion inside the metal does not occur.
Rust is described as a stain, and stainless steel is called stainless because it does not rust. Stainless steel is not only rust-resistant but also strong. It is used in various fields by surface treatment.
Uses of Stainless Steel Surface Treatments
Stainless steel is often used for decorative parts where the luster of the metal is to be utilized, because it can maintain its metallic luster without rusting, without covering the surface with paint or coatings. Examples include automobile interior parts and home appliances.
Stainless steel surface treatment is also widely used in kitchen equipment, clean rooms, guide parts for equipment used to convey products in factories, and conveyance chutes and hoppers for food, medical products, and other products where hygiene is important.
Principle of Stainless Steel Surface Treatments
Stainless steel surface treatment that retains its metallic luster is possible because stainless steel forms a passive film that inhibits the formation of rust on its own. Here, we will explain the passive film of stainless steel.
First of all, iron, the main component of stainless steel, rusts when it is oxidized in the atmosphere. As rust progresses, the iron itself breaks down into pieces. However, when iron is mixed with chromium, the chromium oxidizes more than the iron.
The surface of the chromium oxidizes before the iron, forming an oxide film. This is the passive film of stainless steel. Especially when the chromium content exceeds 11%, the steel is almost rust-free.
Even if the surface of stainless steel is scratched, a passive film is formed immediately. As long as the regeneration of the passive film is not disturbed, stainless steel will not rust.
Types of Stainless Steel Surface Treatment
There are various types of surface treatment methods. Commonly used surface treatments include:
1. No. 1 (Number One Material)
Oxide scale on the stainless steel surface has been removed. The surface has no luster and is silvery white.
2. No. 2D Finish
The product is heat-treated, pickling, and finished with a matte finish.
3. No. 2B (2B Material)
No.2D Finished material with a glossy finish. The surface is smooth and has a slight sheen.
4. BA
This is a surface treatment in which bright heat treatment is performed after cold rolling. This treatment is close to a mirror surface.
5. #400
No.2B is further polished and has a higher luster than No.2B.
6. Hairline Finish
A surface finish, which includes with scratch lines in a vertical pattern. This surface treatment method is often used for building materials.
7. #700 (No.7)
Finished with a finer polish than #400 finish. Also called semi-mirror finish. It is characterized by the fact that it leaves finer scratches than mirror finish.
8. #800 (No.8)
A mirror finish. The surface will have a shiny finish without a single scratch.
Other Information on Stainless Steel Surface Treatment
1. Pickling in Stainless Steel Surface Treatment
Pickling is the process of cleaning stainless steel surfaces by immersing them in strong acids such as sulfuric acid and hydrochloric acid, and is performed to “remove impurities” and “improve corrosion resistance.”
Pickling for the purpose of “removal of impurities” removes burns, black peels, rust, fine scratches, burrs, cutting oil, etc., which are produced during heat treatment, inventory storage, etc., and machining process. This process cleans up the surface condition, and the surface is finished as a base by creating an appropriate unevenness, which improves the quality of the subsequent stainless steel surface treatment.
Pickling for the purpose of “improving corrosion resistance” is to remove the passive film on the stainless steel surface that has already formed, and to form a new, clean one. Passive film is formed on stainless steel as it is, but corrosion resistance is reduced due to destruction of the film by heat treatment, machining, transportation, and storage.
Therefore, acid washing is used to remove the passive film that has degraded corrosion resistance and to re-form a clean passive film on the stainless steel surface.
2. Stainless Steel Surface Treatment for Black Coloring
Stainless steel surface treatment can be blackened by chemical methods such as chromium oxidation, sulfurization, and high-temperature coloring in alkaline solution. Stainless steel surface treatment by black oxidation provides better corrosion resistance, heat resistance, and abrasion resistance than ordinary passivation coatings.
In addition, since reflection can be suppressed, it is used for parts of optical equipment, etc., and as a decoration.