Special treatment of steel parts: passivation, and why we do passivation

Special treatment of steel parts: passivation, and why we do passivation

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If you have ever customized steel parts, you must know the importance of passivation for steel parts. But if you’re just considering steel for your next part, you probably don’t know what passivation is and why we passivate steel.

We will take you to understand the answers to these two questions in the following content, so that you can make a better choice whether to use steel.

What is passivation?

Passivation refers to the process of metal oxidation by strong oxidants or electrochemical methods to make the surface inactive, that is, passivation. It is a method to transform the metal surface into a state that is not easy to be oxidized and delay the corrosion rate of the metal.

The need for passivation

Generally, most of the materials used for steel parts are stainless steel.
This kind of steel has good corrosion resistance, high temperature oxidation resistance, good low temperature performance and excellent mechanical properties. Therefore, it is widely used in chemical industry, petroleum, power, nuclear engineering, aerospace, ocean, medicine, light industry, textile and other departments. The main purpose is to prevent corrosion and rust.
The corrosion resistance of stainless steel mainly depends on the oxide film on the surface. If the film is incomplete or defective, stainless steel will still be corroded. Usually, in the production process, the supplier will carry out pickling and passivation treatment on the parts, so that the corrosion resistance potential of stainless steel can be maximized.
Stainless steel parts may encounter surface oil, rust, non-metallic dirt, low melting point metal pollutants, paint, Welding slag and spatter, etc. These substances will affect the surface quality of stainless steel parts, destroy the oxide film on the surface, reduce the general corrosion resistance and local corrosion resistance (including pitting corrosion, crevice corrosion) of stainless steel, and even lead to stress corrosion cracking.
Cleaning, pickling and passivation of the stainless steel surface can not only maximize the corrosion resistance, but also prevent product pollution and aesthetics.

Different ways of passivation

Passivation after Machining

Stainless steel workpieces usually have iron filings, steel powder and cooling emulsion on the surface after cutting, which will cause stains and rust on the stainless steel surface. Therefore, degreasing and degreasing should be carried out, and then cleaned with nitric acid to remove iron Scrap steel powder, and passivation.

Passivation before and after welding

Since grease is the source of hydrogen, pores will be formed in welds that have not been cleaned of grease, and low-melting-point metal pollution (such as zinc-rich paint) will cause cracks after welding. The surface is cleaned, oil stains can be scrubbed with acetone, and paint rust should be removed with emery cloth or stainless steel wire brush first, and then wiped with acetone.
No matter what kind of welding technology is used in the manufacture of stainless steel equipment, it must be cleaned after welding. All welding slag, spatter, stains and oxidation colors must be removed. The cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing, sandblasting and shot peening, etc. Carbon steel brushes should be avoided to prevent the surface from rusting. In order to obtain the best corrosion resistance, it can be soaked in the mixture of HNO3 and HF, or pickling passivation paste can be used. In fact, mechanical cleaning and chemical cleaning are often used in combination.

Passivation of forgings and castings

Stainless steel workpieces after thermal processing such as forging and casting often have a layer of scale, lubricant or oxide pollution on the surface. Pollutants include graphite, molybdenum disulfide and carbon dioxide. Usually through shot peening, salt bath treatment and multiple pickling treatments. For example, the treatment process of American stainless steel turbine blades is:
Salt bath (10min) → water quenching (2.5min) → sulfuric acid washing (2min) → cold water washing (2min) → alkaline permanganate bath (10min) → cold water washing (2min) → sulfuric acid washing (1min) → cold water Washing (1min) → nitric acid washing (1.5min) → cold water washing (1min) → hot water washing (1min) → air drying.

Hope the above content is helpful to you!

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