The reason of the necessity of passivation for stainless steel

Would you like to know why stainless steel need to passivate? At first, we need to know what is the meaning of the passivation-stainless steel? The passivation of stainless steel refers to make the surface of stainless steel react with a passivation agent to form a stable passivation film which protects the stainless steel substrate from rusting caused by oxidation and corrosion. The rust resistance performance of stainless steel is relatively good. However, in coastal areas or in contact with some acid and alkali chemicals, the generated chloride ions can easily penetrate the passive film of stainless steel. Stainless steel will gradually corrode and rust over time. Therefore, stainless steel needs to be passivated. Passivated stainless steel can improve the original anti-rust foundation by 3-8 years, greatly reducing the chance of stainless steel rust.

 

Hengko stainless steel sintered filter element has precise air pore size, uniform filter hole size and uniform distribution; good air permeability, fast circulation, good interception effect, high filtration efficiency; good corrosion resistance, high-temperature resistance, heat shock resistance, stainless steel material can be 600 Operation at high temperature; the length of the fine filter tube can reach 800mm, the maximum size of the filter plate structure can reach 800mm long * 450mm wide, and the maximum diameter of the round filter structure can reach 450mm. We also provide the passivation of product parts, If you need to apply to the stricter corrosion resistance and rust resistance environment.

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How to do the passivation-stainless steel? Let us tell you.

Because different types of stainless steel use different combinations of passivation chemicals, some details will be different during the passivation process of stainless steel. Passivation needs to custom. However, the basic process often has the same steps: Ensure that the surface of the part is clean. Only when the substrate is exposed can it be better passivated. Put one or several parts to be passivated into a container. Pour the chemical liquid into the container and let the parts soak for a period of time. Wash with running water. Ensure the surface of the part cleaned and without residual passivation liquid on.

Nitric acid passivation is one of the current basic passivation technologies. Before choosing the passivation technology, we need to know the chemical solution required for the passivation reaction of stainless steel to avoid using the wrong solution to cause damage to parts. And the same time choosing the related passivation technology according to the characteristic of different stainless steel. For example, the chromium content of austenitic stainless steel (such as 304 stainless steel) is often higher than that of martensitic stainless steel (such as 430 stainless steel), making austenitic alloys more resistant to corrosion and pitting.

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Before passivation of stainless steel parts, the following points should be considered:

Weldments may not be suitable for passivation. The passive film on the surface of stainless steel makes it corrosion resistant. Arc welding will subject the material to a short period of high temperature thermal cycling, thereby destroying its corrosion resistance.

A custom chemical bath is required. The temperature and type of acid used in the passivation process must be adjusted according to the alloy being passivated. This adds cost and complexity compared to electropolishing. Some alloys cannot be passivated. For example, some stainless steel alloys with low chromium and nickel content will be destroyed. Therefore, they cannot be passivated.

The main advantage of stainless steel passivation is that it can improve the rust resistance of stainless steel parts, and the cost is lower than that of electropolishing. The biggest disadvantage of the electropolishing process is that it is more expensive than the standard passivation process.In addition, electropolishing does not have a significant effect on the protective oxide layer of steel like passivation.

Stainless steel powder sintered filter element-DSC_3924

However, when talking about the surface condition of parts into the main consideration, electropolishing still is the preferred treatment. Passivation is not like electropolishing to make the surface of the part smooth, it also does not greatly change the appearance of the part. Therefore, the passivation is not an ideal method if the product needs a smooth and non-stick surface. The food and pharmaceutical industries tend to use electro-polished stainless steel surfaces because the surface is smooth and easy to clean and disinfect.

 


Post time: Oct-10-2020