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Stainless Steel Parts Satisfactory Performance Oct 23, 2017

Passivation is still a key step to improve the basic corrosion resistance of stainless steel parts. It allows the components to have satisfactory performance, or it can cause premature failure. If the operation is not correct, passivation can actually lead to corrosion.

Passivation is a kind of post fabrication method to improve the intrinsic corrosion resistance of stainless steel alloy workpiece. It is not a process of oxidation to the skin, nor is it like painting.

There is no general argument for the precise mechanism of passivation. But one thing is certain: there is a protective oxide film on the surface of the passivation stainless steel. The invisible film is thought to be very thin, less than 0.0000001 inches thick, and about 1/100 of human hair, 000.

Newly machined, polished or pickling stainless steel parts are automatically obtained by contacting oxygen in the atmosphere. Under ideal conditions, the protective oxide film covers all surfaces of the part completely.

In practice, however, contaminants such as workshop dust or iron particles from knives may be transported to the surface of stainless steel parts during processing. If these foreign particles are not removed, they may reduce the effectiveness of the original protective film.

During the process, it is possible to shed traces of free iron from the tool and transfer it to the surface of the stainless steel workpiece. Under certain conditions, a thin layer of rust may appear on the surface of the part. This is actually the corrosion of steel because of the tool, not the base material. Sometimes a crack in a steel particle from a cutting tool or its corrosive product is likely to cause a chemical-corrosive reaction to the part itself.

Similarly, dust particles containing iron may be glued to the surface of the part. Although the metal may be "processed" shiny appearance, but invisible free iron particles, exposed to the air, may lead to surface rust.

Exposed sulphides can also be a problem. This sulfide is produced in the process of adding sulphur to stainless steel to improve its machinability. Sulfide improves the ability of the alloy to form cuttings, which can be removed from the tool simply by the process. Unless the parts are properly passivated, sulphides may act as the initial site for corrosion on the surface of the processed product.

In both cases, it is necessary to adopt passivation process to improve the natural corrosion resistance of stainless steel. It can remove surface contamination, such as the possibility of forming rust or acting as the first site of corrosion, iron-containing dust and iron particles from the tool. Passivation can also be removed from the sulfide that is exposed to the surface of the freely machined stainless steel alloy.

After complete cleaning, stainless steel parts can be immersed in the passivation acid bath. Any of three methods can be used for passivation-nitric acid passivation, acid sodium dichromate passivation and citric acid passivation. The specific use of which method, according to the stainless steel grades and the required acceptance criteria.

In 20% (volume) of the nitric acid bath, can be passivated more brands of anti-corrosion chromium-nickel stainless steel. As shown in the table, by adding sodium dichromate to the nitric acid bath, the solution has a stronger oxidizing property and can form a passivation film on the surface, which can be used to reduce the number of corrosion resistant stainless steel. Another alternative to adding sodium chromate to nitric acid is to increase the concentration of nitric acid to 50% (by volume). The addition of sodium dichromate and the concentration of progressive nitric acid can reduce the chance of rapid corrosion without need.

The process of passivation of stainless steel is different from the process of passivation stainless steel. This is due to the sulfur-free processing of sulfide in stainless steel, in the passivation process in a typical nitric acid bath partially or thoroughly, thus forming a micro-discontinuity on the surface of the machined parts.

Even if it is normal enough to wash, after passivation, the remaining acid may be trapped in these discontinuous places. The acid may then react with the surface of the part, unless it is already in and out.