Passivated Stainless steel bearings can go through salt spray tests for 168 hours

Anti-corrosion is crucial for stainless steel bearings in wet and corrosive applications. It is determined by the chemical composition of the bearing material. The stainless steel itself has a lot of chromium, nickel as well as other metal elements. These components are easy to be oxidized into a high-density passivation film, thus making the stainless steel bearings appear with the rust resistance in a natural way. However, stainless steel bearings still need the passivation.

Comparison of unpassivated and passivated stainless steel bearings after 168-hour salt spray test

The natural passivation film of stainless steel bearings is only 1 to 3 nanometers thick. The natural layer is possible to play a protective role to a certain degree. However, stainless steel bearings are not rust free in all cases. It just rusts more slowly than other steel bearings. Increasingly complex machining processes, such as welding, assembly, scratching, etc., might cause incomplete or cutting of the oxide film, which means that the protective layer of stainless steel bearings might suffer from damage. In some severe cases, iron powder as well as free iron ions might appear on the bearing surface. Free iron ions are precisely the primary culprit which can cause rapid rusting.

In addition, the applications of stainless steel bearings are much more demanding. In cases such as river or coastal areas, submarine fields, food and beverage processing fields and even acid or alkaline chemicals plants, stainless steel bearings still face the risk of being rusted. If the passivation of stainless steel bearings is not done, the effect of rust prevention will be greatly reduced.

Passivation is a non-electrolytic finishing process. It can be used to improve the anti-corrosion and the rust resistance of stainless steel bearings. Rusting needs the iron, therefore if iron ions can be removed from the metal’s surface, then a majority of chromium ions will appear on the metal surface. As is known, chromium ions are naturally inert and are not easily oxidized. The passivation of stainless steel bearings, as a matter of fact, is a kind of chemical reaction. Specifically, in the passivation process, the oxidation from the passivation solution react with the stainless steel. The result is that a passivation film, the thickness of which is about 1-10 nanometers, can appear on the surface of stainless steel bearings. The film is dense and complete and is not easily damaged. Thus, the corrosion resistance of stainless steel bearings can be further enhanced and the surface of stainless steel bearings can be more effectively protected.

On the whole, the passivation for stainless steel bearings includes several steps, that is, pre-cleaning, acid bath, testing and certification, etc. At the beginning of the passivation process, it is necessary to do a thorough cleaning for stainless steel bearings. This step can help remove any surface oils, chemicals, or debris which were left over from machining. Without this step, foreign matters on the metal surface can weaken the effects of passivation. For the acid bath, we use a citric acid bath which is both cost effective and eco-friendly. Through the citric acid bath, can any free iron particles be removed from the surface of the stainless steel bearings. The citric acid bath, which requires no special handling and does not emit any toxic gases, has become a highly accepted method. Testing after the passivation process can provide verification of the effectiveness of the passivation and certify compliance with ASTM, Mil-Spec as well as other various industry standards.

Please see the flow chart below for the specific passivation process.

As has been proved many times, after passivation, the corrosion resistance of stainless steel bearings can be greatly improved. Take Lily’s 440 stainless steel bearings for example. Generally, The time of the salt spray test is 24 hours without any surface treatment. After the passivation process, the salt spray time can reach 168 hours. In fact, about the salt spray test time, there is not only one standard. Different clients have different requirements, such as salt spray with 48 hours, 72 hours, 96 hours, 120 hours, 168 hours and so on. The specific salt spray test time can be located according to the actual working conditions.

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