Influencing Factors Of Passive Film On Stainless Steel Surface

In the actual use process, there are many factors that will damage the passive film of stainless steel and convert the passive state of stainless steel into an active state. The corrosion resistance of stainless steel is reduced.

(1) Chloride ions. Chloride ions are extremely harmful to stainless steel. In the passivation process, the chloride ion content in the passivation solution should be strictly controlled, and the chemical materials used for passivation have limited requirements for chloride ions. The water for preparing passivation solution and cleaning water also has strict water quality requirements for chloride ions to ensure that the passivation products do not adhere to chloride ions, so as to avoid future troubles.

(2) Surface cleanliness. For stainless steel alloys, the lower the surface roughness, the smoother the surface, the more difficult it is for foreign matter to adhere, and the lower the probability of local corrosion of each part. Therefore, stainless steel should be finished with a finished surface as much as possible. In addition, the cleanliness of the stainless steel surface is also very important, and the final cleaning after passivation should be carried out carefully, because the residual acid promotes the cathodic reaction and ruptures the film layer, thereby activating the stainless steel and drastically reducing the corrosion resistance.

(3) Use environmental media. The passive film of stainless steel is a thermodynamically inhibited metastable structure, and its protective efficiency is related to the environmental medium. During use, it should be cleaned regularly to remove harmful substances privately attached to the surface for a long time. Especially in the environment with chloride ions, avoid the chloride ions attached to the surface for a long time and concentrated in water. For example, stainless steel is used for professional food appliances. After each contact with food, it must be washed to avoid the action of chloride ions and damage to dullness. chemical film. Such as containers and equipment used for dairy products, the chloride ions in dairy products have a destructive effect on the passivation film. For example, long-term storage of dairy products will lead to corrosion and perforation of the capacity and equipment. Therefore, regular cleaning is required to restore the passivation film.

(4) Internal factors of stainless steel. The content of martensite and chromium-nickel in stainless steel has a great influence on the passivation performance of stainless steel. The lower the nickel content, the lower the passivation performance. The passivation film performance of martensitic stainless steel is not as good as that of austenitic stainless steel.