How to Pickle and Passivate Stainless Steel？

Pickling passivation method

According to different operation modes, stainless steel pickling and passivation treatment mainly includes six methods: immersion method, paste method, brushing method, spray method, circulation method and electrochemical method. Among them, immersion method, paste method and spray method are more suitable for pickling and passivation of stainless steel chemical ships and equipment.

1. Impregnation method:

stainless steel pipeline, elbow and small parts are most suitable for this method, and the treatment effect is the best. Because the treated parts can be fully immersed in pickling and passivation solution, the surface reaction is complete and the passivation film is dense and uniform. This method is suitable for continuous batch operation, but it needs to continuously supplement new solution with the decrease of solution reaction concentration. Its disadvantage is that it is not suitable for high-capacity equipment and pipelines with long and wide shape due to the limitation of acid tank shape and capacity; If it is not used for a long time, the effect will be reduced due to solution volatilization and other reasons, and special site, acid tank and heating equipment are required.

2. Paste method:

stainless steel pickling passivation paste has been widely used in China and has a series of products. The main components are nitric acid, hydrofluoric acid, corrosion inhibitor and viscous agent in a certain proportion. The advantages of paste method are that it does not need special equipment and site, heating equipment, flexible on-site operation, acid pickling and passivation are completed at one time, and strong independence; The passivation paste has a long shelf life. Each coating treatment uses a new passivation paste for one-time use. The reaction stops after one layer of passivation on the surface, which is not easy to be over corroded and is not limited by the subsequent flushing time. Passivation can be strengthened in weak links such as welds. The disadvantages are poor operating environment, high labor intensity, high cost and poor treatment effect on the inner wall of stainless steel pipeline, which needs to be combined with other methods.

3. Spray method:

It is suitable for pickling and passivation of single products with fixed site and closed environment or equipment with simple internal structure, such as spray pickling process on plate production line and pickling of inner wall of liquid cargo tank for stainless steel chemical ship. The utility model has the advantages of fast continuous operation speed, simple operation mode, little corrosion impact on workers, and the pipeline can be pickled again in the liquid transfer process. The solution utilization rate is high.

Special precautions

Pretreatment of pickling and passivation: if there are dirt such as grease on the stainless steel surface, it will affect the quality of pickling and passivation. Generally, alkaline detergent shall be used to clean the inner wall of the tank first.