Email:sales@castingindustrial.com Qingdao Antai Import and Export Co.,Ltd

Qingdao Antai Import and Export Co.,Ltd

What's white rust, galvanizing life and passivation

White Rust


White Rust or white storage stain is the white or grey deposit formed by accelerated corrosion of the zinc coating when new galvanized products are stored in damp, hot, humid and poorly ventilated conditions, especially when stored for long periods. This is a major problem in hot and humid area. In the cooler climates, white rust does occur, but only if storage conditions are really poor or the galvanizing has been poor. 


Minor white rust is a cosmetic effect and generally has no effect on the performance of the coating if the underlying zinc coating retains the specified coating thickness. Once the parts are removed from storage at this stage and installed they ‘weather’ - this is the formation of a zinc carbonate film which is non-reactive and reduces the tendency for further white rust formation or corrosion. 


However, in coastal area etc, white rust can be quite heavy. Depending on how pronounced the white rust is, the galvanizing life could be affected sharply. Sometimes parts can be salvaged by cleaning before use to remove the white rust (using approved methods). If deep corrosion has occurred where the sub layer of zinc has less than acceptable thickness or rust spots have formed, the parts need to be re-galvanized. Conspicuous presence of white rust that has turned dark grey is also an indicator of unacceptable quality.

 

White rust in salt spray testing


 


Salt spray testing tank



Galvanized Coating Life


Coating life is normally thought to be directly correlated to coating thickness. For any given passivation process, coating thickness will affect the life of the galvanized coating. We measure the galvanized coating thickness randomly on every batch to ensure that process parameters are being maintained and that our coating thicknesses are better than that specified by standard. 


However, from our experience with white rust corrosion and salt spray testing, we found that merely maintaining galvanizing coating thickness to Standards is not adequate as a ‘reactive’ zinc surface corrodes rapidly. Consequently we have been experimenting over a period of 6 months, with various passivation chemicals in an effort to offer class leading galvanized coating life in castings and forgings.

 

Passivation


After galvanizing, the castings/forgings are quenched in water and then immersed into a tank containing a passivation chemical mixed in water to a desired concentration. Typically this is a 0.15% solution of Sodium Dichromate. This passivation process develops an ‘inert’ zinc surface on the part. 


We are now adopting a new ‘Double Action’ passivation process. This expensive process dramatically increases the resistance of our products to the formation of white rust and improves the life of the galvanized coating compared to our previous passivation, even though the zinc coating thickness is the same. 


Discolouration


The passivation chemical leaves a stain where the solution drips and dries off the casting/forging. Our double term passivation colour is stronger than the normal chromate passivation colour.

          


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