Where the
world gathers for metal finishing
Q&As since 1989


-----

Corrosion of Al Alloy

 

Pretreated and powdercoated Al components ( Both have undergone identical condition rather it had been a mixed production situation):

Pretreatment/coating sequence : degreasing - water rinse - etching - water rinse - 2nd water rinse with running water- chromate conversion treatment -water rinse- chrome passivation- forced drying - coating with high quality polyester/epoxy powder - heat curing. 1. Al sheet of 99% purity. 2. Al Alloys conforming to BS 6496.

Components had been exposed to harsh atmospheric condition Rain and Sun light with highest day temperature rising up to 40 to 44C.

After a period of 12/18 months the paint flaked on the Al alloy components and severe white rust corrosion was noted on the base metal. Where as there had been no failure of paint film or any sign of corrosion on the pure Al components.

What could be the reason , if the Al alloy had been of sub-standard quality then what could be responsible constituent in the alloy for formation of white rust and paint flaking.What should be direction for finding the reason of fault?

Shall be obliged to have your views and valued guidance.

Regards,

Swapan Sarkar
- Calcutta, WB, India

---

 

Hi Swapan,

I am not familiar with the details of your Al alloy #2, but in general, the quality of chromate coating is dependent on the steps preceding it.

In your process, you go through a cleaner followed by an etch. If this etch stage produces an appreciable amount of smutting, the surface will have to be deoxidized before a uniform chromate conversion coating will form.

Not all Al alloys need to go through etch and deoxidation steps. Can you run your process without the etch and see how that affects the painted surface?

George Gorecki
- Naperville, Illinois