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Sealing Alodine CCC

Dear members,

Does Alodine CCC requires sealing for a better corrosion protection in aqueous solutions?
For example, will treating with KMnO4 or K2Cr2O7 seal the cracks and pores in CCC?

thank you!

Göran Wängström
PD student - Göteborg, Sweden
March 19, 2011

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Hi, Göran

I haven't heard of "sealing" a chromate conversion coating on aluminum, but most chromate conversion coatings today must be trivalent in order to comply with RoHS requirements. They will no longer comply if the parts are dipped into a solution containing hexavalent chromate. Indeed, it wouldn't surprise me if the potassium permanganate ⇦this on eBay or Amazon [affil links] were able to oxidize the trivalent coating to hexavalent.

I don't understand exactly what you are looking for in "better corrosion protection in aqueous solutions", but can you paint or powdercoat the item?

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 21, 2011

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March 22, 2011

Dear Ted,

in this particular application RoHS restrictions don't apply. So I literally aim at better corrosion protection: a system, containing galvanic couple /aluminium - copper/ circulates coolant which is weak electrolyte. Hence galvanic corrosion is a real challenge.

I cannot powdercoat or paint inside the cooling radiator. I cannot anodize as current will only strike the edges and not between the plates.

I found Alodine C6100 (hex. chrome) a good option that reduces galvanic currents initially to almost zero; but according my measurements, currents increase on 4th day.
I associate this with aging and development of cracks.
Also Alodine is not good at all in near-intergranular regions:

*Pls. see 'Formation and characterisation of a chromate conversion coating on AA6060 aluminium'
O. Lunder, J.C. Walmsley, P. Mack, K. Nisancioglu
Corrosion Science 47 2005 1604-1624

Hence, I seek for options to 'seal' these flaws.

One of ideas was to use Potassium Dichromate for this, applying it after Alodine film has aged already. According articles, hex chrome should attach to Chrome 3 polymer matrix and then slowly age to Chrome 3 too. However, I didn't find any further articles on this topic, so I addressed the issue to Finishing.com.

I thought you all guys are great practicing engineers, so I call for your rich experience :)

Any ideas?

Thank you for immediate reply!

Göran Wängström
- GöTeborg, Sweden

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Hi, Göran. Describing the system as a radiator helps me visualize it a bit, but not very exactly. Anodizing builds an insulative film that tends to distribute the current to uncoated areas. Are you sure that anodizing won't work? Electrocoating tends to distribute current to uncoated areas the same way. Is that a possibility?

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 22, 2011

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Dear Ted,

to help use visualize, imagine something like car engine radiator, which is enclosed into a (copper) box. Right, I cannot 'reach inside there' but, I built a model to experiment with. Unfortunately, anodizing didn't work - well, I am not experienced with but, general skills I mastered :) The coating only appears at the edges of the plates, and not on surface between the plates, maybe max. 3 mm away from edges. So I cancelled this idea and focused on CCC, yet faced the above mentioned 'cracking' problem.

To seal the aged CCC would be a solution, the question is, what do I use to seal aged CCC? Potassium dichromate was a candidate.

Thank you!

Göran Wängström
- Göteborg, Sweden
March 23, 2011