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topic 61206

Micropitting issue chromate conversion coating of Aluminum

February 4, 2021

Q. Hi, I am Megha, Senior Engineer at Titan Engineering and automation Limited- Aerospace and Defense Division.

We have been facing issues with Type 1 Class 1A- conversion coating. We suspect the micropitting in the alloy is due to long immersion time in alkaline bath / issue with nitric acid deoxidizer.

Could anyone elaborate on what would be the cause for micropitting? No corrosion inhibitors have been used till date.

Megha Raman
- Tamilnadu, India
^-- Answer or Comment on this question --^
none
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February 2021

A. Hi Megha. While awaiting replies, please get back to us with which aluminum alloy you are processing. I assume it's a copper-bearing one?

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


February 5, 2021

Q. The alloy processed is Al - 2024, Cu bearing alloy only Ted.

Megha Raman [returning]
- TamilNadu, India
^-- Answer or Comment on this question --^

February 6, 2021

A. 1. Let us know whether it has been heat treated.
2. Let us know if the panel is over one year old.
3. Meanwhile, look for chlorides and fluorides.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner


February 9, 2021

A. Hi Megha,
When you say 'long immersion in the alkaline bath', are you talking about an alkaline detergent or a hot caustic etch tank?

If the latter, you really shouldn't have to leave parts in there for long- only until you see a uniform reaction over the whole surface. Assuming the parts go into Caustic Etch completely clean and free of cutting oils, in my experience it takes about 10-30 seconds on a slightly oxidized unbroken mill finish, and mere MOMENTS on a freshly milled or AlOx (NEVER Silicon carbide abrasive) scotch-brite scrubbed one, then move FAST to the rinse and deox, and don't leave the parts in deox too long, either- just until you can wipe them with a tissue or white glove, and not get any residue. Make sure you have LOTS of agitation in the deox tank! Better mixing means less immersion time required to wash away the smut. Air sparging works awesome for this, far better than just a prop mixer.

If you're having to soap them in alkaline cleaner excessively, what can you do to minimize contaminants prior to entering the process? Hand-wiping, degreasing, etc.

The cleaner they go in, the less time you have to expose them to aggressive chemicals, thus reducing the possibility of over-etching them.

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist /
Industrial Waste Water Treatment / Lab Rat - Brattleboro, Vermont



February 13, 2021

A. My first rule of anodizing applies here.1- "Do not unless it is absolutely necessary and if so use the minimum time to do it". That is a good starting place since "etches " remove Aluminum and leave behind other metals.It may not solve the problem but it is a start!

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina
anodizeusa1


February 15, 2021

Q. The panels/parts have been heat treated. The panels are one year old. The chlorides and fluorides are within limits. The panels are cleaned using scotch brite and then processed in alkaline degreaser.

Megha Raman [returning]
- Tamilnadu, India
^-- Answer or Comment on this question --^

February 2021

A. Hi Megha. All I know is what I read here, but I've read here at least a dozen times that the first thing to do is buy new test panels.

Checking your process via 2024 test panels is 10% a check of the quality of your process and 90% a check of the quality of your test panels :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


February 15, 2021

A. Copper walks with time, and walks faster with temperature.
"Chloride in spec", really dos not tell us whether we have chloride pitting.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner


February 16, 2021

Can you please elaborate on what do you mean by chrome ^chloride in spec ?

Megha Raman [returning]
- TamilNadu, India
^-- Answer or Comment on this question --^

February 2021

A. Hi Megha. There is a typo in your question. Robert said "chloride" not "chrome". You said your chloride and fluoride were "within limits", and Robert is saying that that does not answer his question of whether there is enough chloride to cause chloride pitting :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading

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