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Black spots on aluminum from anodizing

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Q. Dear sir,

We are facing problems after Anodising: some black pits & patches, but they're not there before anodising. Please help me sir.

Karthik Mechonz
Employee - Chennai Tamil nadu
April 6, 2022

A. Hi Karthik!

Readers have already suggested possible porosity, poor quality castings, poor cleaning, and lack of proper de-oxidizer/desmutter as the most likely causes ... but without pictures, or a listing of your process steps, or knowing the alloy and type of parts, there probably isn't much more to say :-(

Please send pics to mooney@finishing.com along with alloy and part type, and treatment sequence. Thanks!

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.
April 2022

April 11, 2022

Q. Dear sir,
Thanks for your reply.

We are Facing the problem for black pits found only in particular area in vision camera & boroscope inspection.

black pits after anodizing black pits after anodizing 2

Before Anodising the part is visually inspected through cutting section & penetrant test part will be OK

But After Anodising we face that problem in particular area

Regards

Karthik Mechonz [returning]
Employee - Chennai Tamil nadu

⇩ Related postings, oldest first ⇩

2007

Q. We do AC2A and we are doing anodizing after machining.
Anodizing process:
Temp 8 °C
sulfuric acid :150 g/litre
Precleaning : non alkaline degreasing agent
Current density 1.5A/dm2
Using distilled water with pH 6.5 ~ 7.5
TDS <30 PPM

We face a peculiar problem of black spots after anodizing on our brake parts.
Why is this happening and what could be the countermeasures?

R Vasudevan
- Chennai, Tamilnadu, India

A. Guess #1 is a porous and poor quality casting.
Guess #2 is the cleaning cycle is inadequate.

James Watts
- Navarre, Florida
2007

A. Does the spot happen on any parts you do or just only happen on a certain part?
1. If happens on any parts you do, more information on your facility as well as chemistry needed.
2. If only happen on a certain part, then tell us what the material is? Where is the spot on the part?

John Hu
- Singapore
2007

A. Degreasing alone as pretreatment for anodize is totally inadequate. All aluminum alloys have a natural occurring oxide layer on them that must be removed prior to building a fresh, dense aluminum oxide (i.e. anodize) layer. These are typically acid mixtures such as nitric / bifluoride or chromic / sulfuric. Without one of these etches, deoxidizers, and / or desmutters, you'll get a patchy, non-uniform, spotted anodize!

Milt Stevenson, Jr.
Plating shop technical manager - Syracuse, New York
2007

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