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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 along with alloy and part type, and treatment sequence. Thanks!

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
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


Karthik Mechonz [returning]
Employee - Chennai Tamil nadu

⇩ Related postings, oldest first ⇩


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

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

simultaneous replies

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

James Watts
- Navarre, Florida

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

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.
Milt Stevenson, Jr.
Syracuse, New York

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