Summary for black spots on aluminum anodizing Discussion
Black spots on aluminum from anodizing are most often traced to the substrate or pretreatment rather than the anodize itself. In this thread, Karthik Mechonz reports black pits and patches appearing only after anodising. The reply notes that, without photos, alloy details, part type, and the full process sequence, diagnosis is limited, but the likely causes already suggested by readers are porous or poor-quality castings, inadequate cleaning, and missing or insufficient de-oxidizer/desmutter steps. James Watts says his first guess is a porous, poor-quality casting, with a second guess of an inadequate cleaning cycle. John Hu adds that if the issue appears only on certain parts, the material and location matter. Another answer stresses that degreasing alone is not enough for anodize; the natural oxide must be removed with proper etch/deoxidizer/desmutter chemistry or the finish can become patchy and spotted.
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⇦ Tip: Readers like to learn from others’ situations;
some readers skip abstract questions.
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 finishingdotcompublic@gmail.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
Need quick confidential answers? $25
Need project assistance? $100/hr.
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.
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
⇩ Similar, related, Q&As — 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. James Watts A. Does the spot happen on any parts you do or just only happen on a certain part? John Hu |
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