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topic 48164

Anodizing causes pitting?

March 14, 2008

Q. We have recently run into a snag with our anodizing process. We are getting pitting on after the anodizing process. I'd like to know where to look to solve this problem. We've changed out our anodize solution but still have the issue. It seems that it's related to one customer and only polished parts... but the pitting is nowhere to be found on the non-anodized parts. All other customer parts seem fine, although the finish is not as high of a polish on most, but we did 4 batches of polished parts prior to this customer's parts and had no visible pitting; but on this customers parts there was severe pitting prior to dye.

Justin Martin
metal finishing shop - Liverpool, Pennsylvania

March 14, 2008

A. Let me guess, the parts were cast or extruded, then machined and then polished.
I will bet you a cheap cup of coffee that if you look at those parts after cleaning, etch and desmut, you will see pits with a 3 power glass.
I think that the pits have been smeared over and you are removing the smear.
Best effort would be to go to a non etch process for those parts. It is available from most chemical solution vendors.

James Watts
- Navarre, Florida

March 15, 2008

A. Justin

You do not provide much information to be able to trouble shoot your problem remotely.

However, if you believe the material is root cause, have one of your competitor buddies run some of the parts and see if he has the same results.

Willie Alexander
- Colorado Springs, Colorado

March 15, 2008


Can you you please post some photographs of the pitting?


Srimay Basu
- Dubai, U. A. E.

March 17, 2008

A. What Mr Watts said PLUS: If wrought 7075, that is old or heat treated, the zinc has migrated into the grain boundaries and when you anodize you "de-plate" the rich zinc spots and leave a pit - and there is nothing you can do about that.

Or, what ever you mean by "polishing"? Are you imbedding iron particles from polishing media that has been used on iron? When you anodize a red spot or a pit is left behind.

Or, Chloride contamination in your new sulfuric anodizing bath. How pure was the acid? Was it yellowish or brownish? This would affect all parts but more pronounced on high polished parts. Also consider chlorides in the deox.

When you come back, and always, the first thing we want to know is the alloy !

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

April 20, 2008

A. We ran into this problem a couple of years ago, it was very frustrating. We did however track the source of the problem and correct it. When we Polish or parts we first wet tumble, then dry polish. It turns out that the city had added a large dose of chlorine to counteract contamination from the runoff from a snow storm. It was this chlorine that leached into the parts after tumbling for several hours. We added a RO system for all incoming water and Voila, problem solved. Best of Luck

Dennis Paulson
- Victoria, BC, Canada

April 2, 2008

Q. I have encountered this problem on and off for the past six months with bare 2024T81 sheet during phosphoric acid anodizing. The pits occur in clusters where the ink stenciling used to be (as applied by the mill). After PAA, you could still read the raw material ID. I suspected that the heat treater did not remove the ink prior to aging from T3 to T81 or it was simply left on the part prior to processing in PAA.

If anyone else has encountered this, I'm curious to hear what you think the root cause really is.

Jonathan Durocher
- Winnipeg, Manitoba, Canada

July 20, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi all...
We have a strange problem of pitting after anodizing. This problem occurs sometimes on some racks (not all racks). Also we see this pitting on racks stick. So I don't think it's related to source of extrusion. Is there any comment?

Alaattin Tuna
- Sakarya, Turkey

December 21, 2013

Q. I have a problem about pitting with some anodized profiles. These pits do not occur on every rack. I realize this problem before the anodizing bath. Also we can these defects on racking rods, so I suppose it is related to the etching bath.

pitting on anodized profiles

But than I ask myself: why doesn't it occur on all racks? Also sometimes I think about metallurgical reasons like corrosion or extrusion quality. So, I have a dilemma ... how can I get rid of this trouble?


Alaattin Tuna [returning]
- Turkey

December 23, 2013

A. As per the photograph it appears like chloride corrosion. Please get your anodising bath solution analysed for chloride. It may have been contaminated. At the same time check all the incoming water to your anodising line for chloride content as well. Perhaps you can apply a simple straight forward check on your incoming water by using few drops of 10% Silver Nitrate solution in to the water sample which will tend to milky/ cloudy precipitation if there is chloride even in traces.
The only remedy if chloride contamination has taken place then clean the entire anodising line and replenish only with fresh solutions.
Anything from 65 ppm of chloride content corrosion would begin without any specific pattern; but if the content escalates around 120 ppm then problem would occur quite vigorously in every load of anodising.
It has been found sometimes that whenever any anodising line uses city water in the pretreatment baths without proper control those lines usually get contaminated.

Srimay Basu
- Dubai, U.A.E.

simultaneous December 26, 2013

Q. Dear Mr. Basu,

I won't agree with you because firstly, I'm confident of anodizing bath's solution. We use RO water; we check it weekly. Also I mentioned about problem; it doesn't happen every rack. If we think your thesis right, all racks must be like that? So we couldn't produce any. I don't know, do you think I'm wrong?


Alaattin Tuna [returning]
- Sakarya, Turkey

December 27, 2013

A. First of all I would check if pitting occurs on preferential surfaces pr all around the profiles. In your case, most probably it occurs on preferential ones. If this is the case, you may have a storage problem and the action of alkaline fumes, both from the ambient and from proximity with etching baths which will deposit tiny corrosive particles on the exposed surfaces of mill finished bundles of profiles or on skips. Acidic etching even would not remove the defect and the only solution, should you identify the problem by close examination before anodising is to polish the contaminated surfaces, at time deeply. Same solution in case you strip the anodised metal: polish it before re-anodising.

Richard Reed
- Naples, Italy

December 28, 2013

A. Dear Alattin,

I am sure that you must be using RO water in your anodising baths and so the others. But as I have said in my previous reply that there will be no specific pattern of occurrences when Chloride content is low; but when it exceeds 150 ppm then one would expect in almost every load. Anodising bath gets contaminated through carry over from each load and gradually it gets build up over a period of time.
It will be better if you get your bath analysed for chloride content only perhaps to be sure.

Good luck....
S. Bassu DUBAL

Srimay Basu
- DUBAI, U. A. E.

January 22, 2014

A. The reason for pitting may be cell effect (galvanic corrosion) if the flight bar / work piece comes in contact with the process tanks.

Timur Ulucak
- Istanbul, Turkiye

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon

March 5, 2014

A. In my 30 years of experience building, managing and operating anodizing facilities, this appears to be galvanic corrosion. This can occur when a cleaned load is allowed to sit in a rinse tank containing low pH rinse water for an extended period of time. The fact that it is not every rack or piece does not always rule out this effect, as there are slight differences in the resistance of the connections to the work bar from the racks. If you need to store cleaned loads prior to anodize, make certain the pH of the rinse, especially after desmut is neutral, preferably low in chlorides, as the chlorides act as an accelerator to the galvanic effect. We are talking very slight voltages between the work bar and the tank solution, often expressed in millivolts and milliamps of voltage and current. Proper grounding of the rinse tanks helps solve this problem, as does storing loads in an RO water rinse tank, using polypropylene tanks, or filtering the city water to remove the chlorides, or all the above. When properly designed, a load can be safely stored in a rinse prior to anodize for up to 15 minutes without issue, but an improperly designed system may exhibit this issue in as little as 2-3 minutes or less if the pH is very low and the work bar rests on a metal support on the tank providing continuity. Insulation of the work bar from the tank at the tank set down point is the first thing to verify.


Scott Turner
- Green Bay , Wisconsin

June 29, 2015

A. I agree with Scott.
Seen this condition or similar many times. Usually when you see a defect it is rarely attributed to the tank it just came from. First rule is generally always look back. I have seen this condition also caused by contaminated rinse tanks that are either not grounded properly and a small amount of current is present and causing an anodic reaction.
What had peaked my interest and hence my response is that the defect is not prevalent within the entire load. Albeit the question did not state if it was random, or exhibited to one end; I have seen one side of the rack exhibit these characteristics and after confirming all rinse and or process tanks in specification, it took following some loads around the line to determine source. That took some time as on both manual and automatic running lines, there are delays that are not necessarily consistent form load to load. Particularly manual lines, difficult to pin point this cause as operators are more on the ball moving loads in a timely fashion when the Manager is monitoring. What we found on warm days was the operators would open an overhead door on one end to let in fresh air. If the wind was blowing in the door, it was partially drying the load closet to the door and even if timely movement of the load to next tank took place, it was too late.

Just another item to look at.

Terry Paskal
- Calgary Alberta Canada

July 14, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread



July 2015

A. Hi Kataru. We appended your inquiry to a thread which should offer you some good hints about the pitting issue, while also suggesting the kind of information readers would probably need to comment further. The aluminum alloy which you are anodizing, and the form (cast, extruded, machined) would be vital info. As you see, a photograph of the pitting would help. Also, is this a new problem on an anodizing line which has been running successfully for a long time, or is it a new installation which has never yet yielded satisfactory production?

Regarding the dissolution of your anodizing racks at the solution level, are you certain that this is happening in the anodizing tank per se, and not in an earlier tank like the etch tank or de-smut tank? Thanks.


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

July 18, 2015

Q. Hi Ted Mooney,
regarding my question of dissolution of racks in anodising bath, bath was new, and there is no problem in etching, & desmutting. The components are machined, small aluminium plain washers. The problem is the components are getting pitted and along with racks. We are not facing the pitting problem in other components.


Pitting after anodising

September 22, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Can anyone tell me why a batch of 6 parts that are anodised, 3 of them would come out with pitting and a crack on them and the other 3 came out okay. They were plated all at the same time; the faces that had the problems on them were all masked with lead tape and plated at temp of 40 °C for 42 minutes.

andrew thomas
plating shop worker - uk

simultaneous September 23, 2015

A. Lead tape is not good for a mask in the anodizing process.

Using lead tape as a mask during anodizing can very well cause burning or pitting if electrical contact is made from the lead to the part through the tape adhesive. This is what I'm fairly certain happened to your parts.

Add to that, the cost of lead tape is extremely high compared to other masking materials which work much better.

Also, 40 °C? That is an extremely high temperature for anodizing. I'm not sure, but that may exacerbate your problem as well.

Marc Green
Marc Green
anodizer - Idaho

September 22, 2015

A. The temperature was too high for one. But give us more info: free acid, dissolved aluminum, ALLOY, heat treatment, then come back,

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

Temperature for chromic anodising

September 29, 2015

Q. Further to my post about pitting and crazing during plating, I said that we use lead tape. What would be a better solution? Also, we plate at temperature 40-42 °C, which from the reply I got, was too high. What should be the correct temperature?

Foot note: I've only been on this process for 18 months and that's the temperature that they have always plated at. Thanks.

andrew thomas [returning]
plating shop worker - uk

September 2015

thumbs up signHi Andrew. Your temperature seems a bit high for chromic acid anodizing -- it's normally 95 °F ±4°F (35 °C ± 2 °F -- but not outlandish. I think the issue there was that I and the other readers assumed you were doing sulphuric acid anodizing rather than the ever rarer chromic acid anodizing.


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

September 30, 2015

A. Ted is correct, I assumed you were talking about sulfuric acid anodizing. My knowledge of chromic acid anodizing, and proper masking materials is practically non-existent.

Marc Green Marc Green
anodizer - Idaho

October 7, 2015

A. Hi Andrew,

40 °C is pretty standard in the UK for chromic anodising, usual range you find for wrought materials is usually 38-42 °C, for cast materials you would want to drop the temperature to 32-35 °C.

What alloys are you anodising? 2xxx series alloys with relatively high copper content (4.4% nominal for 2014 and 2024 alloys) and commonly suffer from localised corrosion, which appears as little black spots. If the corrosion is present in the material before anodising the anodising can emphasise the fault.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Corrosion of 7075 Aluminum after Alkaline Cleaning

February 22, 2016

Q. We have been experiencing some trouble with 7075 aluminum. The parts were received for anodizing and when the parts are coming out of the mild alkaline cleaner/soap, they have been dotted with corrosion markings on them that were not evident when racked for processing. This only seems to happen with 7075 and has happened a few times lately. Has anyone experienced this or does anyone know what may be the cause?

Charlene NAUSS

February 22, 2016

A. Until you send more information I will wild guess what is going on. Apparently the cleaner is slightly etching. Apparently the 7075 has been heat treated or the surface has been heated by machining or forging. The heating brought some of the zinc out of alloy as a molecule, then on the surface the cleaner etched away at the exposed molecular zinc, showing you a corrosion spot. Very common with Chinese/Mexican metal and/or with forged gun receivers.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

February 23, 2016

I have attached 2 photos showing the problem we have experienced on 2 different parts within the last 2 weeks.

48164-2a  48164-2b

I want to thank Robert for his reply. We have not had this happen before but now that it has happened twice on 2 different clients parts, we want to try to figure out what may be the cause.

We suspect a bad shipment of aluminum but there is no way of knowing that. Then we thought it may be contaminated at the manufacturing of the parts with cutting fluid.

We had to hand sand using Scotch Pads and just did a light etch and black dye and they turned out acceptable, but what a nuisance to have to do 300+ parts and to keep our clients satisfied with our service.

Charlene NAUSS [returning]

February 23, 2016

A. Your pictures are typical of what I have seen before on some batches of 7075. Thee is really nothing you can do, but anyway, just try a slower ramp and let us know. Also let us know the free acid and the dissolved aluminum.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

simultaneous February 24, 2016

A. Good day Charlene.

I find it odd, that from 2 different client lots that you are experiencing the same defect directly after soak. Have you done a cleaner loading test? What is the ratio of total/free alkalinity/pH as compared to a new make-up?
Try a bucket test with a new cleaner.
Hope this helps.


Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada

February 24, 2016

A. Charlene

The pictures suggest to me that it is possibly machining related; the spots follow the arc of the end mill. If in the machining process that parts are stacked wet as they come off the machine, it is possible to get this condition on the heavily alloyed aluminums it they sit too long. It is comparable to screw machine parts (2xxx aluminum) that are rinsed and bagged wet. Let 'em sit a few days and you can see the corrosion sites.

You don't say if it is all 7075 parts or just occasionally. If it occurs 'sometimes' it suggest the material as opposed to your process is suspect.

Do the spots appear on all surfaces when it happens?

Willie Alexander
- Colorado Springs, Colorado

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