Pitting and black spots during Anodizing

A. Hi Michael
You should follow this advice:
1- Check your anodizing bath for chloride contamination. Also check your rinsing baths before anodizing. Chloride concentration must be below 50 ppm.
2- All baths's grounding should be done, so check all baths's grounding connection. Also cranes's & hooks should be insulated.
3- You should not use rinse bath after neutralize bath before anodizing. Neutralization bath's rinse bath forms corrosion

A. This problem of black spots may be due to corrosion before anodising.

How do the parts come in to you - are they dry (not oiled)?

Are they degreased and left hanging about?

Try answering these questions and if you find the answer is yes to either of them you may have found your problem.

Walk the line with then and see if there is any step were corrosion could happen.

The corrosion has been halted by the anodising process but the parts need a concession, strip and re-process if you have tolerances to allow for the loss of base material from the strip op.

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.

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.

? Hi,
Can you you please post some photographs of the pitting?
Thanks.
Srimay Basu
- Dubai, U. A. E.

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 !

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

? What is the thickness of the anodic coating? Is this thickness uniform all over the thin panel? Dimensions of the panel? How is it sealed? How reliable is the seal? Does it pass the ASTM B136 seal test?

It is very difficult to get thickness on thin panels.

A. Hi Fons,
I would like to ask more questions about the nature of the black spots. For example, are they smooth and hard or of a "fluffy" nature like corrosion products often are? However, here a some comments and suggestions.

Perhaps the Bahrain techies are right and there spots on the aluminium which the acid fails to remove. If so, perhaps a light alkaline etch might be effective.

My own suspicion is that the problem lies in corrosion occurring in the sealing stage. Sometimes intermetallic compounds within the aluminium form electrolytic cells causing holes (pits) in the coating. The effect is usually most severe if racks have electrical contact with tanks and cooling coils. Titanium racks (where electrical contact is unavoidable) can also exacerbate the problem. However the electrical potential from the intermetallics can be such that corrosion occurs with the surrounding aluminium.
Actions to take are to electrically isolate racks from tank: strips of plastic will do the job. Keep the seal water at a low conductivity.

Whatever the cause, as the problem only happens when you send parts to hot climates, I wonder if they are completely dry when packed? If not, you will be dispatching them in a mini corrosion cabinet complete with heating to accelerate any corrosion. This will be particularly troublesome if you are wrapping in polythene.

Of course, even if you can avoid the problem during transit, the question of whether the product will corrode in use remains. The questions raised in the previous reply by Robert Probert take you back to fundamentals.

Thank you Mr Parkes and Mr Probert for your response. Sorry it took a year to respond but I am new to this internet game.
The shipping in a mini corrosion chamber probably was the culprit. these aluminum panels had acrylic faces on them, which in turn sealed in moisture and slightly moved around during transport thus creating bold spots at the high points on the alum sheet, moisture got in and the rest is history. happy days. we inserted paper in between and no more spots.
Stay tuned for my next adventure into the insane.

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.

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.

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

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

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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

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.

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.

Regards,

Hi 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 sulfuric acid anodizing rather than the ever rarer chromic acid anodizing.

Regards,

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.

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.

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.

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.

A. Spend 2 or 3 minutes ramping up to 21 volts, otherwise same as any other alloy.

A. The first thing I would do is reduce the etch time to as short as you can get away with. You are seeing the 'grain' of the stock material, which is made more apparent as the etch process removes various components of the alloy at different rates and exposes/leaves behind material that will not anodize, and therefore not color, evenly.

The higher the Aluminum content in the starting alloy, the less it will be prone to this problem. And related to that, alloys that are high in Magnesium or Copper are quite difficult to get the very even finish you are looking for after dyeing.

A. Hi K Tarun.
You will see that we appended your inquiry on the subject going back to 2001. Also, you'll see lots of photos very similar to yours on thread 45421. Please do a brief review of the two threads and you will either quickly solve your problem or at least be able to offer a detailed follow-up that readers will be able to help you with.

Luck & Regards,