plating, anodizing, & finishing Q&As since 1989
Surface corrosion of 2024 T3 bare material in process rinse waters
Q. I am a materials and Process engineer who supervises a phosphoric anodizing clean line. We process aluminum in this clean line both bare and clad material.
The problem I am experiencing is the following: 2024 T3-bare aluminum material is processed in our alkaline cleaner bath (TURCO 4215) for a 12-14 minute period; once this period is up the material is then rinsed with a hose using city water; it is then placed in a immersion water/rinse tank comprised of city water. After approximately a 3-5 minute period surface attack of the metal occurs (corrosion). This attack is very aggressive and makes for unacceptable parts aesthetically not to mention the corrosion issue. This appears to occur along the grain boundaries and is not localized. I have checked the chlorides and fluorides in our rinse waters and they are within specification requirements. The clad aluminum material does not exhibit corrosion due to the extra protection of the material.
Note: this phenomenon will not occur in deionized or distilled water when processing bare material. Help please!Steve Wallace
- Tulsa, Oklahoma
A. You do not mention the temperature or concentration of your Turco tank, but it seams like your soak is excessively long.
My suspicion is that the surface is significantly etched, the hose rinse is only removing a fraction of the cleaner and the rest is going into the still rinse tank. It does not take long for this tank to be strong enough to add the minute amount of additional etch so that you get pitting.
Have you checked the pH of that rinse tank?James Watts
- Navarre, Florida
Q. Mr. Watts,
The temperature of the alkaline tank is 120-140 °F. The phenomenon as described in the initial letter will occur from fresh rinse water placed in a 5 gallon plastic container (the pH of our water supply is in the range of 5.5-8). This eliminates the possibility of stray current corrosion etc. I do agree that the amount of time in the alkaline cleaner affects the severity of surface attack, by reducing the amount of time in the alkaline cleaner the initial onset of corrosion is delayed. This phenomenon will occur on most 2024 T3 bare material, however it does not occur on all bare material.Steve Wallace [returning]
- Tulsa, Oklahoma
A. You might consider an acid based cleaner.
We have a proven mildly acid, inhibited, liquid, non-etch cleaner for aluminum that is successfully used in anodizing lines.
Remember, your tank must be stainless or lined.Tom Walen
First of three simultaneous responses.
A. Hi Steve,
Your situation is perplexing. I would cut down the soak time to a minimum to do the job. Right now, you are wasting chemical, increasing waste for disposal, and probably creating a problem.
Buy some distilled water and use that for a still rinse tank water in the bucket. This will tell you if it is a water related problem.
Letting any type of part set in a rinse tank after cleaning is a poor practice. Sometimes it is absolutely unavoidable, but most times it is only done as a convenience or because that is the way it has always been done.
Please let us know what the distilled water does vs. tap water.
- Navarre, Florida
Second of three simultaneous responses.
Do you circulate your rinse water. If your immersion rinse is not continuously agitated, pitting can occur from localized changes in chemistry along the grain boundaries. This copper bearing alloy, 2024, can be particularly troublesome. You will notice gas formation on the part if this is occurring.
If you are circulating, then the acid cleaner or slightly acidic rinse may help. DI water tends to be mostly on the acid side and would be a good choice. Or simply removing the all chlorine from the tap water with a carbon filter may be enough.Earl Woolsey
- Grand Forks, North Dakota
Third of three simultaneous responses.
A. Steve, it sounds as if the obvious things have been looked at. You already have looked at stray electrical currents. Have you thought about galvanic corrosion from some dissolved metal in the rinse waters coupling on the aluminum surface? It sounds like the problem is associated with the city rinse water. It seems as if it would have to be a dissolved metal with a large disparity from aluminum in the electromotive series perhaps dissolved copper. Small amounts of dissolved copper strongly affect bright dip chemistry for aluminum in an acidic environment.
Iron doesn't fit that scenario but I have worked in one shop where dissolved iron from drag-out of an iron based deoxidizer caused tiny pitting during the subsequent anodizing cycle.
I know that you are limited by QPLs to the chemicals you use in your cleaning line. Usually hoofbeats heard over the horizon are horses, maybe this time it is zebras.
I am sure that the readers will be interested in the cause once found.Ward Barcafer, CEF
aerospace - Wichita, Kansas
Q. I'd be very interested to hear how you solved your problem. One of our suppliers is experiencing almost exactly the same problem with the same alkaline cleaner.David Bahrani
- Belfast, N. Ireland
A. I am a process engineer and I'm am experiencing severe corrosion of our 2024-T3 aluminum parts as well. We soak our aluminum parts in a chlorine bath for specific reasons that can't be avoided and then the parts are rinsed extremely well in a renewing bath of city water. Our severe corrosion problem usually happens every couple of months and creates quite a headache ... The severe reaction only seems to effect the 2024-T3 aluminum and doesn't present a problem with the 2024-T4 aluminum that we put through the same chlorination and rinse ... So many variables ... It may be a vendor problem, it may be a city water problem, it may be an excess chlorination problem ... I just want to say that I sympathize with other people who are experiencing the same problem and have the job of solving it ... It's quite the headache.Craig Schmiesing
Engineering - Brighton, Michigan
A. We have been experiencing problems with 2024 T3 bare as well. We are seeing pitting in the raw material, it has effected heat treating, Alodine and anodizing. Check you raw material for black streaking under a microscope. It is also visible to the naked eye.Steve Davis
- Victoria, B.C., Canada