Chromic Anodize Pitting Indications-2024 and Rack Stock
In our recent experience, 2024 aluminum is successful through Type I, chromic anodize providing the rack is aluminum. Racking 2024 on titanium appears to cause pitting. Why? Dissimilar metals and the high copper content of 2024 are obvious factors, but what is the specific connection?
Additionally, why does titanium maintain conductivity between runs without stripping while aluminum must be stripped between each load?
You are actually experiencing a traditional problem in anodizing. In your case, pitting was mainly caused by galnavic effect between aluminum parts and titanium rack in the chromic acid solution with a certain amount of chloride ion. For some reasons, your chromic acid has been contaimnated with cholrides. Titanium has a more noble potential than aluminum, which leads to a galvanic cell in an electrolyte. Since aluminum rack has almost the potential same as the aluminum parts, there is no galvanic effect between the aluminum parts and the aluminum rack. However, if chloride content is high enough, you will still encounter the pitting problem even if aluminum rack is used because chloride is very aggressive to attack aluminum base metal and anodized aluminum parts localizedly. Do not leave you racks in the anodizing tank without power and in the rinse tanks too long!
During anodizing, aluminum rack fingers are anodized along with workpieces and aluminum coatings are nonconductive and relatively hard. As a result, it is necessary to strip the anodic coatings on the rack fingers for the subsequent anodizing use. In contrast to this, as long as a very thin and pore-free oxide forms on titanium rack finger surface, titanium fingers no longer anodize along with workpieces. Thin titanium oxide is still conductive due to electron-tunnel effect and also it is very soft. Furthermore, the thin titanium oxide on the rack fingers can be easily removed by loading and unloading. Therefore, it is not necessary to strip titanium racks for subsequent use.
Hope this can give you a little help.
- Grand Rapids, Michigan
I'd like to add a few comments to add to Mr. Hao's advice. It should not be too difficult to determine at what point the pitting is occurring- take parts off the racks after each step and inspect for pits. If indeed the pitting is occurring in the anodize tank, then chlorides are probably responsible. One reference I checked calls for .010 gm/l maximum chlorides. If chlorides are the problem, you may need to change your source of chromic acid, and get a chemical analysis from any potential source.
The parts are most susceptible to galvanic corrosion in the rinse prior to anodize, where you have deoxided any atmospheric oxide, and the surface is as close as it can get to bare aluminum- with copper present at the surface in the case of 2024. As Mr. Hao says, don't leave racks in rinse tanks (or any tank) longer than necessary, and especially this one.
Any anodizer should follow this axiom: all stainless or steel tanks should have have a pad of plastic or other non-conductive material at both ends of the tank where a rack would sit.(not wood or other porous substance which would become conductive if it absorbs moisture). A Buss bar in contact with a metallic tank is guaranteed to have some galvanic current flow.
The pitting could be occurring in the seal bath. (Again, check parts before and after seal for pits). I experienced elimination of pitting of black dyed sulfuric anodized 2024 parts on titanium racks by hanging magnesium rods next to the racks- magnesium is even more anodic than aluminum, and robs the galvanic currents away from the parts. If the pitting is occurring in the seal, and if you don't have chlorides in your seal, this trick would probably work in your case. Hang the Mg rods on after anodizing.
- Madison Heights, Michigan
The above comments really cover the story. I have used magnesium bars also for aluminum color anodizing on titanium racks, and they work well. Be sure that the mag bars do not contact the tank floor or sides. There is also a product that is available for this purpose which actually reverses the flow of electrons which are created by galvanic coupling reactions. I have seen it used and is called Pitstop. One must be sure to correctly size it to work. It is similar to a trickle battery charger.One further note, I have usually only experienced galvanic coupling in rinses when rinse tanks were used for staging purposes. Typically, the pitting that aggravated, was in the seals.
The pitting I have seen from chlorides occurred at 45 ppm, or 45 mg/l, during a 50 minute anodize cycle.Ward Barcafer, CEF
aerospace - Wichita, Kansas
Hello, What have you done for the Titanium surface pretreatment?Carol R. Kalil
College of William and Mary - Williamsbueg, Virginia
April 6, 2010
Phil is right. Your first two steps will need to be (1)identifying the step at which the galvanic corrosion first appears (2) putting a plastic pad at each end of your dye, seal, and rinse tanks. This pad will allow you separation between the tank and the bar of racks. These pads can be cut to an exact fit by your maintenance staff, and this is the least costly (and most likely) way to begin the process of eliminating the problem. I have experienced dissimilar metal corrosion (galvanic corrosion) before, and both times the problem was eliminated in a seal bath and a dye bath by the addition of pads.
I have also seen this problem before with 7075 being racked on Titanium for both Type I and Type II Anodizing.
Automatic Anodizing - Illinois
March 12, 2011
how to reduce chloride content in chromic acid anodizing bath?
( we have to maintain the chloride < 0.20 g/l but we observed 2 g/l )
plating shop employee - Chennai
March 16, 2011
Two things you need to do, firstly find the source of chloride contamination (2 g/l is massively high and will be detrimental to the anodising process) and secondly, rather than trying to play around with processes to reduce the chloride content just discard the bath and make up a fresh solution (ensure you use distilled or demineralised water to make up the solution).
aerospace - Yeovil, Somerset, United Kingdom
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