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Large Chromic Acid Additions Suddenly Needed in Anodize Tank
October 13, 2010
Hello all,
I'm a part time chemist at a metal finishing shop in the aerospace industry. I'm only at our plant about two days a week so we also employ someone to do an outside analysis of our tanks.
Approximately a month and a half ago our Chromic Acid Anodize tank was completely emptied and our dilute chromic seal water used to replace the water. Approximately 660 lbs of chromic acid flake was used to make up the tank (It holds approximately 1650 gallons). We also use a fume suppressant mixed in with the tank.
Since this tank was created, we have consistently (read, every week, once a week) received recommendations from our outside analysis of additions in the 150 lb range. This is almost one fourth of the total tank make-up!
There is almost no possibility of drag over considering the only other tank on the line past the CAA is a conversion coating. Racks that are being unloaded on that side pass over the tank, but they're coming from a seal tank that is then put into rinse tanks. The only racks actually going into our CAA are coming directly from a deox rinse with a pH of about 4 - 4.5 and a ppm kept within limits. Long story short, everything going over it is either coming from a rinse tank we use for our chromic tanks, or rinse tanks for our deox tank.
We have separate rinse tanks for our alkaline tanks, deox tank, boric and sulfuric anodize tanks, and our chromic tanks. (Our conversion coating, chromic acid anodize, and sodium dichromate and dilute chromic seal tanks all share). There are therefore no common rinse tanks.
The number of parts that we've been running through the tank hasn't greatly increased. Our chemist tests with sodium thiosulphate [on
eBay or
Amazon]
that has been standardized weekly. His test was also verified with a pH meter and another set of eyes.
Chelsea Flaum
plating shop - Macon, Georgia
|
The most probable reason is the operator on the night shift is leaving a hose in the tank while he is on break and is running the tank over. Option 2 is the fill system is operating as a siphon. Without a siphon break or double valve it is possible. I have had it happen to me. - Navarre, Florida October 15, 2010 Your arithmetic in order. I figure the 660 pounds is 6.4 oz/gal which is good, the 150 pound additions is 1.4 oz/gal, but how often? You have my book, try the analysis procedure I use and verify the outside chemist. Be reminded that if you drag that sulfuric acid over the tank you will quickly get over 1/2 gram per liter sulphate, fail salt spray and skew the analysis with the presence of sulphate.  Robert H Probert Robert H Probert Technical Services  Garner, North Carolina  October 15, 2010 Hello, Chelsea. I didn't see an actual question in your narrative, but it sounds like you have doubts about the analysis or perhaps the math involved in generating an add sheet. There really isn't enough information about either one to tell you if you should make the 150 pound add or not. As you know for aerospace chromic anodize, you need to maintain a minimum amount of "free" chromic acid with out exceeding a maximum amount of "total" chromic acid. So you don't want to make an add that isn't necessary. One thing that you might do is to make a controlled addition to a small volume in the lab, say a 1-liter sample, then reanalyze it to verify that the analysis is showing the additional free chromic acid (and "total") that you just added. Once you are satisfied with the lab scale test, there is not any reason why you should hesitate to make the full proportional addition to the tank.  Jon Barrows, MSF, EHSSC GOAD Company Independence, Missouri  October 15, 2010 |
I have seen a similar problem caused by the operator not stirring the tank thoroughly after each addition. Your 'lost' chromic could be sitting on the bottom!
Worth checking before you add any more
Geoff Smith
Hampshire, England
October 18, 2010
October 20, 2010
Thank you all for your help!
I'm going to respond to all and hopefully add some more fill in information to figure out why these adds would be so high. Where is our chrome going? (Sorry, that was the actual question Jon!)
First of all, there is no spill over or hose left running. The volume has been pretty much consistent in the tank and we don't have a drainage system in the floor or built in automatically to the tank, so any spill over would be evident and need to be cleaned. We have a check valve installed on the water pipe leading to the tank and that is in working order.
The chromic coating weights and salt spray results are still looking very good.
The tank has a pump on it to agitate it at all times when it is in operation (not air agitation, cycling the liquid itself through). When we've been making the adds, we've introduced the chromic acid flakes slowly and used air agitation to spread the chemical through the tank.
My concern is that the "free" and "total" acid both seem to be staying in the same place. You'd think that the total would be increasing and staying higher than the free would be over time. This makes me feel like we're actually losing some of the chrome content somehow.
Last thought (which I didn't know at the time I wrote this first posting): At the same time we made up this new tank, we also added three more cathodes to the tank. We had two 3 inch by 47 inch cathodes, and added three more of the same dimensions, effectively raising our surface area of cathode by 150%. This is when we started requiring large adds in the tank. Also, another very odd thing is that when we don't make the add for one week, the add amount requested by our outside chemist for the following week is approximately the same as it was before. In other words, it seems like our analysis is never going below a certain range, or ratio, and I don't see how that is possible.
I don't have an entirely accurate idea of what the surface area of our parts going into the tank are. I've been trying to work on that.
Thanks again all!
Ps. Mr. Probert, I'm not sure that we do own a copy of your book but I will look into that. Right now I've tried two methods of titration. One is to titrate the solution with 0.5 N NaOH
[on
eBay or
Amazon]
to a pH of 3.2 (the free acid) and then 4.8 (total acid). The other method involves potassium permanganate [on
eBay or
Amazon] and sulfuric acid, but I believe this only gives me the total acid endpoint.
Chelsea Flaum
plating shop - Macon, Georgia
Hello, Chelsea. The 3.2 endpoint is good for free chrome. For total chrome, I am accustomed to the iodometric method that uses sodium thiosulphate as the titrant, but your permanganate method could be equally valid. When you make an add, the free and total should both go up by the same amount. I still like the idea of doing a lab scale test first to rule out a potential lab error. If it were me, I would add 2.00 grams of chromic acid to 1.00 L of bath sample and then titrate to see if I measure a 2 g/L increase in both free and total acid. It's probably a good idea to analyze freshly prepared standards as well in the range of 45 g/L. Free and total should both come out at 45 for that fresh standard (within some reasonable amount of error). If your lab methods check out, then you have a problem at the tank somewhere-- either undissolved salts in the bottom of the tank, tank runovers, significant solution dragout, or something. Your extra cathode area is not a potential cause for concern in my view. Good luck.
Jon Barrows, MSF, EHSSC
GOAD Company
Independence, Missouri
October 20, 2010
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