Metal finishing Q&As since 1989
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Chrome plating solution problem: hydrometer reading is low? Low S.G./low solution density
Q. Our hydrometer reading in our hard chroming tank is reading 22. Our control is supposed to be 35. I am trying to figure out how much Chromium to add to our tank. Our ratio we use is 100:1. Our tank is 15 gallons.
Greg Brimermachinist - Mayfield, Kentucky, United States
2003
A. When a chrome tank is new, a hydrometer is a reasonable way of controlling chrome. As it gets tramp metals in it, it is inadequate as it measures the ions of everything, not just the chrome. This affects your ratio also as you gradually get a false ratio from the chrome being less than indicated. Note that you have to let the chrome solution cool to the temperature of the hydrometer calibration, or, you will get a false number. How do you check your sulphate?
In answer to your question, it would take a small chapter of a book. See if the guide book is still on line at metalfinishing.com If not, consider a subscription to metal finishing. It is free with the subscription and is a fantastic resource.
Look for a chrome plating book in the local library or see if they can get one on inter library loan. Last resort -- hire someone to make a conversion table for you, for the size of your tank.
- Navarre, Florida
2003
A. Look in the conversion tables in the Metal Finishing Guidebook or the Electroplating Engineering Handbook [on
AbeBooks or
eBay or
Amazon]
and determine the oz/gal for 35 and subtract the oz/gal for 22, then make your add.
More importantly, be aware that the hydrometer is only accurate for a brand new solution, once the solution is old and loaded with all kinds of contaminants, then the hydrometer reads that also, - - in other words you need at least the difference between 35 and 22, and probably a little more. A titration analysis should be used periodically and compared with the hydrometer.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2003
Q. To be honest with you I have no idea how to check the sulphate. We have had the chrome machines for 15 years now and have had very little trouble with them. We have a 15 gallon tank and a 75 gallon tank. The small chrome machine is the one reading low on the hydrometer. We had both tanks tested about two months ago and they said to add 2 lbs of chrome to the small tank. We also had high copper content and iron. The big tank was way off. They said we needed to add 13 lbs of chrome and 2 lbs of barium. It had high copper and iron content also. We found that both of the tables in our tanks had bad spots where the steel was showing. We bought a porous pot and cleaned the small tank. It has been chroming great. I checked the small machine and got the low reading on the hydrometer. The company who tested our tanks won't return my calls for help and I am at my wits' end on what to do. Can anyone tell me who to contact about making a chart for us to use. Also what do I need to check the sulphate content of our tanks.
Greg Brimer [returning]machine shop - Mayfield, Kentucky, United States
2003
A. Hi Greg. For the sulphate analysis, please see letter 44422, "sulphate Analysis of Chrome Plating Solution". If you want personalized or on-site help with this, please see our Directory of Consultants.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Chrome plating solution density is inconsistent
Q. We do hard chrome plating and until recently had horizontal baths with depths not more than 2.5 - 3 feet but lengths up to 8-10 feet. Recently we added another vertical bath with depth 10 feet.
The density of initial bath was made up to 21.5-22 baumé at 25 °C. with good stirring. The bath is working well. However, if we do not run it for a few days, the solution temperature comes down to room temp. (approx 25-30 ° C these days). At this time if we take a sample for density testing from the top, it measures about 14-16. This increases to 21-22 upon stirring. Such behaviour is not seen in older horizontal tanks.
It appears that heavy solution settles down upon cooling. Since the bath is not a saturated Chromic acid solution such settlement is not understandable. Can anyone suggest reason for such a behaviour. Has anyone else also experienced something like this? I am very curious to know the reason ... expert advice will be highly appreciated and thanks in advance.....
Vikram Dogra
Irusha India - Chandigarh, India
April 22, 2014
A. Hi Vikram. We appended your inquiry to a thread which addresses the issue that solution density is only useful for the make-up of a new bath. Once you get iron and other junk in there, your hydrometer doesn't offer useful information.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 2014
A. Hi Vikram,
The solution will not separate into more and less dense parts like for example, oil and water. The answer must lie either with accumulation of water in the surface layer lowering the density or temperature gradient down the bath causing apparent concentration differences. Or, a combination of both.
Harry Parkes
- Birmingham, UK
April 23, 2014
Q. Thanks TED and HARRY for your opinions.
TED I understand that all impurities and junk in tank will give a give a false impression about chromic acid conc., but in this case whatever the impurities are, they shall be same at the top or deeper in the tank. Impurities don't justify concentration gradient.
HARRY I understand the temp gradient theory and appears to be fairly logical. Since the tank is below ground level there may be a temperature gradient as we go deep and hence the conc. difference, but I don't understand "accumulation of water in the surface layer". How does that happen ?
Thanks in advance.
Vikram Dogra
Irusha India - Chandigarh, India
April 24, 2014
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A. Dear Vikram - BELGAUM -Karnataka India April 27, 2014 A. Hi Vikram, Harry Parkes - Birmingham, UK April 27, 2014 |
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