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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 Brimer
machinist - Mayfield, Kentucky, United States
2003

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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

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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

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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

A. Dear Vikram "Few days" is very vague; naturally there is temperature drop across the section of the Tank top cooling rapidly. I have a feeling that the chromic acid concentration at the top is depleted after plating of job. Please verify the condition before and after plating -- that could solve the problem. Best luck Mahendra Gargatti - BELGAUM -Karnataka India April 27, 2014 A. Hi Vikram, Of course, temperature would only have played a part if your failed to adjust the temperature of the samples to a constant value. Not knowing the circumstances it is impossible to say just what might be the source of water. Possibilities include water added to make up, evaporation losses, rain water coming through an extraction system or condensation running back into the tank. This would be my approach to investigation. The differences between the high and low SG values are considerable. Is the gradient uniform down the tank or the low SG part confined to the very top of the tank? If it is just at the top, a sampling vessel might just skim off that very topmost part. What do you think? Harry Harry Parkes - Birmingham, UK April 27, 2014