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"Help with sulfate concentration in a chrome plating bath"



Current question and answers:

April 19, 2021

Q. Hello. Our teacher told us that a dilute bath of CrO3 and H2SO4 will produce hard chromium plating and concentrated bath will produce decorative plating. He said that diluted or concentrated, the acid ratio should be maintained at 100:1 (CrO3: H2SO4). I wanted to know why exactly diluting the bath gives hard coating and concentrated bath gives decorative coating.
I'm sorry if I'm being very vague. And I'm not sure if this has been asked before. I'm having a hard time finding an answer for this.

Humayra Rashid
- Rajshahi, Bangladesh
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April 2021

"Electrodeposition of Chromium from Chromic Acid Solutions"
by George Dubpernell"
from Abe Books
or

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A. Hi Humayra. I think your teacher is correct on one claim and in error on the other...

The conventional "Sergeant" chrome plating bath uses a 100:1 ratio of CrO3 to H2SO4. The precise electrochemical reason is not for most of we mortals to fully understand, although people have won prestigious awards for their efforts to explain it. For us the answer is simply that it doesn't plate if the ratio goes significantly higher or significantly lower.

As an aside though, the Sergeant bath with its H2SO4 catalyst isn't the only type anymore. Some people use fluoride catalysts these days and some use proprietary catalysts reputedly based on methane sulfonic acid. Topic 35184, "Hard chrome plating catalysts" talks in more depth about the various catalysts that can be used in chrome plating baths.

Where I believe your teacher in error though is in the assertion that hard chrome plating uses dilute baths and decorative chrome uses more concentrated ones. To an extent the reverse is actually true because some people still advocate highly concentrated baths 48-54 oz/gal of chromic acid for hard chrome plating, although I think most people use 28-32 oz/gal for both. You may be interested in our "Introduction to Chrome Plating".

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


April 20, 2021

Q. Hello Sir,
Thank you so much for replying. I just finished reading the other post and "Introduction to Chrome Plating". I get that CrO3 and H2SO4 ratio is maintained strictly at 100:1. Fluoride and other baths can be used too as catalysts.
But if we use the 100:1 ratio for both hard and decorative chromium, then why we are getting two different types of coatings? I'm sorry if I'm missing something obvious. As I understand it, decorative plating is done on a nickel electroplated surface and the chrome plating is very thin. And even though the hard chrome plating is called hard, it just thicker than other chrome plating. So is it because we give it more time to develop a thicker coating? I'm coming back with more questions than I started with. But let me clear up how our teacher explained it to us. Being in the pandemic, we couldn't do this experiment in our university. So for now, he only explained this to us on zoom. And he said he did this experiment before the pandemic. He said that for hard chrome plating he used 225g/l CrO3 and 2.25 g/l H2SO4. For decorative coating he used 450 g/l CrO3 and 4.50 g/l H2SO4. He said both the experiment were done for 20 minutes. And he mentioned specifically that the dilute and concentrated baths were the reasons for two types of coatings. Since that's not true, then what really contributes to two different types of coatings? I'm assuming there's little or no effect of time. Is it the effect of temperature and current density then?

15017-1a

15017-1b

15017-1c

Another question I have is that, does the dilute and concentrated baths have different cathode current efficiency? Our teacher showed us a graph. And in the graph the diluted bath had significantly more cathode current efficiency. Sorry for the long read. I feel like I'm missing something really obvious. It'd be really helpful if you cleared it up. Thank you.

Humayra Rashid
- Rajshahi, Bangladesh
^


April 2021

A. Hi Humayra. Although some people sometimes do want the chrome to be as hard as possible, and they sometimes do various things like use different catalysts to accomplish that, and then actually measure the hardness, it's actually called "hard chrome" only out of tradition and because it's thick enough to measure that hardness.

The fundamental differences between decorative chrome and hard chrome are that decorative chrome is always done on top of nickel plating and is always very thin, measured in millionths of an inch, and hard chrome is applied much much thicker for wear resistance, usually on hardened steel, although not necessarily.

Yes, the plating times are vastly different! While your teacher may have done an experiment where he plated both for 20 minutes and called one 'decorative chrome' because it was on top of nickel, and called the other 'hard chrome', the plating time is actually a very fundamental difference between the two finishes :-)
Decorative chrome takes as little as 20 seconds and very rarely more than 2 minutes. Hard chrome can take hours, sometimes days. Another "Intro" here which you might like to read is "Faraday's Law of Electrolysis for Plating", which explains the dependence of thickness upon plating current and time.

Sorry, I don't know the exact relationship between efficiency and concentration for the Sergeant bath.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


April 21, 2021

thumbs up sign Thank you, sir. I'll spend more time studying the topic.

Humayra Rashid [returning]
- Rajshahi, Bangladesh
^




Previous closely related Q&A's, oldest first:

2002

Q. We have been having problems lowering our Sulfate concentration in our chrome plating bath. I have analyzed the bath and determined that the concentration is .14 oz/gal (I know I hate standard too). We have added a total of 1 3/4 lbs of Barium Carbonate and my analysis still shows no change in sulfate. I KNOW that barium precipitates sulfate rapidly so I don't know what the problem could be. If you have any suggestions please feel free to share them.

Thank you,

Hope Bearbower
- Las Vegas, Nevada
^


"Hard Chromium Plating"
by Robert K. Guffie
from Abe Books
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see our Review





"Chromium Plating"
by Weiner & Walmsley
from Abe Books
or

Affiliate Link
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or
see our Review

2002

A. Assuming this is a conventional 100:1 bath, why is 0.14 oz/gal too high? It sounds too low.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


2002

A. What is the volume of the chrome tank? Are you using the centrifuge method for analysis? Are you having a problem with the deposit, poor throwing power? How many mm of coverage do you have in a 267 ml Hull Cell panel plated at 5 amperes for 3 minutes?

tom pullizzi animated   tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania
^


TUTORIAL FOR NEWBIES: You will observe a bit of confusion here about "what kind of catalyst". For an understanding of the various catalysts for chrome plating, please see letter 35184, "FAQ: Chrome Plating Catalysts".

2002

Q. Our tank is 830 gal. our ratio is 224:1. I am getting 55 mm of coverage in the hull cell at 5 amps. I had them add 2000 mls of fluoride catalyst and it seems to have fixed the problem but it is too soon to be sure.

P.S. What is this about a 100:1 ratio. I have heard this a couple times, but on all of the spec sheets that I have seen for chrome tanks it says the ratio should be around 170-230:1? Is this wrong?

Thank you,

Hope Bearbower [returning]
- Las Vegas, Nevada
^


2002

A. I thought that you were using a conventional bath when you said you hated standard, I thought this was what you meant, (without other catalysts like fluoride), 100:1 is correct. With fluoride catalyst in a mixed catalyst bath, the 200:1 ratio for sulfate is usually correct.

55 mm coverage is marginal, I like to see 70 mm. Why would you add fluoride, but try to remove sulfate to cure the same problem? They are both catalysts and too much of each would tend to reduce throwing power. What problem were you trying to solve? I don't know why the barium did not work, filter your sample next time.

tom pullizzi animated   tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania
^


2002

A. The precipitation of sulfate with barium is not as quick as you think. Because the solution has a lot of chromium and relatively only a little sulfate, the first precipitate will be of barium chromate and only with time (in lab it will take at least an hour) it will create the barium sulfate. So you cannot add the barium carbonate mix and recheck the result right away.

If you use a centrifuge for the analysis, you MUST filter the solution prior to the analysis (even prior to the first analysis prior to the addition of the barium). The reason for it is that the solution contains lead chromate which might precipitate when the centrifuge is used.

sara michaeli
Sara Michaeli sara michaeli signature
chemical process supplier
Tel-Aviv, Israel

^


2002

A. Listen to Sara, she has it right. What is your procedure for sulfate analysis? There are a number of things that can go wrong, not filtering your solution is only one. As a rule I give Barium 8 hours to do it's thing, only then will I re-sample.

Steve Delaney
- Ontario, Canada
^


2003

A. Make sure that you stir up the sludge in the bottom of the tank. I use a one foot stainless steel disc, stainless welded to a stainless pipe. Do this every morning of use, and every thousand amp hours, to ensure even solution uniformity, better absorption of sulphate and a true Hull Cell test. Works for me!

Mark Gunn
- NSW, Australia
^


September 18, 2018

A. Hope,
are you talking about hard chrome or decorative hexavalent chrome? If you have hard chrome, chromic acid-sulfate ratio 100:1; if it is decorative chrome ratio 200:1 ,+/- 5 %. After you add barium, you have to wait at least 12 hrs. because chemical reaction is very slow. If you analyze sulfate very quick you get wrong reading and more confusion.

popat patel
Popatbhai B. Patel
electroplating consultant - Roseville, Michigan
^


April 2021

thumbs up sign I appreciate and admire Popat's contributions to this site, and I learn a lot from him. But I am not confident that he is right about a 200:1 ratio for a Sergeant bath for decorative chrome plating. I wonder if the bath in question is a mixed-catalyst bath with chromic acid to sulfate at 200:1, but a fluoride catalyst as well?

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^



2005

Q. Barium chromate is yellow in colour due to the chromium. Barium chromate precipitates sulfate as a barium sulfate. I want to determine sulfate concentration using barium chromate. But I do not know solubility of barium chromate and how long it will take to precipitate sulfate as a barium sulfate at room temperature?

Patel Rahul P
University of Tasmania - Hobart, Tasmania, Australia
^


August 26, 2008

Q. We are trying to find a technical data sheet or procedure on determination of sulfate in chrome tank (HEEF-25).
Any assistance is appreciated.

Joe Faitak
Quality Manager - Warsaw, Indiana
^



June 12, 2012

Q. I want to know how can I increase the throwing power of decorative chrome bath, and how can I eliminate the chrome burning at HCD areas and corners?

Nadeem Shahid
- Pakistan
^


June 13, 2012

A. Hi Nadeem.

Chrome plating has notoriously poor throwing power, and as far as I know there is no way to increase it, although the wrong ratio can decrease it. So get your ratios correct, as described by Sara et al, and check the throwing power as described by Tom.

You can decrease burning at corners and HCD areas by equipping your racks with "thiefs" or "robbers" that absorb the excess current.

Another alternative, if acceptable, would be to convert to a proprietary trivalent chrome plating bath for better throwing power. Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


June 20, 2012

Q. Please explain what are thiefs and robbers that are equipped in the chrome rack to absorb the excess current and purpose is to reduce the burning of chrome at HCD areas.

Nadeem Shahid [returning]
Student - Lahore, Punjab, Pakistan
^


June 20, 2012

A. Hi Nadeem.

Chrome plating is an electrochemical process whereby current causes chromium to be reduced from hexavalent ions to metal, and the current essentially follows the "path of least resistance". So too much current can flow to corners and exterior edges, causing burning. A thief or robber is a piece of scrap metal that is connected to the cathode (it can be part of the rack) and placed close to those corners and edges to absorb some of the current that is flowing to that area. The thief is later stripped or replaced.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^



June 12, 2014

Q. I want to increase the blueness in chrome plating but I am unable to do so. Please help. Also tell me what ratio should be maintained, I mean what is the ratio of chrome oxide and sulphate ... and how many kinds of sulphate can be used? Please mention their names also.

mohd malik
- aligarh,up,india
^


June 2014

A. Hi cousin Malik. I have never heard of such a thing myself. To my eye, chrome plating from a decorative hexavalent chrome plating bath is always exactly the same color. Maybe a highly experienced chrome plater will disagree with me.

But you are posting abstract questions instead of telling us your own situation which leaves me wondering if, for example, you are trying to plate chrome onto nickel. Because in that case nickel is indeed 'yellowish' and lack of a 'bluish' color means no chrome is being deposited, which might well be the case if the ratio you ask about is wrong.

The formulation of a conventional (Sergeant) hexavalent chromium plating bath is simple: about 28-32 oz./gal of chromic acid, and exactly 1/100th as much sulfuric acid.

Luck and Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^



August 14, 2018

Q. Hello everyone.
I am facing problem in chrome tank. Recently I prepared the new chrome bath with the ratio of 250 g/ltr chromic acid; the problem is H2SO4 it should be 2.5 g/ltr mistakenly it got 5 g/ltr. To go back to my original value I precipitated the chrome solution with barium carbonate for 12 hrs. Even after 12 hours of waiting the result is a slight change from 5 g/ltr: it's gone down to 4.7 g/ltr

Note tank capacity 915 ltr.

yaseen khan
flexible packing industries - HYDERABAD INDIA
^


September 11, 2018

? Yasseen,
How much barium carbonate did you put into your 915 liters chrome tank to reduce 5 g/l of sulfate to 2.5 g/l?

Cheah Sin Kooi (ed note: aka our frequent helper SK Cheah)
- Georgetown , Penang , Malaysia
^



Chromium Plating Catalyst Ratio

September 11, 2018

Q. Regarding hexavalent chromium bath: We all know that this type of bath needs a catalyst present in order to form a deposit. In its simplest form it is sulphur alone, at typically 100 : 1 ratio with chromium trioxide (and D.I. water). Also typically, the sulphur is introduced into the bath by way of sulphuric acid.

Since sulphuric acid is not 100% sulphur; 1 molar mass of sulphuric acid is 98.1g, of which only 32.1g is sulphur - the rest being hydrogen and oxygen (please correct me if I'm wrong on this).
So, does the 100:1 ratio refer to the sulphur content alone of the sulphuric acid added to the bath, or does it refer to the total mass of the sulphuric acid added.

In the former case the mass of sulphuric added to the bath would have to be close to 33:1 to achieve a 100:1 chromic to sulphur ratio, whereas in the latter case it would just be by mass 100 parts chromic to 1 part sulphuric acid.

Can anyone verify which out of the two it is please - it is a lot easier to add more acid later than try to remove it with barium carbonate or whatever.

Thanks and Regards,

Nigel Smith
Vehicle restorer as a hobby. - Birmingham, West Midlands, England
^


September 2018

A. Hi Nigel; it's actually not exactly either. The 100:1 ratio is Chromic acid to Sulfate, H2CrO4 to SO4-2. The ratio is important but it's not written in the stars to six digit precision. According to Guffie, people historically used 54 oz/gal of Chromic acid, which made the desired amount of sulfate 0.54 oz, which worked. The weight of the hydrogen is fairly insignificant in the formula -- so you wouldn't be far off if you calculated based on H2SO4 instead of SO4-2 ... but you would be way off if you calculated the ratio based on S+6

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


September 13, 2018

thumbs up sign Hello Ted, thanks for verifying the specifics with the catalyst ratio with your reply.

Cheers,
Nigel

Nigel Smith [returning]
- Birmingham, West Midlands, England
^

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