Chrome Plating: sulphate concentration, throwing power, bright range

A. Hi Will
In a conventional bath like you have, current density is interconnected with the bath temperature. Higher temperature will allow much higher current density, but generally the "bright" range is very wide. *By the way, it looks that you operate at little lower temperature than usually mentioned for the conventional bath (120-140 °F)...

In Hard Chrome Training Course from NCMS, they estimate the "bright" range for your temperature approximately 18-43 A/dm2 (so your first number is perfectly within "the range")

If you look at the bottom of this discussion there is an unidentified chart from Humaira Rashid, that starts bright plating for your temperature at approximately 7 A/dm2.

Unfortunately there is no scientific equation (or I'm not aware about it). My feelings - all those diagrams are very approximate. In addition to the temperature, current may be dependent on purity and contamination of the bath. I would recommend to try the Hull cell. It should give a relatively accurate picture for your specific conditions.
Good luck,

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

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?

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 sulphate is usually correct.

55 mm coverage is marginal, I like to see 70 mm. Why would you add fluoride, but try to remove sulphate 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.

A. The precipitation of sulphate with barium is not as quick as you think. Because the solution has a lot of chromium and relatively only a little sulphate, 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 sulphate. 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.

A. Listen to Sara, she has it right. What is your procedure for sulphate 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.

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!

A. Hope,
are you talking about hard chrome or decorative hexavalent chrome? If you have hard chrome, chromic acid-sulphate 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 sulphate very quick you get wrong reading and more confusion.

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 sulphate at 200:1, but a fluoride catalyst as well?

Luck & Regards,

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 with a Hull Cell.

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 practical, would be to convert to a proprietary trivalent chrome plating bath for better throwing power. Good luck.

Regards,

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,

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,

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

A. Hi Nigel; it's actually not exactly either. The 100:1 ratio is Chromic acid to sulphate, 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 sulphate 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,

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,

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,