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Taper Problem on Chrome Plated Barrel Bores

adv.     u.s chrome


An ongoing discussion beginning back in 2008 ...

January 8, 2008

Q. Hi all. We chrome plate the inside of gun barrel bores. We've recently had a rather odd problem. The barrel in question is approx. 80 inches long, with a 1" bore (rifled). Chrome thickness range is .007-.008".

We have plated these barrels successfully in any of 4 other tanks, with the deposit even the full length of the bore (no taper). We recently constructed a new chrome tank, the same depth as the others, but a little wider. Barrels plated in this tank typically have taper of .002" or even more, with the muzzle end coming out a lot heavier than the breech. All the other tanks, with identical solution chemistries, show almost no taper, and what little we get can be controlled by lowering the plating current, or raising the chrome:sulfate ratio. Barrels are suspended from the common cathode bars, muzzle end down. Anode and cathode connections are only made at the top (breech) end.

The older tanks used a common cathode buss (4 rectifiers on 2 buss bars, 2 on each), with the anode connections separate. The new tank had separate cathode and anode cabling with individual cathode bars (we use a mix of hard bussing and cabling on the other tanks, with no problems. We connected the cathode legs in the new tank together- no difference in results. We use low-pressure air for agitation (although we had installed eductors on a pump system on this new tank, but switched back to air to try to locate the source of the taper problem). The rectifiers are new Rapid units. We checked the output and amp-hr counters, they're perfect. We did connect one of these rectifiers to a barrel in another tank (connecting the cathode to the common cathode buss of that tank, and it plates OK. So basically, the setups are identical, except for the un-common cathode leg. Any ideas what's going on?

George Brackett III
Sr Mfg Engineer - Saco, Maine, USA

simultaneous January 10, 2008

A. Your assertion "the only thing different is the un-common cathode leg" says it all!
If it is true, Logics tells us this MUST be the reason. It is theoretically impossible to have another cause.

Guillermo Marrufo
Monterrey, NL, Mexico

January 10, 2008

A. With a 1" bore, I will assume that you are plating a 25mm chain gun barrel. I also assume that you are using an internal anode. If the anode has a contact at the muzzle, I am going to guess that you have a better contact there, or the breech end has most of the contact out of solution and is heating up allowing more current to flow at the bottom which is kept cool by the solution. If this is not the case, I would take an easy way out and mask off part of the anode at the muzzle end. I assume that you are using some kind of standoff for this anode and the masking could become part of the standoff.
I also assume that this process is proprietary and that you do not really want to get into details on it.

James Watts
- Navarre, Florida

January 11, 2008

A. Hello George,

I have encountered a couple of similar cases in my past projects. One was an oil well drilling shaft and the other was naval gun bores. The solution for these was to increase agitation significantly. In both cases, we were using pumped agitation. For the large naval barrels, properly located eductors were successful. However on the well drilling shafts, eductors would not work due to the small diameter, so high velocity jets were employed.

Our root causes for the taper were metal depletion in the solution and current flow impedence from the hydrogen bubbles. Increasing the flow through the tube caused the solution to be more consistent from top to bottom in the tubes and successful evacuation of the hydrogen bubbles.

I hope this helps.

Best Regards,

Ira Donovan, M.S.F.
Kansas City, Missouri

January 15, 2008

Q. James- The connection is only at the breech- barrels are suspended in the solution muzzle-down, the anode and cathode connections are both out of the solution. No stop-off is used.

Ira- what does your electrical connection look like on your small diameter set-up?

George Brackett III [returning]
- Saco, Maine

January 16, 2008

A. OK, you are getting plating electrons/ions bouncing off the bottom of the tank. Shorten your anodes to initially be at 1" higher than the muzzle. Depending on how close you are to the bottom of the tank, you will probably end up with the anode 3 inches higher than the tip of the muzzle.
I am shocked if you are plating the ID with out using an ID anode and not having a severe run out problem in the middle.

James Watts
- Navarre, Florida

January 22, 2008

thumbs up sign  Am I describing this wrong, James? There is an internal anode (platinum coated, niobium-clad copper, fixtured to be held rigidly centered in the barrel bore. The anode and cathode connections are both in the air, out of the solution. The tank is 11-1/2 feet deep, the barrel is almost 5 feet from the bottom of the tank. Incidentally, it looks like the problem was excessive trivalent, due to solution reacting with the adhesive used to hold the Koroseal panels to the tank. Thanks everyone.

George Brackett IIIret
- Saco, Maine, USA

Chrome plating I.D. of Tube

March 8, 2018

Q. I need to be chrome plated the ID of SS 304L Tube having 57.80 mm dia. and 3400 mm long, wall thickness 3.5 mm. You are requested to suggest about the following:

1. Anode Size
2. Bath Concentration
3. Current in Amps
4. Plating duration
5. Is it possible to achieve in one setup?

Thanks & Regards,

Self - Bangalore, India

March 2018

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

Affiliate Link
(your purchases make finishing.com possible)

A. Hi Suksmith. You have not introduced yourself so it's a bit hard to know the right starting point. May we assume that you have experience in hard chrome plating? If not, this is sure a tough part to start with :-)

1. The anode can probably be a copper or steel rod about 3400 mm long with lead plating and wrapping. It needs to be sufficient diameter to carry the current from question 3; moreover, it should probably be as large as possible per Guffie, with an ideal anode-cathode spacing of 1/4" - 3/4" .
2. I don't see a reason to not use whatever standard Sergeant bath or HEEF-25 you normally use.
3. Figure on about 2 A /in2 of plated area.
4. You have to advise required plating thickness before the plating time can be estimated.
5. The 304L probably needs sulfuric acid cathodic etching, so it may depend on exactly what you mean by "one setup".


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

March 8, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We have a requirement of Hard chrome plating inside the SS 304 Tubes having ID 57. Plating thickness required is 0.01 - 0.10. Can we have following datas -

1) Anode diameter
2) Current required in Amps
3) Bath solution need to be circulated?

Sukshith Kumar
Special Purpose Machineries - Bangalore, Karnataka, India

March 2019

A. Hi Sukshith. Apologies if you didn't previously see my response, but we've appended your inquiry to the previous thread where it was asked and answered last year. Please try to phrase ongoing questions in terms of the suggestions already offered. Thanks!


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

April 20, 2019

A. Hi Sukshith,

Mr. Mooney explained it very well, I would like to add something to it. How will you pre-treat the inner diameter? How is the surface of inner diameter, in example, if it is a cold drawn pipe, you may probably need to machine the inner diameter first, like honing. If it is a polished pipe, than you are okay. You have to have a clean surface before any treatment.

Best regards.

- Istanbul, Turkey

adv.     u.s chrome

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