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Plating both outside AND inside of a tube

June 12, 2021

Q. Rusting inside M10 Thread, 45 mm (1-3/4") through hole, EN1AL Steel Hex, Zinc Plating?

We are manufacturing a steel hex nut with M10 threads inside, the total length of the component being 45 mm.
After zinc plating, more than 50% of the material remains un-plated in the middle of the tapped threads and after 2-3 days, rust appears.
External surface has no issue, it's only the inside thread having the problem.
Can anyone suggest a better method?

Jacob Ebanezer
Jacob Ebanezer
forging & turning - Jamnagar , India

June 14, 2021

A. You have not revealed whether chloride, cyanide, or alkaline. In either case "throwing power" is maxed out, at the expense of efficiency, with higher Cl or CN or Caustic, and lower metal. If such an adjustment does not get the inside, then you must use an inside anode. An inside anode in this case may be a flexible steel wire with beads, the beads prevent shorting and the wire anodes the current on the inside.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

June 2021

A. Hi Ebanezer. In addition to Robert's questions, we don't know if you are attempting to barrel plate this component, which will prove impossible.

The plating goes where the current goes, and pretty much in proportion to it; and the current certainly will not flow to the middle of the inside of this long narrow tube without the auxiliary anodes which Robert mentions.

The problem is not that this component is unplateable, it's that plating the inside of the thread will cost perhaps 20X or 50X what it would cost to barrel plate just the outside :-(

Luck & Regards,

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

June 14, 2021

Yes, we are doing barrel plating
Single-piece plating didn't work out well, as too much zinc plating was stopping go gauge to pass through easily.

Jacob Ebanezer
Jacob Ebanezer
forging & turning - Jamnagar , India

June 2021

A. Hi again. Sorry to hear of your troubles with rack plating but I think you will never get electroplating coverage in the middle of a 45 mm (1-3/4") long M10, 8.38 mm (about 5/16") minor diameter female thread without auxiliary anodes -- but send a drawing to for posting here if you think I'm misunderstanding.

Luck & Regards,

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

⇩ Closely related postings, oldest first ⇩


Q. We have tubing, usually about 3/8" ID to 1" ID that our customer spec calls to be CAD plated or Zinc plated. CAD is better but is becoming harder to find vendors and I am told that it will probably disappear over the next few years, I think due to environmental concerns.

Is there any way to get the inside of the tubing zinc plated or is there another process that I could convince my customer is equivalent to for corrosion protection? Presently, we have to coat the inside with LPS as a crutch.

Is it true that the zinc plating process will actually strip the ID of corrosion protection.


Doug Swanstrom
Aerospace Manufacturer - Duluth, Minnesota


A. Yes, simply running the parts through the necessary pretreatment steps will probably strip any previous cad or zinc plating from them. Electroplating is driven by current, and I'm sure that you can picture that virtually no current will flow from the anodes all the way to a spot on the inside of a tube because it will take the path of least resistance instead.

It is certainly possible to plate the insides but it may be prohibitively difficult depending on the particular parts and their value. Plating the inside requires "stringing" an auxiliary anode through the center of each tube and probably also involves some kind of pumped solution transfer because stagnant solution within the tube may not contain enough zinc or maintain the right balance.

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


A. It certainly is not world class for aero parts but you can zinc plate in and out by hot dip galvanizing. Most commercial pipes and fittings are processed this way and have all their surface protected.

Guillermo Marrufo
Monterrey, NL, Mexico


A. Hi there,

Electrolytic zinc plating will be really hard to accomplish inside the tube.
You can use hot dip zinc plating, or other process where you do not apply current, maybe Electroless Nickel.


Guillermo Castorena G.
- San Luis Potosí, México


A. Another method to plate zinc or cad for that matter, is using a brush plating process. We have experience in running electrodes down the ID's of pipes or tubes in order to get deposits of a variety of metals onto the surface.

There are several factors that must be determined at the onset of work; these include length of the tube and deposit thickness, but the process has been successful on previous applications

Chris Helwig
- Valencia, California


A. Yes, it is possible to plate inside tubes today. You will need in principle 2 main conditions: a, the suitable electrolyte; b, the matching internal anodes. As electrolyte I recommend alkaline zinc electrolyte and an internal auxiliary anode. With both of this you may come close to your target.

Rudolf Kempf
- Haan, Germany

Plating nickel and chrome inside a tube

September 24, 2014

Q. Is it possible to plate inside the tube in nickel and after nickel in chromium baths? The hollow is only 1 or 2 cm long

Bojan Koren
- Bovec, Slovenia

September 2014

A. Hi Bojan. The devil is in the details. In conventional plating with no special internal anodes and without solution pumped through the tube, a tube of 2 cm diameter or more may plate 1 cm deep or a little more, which could total over 2 cm if you count from both ends. But a 1/2 cm dia "straw" will not plate on the inside.

How critical is the chrome? Because I think you have a good shot at getting full coverage of nickel, but only enough chrome coverage to just make it around the bend, not inside the tube. For non-critical components, I think a lot of people would be satisfied with that. Good luck.


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

September 27, 2014

A. If you are after corrosion protection and a good wear surface, consider electroless nickel.
If you want it to look more like chrome, look into electroless cobalt. It is a bit more expensive but will make your life a lot easier.

James Watts
- Navarre, Florida

October 3, 2014

Q. Yes I know that electroless nickel has much better covering power, but we don't have it in our company and I have no experience with electroless nickel.

Bojan Koren
- Bovec, Slovenia [returning]

October 6, 2014

Q. I have a couple of questions about plating the inside of a tube...1) should the flow be continuous within the inner diameter and is there a way to calculate the needed rate and 2) what is the best way to prevent the build up of plating on the bottom of the tube on the outer diameter (aside from masking it)?

Dawn Baeckeroot
- Belliare, Michigan, USA

October 2014

Hi Dawn. The thread has wandered a bit, so we don't know if you are talking about zinc plating, nickel-chrome plating, or electroless nickel. What is the length and inside diameter of the tube? Is it made of steel? Thanks.


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

October 7, 2014

Q. Morning Ted,

I just hired in as new engineer. I have a chem eng degree so I have a decent understanding of chemical process and the mechanics of plating in general. I have a background in flash plating nickel (barrel-type) & yellow chromate but I've never done anything like this. We're dealing with DOM tubes, 32" long with varying inner diameters from 0.75" - 3".

I was just looking for some overall info on plating the inside of tubes just so I can understand what is happening (we are using auxiliary anodes down the lengths of the tubes). This is copper plate by the way. Things I'm trying to learn are 'Is flow supposed to happen through the ID', 'Is there a way to calc what optimum flow is,' 'what are different racking options', etc. This is an old (but very effective) process but there isn't anyone on-site that can truly explain "why" we do what we do. I'm just looking for some perspective.

Part of my responsibility is going to be to evaluate and optimize our current processes but I just need to understand what is happening before I can make any improvements.


Dawn Baeckeroot [returning]
- Bellaire, Michigan USA [returning]

October 2014

A. Hi Dawn. I've never done or even seen ID plating of long tubes like this, and hopefully someone else will tell us both about it. But we do know that as you plate the metal out of the small volume of plating solution inside the tube, it's concentration will decrease. And we know that plating solutions function best at given concentrations. So you can probably start by calculating and/or measuring the concentration at the entry and exit of the tubes and at least make some projections whether the flow rate is adequate to keep the concentration within the target range.

You might also calculate and/or measure whether there will be a temperature rise within the tube that may be problematic. I think the short answer is that there is probably not a flow rate that is too high for successful operation, but there is surely a flow rate that is too low, and you will be able to at least approximately calculate that. Good luck.


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

November 13, 2014

Q. Dear Professionals,

I am trying to zinc electroplate (gold colour) an item which is ID 7" x Length 12". Material is mild carbon steel and around 4 mm thick. We are located in Thailand.
I've found many small facilities which are unable to get the zinc plating to go full length of the OD^ID and one which can do it by adjusting the current on his machinery. As far as I know he has not run any jigs through the centre of the pc.

A few others have said they can do it but will need to make jigs and I suppose put an anode down inside the pc.

My problem is that we are extremely late with delivery and the one supplier who can do this work is painfully slow due to prior commitments. Anyone have a fix they can suggest?

Kirk Riddell
- Banpong, Ratchaburi, Thailand

November 2014

A. Hi Kirk. I assume there is a typo in your posting, and you mean "full length of the ID". An internal anode is a very good idea, and considering the dimensions and the type of part, I think it will be simpler, faster and cheaper than struggling to try to get by without it. But yes this will require special jigging that someone has to pay for.

If the quantity of parts is small, the special jigging can be accomplished in a short lead time but labor intensively with fairly standardized racks, plus copper wires, platinum clad titanium auxiliary anode material, and platers tape or masking materials. If it's a production job, custom designed racks, made of properly plastisol covered copper and stainless are no problem except for cost and lead time.


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

November 13, 2014

thumbs up signDear Ted,

Thank you for your reply and yes, that was a typo, should be ID of course. I'll work with a couple facilities to get some jigs set up. The quantity is roughly 1,200 pcs and we're well late.

40243-1a 40243-1b

Thanks again.

Kirk Riddell [returning]
- Banpong, Ratchaburi, Thailand

November 13, 2014

A. Hi.
Yes, an auxiliary anode inside the ID would do the trick. I'm not sure, but for an ID of 7", I would think an alkaline zinc bath should be able to plate inside the tube.
The beauty of alkaline zinc is its ability to plate inside low current density areas.

SK Cheah
- Penang Malaysia

December 10, 2014

Q. I want to do plating inside tube 101.6 dia * 1.6 thick bent tube, bend angle is 43 degrees, and after bending total length is hardly 300 mm, please guide me and give your suggestion.

exhaust silencer manufacturer - Mumbai, Maharashtra, India

A. Hi Ramesh. You didn't include the units in your dimensions, nor the type of plating. Since you are a silencer manufacturer, you must mean that the tubing is 101.6 mm in diameter (4") and 1.6 mm thick (1/16"). I doubt that you will get consistent full coverage in a 300 mm bent length (11.8") without auxiliary anodes, but are you sure it is required? Zinc plating is sacrificial, and "good" rather than perfect interior plating might be satisfactory for this application ... but are you sure zinc plating will hold up to the temperatures of your application?

Then again you may be contemplating nickel plating?

On the third hand, I thought most silencers these days were made of stainless steel :-)


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
December 2014

January 20, 2017

A. The Manhattan Project needed to chrome plate the inside of pipes for gaseous diffusion plant. They found an old guy and his son working in their garage; they used the pipe as the tank, put chemicals and electrodes in side pipe, capped it, rotated it and viola. I just recall recall reading about in the book Manhattan Project

David Christensen
retired - Minneapolis, Minnesota USA

Q. Sir.
We want to plate zinc nickel plating in 14 mm dia. tube. Plating must be in the ID of the tube. How should I do it?
Please suggest
With regards

Duraisamy Ravi
Plating shop - Chennai Tamil Nadu, India
November 29, 2017

December 2017

A. Hi Ravi. You haven't told us what the tube is made of, or how long it is, but the general principle presented on this page and on thread 4153 is that you need to run an anode through the center of the tube, and probably flow solution through it as well. Good luck.


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

Corrosion proofing the ID of plated tubing

July 13, 2018

Q. Dear Sir,

What would be the best method to prevent corrosion appearing from the inside of small diameter steel tubes (diameter around 10 mm) I wonder about the conventional chrome plating but there is a issue regarding zinc plating on the inside of the steel tube. Other metal plantings like electroless Nickel or Hot dip galvanizing would be more expensive and not practical according to my situation. So please advice what would be the most economical and practical method


Thanura Dilanka
Research and Development Officer - Sri Lanka

July 2018

A. Hi Thanura. I don't understand what you mean by "there is a issue regarding zinc plating on the inside".

An approach I have seen is to do the chrome plating of the outside first, then go into a phosphatizing solution which leaves the chrome plating okay, but reacts with the bare steel inside diameter to create an iron phosphate coating. If you can afford to plug and unplug the tubes, you might have additional processing options.

I think the readers and myself have only a very vague idea of your situation so far.


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

Q. I want to do plate a double ferrule fitting which is made up of carbon steel with the standard dimensions. And which plating do you recommend for this object.


I am not able to do an uniform plating in the internal surface of the part. So, please can you guide me and get me a solution.

Nikhil Sawant.
- Maharashtra, India.
September 21, 2020

January 2021

A. Hi Nikhil. Certainly electroless nickel plating would be easier than any electroplating process, and is well suited towards this. However, please tell us why you want to plate them so we know what platings sound useful and non-useful. What flows through them -- hydraulic oils?

Luck & Regards,

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

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