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topic 36033

Avoiding rust inside zinc plated steel tube

A discussion started in 2005 but continuing through 2018


Q. Gentlemen:
We are seeing severe rust inside the tube we are plating in an alkaline zinc bath on a rack line.
These tubes are 16 inches long and 1 inch in diameter.
We have made several changes since then but problem does not seem to go away. I think it is just simply not possible to provide rust free tubes from the inside since there is no plating inside and the tube passes through corrosive acid media/ air during the plating process.
I would request other opinion from all over the world on this topic. Can the rust inside the tube be completely eliminated. (Of course I am not talking about auxiliary anodes or masking the ends).
Thanks for your time in this matter.

Hemant Kumar
Hemant Kumar
- Florida, USA


A. Nickel-chrome plated tubing is sometimes phosphatized after plating and before unracking to solve this problem. But I don't know if a phosphate chemistry can be found that will be compatible with the alkaline zinc plating and any chromate that you may be applying to it.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


A. A water soluble rust preventive in your final rinse will provide moderate rust prevention.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina


A. A lacquer topcoat applied by immersion after plating will offer some degree of protection against rusting.

John Martin
- Wales


A. Electrical wiring conduit is painted on the inside with an automatic rig that goes all the way through the 10 foot long 3/4 inch tube!

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


A. Try to maintain bath composition 10 gm metal and 100 gm caustic with good brightener system, with some solution movement may improve inner zinc plating there by you protect corrosion.internal ms anode is ideal even achieving thickness of 2 to 4 micron zinc inside. In India we are doing tubes up to 20 feet with proper inner coverage with alkaline bath chemistry.

p gurumoorthi
P. Gurumoorthi
    electroplating process chemicals
Chennai, Tamilnadu, India


Q. Thanks all
We already have a post dip (lacquer sort) at the last step. It does not eliminate the problem, only helps a little. I can not start using auxiliary anodes inside the tubes.

Mr. Gurumoorthy, I was surprised to see that in India you do 20 foot of tubes in alkaline chemistry without rusting. Please share your thought in detail as how do you achieve this.
What extra steps do you follow. What pH you use in the rinses after Pickle step. What kind of chromate and what pH. What concentration you maintain on Pickle and Nitric bright dip, what is the zinc metal level in the plating bath etc...I would appreciate your timely response.

Hemant Kumar
Hemant Kumar
- Florida, USA


A. Skip electroplating and just use hot dip galvanizing?

Thomas Hanlon
Thomas Hanlon, Materials Engineer
aerospace finishing - East Hartford, Connecticut, USA


A. Dear Thomas.

You really provide an answer, that has made my ordinary day at work a little more pleasant; after a long, long time smile has come on my face :)
But none the less, I rather wear rubber gloves than asbestos one :)

Nice day to all of you from Slovenia!

Dimitrij Us
- Kranj, Slovenia (EU)


A. Dear Sir,

In Chennai we plate 20 feet length tube with 3/4 inch dia and some times 1/2 inch. This tube is a fuel conducting tube for major automobile heavy vehicle manufacturer. Bath chemistry as I already mentioned 10 to 12 gm metal 100 to 120 caustic. With good world no one brightener system. Also we provide internal 2 mm to 3mm auxiliary ms anode with pp bushes, we had jig designed in such a way in each jig 6 tubes were loaded with solution movement. Also we maintain higher addition rate of brightener and we use some booster for extra throwing. This system we are running for last 4 years consistently without any problem. Earlier we used cyanide base bath. Use of good inhibiter in acid and post rinse with 7 to 8 pH water, bright dip we use .5% to 1% nitric acid, chromate we use hexavalent yellow. No sealer, no lacquer.

With best regards.

p gurumoorthi
P. Gurumoorthi
    electroplating process chemicals
Chennai, Tamilnadu, India


A. Hi Hemant,

We plated power steering tubes for several years where NO plating was allowed on the internal, as platers we find that murphy's laws as well as faraday's always seem to come into play, we found the best solution was to use a Macdermid product called "ferrohib" which is water soluble this provided enough protection until the part was assembled.

Steve Jennings
- Adelaide, South Australia

October 27, 2010

A. This is always a major concern with tubing, particularly the longer the tube gets. The looming questions are typically to plug or not to plug, what to apply after plating and how? We plate tubing from very short pieces up to 21 feet in length. The best solution is a rust inhibiting post-dip, which does fairly well for the short term. Other methods such as spraying an inhibitor through the tube is a more time consuming and messy method, often leaving an unwanted appearance on the outside of the tubing.

Shannon Kelty
- Cedar Rapids, Iowa USA

January 31, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We do leakage taste of ERW tube 15.88 x 1.6 x 1500 For compressor pipe in normal water, then sent for green zinc plating. After Plating there was found rust inside of pipe. Please give me right procedure, thanks,

kamal kant yadav
- Pant nagar, Uttrakhand, India

April 21, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear All,
We got new project for zinc plating with trivalent rainbow chromate. Part to be plated is pipe. We are using alkaline zinc plating line with pickling and cathodic acid cleaning in it. We found difficulty to process this pipe because customer ask that the inside of pipe should not rusty. At first trial, the inside of pipe already rusty when part came out from drying.
Could any one help us to get the result as asked by customer?

Best Regards,

Joko PY

Joko Prasetyo Yuniartanto
plating shop employee - Bekasi, West Java, Indonesia

September 13, 2013

Q. Hello every body.

We are down here in Mexico and we manufacture electrical wiring conduit.

Since we are trying to get the UL certification, we are looking for a process which allows us to plate zinc on the inside of the tubes.

I read Mr. Robert's comment regarding the "automatic rig". I was wondering if someone can provide me with more detailed information on this concept?

Any help will be greatly appreciate.

Isaac Diaz
Tube and Piping - Monterrey, NL, Mexico

December 16, 2018

Q. Hello everybody,
I have zinc plating near chennai, i have facing issue of inner area rust, in hexcan slot -- give me some solution.

Balasubramani Gopal
- CHENNAI Tamilnadu India
  ^- Privately contact this inquirer -^

December 2018

A. Hi Balasubramani. Please email a pic of the part to and tell us whether you are rack or barrel plating, using acid, alkaline, or cyanide zinc, and perhaps the dimensions of this "slot". Usually there are answers but not easy answers :-)


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

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