Letter 36033

Avoiding rust inside zinc plated steel tube [Kentucky] 

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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
- Florida, USA

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, P.E.  finishing.com Brick, NJ

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

Jeffrey Holmes, CEF - Spartanburg, SC, USA

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

John Martin
- Wales

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 H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Try to maintain bath composition 10 gm metal and 100 gm caustic with good brightner 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 alk.bath chemistry.

P. Gurumoorthi     electroplating process chemicals Chennai, Tamilnadu, India

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

Mr. Gurumoorthy, I was surprised to see that in India you do 20 foot of tubes in alakline 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
- Florida, USA

Skip electroplating and just use hot dip galvanizing?

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

Dear Thomas.

You realy 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 glowes than asbestos one :)

Nice day to all of you from Slovenija!

Dimitrij Us
- Kranj, Slovenija (EU)

Dear Sir,

In Chennai we plate 20 feet lenth tube with 3/4 inch dia and some times 1/2 inch.this tube is a fuel conducting tube for major automobile heavy vechile manufacturer. Bath chemistry as I already mentioned 10 to 12 gm metal 100 to 120 caustic.with good world no one brightner system. Also we provide internal 2 mm to 3mm auxilary ms anode with pp bushes, we had zig designed in such a way in each zig 6 tubes were loaded with solution movement. Also we maintain higher addition rate of brightner and we use some booster for extra throwing. This system we are running for last 4 years consistantly without any problem. earlier we use cyanide base bath.use of good inhibiter in acid and post rinze with 7 to 8 ph water,bright dip we use .5% to 1% nitric acid,chromate we use hexa yellow. no sealer, no laquer.

With best regards.

P. Gurumoorthi     electroplating process chemicals Chennai, Tamilnadu, India

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

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