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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
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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.
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Ted Mooney, P.E.
finishing.com
Brick, NJ
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A water soluble rust preventive in your final rinse will provide
moderate rust prevention.
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Jeffrey Holmes,
CEF
- Spartanburg, SC, USA
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A lacquer topcoat applied by immersion after plating will offer
some degree of protection against rusting.
John Martin
- Wales
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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!!!
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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.
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P. Gurumoorthi
electroplating process chemicals
Chennai, Tamilnadu, India
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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
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Skip electroplating and just use hot dip galvanizing?

Thomas Hanlon, Materials Engineer
aerospace finishing - East Hartford, CT, USA
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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)
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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.
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P. Gurumoorthi
electroplating process chemicals
Chennai, Tamilnadu, India
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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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