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topic 12427

I.D. rust in plated hydraulic tubes


My company bends and assembles steel hydraulic tubes that get zinc plated. During the zinc process acid is used which removes the oil rust protection from the inside of the tubes. This causes the inside to rust. We then have to sandblast the id of the tubes to remove the rust as our acid cleaning would remove the zinc from the outside of the tubes. Is there a better way to clean the tubes after plating? These tubes are 1/4" to " diameter and 1 to 8 feet in length.

Gary Rickert
Morton, Illinois, USA


It is not uncommon when nickel plating tubing work to finish the cycle with a dip in a phosphatizing process so the inside of the pipe gets rustproofed whereas the phosphating does nothing to the plated surface. I am not sure exactly how this would work in the case of zinc plating if the zinc surfaces need a subsequent chromate conversion coating, but hopefully it's food for thought.

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



I have one crazy idea and that's to employ what a professional pipe bender uses for tight radius bends in metals, particularly copper.

They use a material called 'ROSIN' ... it is poured into the pipe and sets rock hard. It is removed, after the pipe has been heat formed, going by memory, by just using hot water, ie. it's a water miscible material.

I'm not sure of the affect the plating acids might have but at least it's worth a try as it is as cheap as wurst, can be easily applied & removed. In other words it could act as a plug at each end.

May I suggest you approach a local specialist in pipe bending. And if this works, let us know, OK?

freeman newton portrait
Freeman Newton
White Rock, British Columbia, Canada

(It is our sad duty to
advise that Freeman passed away
April 21, 2012. R.I.P. old friend).


Some other fabricators of tubing that you describe have found that alkaline zinc gives them better results than parts plated in Acid Chloride Zinc. The main reason given, is that the Acid Zinc baths form an active surface on the inside of the tubes, which flash rust rapidly. The Alkaline Baths keep a passive surface which has less tendency to flash rust.

Make sure, that the tubing is free of rust prior to going into the plating tank. Many times,the tubing arrives at the plater with considerable internal corrosion. Preplate pickling does wonders for such tubing. Post Plate corrosion on the insides usually necessitates stripping of the zinc, pickling to remove the oxide and replating. A costly and inefficient method.

Some firms specify proprietary silicated compounds after the plated part is chromated, to give longer storage times after plating. A good chemical manufacturer can assist you with that selection or option.

ed budman
eb sig
Ed Budman
- Pennsylvania


Hello Gary,

After chromate passivation, a good rinse followed by a water soluble lacquer dip may give temporary protection from flash rusting inside the tubular surface.Once dried, rust preventive oil may be applied.

Subramanian Ramajayam
Subramanian Ramajayam
Bangalore, Karnataka, India

September 7, 2009

There are 2 issues at play here.

1. The main issue is stop putting the finish, acid and sand blasting on the inside. This is a big no-no in our world of fluid power. You are removing the tubing dia. The ID is just as important as the OD of the tubing. And if I call out for a .032 wall thickness it is not for the dia. it is for the working pressure. And so on; I can go on with the hydraulic issues but it will go on and on.

2. The answer is when you form the tubing and install the flare, nuts, etc. If present just install end caps so the inside is not effected. Or should I say contaimnated. If no ends on the tubing then install plugs. Bottom line do not get any contamination inside of the tubing.

If you need help call a local hydraulic shop and get some advice. Remember you must not contaminate the inside of the tubing. It is important nothing is inside or whatever equipment your tubing is going onto will be effected. And always clean the tubing with the asap hydraulics hose/tubing cleaning system. This will remove any contamination left inside from forming. This is a bullet projectile that cleans the tubing inside.

Mike O'Neill
- Downingtown, Pennsylvania

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