Galvanized iron

Vanessa Roth <vanessaroth [at] hotmail__com> writes

>I am interested in finding out what other conservators are using to
>treat rusting galvanized iron.

Vanessa Roth asked for opinions on treating rust spots on galvanized
iron. I am assuming that the object is unpainted. The chemical rust
removers I know of have ammonium citrate, oxalic acid, phosphoric
acid or hydrochloric acid as a base. According to John G. Waite's
article on Iron and Iron Alloys: Preservation and Repair (Metals in
America's Historic Buildings, 1980, U.S. Dept. of Interior), rust
removers composed of hydrochloric acid are not as desirable as those
containing phosphoric acid because the hydrochloric acid leaves
chlorides on the metallic surface which will cause corrosion in the
future. (Also reference

    Robert M. Organ.
    "The Corrosion of Tin, Copper, Iron and Steel, and Lead"
    in Preservation and Conservation: Principles and Practices
    (Smithsonian Institution Press and Preservation Press of the
    National Trust for Historic Preservation, 1976), p. 252)

I have used a phosphoric acid based prep for treating rusting
galvanized iron. The product I have used is called "Ospho,"
manufactured by Skybrite Chemical Co. of Cleveland, OH. It's
typically available in most paint and hardware stores. It has
wetting agents and dryers also, which make it good as a pre-primer
under a paint coating. Since the phosphoric acid changes the ferric
oxide into a ferric phosphate, I don't believe that the underlying
sound metal is affected except perhaps for some minor etching.

If it is used on galvanized metal which is to be left unpainted,
care must be taken not to get the solution on the galvanized
coating. The phosphoric acid will etch the galvanized coating and
can accelerate deterioration of the intact galvanizing. A small
brush should used to control application of the solution to only the
rusted area. It should be applied in very thin coats. Depending on
the depth of the rust, thick applications will leave a brittle
gray-black residue which is difficult to remove. A powdery
efflorescence can also appear with too heavy an application. Thin
coats, blotted while wet will eventually turn the rust a
grayish-black color.

Once dry, the area should be wiped down with mineral spirits. Unless
the object will be protected from moisture and humidity, the treated
area will eventually rust again. I suppose that a coat of wax would
help protect the piece. Depending on the aesthetics, a careful spot
application of a cold-galvanizing paint to the treated area would be
a more permanent treatment.

John Horton, RA
Restoration Specialist
NC State Historic Preservation Office


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                  Conservation DistList Instance 13:42
                Distributed: Wednesday, February 2, 2000
                       Message Id: cdl-13-42-002
                                  ***