Commercial Rust Converters: Surface Protection for Rusted Iron
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By: Nancy Binnie
Publication Date: 4/1/1992 12:00:00 PM
Between 1987 and 1992, researchers in CCI's Conservation Processes Research Division investigated the durability of nine commercial rust converters. This was done at the request of several agricultural and industrial museums across Canada that have been experiencing problems with rusting in iron artifacts that are exhibited or stored outdoors.
In industry, the paint-like primers are applied directly to rust-covered iron surfaces after only minimal surface preparation. Usually, paint is then applied over the surface that has been treated with a rust converter. The manufacturers state that the rust converter will stabilize corroded surfaces without the need to remove the rust. Rust converters thus offer an alternative to the approach used for conventional metal primers, where a corroded surface must first be abraded down to bare metal. A typical formulation contains tannic acid (which reacts with the rust to form a blue/black ferric tannate) and a polymer (which consolidates the corroded surface).
For CCI's investigation, nine rust converters were obtained from a variety of sources. Mild steel plates were selected for use as the test surfaces because many agricultural and industrial machines employ this material. The plates were air-abraded, were placed outside for two to three months to rust, and then were coated with the rust converters. They were not given any additional coats of paint before testing.
Outdoor exposure tests were carried out for five years at three locations across Canada: on the rooftop of the Maritime Museum of the Atlantic in Halifax, in the back parking lot behind the CCI building in Ottawa, and in the middle of a hay field at the Ukrainian Cultural Heritage Village near Edmonton. The annual precipitation, the mean monthly temperature, and the amount of bright sunshine varies considerably in each location. The ageing properties of the coatings were rated semi-annually. Ratings included evaluating any changes to the surface of the metal.
A "30-day salt spray test" was carried out on a separate set of plates that had been coated with the rust converter.
Before the plates were coated, corrosion products on the rusted plates were analyzed in the Analytical Research Services Division at CCI using x-ray diffraction. It was found that the rust on the plates exposed in Ottawa and Edmonton was mainly composed of lepidocrocite (gamma-FeOOH), whereas the rust on the plates from Halifax was mainly magnetite (Fe3O4) with only minor amounts of lepidocrocite.
After the results from the outdoor and salt spray testing were analyzed, three rust converters were found to have useful properties: Conquest (available from National Chemsearch, Brampton), Rust-Oleum Rust Convert (available from Rust-Oleum Canada, Downsview), and Neutra Rust (purchased at the time from Bernard Marks Company Ltd, Toronto, but no longer available from this distributor).
In general, rust converters are more expensive than paint. When used without a top-coat of paint, the rust converters gave approximately two years of protection from further rusting. After two years of exposure, the surfaces began to deteriorate and the rust became active. In the marine climate of Halifax, the rust deteriorated more quickly than at Ottawa or Edmonton.
For museum artifacts, it is often inappropriate to remove corrosion products when cleaning. However, inadequate surface preparation is often identified as a primary cause of paint adhesion failure, and most conventional paints do not adhere well to rusted surfaces. Applying a conventional, compatible, long-lived protective paint onto the primer should further enhance the protection of the metal surface, although this was not investigated in this project.
It is hoped that conservators will make use of the long-lasting rust converters identified in this study as primers for corroded iron surfaces. Further work is planned which will investigate the effect of rust converters coated with various paints. Staff involved in this project at CCI included Mark Gilberg, Deborah Rennie-Bisaillion, Lyndsie Selwyn, Carl Schlichting, Bob Barclay, and Nancy Binnie.