NCPTT In house Research: Comparative Study of Commercially Available Rust Converters

Conservator Eric Schindelholz applies a rust converter to a historic iron fence.

Conservator Eric Schindelholz applies a rust converter to a historic iron fence.

Starting in the summer of 2010 Materials Conservator Jason Church and Materials Conservation Intern Anna Muto began a study comparing commercially available rust converters. A rust converter is a chemical treatment that turns iron oxide into a more stable form such as iron tannate. The advantage of a rust converter is the treatment is less invasive and the only surface treatment needed is a cleaning of loose rust and soiling generally with a wire brush. Rust converters have wide stretching usage in conservation, they are used from the stabilization of iron based museum artifacts and collections to iron architectural elements and outdoor sculpture.

This study is based on a widely read and quoted study done by CCI and published in 1995. Besides the numerous positives that this respected study has there are three aspects that we wanted to change. The first is the issue of age, this study was started in 1987 and has become outdated; the chemicals recommended are no longer available. The second issue is that the study did not go into much detail about quantifying the results of their testing. In this aspect we hope to improve on the CCI study. The third issue is timing; the original study was done with natural weathering over an eight year period. This is not actually a negative but a positive for the results of the study however our study will utilize NCPTT’s capabilities with accelerated weathering.

For this study we are using naturally weathered mild steel coupons cut with a mechanical sheer (as to not alter the surface of the metal) to 3”x6” the size of the QUV plates. Each of the front face of the plates will be treated with a rust converter, tested weathered and tested again. The products will be evaluated and ranked by rate of failure.

The samples were evaluated by a variety of techniques before being treated with the rust convertor, after being treated, and every 200 hours (for a total of 1000 hours) of artificial weathering. In this study, naturally rusted samples were treated with five commercially available rust converters that were chosen for accessibility and chemical variance. With the exception of the “CCI Recipe” all of the chemicals are commercially available. The “CCI Receipt” was added to the study after a survey of chemicals used was sent out to metals conservators and the results were found that a majority of the conservator polled used this recipe as opposed to a commercially available product. To evaluate which rust converters function best over an extended period of time, NCPTT used several analytical techniques. Condition was assessed before treatment, after treatment, and at several points during artificial weathering. Photography was used to document physical appearance throughout the experiment. Colorimetry, magnetic induction thickness measurements, gloss measurements and laser profilometry together charted changes in the surface and coating of the samples. Finally, Fourier Transform Infrared Spectroscopy helped to characterize the primary chemical compounds present on the surface of the treated metal during the study.

The five chemicals chosen for this study are listed below. The chemicals were chosen to represent each of the major types of rust converters. The chemicals in each category were selected based on their popularity in the commercial market place.  The goal of this study is to provide the end user be it home owner or museum conservator with the knowledge needed to make an informed decision when selecting a rust converter.

Rust-oleum® Rust Reformer® is a tannic acid based rust converting product. It also has an acrylic vinylidene chloride copolymer additive. Tannic acid acts as a chelating agent while the copolymer creates a protective coating. The product has a pH of 2.13, falling in the mid-range pH of this study’s converters. Rust Reformer® has a relatively thick consistency, and is blue-ish white in appearance.

OSPHO®, manufactured by the Skybryte Company, is a phosphoric acid based rust converting product balanced with dichromate and wetting agents. OSPHO® is by far the most acidic rust converter of those tested. The product’s pH is 0.08, and is a thin, translucent green liquid.

Corroseal® is a rust converting product based in gallic acid. This converter is composed of gallic acid, ethylene glycol and acetate. It is relatively acidic, both generally and in comparison to other tested converters, with a pH of 1.50. Corroseal® is creamy white and has a thick consistency

Intern Anna Muto applying rust converter to test samples.

Intern Anna Muto applying rust converter to test samples.

RCx427 is a product of Enviro-Safe Services, Inc. that uses oxalic acid as the rust converting compound. Like Corroseal®, it also incorporates ethylene glycol in the chemical composition. Oxalic acid as an active ingredient—a compound that exhibits different physical properties after conversion. Instead of causing the iron oxide layer to darken, it instead turns a light gray. Of the tested rust converters, this product is the least acidic with a pH of 3.11. RCx427 has a thick consistency and is a blue-gray color.

The 10% Tannic Acid solution stands out from the other rust converters in that the solution is not commercially available but must be prepared. However, a survey of metal conservators done by conservation graduate student Rose Daly indicated that the “CCI Recipe” for tannic acid solution was used more often than commercially available rust converting products. As such, it was deemed important to include in the study. The preparation method outlined in the CCI Note “Tannic Acid Treatment” was followed (CCI Note, 1989). As directed, 2.75 mL of phosphoric acid was added to lower pH to 2.39, and fell into the mid-range of acidity in this study’s converters. The solution is thin and appears a translucent, orange-red color.

At the end of the accelerated weathering only the samples treated with Rust-oleum® Rust Reformer® showed no or only slight signs of any failure and no active corrosion. This research was completed in the summer of 2012, afterwards the results were presented at ICOM CC Metal 2013 in Edinburgh, Scotland. The results of the study were also published in the conference proceedings. Click here for pdf of paper.

The next phase of the study began in the summer of 2013 with Jason Church and intern Sarah Hunter looking at how rust converters react in a salt environment. In this current research the samples will run 200 hours accelerated weathering followed by four cycles of the SWAAT test.

For this phase of the study we are repeating the same tests and testing parameters on Rust-oleum® Rust Reformer® and four other commercially available rust converters. One of the main questions for this phase of the study is how do other rust converters chemically similar to Rust-oleum® Rust Reformer® preform and compare. For the phase two of the study we are comparing

Rust-oleum® Rust Reformer® is a tannic acid based rust converting product. It also has an acrylic vinylidene chloride copolymer additive. Tannic acid acts as a chelating agent while the copolymer creates a protective coating. The product has a pH of 2.13, falling in the mid-range pH of this study’s converters. Rust Reformer® has a relatively thick consistency, and is blue-ish white in appearance.

Intern Sarah Hunter photographing test samples.

Intern Sarah Hunter photographing test samples.

Fertan® is a tannic acid based rust converting product. It also has an zinc nitrate additive. This product has a pH of 1.4-2.0.

Conquest® is a tannic acid based rust converting product. It has a vinyl acrylic polymchloride copolymer additive. Tannic acid acts as a chelating agent while the copolymer creates a protective coating. The product has a pH of 1.3

Jasco Prep and Prime®, is a phosphoric acid based rust converting product. This thin green converter has a pH of 1.

Krud Kutter® is a phosphoric acid based rust converting product. This thin green converter has a pH of 1.

This phase of the project is expected to finish around the beginning of 2015. The project’s final goal will be to test the best two rust convertors with the addition to a painting system and test this until failure. More information will be blogged as the research progresses.

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