Page 50: of Maritime Reporter Magazine (December 1994)

Rust Converters For Heavy Marine Use

As environmental laws escalate and maintenance costs climb, many marine operating companies are incorporating rust converters. Rust converters reportedly offer several advantages: reduction of metal preparation costs, easy application, environmental improvements, im- proved OSHA and EPA compliance — while still giving satisfactorily adhering to marine performance standards.

But there are also some disad- vantages: performance is only as good as the top coats used to cover the converter/primer; durability is somewhat less than a zinc primer's (but cheaper); and they are not foolproof. There are more than 80 brands of coating products that make some claim to deal with rust removal or rust conversion. Some are better than others.

There are two types of basic rust converters available in the marine market today: phosphoric acid con- verters or tannic acid converters.

Phosphoric acid converters convert rust to an inert substance thought to be iron phosphate. Tannic acid based products convert rust to a different substance which research has shown to be magnetite or other inert forms of iron. There are, within each type, brand differences in performance claims and cost per gallon. Most are cost effective compared to sand blast- ing but the results vary by brand for heavy marine work.

Tannic acid converters mimic the natural chemical path of corrosion of steel. As steel comes into contact with moisture, a source of oxygen, it starts an electron/chemical reaction whose main product is rust. How- ever, over time, generally 10 years or more, the rust continues to change to its original ore form. Research has shown that mild steel will natu- rally weather to produce either Go- ethite FeO(OH); Lepidocrocite y.

FeOOH; Hematite, Fe20Q; or Mag- netite, FegO^ Of these, the hema- tite and magnetite layers appeared to play a key role in the mechanism of corrosion resistance for weather- ing steels in marine, industrial and rural environments.

The biggest fans of the tannic acid converters are vessel crews and drydock personnel who must remove the rust or apply the rust converter.

The products are non-flammable and reportedly require less surface prep time. Most brands work over a wide range of temperatures and surface conditions. At drydock, welders can continue to work near areas being converter primed.

Nearly all the tannic based con- verters offer little or no release of

Volatile Organic Substances (VOCs) into the atmosphere. Therefore, used in combination with normal coatings, they offer a reduction in

VOCs.

Tannic based converters can of- fer the vessel owner the chance for reduced maintenance costs if direc- tions are followed. The savings can be very dramatic, as much as $1.00 to $1.50 per square foot over con- ventional white metal sandblasting costs. Metal prep still remains the king-sized portion of the coatings bill, not the paint costs.

The most common use of tan converters is to ship maintenar

Common applications are dec houses, bilges, ballast tanks, he ing tanks, double bottom voids, ste ing compartments, shaft tunnt chain lockers and engine rooms. L is especially popular with crews smaller vessels such as tugs a fishing vessels. The lack of fla mable vapors make these produ< especially useful for safe below-dec painting. The ease of applicati helps avoid application and mixii problems reportedly experience with two part products. Bar) owners use tannic converters f barge rakes, void areas and balla tanks, decks, container racks ar cranes. However, for successful coa ing life ballast tanks must be to] coated. Working dredges, crane and trawlers have commented o added economies after using coi verters.

Cost savings occur when equij ment bearings do not require rt placement after using converters as is reportedly the case with san blasting.

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