The Short Happy Life of Waterborne Zinc

http://www.c4tx.org/ctx/pub/znsi.pdf

Shop primers aside, nearly all coatings used on

tankers today are based on organic polymers. These

coatings have several things in common.

1. They are porous to water. If the water

molecules that reach the steel encounter contaminates,

they blister.

2. They are mechnically weak, easily damaged by

wires, fenders and the like.

3. They are poor at resisting damage. When they

are damaged, the edges are subject to undercutting

and accelerated corrosion.

4. They are sensitive to temperature. Many of

these coatings have a Glass Transition Temperature

(GTT) less than 50C. Such temperatures

are often exceeded on tanker decks and in

the top of tanks. If an organic coating is cycled

through its GTT, it quickly looses strength and

elasticity.

Zinc silicate coatings originated in Australia in

the late 1930’s where Victor Nightingall experimented

with combinations of zinc and sodium silicates

in an attempt to emulate zinc-iron silicate

ores. He finally came up with a combination of zinc

dust, alkaline sodium silicate, and a little lead which

was was applied to a 250 mile pipeline in 1942.

1 To get this job, Nightingall offered a 20 year guarantee. The guarantee was never called in

This process was largely an American phenomenom. Chuck Munger, then technical director of Ameron, joined forces with Nightingall’s company Di-met to produce Dimetcote. Exxon develped a similar product called Rustban.

Exxon USA speced waterborne zinc for its tanker decks. Ludwig used waterborne zinc just about everywhere on his ships. The entire external hull of his tanker was waterborne zinc, topcoated with vinyl with outstanding results.[2][page 164] Most of the zinc coated tankers were built in Japan.

Failures are obvious Waterborne zinc is unforgiving. It must be applied on an excellently prepared surface. It must be carefully and continuously mixed. Pot life is short. Weather must be right. Thickness must be right. And most importantly for the yards, errors are almost always immediately apparent. If you do a poor job applying an organic coating, there’s a good change it will still pass inspection. This will be a big problem for the owner down the road, but the yard is off the hook. Waterborne zinc on the other hand is self-inspecting.

IC ended up with a 5.3:1 silicon/potassium ratio, and called its product IC 531. The product quickly developed a market in the United States, especially among bridge builders. It was also used extensively in Australia on offshore structures, As we shall see, we used it very successfully on tankers. Properly applied it was magic. We sprayed it on badly pitted topsides and ballast tanks with no problem.

We used in chain lockers, going to thickness well above 200 microns without mudcracking. The coating was so strong and anodic that it actually protected the chain lockers, something no organic coating could have done. Our crews became so proficient at laying down 531 that they made it quite clear that they would not go back to organic coatings, if we had been silly enough to try and make them. The decks we laid down in the early 1990’s were still in near perfect conditions ten years later when we had to scrap these ships because of age restrictions.

In late 1991, IC had started switching from the Polyset binder to a simpler process. This process started with a glass that already had a 5.3:1 silicon/potassium ratio. The idea was to simply grind this glass up very finely and disperse it in water, eliminating the costly, hard to scale, blending step. IC made the switch without telling any of its customers. It turned out to be a terrible mistake. In many cases, the coating simply didn’t cure, and washed off in the first rain. In other cases, usually involving salt water, the coating cured; but the surface was so rough that, if it got contaminated before topcoating, it could not be cleaned. IC refused to admit any responsibility, putting all the blame on its customers’ poor application. The result was a series of law suits, and a massive black eye for waterborne zinc from which it has not recovered.4