Condition of Preservative-Treated Cooling Tower Slats After 10-Year Service

Slats of redwood, Douglas-fir, hemlock, and ponderosa pine were treated with six preservatives to evaluate the effectiveness of the preservatives in controlling decay by typical white- and brown-rot fungi and by soft-rot fungi under actual service conditions in a cooling tower. Analysis of residual chemicals during and after 10 years of service indicated that waterborne preservatives are more resistant to leaching than airborne preservatives. It was also observed that chemical additives in the water used to control corrosion and the growth of algae and slime are absorbed by untreated slats and by those treated with waterborne preservatives. Toughness tests conducted at several intervals during the 10-year exposure generally indicated, with redwood an exception, a greater loss in strength in untreated slats than in treated. However, the results are highly variable, and only slats treated with creosote are significantly higher in toughness than untreated controls. Leaching of preservatives from the slats and the loss of slat toughness were high during the initial 1-1/2 years of exposure. During the remainder of the 10-year exposure, there was a pronounced decrease in the rate of loss of preservatives and of toughness. Because of these trends, apparently all slats still have the potential for considerable service life.

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