Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Service Life Increase Through Galvanization Hubbell Power Systems, Inc. bulletin 01-9204, “Anchor Corrosion Reference and Examples,” contains extensive metal loss rate data on galvanized steel derived from Romanoff’s work. It is recommended that this information be used to determine the service life of the hot dipped galvanized coating in disturbed soil. When hot-dip galvanized steel is used, the total service life should be increased by the time it takes the zinc coating to be lost due to corrosion. Another method for estimating service life increase is presented in the following paragraphs. The results of the studies conducted by the National Bureau of Standards and by Porter indicated that a galvanized coating (zinc) was effective in delaying the onset of corrosion in the buried steel structures. Typical conclusions drawn from this study for 5 mil (3 oz/ft 2 ) galvanized coatings include: • It is adequate for more than 10 years corrosion protection for inorganic oxidizing soils. • It is adequate for more than 10 years corrosion protection for inorganic reducing soils. • It is insufficient for corrosion protection in highly reducing organic soils (pH<4), inorganic reducing alkaline soils and cinders, typically offering 3 to 5 years of protection in such cases. It was also noted, however, that the use of a galvanized coating significantly reduces the rate of cor- rosion of the underlying steel structure once the zinc coating was destroyed. The observed rates of corrosion for the galvanized coating were different (less) than that for bare steel in the NBS study. For galvanized coatings (zinc) of 5 mils, Equation A-3 can be used to estimate the corrosion (weight loss) rate. CL 1 = 0.25 - 0.12 log 10 (R/150) Equation A-3 CL 1 = Weight loss (oz/ft 2 /yr) R = Soil resistivity (ohms/cm)

CORROSION

NOTE: For thinner galvanized coatings, the rate of galvanized coating loss is two to three times the rate determined from Equation A-3.

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