Regression Analysis for Corrosion Loss, W, of Zinc against Time, t, in Indoor Atmospheres into a Formula of W=atn

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Literature data for zinc coating in outdoor exposure tests were analyzed into an equation of W=atn where W is a corrosion loss in g/m2 after a period of t years. The coefficient a varies widely from 0.3 to 100g/m2, while the exponent n is found to be close to 0.5 in marine and less corrosive rural environments.An indoor corrosion test was carried out sheltered from rain and sunlight. Zinc coated specimens were given 0.1 or 1.0g/m2 of sea salt and 10g/m2 of Kanto loam deposited on each and placed horizontally inside a box with ventilation holes to make the inner condition in temperature and humidity similar to the outer one. This test determined the n value as 0.37-0.39 for zinc coating, which combined with the outdoor exposure data made it moderate to assume the n value to be 0.5.The expression of W=at0.5 was applied to several W data after period, t, as long as 30-40 years obtained from galvanized ducts in some buildings. The analyzed values of a are found to depend highly on the amount of chloride ion deposited on each duct and considered as a good characteristic to corrosivity of the environment to which ducts have been exposed.
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