Abstract
ASTM A588 weathering steel, Grade A (two varieties) and B, as well as ASTM A36 structural steel were exposed in three different accelerated laboratory corrosion tests including exposure to wet-dry cycling and salt- fog conditions and the rusting characteristics and kinetics of weight loss were studied for 2200 cycles, equivalent to six years of atmospheric exposure. An acceleration factor of 50 applied did not lead to distortion of results. The average equivalent thickness loss was about 35 to 55 mpy in the salt-fog test, 1 to 1.5 mpy in the simulated, accelerated sheltered location atmospheric exposure test and 0.2 to 0.5 mpy in the accelerated bold location atmospheric exposure simulation test. These values match well with the data obtained from field coupons. Tannic acid treatment was found to provide the best corrosion resistance in wet-dry cyclic tests, whereas double surface treatment with tannic acid and phosphoric acid solutions provided for the best corrosion resistance in the salt-fog test. Magnetite formed in the salt-fog test and got converted to γ-Fe2O3.H2O and/or ∝-FeOOH.∝- FeOOH was the most stable phase in the wet-dry cycling test without rain simulation at slightly elevated temparatures, whereas more of the amorphous rust was stabilized due to the action of simulated rain. The characteristics of the rust phases formed in the various tests are discussed.
