Rust Formation Behavior on Carbon and Weathering Steels in Wet/Dry Cycles with and without Na₂SO₄ Solution Spraying Combining NaCl or MgCl₂ Solution Dripping

Abstract

Initial stage of rust formation behavior of carbon and weathering steels was investigated under wet/dry cycles with spraying of Na₂SO₄ solution and dripping of NaCl or MgCl₂ solution. With spraying of Na₂SO₄ solution, the weight loss of both steels increased directly with increasing time during the dripping of either NaCl or MgCl₂ solution. On the other hand, in the absence of spraying, the rate of weight loss decreased with time. The weight loss with dripping of MgCl₂ solution was smaller than with NaCl solution after 28 days regardless of spraying. X-ray analyses of the rusts showed that the amount of α-FeOOH was greater with dripping of NaCl solution than with MgCl₂ solution, and with spraying than without spraying. The amount of X-ray amorphous phase was greater with dripping of MgCl₂ solution than with NaCl solution, without spraying than with spraying, and for weathering steel rather than carbon steel. Without spraying a dense inner dark layer, including a black region, was formed in the rusts on both carbon and weathering steels subjected to the dripping of NaCl or MgCl₂ solution after a test period of 28 days. On the other hand, with spraying the rusts formed were all porous and not dense. The surface potential measured by Kelvin probe showed that the rusts on both steels after 28 days exposure without spraying are more protective than those with spraying.