Incubation and development of atmospheric corrosion in the microstructures of low alloy steels

Initial corrosion behaviors of three low alloy steels in simulative marine atmosphere were analyzed by optical microscopy along with electrochemical impedance spectra. It is found that the microstructures of the steels not only affect the initial corrosion behaviors directly, but also influence the long term corrosion behavior indirectly. In the early stages of corrosion, large-angle boundaries in ferrite are easy to be corroded, on the contrary, low-angle boundaries in bainite are difficult to be corroded, while preferential etching in ferrite + pearlite steel always concentrates on pearlite phase and its boundaries. The corrosion resistance of bare samples of ferritic steel is fairly good, whereas the resistance increment of rust layer formed on the steel is little when prolong the test. However, the resistance increment of rust layers formed on bainitic steel and ferrite + pearlite steel is large when prolong the test, which means the steels will have good corrosion resistance in a long term, despite of their lower resistances of bare steel. The results indicate that, by proper alloying, bainitic steel can become a potential candidate for a new type of low cost and high strength weathering steel.