Effect Of Copper On The Corrosion Resistance Of A Ultra-low Carbon Bainitic Steel

Developing atmospheric corrosion-resistance steel with high strength and high toughness has become a hotspot in recent years. The coastline of China is very long, and the coastal region is developed area, where a large quantity of steel structures is needed. Thus,the marine atmospheric corrosion resistance has become a big problem,it is urgent to research and exploit high strength weathering steel.In the present study, three Ultra-Low Carbon Bainitic weathring steels with different copper contents have been designed, and tempering at different temperature (400℃, 500℃, 600℃1h) have been carried out. The microstructures of the samples after tempering has been observed by TEM. It was revealed that the disperseε-Cu precipitations was obtained after tempering and theε-Cu became coarser with increasing the tempering temperature.The copper bearing steels after different heat treatments exposed to cyclic wet/dry salt spray (5 wt.% NaCl) 45 days in order to study the erosive rule. The electrochemical behavior of the steels are investigated through polarization curves. The weitght loss and polarization curves results indicate that the corrosion resistance of the steel temered after low temperature(400℃1h) exhibit higher corrosion resistance than that of steel tempered high temperature(500℃, 600℃1h).The camera was employed to observe the growth and development of the rust layer. The sectional morphologies of corrosion products were observed by optical microscope and SEM. It can be found that a compact rust layer forms on specimen tempered at low temperature(400℃1h), but there are many cavities and cracks in the rust layer of steels tempered at high temperature(500℃, 600℃1h). No maked distinction can be detected in X-ray diffiraction results, they are all composited of Fe₃O₄,α-FeOOH,γ-FeOOH. The smaller the grain size ofα-FeOOH is ,the better the corrosion resistance is. EDS results apparently showed that the copper alloy were enriched as several particles at the inner rust to protect substrate form deeper corrosion.