Research On Pitting Behavior And Mechanism Of A High Strength Low Alloy Steel In Marine Splash Zone

Currently, a large number of high strength low alloy steel are obtained with the low carbon w(C) 0.2%- 0.45% and appropriate alloying elements by quenching and tempering processing such. Taking into account the increase of corrosion resistance, alloying elements Cr, Ni, Mo, Cu, P, Si and so on are added to is high strength low alloy steel used in the marine environment. As the development of high performance materials,properties of high strength steel will continue to increase and high strength low alloy steel used throughout the marine environment will be possible and have broad application prospects.Localized corrosion of steel materials in the marine environment, particularly pitting is an important factor affecting the strength and durability of steel materials. Steel surface are almost continuously wetted by seawater in the marine splash zone, where drying time is short and the water film stay for a long time with alternating wet and dry high-frequency and large sea-salt particles. Since the wave and splash and the seawater in contact with air, the oxygen content of the seawater reaches the maximum level. Attack of spray in the marine splash zone also contributed material destruction. When the material surface is covered with rust, the corrosion process is no longer a simple metal anode dissolution and oxygen cathodic reduction, but a very complex variety of substances involved in the reduction reaction. The corrosion behavior of steel is associated with porosity, electrical conductivity, ion selective and reducing of surface rust. For the mechanism of localized corrosion in the splash zone is relatively few. Corrosion in the splash zone of the marine environment is most severe with different rust layer structure unlike others corrosion zone. Study on pitting damage and mechanism of high strength steel covered with rust is important for the improvement of high strength steel in the promotion and safety assessment of marine environment.Based on above review, this work takes low alloy high strength steel AISI 4135 steel as the research object, observing the development process of surface rust layer on low alloy high strength steel in the marine splash zone, including two parts of actual sea experiments and laboratory simulation test. Analyzing the composition of rust corrosion at different stages of real sea experiments can obtain major environmental corrosion factors. Based on these results, simulating the state of rust and steel interface create conditions for electrochemical method to research pitting mechanism on the bottom layer of rust. Adjusting the waves splash zone temperature and alternating wet and dry study its effect on the development of pitting mechanism by laboratory simulation experiments. The main results and conclusions are as follows:1.Heat treatment on AISI 4135 steel can improve the strength of steel and improve mechanical properties. The temperature increase lowers the open circuit potential and increases the corrosion rate for all heat-treated samples. Heat treatments affects the mechanical properties of the tested material but have no significant effects on corrosion electrochemical behavior under natural seawater film at same temperature.2. The results of the exposure experiment in the marine splash zone show that initial temperature affected corrosion behavior of AISI 4135 steel. The higher initial temperature, the larger corrosion rate. After a year of temperature change, the sample exposed at high initial temperature suffered severer corrosion than that at low initial temperature. In addition, AISI 4135 steel exposed in the marine splash zone for 2 years occured pitting perforation.3. The area of hysteresis loop was greater under seawater film at lower temperatures than that at high temperatures, indicating that the pitting corrosion sensitivity is high at lower temperature. The passive(oxide) film exists at lower temperature and will become unstable or totally disappear when temperature is increased.4. The daily and yearly cycles of temperature and humidity change, the wet-dry-wet cycles of the environment, influence both the structure and properties of rusts in the marine splash zone. Cracks and voids that absorb seawater from the splash and water condensates are formed when AISI 4135 steel was exposed in the marine splash zone for 1 year. Significantly, these cracks provide channels for ingression of electrolyte and O2 to inner rust layer forming an underlayer of rust.5. γ-FeOOH was present in the outer rust layer with cracks forming in the AISI 4135 steel exposed in the marine splash zone. γ-Fe OOH is an electrochemically active species that accelerates corrosion in the cracks. Notably, the AISI 4135 steel exposed to the marine splash zone for 1 year contains high concentrations of chloride in the inner rust layer, yielding a low pH that promotes the formation of β-FeOOH that is easily reduced and capable of promoting corrosion under the environmental conditions of the marine splash zone.6. The pH of AISI 4135 steel covered with rust during a wet and dry corrosion cycles showed a minimum in the drying process. In the narrow anode channels, the strongly acidic condition in contact with steel surface results in an increase of corrosion rate, and accelerates local corrosion under the rust with cracks and voids.