Mechanism Of Wear And Corrosion Properties And Protection Of FH36 Steel Used In Shipbuilding

The polar shipping is tremendously important because of the rich natural resources.Exploration and transportation of polar resources in polar regions prompted the icebreaker technology to the research highlights.However,polar ships were exposed to the harsh and complicated environment over a long period in such as low-temperature,sand and ice.As a result,enormous challenge caused by continuously impacted and rubbed by seawater and ice,which will bring about wear and corrosion damage of hull steel plates of polar ships.In addition,the existing key structural materials of icebreaker with low temperature resistant in China are still mainly imported.Efficient and reliable protection technology on low-temperature steel for polar ships is a critical problem that needs to be solved urgently at present.Therefore,it is of great theoretical and practical significance to study the wear and corrosion mechanism and protection of low temperature steel for polar ships.Based on the National Key R&D Program of China-Study on service perform evaluation of ship steel in extremely cold and ultra-low temperature environment,the thesis aims to investigate the influences of both the wear and corrosion resistance of the FH36 steel in Baoshan Iron&Steel Co.,Ltd.The mechanism of erosion-corrosion and tribocorrosion behavior of FH36 steel was also explored.Moreover,the protection method of the heat treatment,microbial mineralization coating as well as Co-WC coating on their wear and corrosion resistance was investigated,so as to make it more suitable for the polar environment.It can provide some theoretical and experimental data support for the development and application of low-temperature steel for polar ships in China,and provides new ideas for the protection technology of low-temperature steel for polar shipbuilding.The main contents of the present thesis involved the following aspects:（1）The influence of erosion velocity and erosion angle of sand-seawater erosion-corrosion（E-C）properties of FH36 steel at-2℃were investigated,through a simulated working condition of polar ship navigation.It was found that the E-C damage of FH36steel was more sensitive to the erosion velocity than the erosion angle in sand-seawater.The max corrosion pits depth increased from 6.067μm to 10.063μm when the velocity increased from 1m/s to 3m/s for 6 h.The descending order of corrosion pits depth was45°,90°,30°and 0°when erosion at 3m/s,and the largest mass loss and the most serious corrosion of FH36 steel was under 45°.The maximum depth of corrosion pits was about3 times compared with 0°.The synergism between wear and corrosion existed in the E-C process,and wear was the main influencing factor.There was a positive synergistic relationship between wear and corrosion.（2）The wear and corrosion mechanism of FH36 steel was studied using a tribocorrosion tester.It was found that the coefficient of friction（COF）of FH36 steel decreased and the wear loss increased with the increase of applied potential.Under the same applied potential,the COF of FH36 steel in seawater was smaller than that in deionized water.Corrosion products were beneficial to reduce the COF of the samples.The wear mechanism of FH36 steel under cathodic protection potential on-0.8V was mainly abrasive wear.Moreover,the wear mechanism under anodic potential on-0.4V and-0.1V was corrosion wear and corrosion pitting.In deionized water and seawater,the wear promoted by corrosion played a leading role in the synergetic effect between the wear and corrosion of FH36 steel.（3）To improve the mechanical properties and wear and corrosion resistance of steel for polar ships,the effect of heat treatment on FH36 steel at different temperatures was prepared to explore the effect between the microstructure and wear,corrosion behavior.It was found that the Martensitic steel（TM）and sorbite steel（TS）were prepared through microstructure transformation at 250℃and 650℃tempering process,respectively.The COF of TM was lower than TS during friction and tribocorrosion.The acicular martensite structure of TM and the tempered sorbite structure of TS were beneficial to improve the wear resistance.TS showed the best corrosion resistance in corrosion and tribocorrosion tests.The dispersed and precipitated carbides produced by heat treatment were beneficial to improve the wear and corrosion resistance.（4）In order to meet the environmental protection requirements for the coating for polar ships,the effect of microbial mineralized coating（MMC）on wear and corrosion resistance of FH36 steel was studied.It was found that B.subtilis can reduce the corrosion rate of FH36 steel in seawater.The uniform and dense biofilm composed of fine and uniform triangular cone shaped particles after 14 d immersion with B.subtilis,which was mainly composed of calcium magnesium carbonate,reduced the corrosion rate of FH36 steel in seawater.The corrosion resistance of the MMC sample soaked at37℃,25℃and 4℃for 7 days was 12.8,25.3 and 21.6 times of the original FH36steel by R_ct,respectively.The hardness of the MMC was higher,and the COF,and the wear volume of MMC were lower than original FH36 steel at-20℃,indicated that the MMC was also helpful to improve the wear resistance of the sample.（5）To make further efforts to the surface protection requirements of heavy icebreaker,the Co-WC coatings were prepared on the substrate of FH36 steel with three WC contents by plasma transferred arc welding.The results showed that the grain size of the Co-WC coating decreased as WC content increase.Due to the formation of WC,W₂C and other carbide hard phases,the hardness of the Co-30WC and Co-60WC coating was about twice of the substrate of FH36 steel,and the wear resistance of Co-60WC coating was improved 13 times than substrate through wear volume.The wear mechanism of Co-WC coating was primarily abrasive wear,while some adhesive wear and oxidation wear still occur in Co-0WC coating.Co-WC coating was able to produce a passive film in seawater,which improved the corrosion resistance of FH36 steel.However,the increase of WC content led to the increase of defects such as holes and cracks in the coating sample,which have negative effect on the corrosion resistance.Co-30WC coating showed the optimum corrosion resistance in three Co-WC coatings.