Corrosion Performance Investigations Of Thin-Wall High-Strength Tube Pile Steels

With the economic development of coastal areas and the rapid expansion of bridge-building projects, the research and application of high strength cube pile steel attract great attention. Since the most severe damage to steel tube pile in its application is corrosion, the research and development of inexpensive anti-corrosion high-strength tube pile steel seem to be quite necessary, showing great importance for the building structures, such as high-rise buildings and bridges. In the present work, the effects of alloying elements Cr/Cu and heat treatments on microstructure and properties of tube pile steel and its corrosion behaviors in high salinity service environment were studied by many analysis and testing methods, including optical microscope(OM), X-ray diffractometer(XRD), scanning electron microscope(SEM) with energy dispersive spectrometer(EDS), tensile testing, polarization and electrochemical impedance spectroscopy measurement, which could provide theoretic guidance for the research and application of inexpensive thin-wall high-strength tube pile steels.The results show that, low alloy tube pile steel dipping into saturated salt aqueous environment was mainly subject to electrochemical corrosion, and the air-liquid interface zone of steel showed the most serious corrosion. With the addition of 0.2% Cu and 0.2% Cu+0.2% Cr in hot-rolled carbon steel respectively, pits of the air-liquid interface zone were fine and densely distributed, and the maximum pit depth reduced gradually from 0.44 mm of carbon steel to 0.11 mm of 0.2Cu-0.2Cr steel, with the corrosion type transforming from pitting corrosion to uniform corrosion gradually. The overall corrosion rate of 0.2Cu steel was 0.0388mm/y, 71% lower than that of the carbon steel. The best result was obtained in 0.2Cu-0.2Cr steel, with the lowest rate of 0.0346 mm/y and 74% lower than that of carbon steel. The polarization impedance increased form 482.6 ??cm2 to 854.5??cm2、1136 ??cm2 respectively after added 0.2%Cu、0.2%Cu+0.2%Cr in carbon steel. Rust layer containing Cu/Cr inhibited the anodic process of electrochemical reaction and the enrichment of Cu/ Cr was mainly in the interface zone between rust layer and substrate. The slow corrosion regions in substrate were always accompanied with high content of Cu and Cr while the fast corrosion regions with less amount of the two elements.With the Cr addition ranging from 0.5 to 2.0 wt.% in TMCP(thermo-mechanical controlled processing) steel, its microstructure changed from polygonal ferrite and little pearlite of 0.5Cr steel to 2.0Cr steel consisted of granular bainite and ferrite as dominant phases with a small amount of M-A(martensite-austenite) islands, and improved the yield strength from 304 Mpa to 503 MPa.With increasing the chromium content, the electrical potential of matrix increased from-0.646 V to-0.544 V while the polarization impedance Rp decreased from 1586?·cm2 to 795.5?·cm2. Meanwhile, the corrosion rate slightly increased from 0.0292mm/y of 0.5Cr steel to 0.0304mm/y of 2.0 Cr steel, which means the increasing Cr content did not lead to higher corrosion resistance of steel during 5 months’ half-immersion corrosion due to the generation of perishable bainite and M-A islands. Enrichment of Cr in rust layer was observed at the interface zone between rust layer and substrate, which was more obvious and made the rust layer more compact with the higher Cr addition in substrate.With the Cr addition ranging from 0.5 to 2.0 % in quenched and tempered steel, the microstructure of each sample consisted of tempered sorbite. The ferrite in 0.5Cr steel was polygonal for its full recrystallization while many ferrites in 1.1Cr steel and 2.0Cr steel remained lath-shaped for the recrystallization delay in temper process by more Cr element. The yield strength increased from 391 Mpa of 0.5Cr steel to 510 MPa of 2.0Cr steel. Specimens remain active state in 3.5%Na Cl solutuion, and their self-corrosion potential were all about-0.57 V while their current density decreased from 1.657×10-5 A·cm-2 to1.391×10-5 A·cm-2 and the polarization impedance Rp increased from 1014 ?·cm2 to 1611 ?·cm2. Relative to hot-rolled or TMCP steel, the corrosion degree of quenched and tempered steel at air-liquid interface zone reduced significantly. The specimen with 2% Cr had the best corrosion resistance, and the corrosion rate was 0.0234mm/y, which was 8.6% decreased than that of the specimen with 0.5% Cr. Cr element enriched in the rust layer in different way at immersion zone and air-liquid interface zone. Cr enriched in the inner area of rust layer at immersion zone, while enriched in the outer area of rust layer at air-liquid interface zone.The rust layer was mainly composed of α-Fe OOH, β-Fe OOH, γ-Fe OOH and Fe3O4, and a small amount of alloy elements had no apparent influence on the phase composition of rust layer. Relative to air-liquid interface zone, the corrosion rates of various steels at liquid-solid interface zone were all tiny, and less than 0.004 mm/y. As a result, the differences of corrosion performance among various steels caused by composition and heat treatment could not be reflected. Corrosion rate was controlled by environment factors, especially oxygen content. Meanwhile, the rust layer generated on steel which immersed in soil was compact than that in aqueous solution. Polarization resistance Rp of tube pile steel in saturated water soil increased with time, which means the rust layer on steel surface expanded continuously, and enhanced the protection to substrate.