Dissimilatory iron reduction is an important form of iron cycle on the earth, and the iron-reducing bacteria (IRB) is one of the main participants. Based on the influence of IRB on corrosion of iron and steel materials in the ocean, this paper reviewed and analyzed the related reports at home and abroad, and put forward some suggestions. Firstly, this paper expounded the diversity of IRB species and metabolic modes, introduced the modes of extracellular electron transfer in the metabolic process. And then, the current research status for theories of the IRB influenced corrosion of metal in seawater was summarized, and the reasons for the differences in the conclusions of different researchers were discussed. Finally, the common research methods of the IRB influenced corrosion were introduced, and suggestions for future research were put forward as well.
The phase constituent and microstructure morphology of a quenching-partitioning-tempering (Q-P-T) steel after heat treatment were studied by XRD, SEM and TEM, while taking a quenching and tempering (Q&T) steel and a carbide-free quenching and partitioning (Q&P) steel of the same composition as references. The corrosion behavior of Q-P-T steel in simulated seawater namely 3.5% (mass fraction) NaCl solution, was examined via weight loss method and electrochemical means. The results showed that the retained austenite in Q&T steel was mainly distributed between martensite laths as thin-films with a volume fraction of less than 3% (volume fraction), while the retained austenite in Q&P steel and Q-P-T steel was distributed as flakes with an average volume fraction of about 15.35% and 14.6%, respectively. In addition, the Q-P-T steel had better corrosion resistance than the carbide-free Q&P steel, whereas the Q-P-T and Q&P steels had slightly better corrosion resistance than Q&T steel. The evolution of corrosion products on the four steels was basically consistent, that is, the initial corrosion products composed mainly of β-FeOOH and γ-FeOOH. With the prolong of the immersion time, α-FeOOH and Fe3O4 were gradually formed. Corrosion pits of various sizes were distributed on the surface of the four steels, which then gradually turns to be uniform corrosion with the prolong of immersion time.
The hydrogen embrittlement of a 1000 MPa grade high strength steel in simulated 800 m deep-sea environment and shallow sea environment by open circuit potential or applied various cathodic potentials was comparatively studied by means of slow strain rate tensile tests, potentiodynamic polarization measurements and SEM. Results showed that for the high strength steel, the open circuit potential was approximately -708 mV (vs Ag/AgCl/seawater) and the hydrogen evolution potentials was about -1000 mV (vs Ag/AgCl/seawater) in the simulated 800 m deep-sea environment. While the open circuit potential was approximately -645 mV (vs Ag/AgCl/seawater) and the hydrogen evolution potentials was about -910 mV (vs Ag/AgCl/seawater) in the shallow sea environment. With the cathodic polarization potential dropped from -800 mV to -1000 mV, the toughness of the tested high-strength steel was reduced, i.e. the brittleness was increased, implying that the susceptibility to hydrogen embitterment was enhanced. When the polarization potential was higher than -900 mV, the hydrogen embrittlement coefficient of high-strength steel was less than 25%, namely, the steel is still in the safe range. When the polarization potential reached further to -1000 mV, the hydrogen embrittlement coefficient increased to about 50%, which was in the brittle fracture range for the steel.
The long-term corrosion behavior of 690 MPa high-strength bridge steel (referred as Q690 steel) in a simulated rural atmosphere was investigated via wet/dry cyclic test with wetting in distilled water and drying in air, as well as electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), electron probe (EPMA) and other surface testing techniques. The results indicate that the corrosion process of Q690 steel can be differentiated as two stages during the whole corrosion process, namely the accelerated and the decelerated stages. In the early stage of corrosion, the corrosion resistance of Q690 steel with microstructure of lath bainite (LB) is better than that of Corten-A steel with microstructure of ferrite (F) and pearlite (P). In the later stage of corrosion, the enrichment of Cr element and the increase of α-FeOOH in the rust scale of Q690 steel have enhanced the protectiveness of the rust scale, leading to the decrease of the corrosion rate of Q690 steel, hence which shows significantly better corrosion resistance than the Corten-A steel.
The pitting behavior of X100 pipeline steel in an artificial solution, as a simulation of acidic soil environment was studied under fluctuated cathodic protection potentials with varying fluctuation parameters, such as potential fluctuation frequency (f), fluctuation amplitude (E), duty cycle (δ) and total loading time (tt). While the fluctuated cathodic protection potentials were simulated by means of square wave polarization (SWP) technology. The results show that the influence intensity of the potential fluctuation parameters can be ranked as follows: tt＞δ＞f＞E. However, among others, when the test lasted for 3 d with the following potential fluctuation parameters, namely f=0.5 Hz, E=-0.95~-0.7 V and δ=50%, the X100 pipeline steel presented the worst resistance to localized corrosion. Furthermore, the pitting density would rise with the increase of f, E and tt, and reaches the maximum when δ is 50%.
Al-Mg alloys are considered as the ideal electromagnetic shielding materials and widely applied on electronic and communication servicing in marine environment due to their excellent comprehensive properties, such as high specific strength, low electric resistivity and excellent corrosion resistance. The microstructure and corrosion behavior of Al-3.0%Mg-0.12%RE alloy wires subjected to different cold drawing deformation was investigated by means of optical microscope (OM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and electrochemical measurement. The influence of cold-drawing strain and microstructure on corrosion resistance of the alloy was also examined. The results indicated that the equiaxed grains of the alloys were elongated and the skeletal second phase particles were fragmented during the cold-drawing process. The corrosion behavior of the Al-3.0%Mg-0.12%RE alloy showed typical characteristics of pitting corrosion at the position of second phases. With the increase of strain, the uniform corrosion sensibility increased, pitting corrosion sensibility declined and the corrosion resistance of the alloy improved.
Ni-Co-B coating is prepared on Cu-sheet using electro-deposition method. The effect of current density on the microstructure, hardness, wear resistance and corrosion resistance of the coating is examined by means of SEM, EDS, ICP-MS, microhardness tester, friction and wear tester and electrochemical workstation. The results demonstrate that, as the current density gradually increased from 1 A/dm2 to 7 A/dm2, the prepared coatings consist of a single face-centered cubic phase with Ni (111) preferred orientation, while the grain size decreases first and then increases. Furthermore, when coatings were made with the increasing current density within the range 1~7 A/dm2, they present the following features: the content of Co and B gradually decreased; the thickness increased from 17.6 μm to 50.1 μm; the hardness increased from 780 HV100 g to 852 HV100 g; the free corrosion potential shifts positively in 3.5%NaCl solution, and among others, the one prepared with 5 A/dm2, presents the smallest corrosion current density. By dry wear test in air, the friction coefficient and wear mass loss decreases first and then increases, whilst with the increasing applied load, their friction coefficient decreases but the wear mass loss increases, and the wear mechanism is mainly abrasive wear and fatigue wear. Accordingly, in 3.5%NaCl solution, their friction coefficient and wear mass loss decreases first and then increases and the increase of applied load results in that the friction coefficient increases first and then decreases but the wear mass loss increases, and the wear mechanism is mainly abrasive wear. The research shows that the increased current density is helpful to improve the microstructure, as well as enhance the hardness, abrasion resistance and corrosion resistance of the Ni-Co-B coating, which provides a reference for the development of alternative chromium plating technology.
The corrosion behavior of bridge steels Q345qENH and Q420qENH in deicing salt was studied by means of wet-dry cyclic corrosion test, immersion corrosion test and rust scale characterization, while taking Q345qE steel as comparison. The results indicate that the corrosion resistance of the Q345qENH and Q420qENH weathering bridge steels was better than that of the Q345qE steel. The dry-wet cyclic corrosion test results reveal that the Q345qENH, Q420qENH and Q345qE steels show corrosion rates of 26.88, 27.5, and 33.75 times higher than those measured in immersion corrosion test, respectively. During dry-wet cyclic corrosion test, the structure and composition of the formed rust scales changed and the amount of α-FeOOH phase increased gradually with time, which may be an important reason leading to the decrease of corrosion rate at the end of dry-wet cyclic test.
7050 Al-alloy is often used for medium and heavy plate extrusions as aircraft components. Based on large-scale simulation software COMSOL Multiphysics, a corrosion simulation model for the galvanic couple of 7050 Al-alloy and AerMet100 steel in atmospheric environment is established. The effect of salt deposits on the surface of galvanic couple, the relative humidity of atmospheric environment, and the area ratio of the anode to the cathode on the corrosion behavior was investigated respectively. The results show that the corrosion rate is the fastest when the relative humidity of the atmosphere is 0.91. When the surface salt deposits exceeds 5.7 g/m2, severe corrosion will occur. Changing the ratio of cathode to anode will not cause the electrode polarity reversal. The corrosion rate of 7050 Al-alloy are positively correlated with the salt deposits and the area ratio of cathode to anode.
The EH36 ship plate steel was exposed for 15, 30, 90, 180 and 360 d respectively in the atmosphere with high humidity, high heat, high salinity and strong radiation at the Zhanjiang marine atmospheric corrosion test station situated in the south of China. The corrosion behavior of different exposure periods were characterized by means of corrosion weight loss method, polarization curve measurement, scanning electron microscopy with energy dispersive spectroscopy and X-ray diffractometer. The results show that the corrosion rate of EH36 ship plate steel increases first and then decreases. After exposure for 360 d, Cr, Ni and Si elements diffuse into and uniformly distribute in the rust scale, which then improves the corrosion resistance of the steel. After exposure for 180 and 360 d, the formed rust scales composed of γ-FeOOH, β-FeOOH, Fe3O4 and α-FeOOH. While the one corresponded to 360 d exposure has more α-FeOOH and less β-FeOOH. In the rust scale of EH36 ship steel exposed for 360 d the ratio of α/γ=0.615, however, a stable protective rust scale has not yet appeared by this time.
The effect of second phases and alloying elements on stress corrosion cracking (SCC) behavior of oil well steel T95 in sour H2S environments was investigated by means of electrochemical polarization measurement, constant tensile test, optical microscope and transmission microscope. Results showed that solutions of pH=2.3 and pH=2.8 were SCC sensitive environment to T95. With the decreasing pH, cracking time of the prestressed rings decreased, namely, the SCC sensitivity of T95 increased. With the decrease of pH, the IH+ (input current from the environment) increased, while the cathode reaction was enhanced, as a consequence, the hydrogen induced cracking (HIC) occurred. Meanwhile, the aggregation of H+ on the crack tip might promote the crack expansion, accordingly, the anodic dissolution was enhanced. The crack propagation was controlled simultaneously by anodic dissolution (AD) and hydrogen embrittlement (HE) in the solutions of pH=2.3 and pH=2.8.
The deposition rate of Cl- is the important influence factor for the corrosion of high-altitude marine engineering equipment in marine atmosphere. For sampling the Cl- deposition rate at different heights, a so called “kite hanging wet-candles”method was invented, while the Cl- concentrations of the collected samples were analyzed by ion chromatography. The effect of environmental factors on the Cl- deposition rate was analyzed by the Pearson correlation coefficient method. At the Hainan island of the South China Sea, the distribution of the Cl- deposition rates exhibits a reverse “S”-shape in the range of 10 m to 100 m vertical scope in the summer time, however, the Cl- deposition rates were gradually reduced in the period from June to August.
