Sulfate-reducing bacteria (SRB) are a group of diverse anaerobic microorganisms omnipresently in natural habitats and engineered environments, they use sulfur compounds as the electron acceptor for energy metabolism. SRB corrosion is one of major cause for corrosion damages and facility failures, making it an important research topic. Due to the complexity of microbiological activities and that there is a lack of deep understanding of the interaction between biofilms and metal surfaces in the present, therefore, it is still hard to predict and interpret the occurrence and the relevant mechanism of the SRB corrosion. In this review, the ecological characteristics and anaerobic respiration of SRB are introduced, focusing on the SRB corrosion mechanism, including cathodic depolarization, metabolite corrosion, concentration battery action, and extracellular electron transfer theories. Finally, the methods and tools of MIC research are briefly introduced.

Recent years, the corrosion of Al-alloy turned to be a serious issue, which directly influence the safe operation of engineering facilities made of Al-alloy. Based on the recent research results related to the mildew induced corrosion of Al-alloy, the representative species of mildew as well as the main factors close to the mildew activity were illustrated. Meanwhile, the relevant corrosion mechanisms were emphatically discussed, including acid corrosion, oxygen concentration cell, the possible direct electron transfer as well as the direct interfacial interaction between Al-alloy and mildew. Mildew can produce large amount of organic acids through metabolism, causing the decline of the pH values in test solution and biofilm, then leading to localized corrosion. As analyzed, the potential direct electron transfer and interfacial interaction between Al-alloy and mildew can be one of important causes, leading to the localized corrosion. Furthermore, the common control methods for mildew induced corrosion of Al-alloy were also reviewed. Finally, the future research focus of Al-alloy corrosion in the presence of mildew was also prospected.

The recent research progress of cold spraying technology for Mg-alloys at home and abroad is summarized in terms of the cold spraying coatings of Al-Zn, pure Al, Al-based composite coating, Al-based amorphous coating, Zn-based alloy, Ni and stainless steel etc. Meanwhile, the process parameters (such as pressure, temperature) and the selection of spraying materials, the influence of the coating on the corrosion resistance of Mg-alloys are discussed, and finally the future development trend of Mg-alloys in automotive- and aerospace-lightweight technology innovation and application is also put forward.

A ZrB₂-SiC-La₂O₃/SiC dual-layer coating was prepared on siliconized graphite by the combination of slurry method and pack cementation. The mechanical properties of the coated siliconized graphite before and after thermal shock were investigated and compared with the bare siliconized graphite. The mass loss was observed and its value of per unit area of the bare siliconized graphite was 52.1 mg/cm², and the flexural strength retention was only 52.0% after thermal shock test from 1500 ℃ to room temperature for 10 cycles, while they were 5.6 mg/cm² and 78.5% for the coated ones, respectively. The high strength retention of the coated siliconized graphite after thermal shock could be attributed to the formation of a protective oxide scale on its surface, which protected the graphite substrate from oxidation and avoided the formation of defects in the interior regions of the coated siliconized graphite.

The effect of Bacillus flexus on degradation and corrosion behavior of polyurethane varnish coating in marine environment was investigated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The results showed that the Bacillus flexus did not exhibit significant effect on the degradation of the polyurethane varnish coating after immersion in Bacillus flexus containing seawater for 1 h. With the extension of immersion time, the corrosion resistance of the coating was significantly lower in Bacillus flexus containing seawater than that in sterile seawater, indicating that Bacillus flexus could cause the degradation of the coating. The coating resistance was about 10⁸ Ω·cm² at the initial stage of immersion both in sterile seawater and Bacillus flexus containing seawater. However, the corrosion resistance of the coating dropped to 5.22×10⁶ and 5.46×10⁶ Ω·cm² after immersion in sterile seawater for 13 and 35 d respectively. As comparison, the corrosion resistance of the coating decreased to 2.16×10⁶ and 7.96×10⁵ Ω·cm² after immersion in Bacillus flexus containing seawater for 13 and 35 d, respectively. The facts showed that the corrosion resistance of the coating in Bacillus flexus containing seawater decreased larger than that in sterile seawater. SEM observation result showed that after immersion for 35 d in Bacillus flexus inoculated seawater, numerous pores and pulverization signs could be observed of the coating surface. From the result of FTIR, the absorption peaks of N—H bond and C—O bond of coatings after immersion in Bacillus flexus containing seawater were significantly lower than that in sterile seawater, indicating that Bacillus flexus can clearly degrade the polyurethane varnish coating.

Materials served in marine environments are not only subjected to the Cl^- and ocean currents, but also suffered from biological fouling. The effect of sulfate reduction bacteria (SRB) metabolism on the stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in 3.5% (mass fraction) NaCl solution were investigated by means of potentiondynamic polarization technology, slow strain rate test (SSRT) and scanning electron microscopy (SEM). The results showed that compared with the sterile solution, the presence of SRB promoted the anodic dissolution process of 2205 DSS in the bacterial solution and induced pitting, which facilitated cracks initiation. The SCC sensitivity of 2205 DSS was positively correlated with the concentration of SRB activity. The SRB activity concentration was the highest during their stable growth period, and the resulted metabolite H₂S could induce the increase of brittleness of the steel. In this case, SCC sensitivity of 2205DSS was the highest. The SCC mechanism of 2205DSS in 3.5%NaCl solution containing SRB was the mutual control of anodic dissolution and hydrogen damage. In the presence of SRB, the ferritic phase in 2205DSS exhibited transgranular cleavage, while the austenitic phase exhibited ductile tearing, and the ferritic phase had higher SCC sensitivity.

The effect of pH value on the electrochemical corrosion behavior of 14Cr12Ni3WMoV steel, which is often used for steam turbine final blade, in acidic chlorine-containing solutions was investigated by open circuit potential measurement, potentiodynamic polarization, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results show that with the decrease of pH value of acidic chlorine-containing solutions, the impedance of the steel surface decreases, while the corrosion rate and pitting sensitivity increase. When pitting occurs, the vertical growth of pits on the steel surface slowed down, while their lateral growth accelerated without obvious uniform corrosion. The passivation film of 14Cr12Ni3WMoV stainless steel is mainly composed of oxides and hydroxides of Fe and Cr in the solution of pH5, while the passivation film is mainly composed of hydroxides and oxides of Cr and corresponding compounds of high valence state Mo⁶⁺ in the solution of pH2. With the decrease in pH value of the acidic chlorine-containing solution, the dissolution of Fe in the passivation film on the steel is accelerated significantly, so that the stability of the passivation film decreases, thereby the corrosion resistance of the stainless steel decreases.

The effect of the thiourea base imidazoline quaternary ammonium salt corrosion inhibitor on the corrosion performance of X80 pipeline steel in three simulated oil-field waters with different pH was assessed by means of polarization curve measurement, electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET) as well as characterization of corrosion morphology and corrosion products. Polarization curve measurement showed that the corrosion current density was the lowest in the water of pH7.2, followed by pH10.5, while the corrosion current density was the highest in the water of pH3.5，and as the temperature increased, the corrosion current density also increased. The EIS results showed that the diameter of capacitive reactance arc was the largest in the water of pH7.2, accordingly, the R_ct in the fitting result was significantly higher than those in the other two waters. The SVET analysis revealed that the adsorption film formation on the surface of pipeline steel in the water of pH7.2 was better than in the other two waters, while the ion current density decreased with time, indicating that the corrosion inhibitor molecule is more suitable for the case in the water of pH7.2. The film formation by adsorption reduces the ion current density, thereby effectively reduces the corrosion reaction rate. In conclusion, the corrosion inhibitor is much suitable for use in neutral waters and it has good corrosion inhibition effect in the temperature range of 40~60 ℃.

To improve the corrosion resistance of Ti-alloy applied to human implants, Nb coating was deposited on the surface of Ti-6Al-4V (TC4) alloy by double cathode plasma sputtering technique. The composition and the cross-sectional morphology of the coating were characterized by XRD, XPS and SEM. The electrochemical properties of the coating and the semiconductor properties of the passivation film for the coating and the substrate were studied in Ringer's simulated body solution at 37 ℃ by electrochemical workstation. The results show that the Nb coating exhibits a preferred orientation on the (200) crystal plane, and the coating thickness is about 18 μm without defects such as holes and cracks. The passive film on the Nb coating is mainly Nb₂O₅. Comparing with the TC4 substrate, the coating has higher open circuit potential (E_OCP), corrosion potential (E_corr) and lower corrosion current density (I_corr). Both of the TC4 alloys with and without Nb coating exhibit a single capacitance loop, however, the coated alloy exhibits higher impedance and lower effective capacitance rather than the substrate. The passivation films of two samples exhibit n-type semiconductor characteristics. By different formation potential (E_f), the flat band potential (E_fb), the donor density (N_d) and diffusion coefficient (D₀) of the passivation film on the coating are always lower than that on the bare TC4 alloy.

The effect of Mg on purification of molten salts MgCl₂-NaCl-KCl (MNKC) and corrosion behavior of 316H stainless steel in MNKC molten salts were investigated by means of immersion test and electrochemical means. Results indicated that the corrosion of 316H stainless steel in MNKC molten salts at 600 ℃ was well inhibited when the addition amount of Mg exceeds 500 μg·g^-1, correspondingly, 316H stainless steel presents the free corrosion potential of E_corr＜-0.80 V vs NiCl₂/Ni, corrosion current density of I_corr＜25 μA/cm², and linear polarization resistance of R_p＞800 Ω·cm². The purification and corrosion inhibition mechanism of Mg for the MNKC molten salts were discussed simultaneously.

The underwater friction stud welding (FSW) joint was obtained with X65 steel as substrate and 16Mn steel as stud. The electrochemical corrosion performance of the FSW joint and X65 steel in NaCl solution saturated with CO₂ were studied by means of electrochemical methods, macroscopic- and microscopic-metallography, scanning electron microscope (SEM) with energy spectrum analyzer (EDS) and X-ray diffractometer (XRD). The results show that the open circuit potential of FSW joint is more positive, the impedance is larger and the corrosion current density is smaller, which indicates that the corrosion resistance of FSW joint is better than that of X65 pipeline steel. According to the microscopic metallography observation for every zone of the FSW joint, it is found that the corrosion in central welded zone is lighter, while the corrosion in upper heat affected zone, lower heat affected zone and base metal zone is more serious. The EDS and XRD results showed that the corrosion product of FSW joints is Fe₂O₃.

A Ni-plate was firstly electrodeposited on X65 pipeline steel surface, which then was chemically etched by salt solution, and finally subjected to modification treatment with perfluorohexylethyltrichlorosilane, so that the surface of Ni-plate was endowed with super-hydrophobic and oil-phobic performance. The corrosion behavior of the modified Ni-plate was studied by potentiodynamic scanning. The result showed that after being etched for 6 h at 80 ℃ in the solution containing cobalt and nickel ions, and modified with perfluorooctyltrichlorosilane for 2 h, the modified Ni-plate surface presented contact angles of 160° and 152° for deionized water and glycol respectively. Compared with the original Ni-plate surface, the modified ones presented significantly lower corrosion rate, namely, better corrosion resistance.

A Cr-free Zr-based conversion coating (ZrCC) was prepared on 2A12 Al-alloy, which then was characterized in terms of the surface micromorphology evolution and composition of 2A12 Al-alloy during pretreatment and conversion process, especially, the influence of the second phase of the alloy on the conversion process. The results show that first, the pretreatment process resulted in a very rough surface of the 2A12 Al-alloy, on which there exist many pits and residual second phases; next, the second phase particulates in 2A12 Al-alloy were unfavorable to the uniform nucleation and growth of conversion coating granulates during conversion process; and third, after conversion process, the second phase particulates on the 2A12 Al-alloy surface were seriously destroyed, that brough strong impact on the compactness and uniformity of the coating.

The corrosion behavior of Q235 steel/conducting concrete in saline-alkali soil, yellow-brown soil, and red soil respectively was studied by means of potentiostatic scanning and electrochemical impedance spectroscopy (EIS) techniques, so that to reveal the influence of soil environmental factors on the corrosion process. Based on the grey correlation theory, the influence weight of each ion in soils on the corrosion process of Q235 steel in conductive concrete was calculated. The results show that after 45 d of accelerated corrosion, holes and fine cracks appeared on the surface of Q235 steel/conducting concrete. The corrosion rate of Q235 steel/conducting concrete in three typical soil environments may be ranked from small to large according to soil type: saline-alkali soil, yellow-brown soil, and red soil. The calculation results of the grey correlation degree show that when the Q235 steel/conducting concrete is corroded in the soil, the weighting of soil environmental factors may be ranked as follows: pH＞[SO₄^2-]＞[Ca²⁺]＞[Cl^-]＞[HCO₃^-]＞[Mg²⁺]＞[Fe³⁺]. As the pH of the soil environment decreases, the degradation degree of conductive concrete increases, while the corrosion rate increases. H⁺ and SO₄^2- in the soil will directly react with conductive concrete components, resulting in concrete degradation, which has the greatest impact weight. Ca²⁺ can migrate inward to the conductive concrete pore fluid, therewith leading the precipitation of relevant oxides or carbonates there, which act as physical protective means, hence the impact weight of Ca²⁺ is slightly lower. The influence of Cl^- on the corrosion of Q235 steel is inhibited by the insulation effect of the concrete layer and the electric double layer, so the influence weight is also lower.

Though Mg-alloy can be used as a new generation of implant materials with good bio-compatibility and biodegradability, its corrosion rate should be reduced to an acceptable level for the application. For that, at the present, the most commonly measure is to apply a micro-arc oxidation coating on the alloy. However, the porosity of the micro-arc oxidation coating is too large, thereby corrosive species can easy migrate inward through the micro-pores, thus degrade its protective effect. Recently, some scholars have found that pretreatment of the metal matrix can increase the density of the micro-arc oxidation coating. So in this study, Mg-alloy was pretreated by ultrasonic rolling before the micro-arc oxidation. Then the prepared micro-arc oxide ceramic coatings on Mg-alloy with or without ultrasonic rolling treatment were comparatively assessed by means of OM, SEM, EDS, XRD, electrochemical workstation (simulated humoral PBS), to reveal the effect of ultrasonic rolling treatment on the properties of micro-arc oxide ceramic coating. Results show that: after ultrasonic rolling treatment, the treated Mg-alloy matrix presents lower surface roughness, finer grains and higher hardness. In comparison with the Micro-arc oxidation ceramic coating on the un-treated Mg-alloy, the element content of Si, P and Ca is increased in the coating on the pre-treated ones, correspondingly the surface was denser and smoother, and the number of macropores significantly decreased, namely, the surface porosity reduced from 31.7% to 19.1%. From electrochemical tests we can see that, the free corrosion potential was 107 mV higher, the corrosion current density was an order of magnitude lower, and the impedance is much higher for the micro-arc oxidation ceramic coating on Mg-alloy pre-treated by ultrasonic rolling. In conclusion, the pre-ultrasonic rolling treatment could effectively improve the corrosion resistance in PBS solution of the micro-arc oxidation ceramic coatings on Mg-alloy.

The operation efficiency of proton exchange membrane fuel cell (PEMFC) is primarily affected by the electrical conductivity, corrosion resistance and hydrophobicity of the bipolar plate. Zr coating (Zr-TA1) is prepared by double glow plasma method on TA1 commercial pure Ti-bipolar plate. Then the microstructure and properties of the Zr coated TA1 Ti-plate are studied. It follows that, the thickness of Zr coating is about 3 μm and the coating surface is smooth and compact. Its potentiodynamic/potentiostatic polarization curves are measured in the cathodic and anodic environments of PEMFC, which shows that the corrosion potential of Zr-TA1 is increased significantly and the corrosion current density is decreased by 1~2 orders of magnitude in comparison with the bare TA1 Ti-plate. Furthermore, the hydrophobicity of TA1 is improved by Zr coating and the water contact angle is increased from 71^o (TA1) to 94^o (Zr-TA1). In addition, the contact resistance of Zr-TA1 (under the compaction pressure of 140 N·cm^-2) is decreased from 117.3 mΩ·cm² to 15.5 mΩ·cm², namely, the surface conductivity of the Ti-plate was significantly improved.

Starch grafted copolymer (St-g-PAM) was prepared by grafting acrylamide monomer onto starch with ammonium persulfate sodium bisulfite as initiator, which can be used as a new "green" inhibitor. Then the inhibition effect of St-g-PAM on Zn in 1.0 mol/L HCl solution was studied by mass loss method and electrochemical techniques. The results show that St-g-PAM is a mixed inhibitor with good inhibition effect for Zn corrosion in HCl solution, while the inhibition efficiency increases with the increase of St-g-PAM concentration. However, the inhibition efficiency increases slowly when the St-g-PAM concentration exceeds 50 mg/L. At 20~50 ℃, the adsorption process of St-g-PAM on Zn sheet is consistent with Langmuir adsorption model. According to the results of potentiodynamic polarization and EIS measurements, the inhibition ability of St-g-PAM for Zn corrosion in HCl solution in the presence of St-g-PAM can be expressed in both of the decreased corrosion current density and increased charge transfer resistance values.