After decades of development, mechanical properties of pipeline steels have a good combination of strength and toughness. But after welding, in the heat affected zone (HAZ), microstructure of the base plate was erased by the welding thermal cycle. Several subzones with different microstructures were formed in the HAZ due to different thermal histories they went through. Toughness of the HAZ varies due to the heterogeneous microstructure. In this work, toughness of the HAZ of X100 pipeline steel was examined with two notch locations. Low toughness of 51 J was obtained when the notch encountered intercritically reheated coarsen-grained (ICCG) HAZ and high toughness of 183 J when the notch did not contain ICCGHAZ. Meanwhile, different sub-zones in the HAZ were simulated using Gleeble thermal simulation machine. Simulated coarsen-grained (CG) HAZ, fine-grained (FG) HAZ and intercritically reheated (IC) HAZ with uniform microstructure had good toughness of 244, 164 and 196 J, respectively. In contrast, toughness of simulated ICCGHAZ was only 32 J. Therefore, ICCGHAZ consisting of coarse granular/upper bainite and necklace-type martensite-austenite (M-A) constituent along grain boundaries was proved to be the primary reason for low toughness. Instrumented Charpy impact test results showed that ICCGHAZ could notably embrittle the sample and lower the crack initiation energy. Characterization on the fracture surfaces of the as-fractured Charpy impact specimens showed that ICCGHAZ was found to be the crack initiation site of the whole fracture, and M-A constituent in the ICCGHAZ was characterized as cleavage facet initiation. Fracture mechanisms in the CGHAZ and ICCGHAZ were separately investigated using EBSD. The results showed that necklace-type M-A constituent in the ICCGHAZ notably increased the frequency of cleavage microcracks nucleation. Fracture mechanism changed from nucleation controlled in the CGHAZ to propagation controlled in the ICCGHAZ due to the existence of necklace-type M-A constituent. Therefore, the formation of necklace-type M-A constituent in the ICCGHAZ could not only cause notable drop of toughness in the HAZ, but also change the fracture behavior/mechanism. Hence, research on how to control the distribution status of M-A constituent in the ICCGHAZ is the key to improve the toughness of a weld joint.

Recently, low-cost advanced high strength steels (AHSS) with high toughness and fatigue limit have been developed in order to ensure the safety and lightweight of the engineering components. As promising candidates for next generation of AHSS, the bainite/martensite multiphase high strength steels exhibit excellent combination of strength and toughness due to the refined multiphase microstructure and retained austenite films located between bainitic ferrite laths. The previous works showed that the mechanical properties of bainite/martensite multiphase steels can be further improved through quenching-partitioning-tempering (Q-P-T) process. In the present work, the effect of Q-P-T process on the microstructure and fatigue behaviors of steels was investigated, and the relationship between the microstructure and the fatigue crack propagation was discussed in detail. Here, a 20Mn2SiCrNiMo bainite/martensite multiphase steel was treated by Q-P-T processes: (1) quenching to 200 ℃, partitioning at 280 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT200 sample); (2) quenching to 320 ℃, partitioning at 360 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT320 sample). Microstructure observations showed that the QPT200 sample consisted of leaf-shaped bainite, martensite and filmy retained austenite (RA), while some blocky martensite/austenite (M/A) islands were observed in QPT320 sample. The volume fractions of retained austenite in QPT200 and QPT320 samples are 4.5% and 9.8%, respectively. The fatigue crack propagation rate da/dN and threshold value of fatigue cracking ΔK_th were measured using compact-tensile specimens. The results showed that the Q-P-T process parameters had a significant influence on the microstructures and fatigue properties of the bainite/martensite multiphase steels. The bainite/martensite multiphase steel after appropriate Q-P-T treatment (QPT 200 sample in the present work) has higher ΔK_th and lower da/dN, which originates from the resistance on fatigue crack propagation due to the presence of leaf-shaped bainite and nanometer-sized retained austenite films. Furthermore, although the volume fraction of retained austenite in QPT320 sample is higher than that in QPT200 sample, the ΔK_th of QPT 320 sample is lower than that of QPT200 sample. It is suggested that the effect of retained austenite on the fatigue behaviors depends on its volume fraction, size and morphology.

Among the wide variety of recently developed steels, high manganese transformation-induced plasticity (TRIP) steels with low stacking fault energy (SFE) are particularly promising. Outstanding mechanical properties combining a high ductility and a high strength are then obtained. Compared to the static deformation of high manganese TRIP steels, the behaviors of martensitic transformation and mechanical properties of such steels during dynamic deformation may be different. In this work, martensitic transformation of high manganese TRIP steel at different strain rates was characterized by the EBSD technique. The volume fractions of austenite (γ), hcp martensite (ε-M) and bcc martensite (α’-M) were calculated based on the XRD data. Meanwhile, variant selections of martensitic transformation in γ→ε-M and ε-M→α’-M transformation were investigated by theoretical calculation. It is shown that orientation dependence of TRIP effect during tension exists even at high strain rates and can be ascribed to the influence of mechanical work in differently oriented γ grains. The transformation of ε-M→α’-M was promoted, but the total amount of transformed martensite decreased, which means that TRIP effect was restricted at high strain rates. The α’-M variant selection is more obvious during static tension and became weaker during dynamic tensile deformation. α’-M variant selection can be predicted by the calculated mechanical works induced by the local stress in <111>_γ and <100>_γ grains during static tension. However, during dynamic tension, the mechanism of variant selection needs to be explained by analyzing the mechanical works induced by the local stress, the strain energy and the interfacial energy in these grains comprehensively. Compared to the occurrence of a single α’-M variant, a pair of α’-M variants having specific orientation relationship reduces the strain energy, then unfavored α’-M variants appear.

Oxide dispersion strengthened (ODS) steels are the leading candidate structural materials for fast reactor and fusion reactor application due to excellent radiation tolerance and high temperature creep strength. High number density nanoscale oxides play a key role in controlling microstructure and properties. Atomized alloy powders with different ball-milling times were employed to produce 9Cr-ODS steels by hot isostatic pressing (HIP). Nanosized precipitates in 9Cr-ODS steels with different ball-milling times were characterized by synchrotron small angle X-ray scattering (SAXS) together with high resolution transmission electron microscopy (HRTEM). Grain morphology and size were observed by electron backscatter diffraction (EBSD). The effects of nanosized precipitates on grain size and mechanical properties were analyzed. SAXS and TEM results indicated that the size of Y-Ti-O-rich nano-clusters in 9Cr-ODS steels decreases with the increasing milling time, while the distribution density increases. The maximum value of distribution density is about 2.93×10²³ m^-3 in 9Cr-ODS steel ball milled for 20 h. The maximum value of distribution density of pyrochlore structure Y₂Ti₂O₇ is the highest (1.03×10²² m^-3) in 9Cr-ODS steel ball milled for 8 h. Some large-scale Ti-Al-O-rich precipitates are observed and show core/shell structure. Their distribution density increases with ball milling time. With increasing ball milling time, the grain size decreases and the yield strength increases. The contribution of Y-Ti-O-rich nanosized precipitates to yield strength is dominated.

The main purpose of normalizing for traditional high temperature Hi-B silicon steel is to obtain enough inhibitors and ensure abnormal growth of Goss grains during final annealing treatment. While compared with high temperature Hi-B silicon steel, inhibitors in thin-gauge grain oriented silicon steel, which is prepared by low temperature method, are obtained mainly by nitriding other than by normalizing. In this work, two kinds of thin-gauge grain-oriented silicon steel specimens with and without normalizing were prepared. Effects of normalizing on microstructures and textures of thin-gauge grain-oriented silicon steels were investigated by EBSD and XRD techniques. The results showed that there were significant differences in the primary recrystallization textures between the specimens processed with or without normalizing, which were named as normalizing specimens and non-normalizing specimens respectively, and so did secondary recrystallization textures. It could be found that compared with the non-normalizing specimens, the intensities of {411}<148> and {111}<112> primary recrystallization textures are lower in normalizing specimens, while the intensity of Goss texture is higher. The secondary recrystallization texture of normalizing specimens, which had excellent magnetic properties, were characterized as sharp Goss texture, while Brass texture and deviated Goss texture secondary recrystallization textures were obtained in the non-normalizing specimens. Besides, higher proportion of 20°~45° high-angle boundary surrounding Goss grains were shown in the normalizing specimens. However, the average grain size of normalizing and non-normalizing specimens were almost identical (20 μm), and their grain size distribution was similar. For the thin-gauge grain-oriented silicon steel prepared by low temperature method, normalizing exerted crucial effects on magnetic properties by increasing the proportion of Goss oriented “seeds” prior to cold rolling and providing appropriate environment for Goss recrystallied grains.

Ti₂AlNb alloys are considered as a potential structural material for high temperature applications like gas turbine engine components due to their high specific strength and good creep resistance. In this work, pre-alloyed powder of Ti-22Al-24Nb-0.5Mo (atomic fraction, %) was prepared by an electrode induction melting gas atomization process and powder metallurgy (PM) alloys was made through a hot isostatic pressing (HIPing) route. PM Ti-22Al-24Nb-0.5Mo rings and plates were welded by electron beam welding (EBW). The microstructure of the welded joints was investigated by OM, SEM, EPMA and X-ray tomography. The effect of post-weld heat treatments (PWHT) on the microhardness, tensile properties and rupture lifetime at 650 ℃, 360 MPa of the welding joint of PM Ti-22Al-24Nb-0.5Mo plate was also studied. The results show that the HIPing temperature will affect the porosity distribution of PM Ti-22Al-24Nb-0.5Mo alloys. The PM Ti-22Al-24Nb-0.5Mo rings HIPed at 1030 ℃ after 980 ℃, 2 h, vacuum furnace cooling show good weldability. The fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of welded joints show homogeneous chemical composition. The microstructures of FZ, HAZ and BM are different while the microhardnesses of FZ, HAZ and BM show no obvious differences. Tensile and stress rupture lifetime testing specimens all fracture in the FZ. It is found that there are a certain number of micro-porosity in the FZ of the welded joints. However, the porosity reduces after PWHT, which will improve the high temperature ductility and rupture properties of the PM Ti-22Al-24Nb-0.5Mo welded joints.

Owing to the low density, high strength, good creep properties at elevated temperatures, TiAl alloy is considered for high temperature applications in aerospace industries. However, a major issue to the industrial applications is the alloy's intrinsic brittleness at room temperature. Therefore, extensive efforts have been made to overcome this defect by near net shape fabrication techniques. An alternative for fabricating TiAl alloy is the powder metallurgy processing of pre-alloyed powders, and by this technique TiAl alloy with fine and homogenous microstructure can be obtained. In this work, TiAl pre-alloyed powders with nominal composition Ti-47Al-2Cr-2Nb-0.2W-0.15B (atomic fraction, %) and Ti-45Al-8Nb-0.2Si-0.3B are produced by electrode induction melting gas atomization (EIGA). The pre-alloyed powders are consolidated by hot isostatic pressing (HIP). The effects of heat treatment on the microstructure of TiAl compacts and the influence of cooling rate on the solid-state transformations which occurs during continuous cooling of the TiAl compacts have been studied. It is found that the cooling rate of the pre-alloyed powders is between 10⁵~10⁶ K/s. As the cooling rate increases, the martensitic transformation, i.e., β→α' occurs in some fine pre-alloyed powders. The heating DSC curves indicate that the transformation from α₂ phase to γ phase takes place between 700~800 ℃. During the HIP processing, the pre-alloyed powders particles randomly accumulate, and the relative density of HIP compact is increased by the particles moving, rotating and rearranging at the initial stage. As the temperature increases, α₂ phase transforms into γ phase. With further temperature increasing, significant plastic deformation and the following formation of sintering necks occur in the powder particles. With the annealing time increasing, the pores between the particles are closed by means of surface diffusion, volume diffusion and diffusion creep. The microstructure of Ti-47Al-2Cr-2Nb-0.2W-0.15B powder compacts consists of fine γ and a small number of α₂ and β; and the microstructure of Ti-45Al-8Nb-0.2Si-0.3B powder compacts consists of fine γ phase, a small number of α₂ phase and dispersed ξ-Nb₅Si₃ phase. The area fractions of γ phase, α₂ phase and α₂/γ lamellar structures vary with the annealing temperatures, depending on the Gibbs free energies of the phases. The cooling rate has a significant effect on the continuous cooling transformation of both TiAl powder compacts. For Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy, the microstructure is composed of predominant equiaxed α₂ phase after water cooling, but of lamellar structures after air, oil or furnace cooling. For Ti-45Al-8Nb-0.2Si-0.3B alloy, the microstructure is composed of γ_m phase and large α₂ phase after water cooling; after oil and air cooling the alloy consists of a mix of feathery like structures, Widmanst?tten laths and lamellar structures; while furnace cooling leads to fully lamellar structures. Comparing the continuous cooling transformation curves, the increase of Nb can effectively extend the continuous cooling transformation to the diffusionless area.

As a cast nickel base superalloy, K4202 is mainly used in aircraft engines due to its high strengths at elevated temperatures, excellent resistance to hot corrosion and favorable weldability. K4202 alloy is usually fabricated by the conventional casting method and mechanical processing, along with macro-segregation and excessive tool wear. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) is able to manufacture high-performance and complex components. According to the requirement of selective laser melting manufactured metal parts with complex structures in aerospace and other fields, K4202 alloy was used as material for SLM in this research and the forming technology, microstructure and mechanical properties of SLMed and heat-treated samples were studied. The results show that the microstructure of samples formed by SLM is composed of dendrites and isometric crystal. The growing direction of dendrites is nearly perpendicular to melt pool traces in most cases. The dendrite structures disappear completely after solution+ageing heat treatment on account of recrystallization and metal carbide precipitates in grains and at grain boundaries. The precipitates are able to improve the strength of the grain boundary due to the pinning effect. The microstructure has no significant changes after ageing heat treatment, but carbide precipitates at grain boundaries as well. The microhardness of SLM samples is uniform on cross section and vertical section. After solution+ageing and ageing heat treatment, there is a significant improvement on the microhardness. The mechanical properties for as-fabricated samples are superior to those of the cast K4202. Besides, the yield strength and tensile strength increase clearly after heat treatments and the mechanical properties is the highest after ageing heat treatment. This is because of the precipitation of γ' strengthening phases. However, the obvious decrease in the ductility occurs at the same time.

The prediction of the macrosegregation in large ingot is a challenging issue due to the size of the ingots and the variety of the phenomena to be accounted for, such as thermal-solutal convection of the liquid, equiaxed grain motion, evolution of grain morphology by suitably considering a coupled grain growth model in the macroscopic solidification model, the columnar-to-equiaxed transition (CET), and shrinkage, etc.. Each of these phenomena is very important to the solidification pattern, while it is impossible for one model to consider all the phenomena together until now due to the computation power limited. Thus, the model capability and computational cost should be counterpoised for the simulation of large ingot. In this work, a mixed three-phase (simplified dendritic-equiaxed, columnar and liquid) solidification model is described based on Eulerian-Eulerian approach and volume average method. The model considers the thermosolutal buoyancy flow, the movement of equiaxed crystal, and the capture of the equiaxed crystals by growing columnar tree trunks. The mechanical interaction and impingement between columnar and equiaxed crystals are considered which give the capability to predict CET. In order to enhance the model capability without increasing the computational cost significantly, a simplified method is proposed to consider the dendritic of equiaxed crystal. This model is employed to simulate the formation process of macrosegregation for two different steel ingots (3.25 and 25 t). The general macrosegregation pattern predicted by this model includes the cone of negative segregation in the bottom of ingot, quasi-A-segregation in the columnar zone, and positive segregation in the top region, which are quite similar to the classic knowledge. The CET zones are also predicted. Although there is still some quantitative discrepancy, the macrosegregation distribution predicted by this model is quite similar to the experimental measurements. The non-globular equiaxed three-phase mixed model results are compared with the globular-equiaxed mixed three-phase model ones, which indicated that for large ingots the equiaxed dendritic structure plays an important role in liquid flow and it affects final characteristic of macrosegregation. It is predicted successfully that a negative segregation zone would be formed in the upper region due to the formation of a local mini-ingot and the subsequent sedimentation and piling up of equiaxed grains within the mini-ingot.

The Al-Zn-Mg-Cu series alloys has been widely used in the aircraft, automotive, ship-building and nuclear industries for the advantages of excellent combination of low density, high strength to weight ratio, good toughness and high corrosion resistance, etc.. Most of the researchers focused on alloying and heat treatment at aging temperature, however, rare work had paid attentions on the deformation process, and the microstructure evolution and mechanical properties has not been analyzed completely. Grain refinement can not only improve the strength and hardness, but also the plasticity and toughness of the alloy. Thermo-mechanical treatment is an efficient and economical treatment for obtain grain refinement by a combination of the deformation and heat treatment. In the present work, an improved thermo-mechanical processing, double step hot rolling (DHR), including low temperature pre-deformation, intermediate short-term annealing and final hot rolling has been proposed, aiming to investigate the microstructural evolution, strain induced precipitation and grain refinement mechanism of the alloys during the DHR process. A 7055 aluminum plate has also been manufactured by the conventional hot rolling (CHR) route. The corresponding microstructure evolution and mechanical properties were investigated by OM, XRD, TEM, SEM, EBSD and tensile test. The results reveal that the grain refinement is mainly preceded via dislocation rearrangement and low angle grain boundaries migration, which in turn leads to the pinning effects of strain induced precipitates. Low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates. The pre-deformation makes influence on the morphology and average size of precipitates without changing their area fraction. The precipitates generated by the pre-deformation can exert significant drag force to the migration of the grain boundaries and dislocation movements, which subsequently promotes the formation of dislocation cells. Although some smaller particles have been dissolved into the matrix during intermediate annealing treatment, some particles are still fine and can pin the dislocation boundaries. At the same time, the activated dislocation boundaries rearranged to form polygon sub-grains. Grains are further elongated after the final hot rolling. The low angle grain boundaries (like sub-grain boundaries) into high angle grain boundaries transition will be accelerated if the motion of boundaries is impeded by the particles. And the new small grains formed near the original grain boundaries can finally cause the fine-grained structures. The results indicate that the optimum thermo-mechanical treatment of 7055 aluminum alloy may be solid heat treatment+pre-deformation (300 ℃, 20%)+intermediate annealing (430 ℃, 5 min)+hot deformation (400 ℃, 60%). The elongation of the alloy produced by the proposed process can increase by 25% without strength loss comparing with that of conventional hot rolling. And the present DHR process is supposed to be a good alternative manufacturing process for the aluminum alloys to obtain fine grain structured heat-treatable sheets.

As the lightest metallic structural material, magnesium alloys were widely used in automotive, aerospace, electronic equipment and other fields. Among commercial magnesium alloys, AM series were commonly used due to excellent ductility and energy absorption. However, their relatively poor strength greatly restricted their extended use. In order to improve mechanical properties of AM50 alloy, the Zn and Y elements were added into the AM50 alloy in the form of atomic ratio of 6∶1 by the permanent mold casting. The microstructure, solidification behavior and mechanical properties of AM50-x(Zn, Y) (x=0, 2, 3, 4, 5, mass fraction, %) alloys were investigated by OM, SEM, EDS, XRD, thermal analysis and tensile tests. The results indicated that addition of Zn and Y elements with an atomic ratio of 6∶1 to AM50 alloy, the microstructures were obviously refined, and the quasicrystal I-phase(Mg₃Zn₆Y) cannot form. In addition, the granular Al₆YMn₆ phase and fine Al₂Y phase were formed in the microstructure, and the size of Al₆YMn₆ phase increased with increasing the Zn and Y content. The Φ-Mg₂₁(Zn, Al)₁₇ phase with lamellar structure was formed around β phase when x≥3, and its amount increased with increasing the Zn and Y addition. Thermal analysis results show that the Φ-Mg₂₁(Zn, Al)₁₇ phase was formed at 354 ℃ by the peritectic reaction, in which the precipitation temperatures of α-Mg and β phase were decreased with the increase of x content. Due to the formation of Al₆YMn₆, Al₂Y and Φ-Mg₂₁(Zn, Al)₁₇ phases, the size and amount of the β phase was decreased. For AM50-4(Zn, Y) alloy, the microstructure was greatly refined, and the ultimate tensile strength, yield strength and elongation of the alloy reached to the maximum, 206.63 MPa, 92.50 MPa and 10.04%, respectively.

Magnesium alloys are considered as one of the lightest structural metallic materials with excellent properties such as high specific strength, superior damping characteristics and electromagnetic shielding performance. In order to improve the mechanical properties of magnesium alloys, the hot rolling, hot extrusion and other hot forming processes are often introduced to produce the high performance parts. Both of the two softening mechanisms, dynamic recovery and dynamic recrystallization (DRX), occur during the hot deformation. As an important softening mechanism in hot processing, DRX is beneficial to obtaining fine grains structure, eliminating defects and improving mechanical properties for magnesium alloys. In this work, isothermal compression tests of AZ80 magnesium alloy were conducted on Gleeble thermo-mechanical simulator in the temperature range of 200 to 400 ℃ and strain rate range of 0.001 to 1 s^-1. In view of the dynamic hardening and softening mechanisms, the softening behavior of AZ80 magnesium alloy, dominated by dynamic recrystallization, was depicted. Dynamic recrystallization volume fraction was introduced to reveal the power dissipation during the microstructural evolution which was indicated by the strain rate sensitivity value based on the dynamic material model. To quantify the dynamic recrystallization softening behavior by the strain rate sensitivity (SRS) value, the SRS value distribution maps were constructed depending on various temperatures and strain rates. Therefore, the critical conditions and evolution process were studied in terms of temperatures and strain rates, while features of the SRS value distribution maps at different strains were deeply investigated. It can be concluded that the value of dynamic recrystallization critical condition decreases and dynamic recrystallization volume fraction increases when the temperature increases and strain rate decreases during the deformation. The strain rate sensitivity was positive correlated with the dynamic recrystallization volume fraction. It has been verified effectively by the analysis of microstructure that the region in which the strain rate sensitivity value is above 0.21 corresponds to the higher dynamic recrystallization volume fraction and lower strain rate.

The three-layer polyethylene (3PE or 3LPE) coatings have been widely used on long-distance high pressure transmission pipelines in China. The 3PE coating tends to remain high insulating after disbonding from pipelines, and block the function of cathoidc protection (CP), similar to PE tape coatings that caused stress corrosion cracking (SCC) failure of pipeline. Disbondment 3PE coatings have been reported worldwide. Because of the high integrity and dielectric strength of 3PE coatings, SCC under disbonded 3PE coating becomes an important issue for integrity management and operation of high pressure pipelines. A great deal of researches have been conducting over the past 20 years to reproduce SCC of high strength low alloy (HSLA) pipeline in laboratory. Most of these studies were conducted in bulk solution condition. The methodology neglects particularity of the thin-layer electrolyte under disbonded coating which has been identified as one of the primary environmental factors related to SCC. In this context, a research project has been initiated on this subject. The overall goal is to systematically investigate corrosion scenarios and mechanochemical interaction of HSLA pipeline steels under disbonded 3PE coating in different soil environments, particularly to further mechanistic understanding the initiation of SCC on pipelines under disbonded coating. In this work, SCC behavior of API X80 pipeline steel under disbonded coating with defect was investigated in acidic soil solution by a crevice cell specially designed for simulating coating disbondment. The crevice cell was equipped with a multi-sample loading frame, through which multi specimens in the crevice cell can be loaded simultaneously. Electrochemical impedance spectroscopy (EIS) was applied to characterize local electrochemical process of the tensile specimens. Local environment parameters (potential and pH) were monitored by microelectrodes. Surface morphology of the corrosion specimens indicate that corrosion intensity of X80 steel decreased over the distance from the opening. Intensive anodic dissolution and microcrack initiation were preferential at the opening defect, whereas corrosion was markedly mitigated under disbonded coating. CO₂ content gradient is proposed for the special corrosion scenarios under coating disbondment.

With the rapid development of electricity and transport industry, more and more buried pipelines are parallel or cross to the high voltage transmission line and the electrified railway. In this work, microbiological analysis method was used to investigate the effect of alternating current (AC) on the physiology of sulfate reducing bacteria (SRB). Electrochemical methods, including open circuit potential, potentiondynamic polarization curves and electrochemical impedance spectroscopy (EIS) on Q235 steel samples, were performed in soil leaching solution to study the electrochemical behavior with the presence or absence of AC and SRB. The corrosion morphology was observed by scanning electron microscopy (SEM). The results indicate that the AC which current density is 50 A/m² and frequency is 50 Hz has only a small impact on the growth of SRB, but its alternating electric field can reduce the adsorption and promote the desorption of the biofilm. During the initial experiment, the active biofilm can inhibit the corrosion of Q235 steel due to the electronegativity and the physical barrier, but the microbial metabolites would promote the corrosion during the later experiment without active biofilm. AC can improve the corrosion rate and lead the corrosion products loose because of the rectifying effect, the alternating electric field and the self catalytic effect of pitting corrosion.