Pitting corrosion behavior of Cu-P-RE weathering steels

The weathering steels are prone to pitting corrosion in an environment containing chloride ions.The pitting behavior of Cu-P-RE weathering steels and its effect on the corrosion resistance of steels were investigated by multifarious analytical techniques,such as field emission-scanning electron microscopy(FE-SEM),electron probe microanalysis(EPMA),scanning Kelvin probe force microscopy(SKPFM),electrochemical workstation and a series of immersion tests.The results show that the original stripshaped MnS inclusions and Al_(2)O_(3)inclusions with sharp angles are modified into the fine spherical rare earth(RE)inclusions with small average size,which are mainly RE oxysulfides after adding appropriate amount of mischmetal(48.9 wt%Ce-42 wt%La-5 wt%Nd-Fe)into the Cu-P weathering steel.In the environment containing Cl^(-),the pitting corrosion in RE weathering steel is induced by the preferential dissolution of RE inclusions in that the RE inclusions have a more negative potential than steel matrix at the initial corrosion stage.With the increase of corrosion time,the driving force of pitting expansion is weakened as a re sult of the continuous dissolution of RE inclusions,which makes the pitting tend to propagate horizontally around the RE inclusions.Once the RE inclusions completely dissolve,the open corrosion pits with shallow depth are formed in steel.The dispersed pitting pits with small size and shallow depth induced by RE inclusions are conducive to the formation of uniform and dense corrosion products layer on the steel surface,which reveals that the addition of RE can improve the corrosion resistance of weathering steels.

Characterization of Compactness of Rust Layers on Weathering Steels by an Adsorption/Dehydration Test of Ethanol

A new method for evaluating the compactness of rust layers on steels has been proposed in the present study. The method includes adsorption and dehydration process of anhydrous ethanol. The protective ability of rust layers can be qualitatively reflected by the adsorption/dehydration rates. The specific surface area and porosity of rust layers can be calculated by a quantitative model. The results from the present method are consistent with electrochemical tests, N2 adsorption and X-ray diffraction analysis. The method characterizes the compactness of rust layers rather than that of corrosion products removed from the metal surfaces, which is generally practiced in classic N2 adsorption method. Furthermore, the method can reflect the compactness of inner rust layers, to which N2 adsorption is unavailable. The method provides a new approach for the study of rust layers.

Abnormal relationship between rust particles size and rust layer compactness of weathering steels

In order to reveal evolution of the rust layer during atmospheric corrosion, commercial weathering steel （WS） 09CuPCrNi and a recently developed bainite WS were subjected to a salt fog test. The protection and compactness of the rust layer were evaluated by electrochemical analysis, absorption-desorption test, etc. The experimental results indicate that more compact rust layer could be derived from the rust particles with larger size. The well-established notion that the rust particle growth can induce the decrement of corrosion resistance of WS. However, the present investigation shows that the notion might not be universally accurate. The unusual result means that the rust particles might grow anisotropically during corrosion.

Stress-assisted corrosion mechanism of 3Ni steel by using gradient boosting decision tree machining learning method

Traditional 3Ni weathering steel cannot completely meet the requirements for offshore engineering development,resulting in the design of novel 3Ni steel with the addition of microalloy elements such as Mn or Nb for strength enhancement becoming a trend.The stress-assisted corrosion behavior of a novel designed high-strength 3Ni steel was investigated in the current study using the corrosion big data method.The information on the corrosion process was recorded using the galvanic corrosion current monitoring method.The gradi-ent boosting decision tree(GBDT)machine learning method was used to mine the corrosion mechanism,and the importance of the struc-ture factor was investigated.Field exposure tests were conducted to verify the calculated results using the GBDT method.Results indic-ated that the GBDT method can be effectively used to study the influence of structural factors on the corrosion process of 3Ni steel.Dif-ferent mechanisms for the addition of Mn and Cu to the stress-assisted corrosion of 3Ni steel suggested that Mn and Cu have no obvious effect on the corrosion rate of non-stressed 3Ni steel during the early stage of corrosion.When the corrosion reached a stable state,the in-crease in Mn element content increased the corrosion rate of 3Ni steel,while Cu reduced this rate.In the presence of stress,the increase in Mn element content and Cu addition can inhibit the corrosion process.The corrosion law of outdoor-exposed 3Ni steel is consistent with the law based on corrosion big data technology,verifying the reliability of the big data evaluation method and data prediction model selection.

Corrosion behavior and corrosion products of a low-alloy weathering steel in Qingdao and Wanning

A newly developed low-alloy weathering steel has been exposed in two coastal sites （Qingdao in the north, Wanning in the south） in China for one year. The samples in Wanning corroded far more seriously than those in Qingdao. The rust layer formed on the steel was analyzed by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, N2 adsorption approach, polarization curves, and electrochemical impedance spectroscopy （EIS）. The rust formed in Qingdao contains more X-ray amorphous compounds and is more compact than that formed in Wanning. Cr and Cu are enriched in the rust layer near the steel matrix, and the phenomenon is more obvious in Qingdao than in Wanning. The rust layer formed in Qingdao suppresses the anodic and cathodic reaction more remarkably than that formed in Wanning does. The rust layer formed in Qingdao possesses a higher ability to block the permeation of chloride ions than that formed in Wanning does.

CORROSION BEHAVIORS OF THE EXPOSED SIDE AND UNDERSIDE OF LOW ALLOY WEATHERING STEEL IN QINGDAO AND WANNING FOR 18 MONTHS

For 18 months, a newly developed low alloy weathering steel has been exposed in two coastal sites （Qingdao in the north China, Wanning in the south China）. The different corrosion behaviors of the exposed side and the underside of the samples were characterized by X-ray diffraetion （XRD）, polarization curve, electrochemical impedance spectroscopy （EIS）, scanning electron microscopy （SEM）, and N2 adsorption approach. It was found that the samples exhibited higher corrosion rate in Wanning than that in Qingdao. The underside of the samples corrodes more seriously than the exposed side in Qingdao, whereas the result in Wanning is just the reverse. The protection performance of rust layers mainly depends on its compactness and the enrichment of Cu and Cr is a secondary causation. The different compactness of rust layers in the exposed side and the underside originates from different corrosion conditions of the two sides.

Electrochemical Performance Study of Rust Layer on Cu-P-RE Weathering Steel

Corrosion rates of 10PCuRE steels with different rate earth contents and plain carbon steel were studied by dry-wet cycle immersion test. The corrosion resistance of rust layer on rare earths weathering steel and carbon steel was studied through the electrochemical means of polarization test and electrochemical impedance spectra. The difference of corrosion resistance of testing steels was analyzed through electrochemical means. The 10PCuRE steels whose rare earths content is smaller than 0.016% have good performance of corrosion resistance because corrosion potential of the steel is positive and resistance of rust layer is large. The results showed that rare earths of proper content could diminish corrosion tendency and promote the formation of the steady and compact rust layer.

Anticorrosive Properties of Hot-Dip Galvanized Weathering Steel in Atmospheric Exposure

Hot-dip galvanized steel is one of the most used materials in equipment and metallic structures of the Brazilian electric sector. Although carbon steel is the main substrate in the galvanizing hot-dip process, recently, weathering steel has been used as an alternative material to be galvanized. In the transmission line segment of the Brazilian electric sector, for instance, compact towers made of galvanized weathering steel have been installed to conduct energy through urban sites. It is well known that weathering steel, depending on wet and dry cycles and on the pollutants present in the atmosphere, develops a protective patina made of its corrosion products. The patina is dense and strongly adhered to the substrate, blocking the active surface and, thus, reducing the corrosion rate of the base metal. However, when the weathering steel is galvanized, the substrate surface has a layer of zinc and zinc-iron intermetallic alloys. When the sacrificial layer is consumed by atmospheric corrosion, critical questions remain to be answered regarding the underlying substrate. Will the patina of weathering steel be formed? In what condition? Does the hot-dip galvanizing process modify the weathering steel microstructure? The present work carried out an experimental research to shed light on the anticorrosive behavior of hot-dip galvanized weathering steel, after the zinc layer is corroded. This was done by a controlled pickling process, where the zinc layer was removed simulating its consumption during real corrosion processes. The results, obtained through electrochemical techniques and different accelerated corrosion tests, showed that galvanizing weathering steel is a promising technology to enhance the lifetime of structures used in the Brazilian electric sector.

Effect of La on the Corrosion Behavior and Mechanism of 3Ni Weathering Steel in a Simulated Marine Atmospheric Environment

The corrosion behavior and mechanism of 3Ni weathering steel in a simulated oceanic atmospheric environment are investigated in order to comprehend the impacts of La,as determined through electrochemical analysis and rust layer characterization.The results of this study demonstrate that the addition of La enhances the corrosion resistance of 3Ni weathering steel in the marine atmospheric environment,thereby reducing the corrosion rate and improving the protection of the rust layer.The influence of La on corrosion resistance can be attributed to two primary factors.Firstly,La functions as a grain refiner,minimizing the potential difference of the micro-regions on the substrate surface,thereby significantly reducing the corrosion of bare steel in the marine environment.Secondly,La inhibits the process of Fe_(3)O_(4) oxidation back toγ-FeOOH during corrosion at the local site,thus decreasing the formation ofγ-FeOOH and enhancing the charge transfer resistance.This research work may serve as a reference for expanding the application of rare earth elements in the field of weathering steel.

Surface grain refinement mechanism of SMA490BW steel cross joints by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a postweld technique for improving the fatigue strength of welded joints. This technique makes use of ultrasonic vibration to impact and plastically deform a weld toe and can achieve surface grain refinement of the weld toe, which is considered as the main reason for the improvement of fatigue strength. In this paper, the microstructure of the surface of a treated weld toe was observed by metallographic microscopy and transmission electron microscopy (TEM). The results show that UIT could produce severe plastic deformation on the surface layer of the weld toe and the maximum depth of plastic deformation extended to approximately 260 μm beneath the treated surface. Repeated processing could exacerbate the plastic deformation on the surface layer, resulting in finer grains. We can conclude that the surface grain refinement mechanism of SMA490BW welded joints is related to the high density of dislocation tangles and dislocation walls. © 2017, University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.

Enhancing strength of an ultra-low-carbon weathering steel to 700 MPa by adjusting Ti content

The microstructure and mechanical properties of a traditional Ti-microalloyed weathering steel were analyzed,and the strength was improved by proposing an optimized Ti content.The yield strength and elongation of the steel were 640 MPa and 25.5%,respectively.The microstructure was ferrite and pearlite,and the average grain size was 5.4μm.The precipitates were mainly TiC with the size below 20 nm,and the average diameter was 18.2 nm.The yield strength of the newly proposed weathering steel with Ti content of 0.018%higher than that of the traditional steel reached up to 709 MPa,and the elongation was 23.5%.The ferrite grain was refined to 3.8μm,the fraction of TiC under 10 nm was obviously increased,and the average diameter of particles was 9.8 nm.The increase in Ti also promoted the recrystallization process,thus leading to the reduction in dislocation density.The yield strength of the newly proposed weathering steel was increased to higher than 700 MPa by adjusting the Ti content mainly resulting from three aspects:grain refinement,precipitation and dislocation strengthening.The contributed values were 45,64 and–40 MPa,respectively.

Secondary work embrittlement of cold-rolled 05CuPCrNi sheet at low temperature

Due to the rapid development of China＇ s rail transportation equipment manufacturing industry and related international distribution, important material suppliers such as Baoshan Iron ＆ Steel Co., Ltd. carries on investigating the stability of material under extremely cold conditions. In this study, mechanical properties and secondary work embrittlement of weathering steel 05CuPCrNi were tested at low temperature. Compared to the mechanical properties at room temperature, the yield and tensile strength increase slightly with decreasing temperature. However, the variation of elongation is not obvious. The experimental results also show that the secondary work embrittlement transition temperature of 05CuPCrNi is lower than - 60 ℃. These results provide the basis for the use of this train body material in extremely cold regions.

Effects of rare earth on inclusions and corrosion resistance of 10PCuRE weathering steel

The types, morphologies and distributions of nonmetallic inclusions in Cu-P weathering steels with and without rare earth were analyzed through a quantitative image analyzer, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) attached to SEM. Solid-soluble content of rare earth in the steels was analyzed by non-aqua electroanalysis and ICP. The results showed that rare earth modified the types and the morphologies of inclusions in the weathering steels. The small spherical rare earth oxysulfides and rare earth sulphides replaced the elongated MnS inclusions in the RE weathering steels. The rare earth inclusions dispersedly distributed and most inclusions were smaller than 2 μm in size. The optimum content of RE was 0.0065%-0.016% for 10PCuRE weathering steels containing about 0.002% oxygen and 0.004% sulfur. Solid-soluble content of rare earth in steels was (14-20)×10-6, which can act as a micro-alloying element. The corrosion resistance of 10PCuRE weathering steels and Q235 were studied by dry-wet cyclic immersion test. Their corrosion rates were obtained respectively. The polarization curves and pitting corrosion behaviors of weathering steels with and without rare earth were measured by electrochemical methods. The corrosion resistance of Cu-P weathering steels was improved by adding an appropriate amount of rare earth. Less and fewer rare earth inclusions largely decreased pitting susceptibility and rate of pit propagation. The pitting potential and the resistance against pitting corrosion of the RE weathering steel were significantly improved due to the modification of rare earth to inclusions.

A new understanding of the effect of Cr on the corrosion resistance evolution of weathering steel based on big data technology

In this work,we studied the effect of Cr element on the corrosion resistance evolution of weathering steel based on corrosion big data technology.It suggested that corrosion big data technology is suitable for evaluation of the effect of microalloying Cr element on the corrosion evolution behavior of weathering steel.New understandings prove that the effect of Cr on the corrosion process is dynamic rather than static,the processes is affected by both of the environmental factors and the electrochemical or chemical reactions in the rust layer.Besides,Cr element has both beneficial effect and detrimental effect on the corrosion resistance of weathering steel.The beneficial effect is that the general corrosion resistance of Cr-additional steel is better than that of Cr-free steel,while the detrimental effect is that localized corrosion is intensified as the increase of Cr content in the Cr-additional steel.

Static Recrystallization and Precipitation Behavior of a Weathering Steel Microalloyed with Vanadium

The static recrystallization （SRX） and precipitation behavior of a weathering steel microalloyed with vanadium were investigated through double-pass compression tests under controfled conditions using the MMS-300 thermal-mechanical simulator. The deformation temperatures ranged from 800 °C to 1000 °C, and the inter- pass time from 1 s to 500 s. The simulation results showed that SRX occurred after 5-10 s at the first compression deformation. The softening fraction of SRX was found to increase with increasing the deformation temperature and the pre-strain. However, the softening fraction scarcely changed during the process of strain-induced precipitation. In addition, the kinetics of SRX was described by the Avrami equation, and the Avrami exponent appeared to be closely associated with the deformation temperature. The microstructure evolution was investigated at the initiation and completion of recrystallization. The amount and distribution of the precipitates were analyzed. The relationship between the driving force of SRX and the pinning force of precipitation was discussed. Besides, the recrystallization inhibition was detected at the early stage of precipitation, and the pinning forces were found to be of a magnitude comparable to the driving force. Moreover, the pinning forces were found to increase with the degree of precipitation and reach a peak at the intermediate stage of precipitation, and finally reduce as the particles coarsened.

Effects of Alloy Element and Microstructure on Corrosion Resistant Property of Deposited Metals of Weathering Steel

Alloy element and microstructure are key factors that dominate mechanical and corrosion resistant properties of weathering steel. The effect of Mo on microstructure, mechanical properties and corrosion resistant property of depos ited metal was investigated. Experimental results show that with the increase of Mo content in deposited metals, the phase transformation temperature decreases, and the ferrite zone in CCT diagram moves rightward, resulting in en larged bainite zone and reduced ferrite and pearlite zone. The addition of 0.24 mass% Mo in deposited metal results in the increase of tensile strength, more M-A constituent and less high angle grain which reduce the low temperature toughness. It is found that Mo can raise the weathering resistance of deposited metal in industrial atmosphere. Analy sis indicates that Mo may enrich in the inner rust layer, produce MoO3, enhance the formation of compact rust film and impede the anode dissolution reaction. Granular bainite in deposited metals displays better corrosion resistance than acicular ferrite during the initial corrosion stage, but its long-term influence on the corrosion resistance is limited.

Effect of Carbon Content on Mechanical Properties and Weather Resistance of High Performance Bridge Steels

The influence of carbon content on the mechanical properties of high yield strength bridge steel has been in- vestigated. The results show that the excellent mechanical properties and corrosion resistance are obtained for the steel with carbon content of 0.03% -0. 05% （mass percent）. According to the results, a new weathering bridge steel plate with carbon content of 0. 045% （mass percent） has been developed. The appropriate controlled cooling process should be taken due to the results of CCT （continuous cooling transformation） and TTT （time-temperature-transformation） to ensure both microstructure and mechanical properties. CCT curve of the newly developed steel shows that when accelerated cooling speed is higher than 5 ℃/s, the intermediate transformation products can be formed. The TTT curve displays that the intermediate transformation temperature ranges from 600 to 530 ℃. Yield strength of the newly developed steels reaches 500 MPa, and their elongation and toughness are excellent.

Atmospheric Corrosion of Q235 Carbon Steel and Q450 Weathering Steel in Turpan,China

Q235carbon steel and Q450 weathering steel were exposed to the hot and dry environment of Turpan,China for three years.The corrosion rates of both steels were calculated and compared.The morphologies of the rust layer products were observed by optical microscopy and scanning electron microscopy.Analyses of the rust layers were performed by X-ray photoelectron spectroscopy,X-ray powder diffraction,and Raman spectroscopy,and analysis results indicate that the compositions of rust are main iron rich oxide such as FeOOH,Fe_3O_4,and Fe_2O_3.The iron oxide layer content proportion was calculated through a semi-quantitative algorithm.The resistance elements（Cr,Ni,and Cu）enhanced the resistance properties of the Q450 weathering steel matrix.Moreover,the resistance elements increased the proportion of goethite crystals in the corroded rust layer.

Industrial Atmospheric Corrosion Resistance of P-RE Weathering Steel

The corrosion behavior of low carbon steel （CS）, P-bearing steel （PS） and P-RE weathering steel （P-REWS） exposed for two years in Jiangjin of China was investigated. The results showed that during 2-year exposure test, corrosion data of the experimental steels followed the bilogarithmic equation, and the average corrosion depth of PS and P-REWS was decreased by 19.5% and 28.2% respectively compared with that of CS. Scanning electron microscope, electrochemical impedance spectroscope and Fourier transform infrared spectroscope were used to characterize the corrosion products. The research results showed that P in steel could promote the formation of an amorphous ferric oxyhydroxide layer near the substrate. The addition of RE could effectively increase the charge transportation re sistanee of rust.

Numerical Approach for Atmospheric Corrosion Monitoring Based on EIS of a Weathering Steel

Electrochemical impedance spectroscopy （EIS） and film thickness measurement have been employed to study the atmospheric corrosion of a weathering steel covered with a thin electrolyte layer in a simulated coastal-industrial atmosphere. The results indicate that the corrosion rate is a function of the covered electrolyte thickness and the wet/dry cycle. Within each wet/dry cycle, the increased corrosion rate is related to the increased Cl^- and SO4^2- concentration and an enhancement of oxygen diffusion rate with the evaporation of the electrolyte. In addition, the corrosion rate increases during the initial corrosion stage and then decreases as the wet/dry cycle proceeds. Moreover, one mathematical approach based on the numerical integration method to obtain corrosion mass loss of steel from the measurements of EIS has been developed, and this would be useful for the development of indoor simulated atmospheric corrosion tests.