Improvement of Corrosion Resistance of Simulated Weld Heat

ISIJ International, Vol. 60 (2020), No. 9

© 2020 ISIJ2015

ISIJ International, Vol. 60 (2020), No. 9, pp. 2015–2023

* Corresponding author: E-mail: [email protected]: https://doi.org/10.2355/isijinternational.ISIJINT-2019-565

Improvement of Corrosion Resistance of Simulated Weld Heat Affected Zone in High Strength Pipeline Steel Using Electropulsing

Shengli DING,1) Siqi XIANG,1) Xin BA,1) Xinfang ZHANG1)* and Yabo FU2)

1) State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083 P.R. China.2) Key Lab of Printing & Packaging Materials and Technology, Beijing Institute of Graphic Communication, Beijing, 102600 P.R. China.

(Received on September 3, 2019; accepted on March 23, 2020)

Subjected to quenching processing, the samples of weld heat affected zone containing microstructures of martensite in high strength pipeline steel were simulated and prepared. Effects of electropulsing treat-ment on the corrosion resistance of simulated samples were studied through electrochemical detections and immersion corrosion experiments. It was found that the corrosion resistance of pipeline steel decreased sharply due to the high lattice strains/dislocation densities and residual tensile stress developed after martensite transformation by water quenching. Interestingly, treated by electropulsing the corrosion resistance of simulated weld heat affected zone samples was dramatically improved, and even exceeded that of the base metal when the current density achieved 5.2 kA/mm2. After electropulsing treatment, the dislocation density and residual stress of the investigated samples were reduced largely, and the rust layer generated after corrosion was more compact, so that its corrosion resistance was improved.

KEY WORDS: pipeline steel; welding; corrosion resistance; electropulsing; heat affected zone.

1. Introduction

High-strength pipeline steel is widely used in the con-struction of long-distance oil and gas pipelines worldwide due to its excellent performance.1) Welding technology is often used to connect long-distance oil and gas pipelines. Owing to the high temperature of the welding process, the structure and properties of the welding joint are greatly different from those of the base material.2,3) Usually, the welded joints are divided into three areas,4,5) depending on the degree of influence of Joule heat during the welding process. Because the distance from the welded joint is too far to be affected by the Joule heat, the microstructure of the base metal is not changed. Due to the heat generated by the welding process and subsequent cooling, the micro-structures and properties of the heat affected zone (HAZ) have changed significantly. The chemical composition of the weld metal is almost the same as that of the base metal, but the fast melting and solidification process occurred during the welding process causing the microstructures and proper-ties of this zone are also different from that of base metal.

Welding treatment will lead to increased corrosion activity and stress corrosion cracking (SCC) sensitivity in the welded area of high-strength pipeline steel.6,7) Local corrosion at the welded joints is the most common failure form of high-strength pipeline steel buried underground. In

the process of pipeline operation, due to corrosion and the joint action of internal and external pressure, cracks grow from the initial small size to the critical size, and eventu-ally lead to leakage. Especially, because of the presence of weld defects, corrosion occurs in the welded joints firstly. Gradually, corrosion causes SCC, and then lead to sudden failure of pipeline or fracture of welded joints.8) Corrosion and SCC of pipelines are very important issues in oil and gas transportation pipelines, because once the pipeline leaks, ruptures or fails, it usually causes catastrophic accidents to human beings and the environment.9,10) In order to ensure the safe operation of pipelines, welding, coating and sub-sequent maintenance need to spend about 20% of the total cost of pipeline construction.11) Although great efforts have been made to develop alternative welding technology, ring metal arc welding technology is still the main means used to connect large-diameter pipes.12)

Due to phase transformation and the unevenness of heat, a lot of dislocations and residual stress will be produced in the heat affected zone, which will reduce the corrosion resistance.3,6,7,9,10) Affected by composition and thermal cycle, ferrite, bainite, martensite and other microstructures will be formed after the phase transformation in the heat affected zone.5,12) In these microstructures, the martensite microstructures are generated by rapid cooling, and the structures are unbalanced, with high dislocation densities and large residual stress, becoming the areas with the worst corrosion resistance.13) Because the HAZ of welded joints is generally narrow and its microstructures are complex, it


© 2020 ISIJ 2016

is difficult to obtain effective samples from actual welded joints to analyze the influence of a specific microstructure on corrosion performance in the heat affected zone.14) Cur-rently, heat treatment is generally adopted to simulate the welding process to prepare samples for investigation.12,15) Although the corrosion resistance is improved to some extent by adjusting the microstructure of the heat affected zone through complex normalizing and tempering heat treat-ments, its corrosion resistance is still far lower than that of base metal.3,15,16) While many efforts have been made to study the corrosion properties of high-strength pipeline steel, the research on the recovery of corrosion properties in the heat affected zone of welded joints of high-strength pipeline steel is far from being enough.4,12,17)

As a convenient, energy-saving and efficient instanta-neous high-energy input method, electric pulse can modify the microstructure of metallic materials in a very short time.18) With the development of electroplastic theory and electromigration theory, electric pulse processing technol-ogy has been paid attention again, and has been more and more widely used in improving the performance of metal-lic materials.19,20) Electric pulse treatment has an important influence on microstructures evolution and corrosion resis-tance of metallic materials. It was found that electric pulse treatment could induce grain refinement of low carbon steel, and smash the carburized strips in strain pearlite steel to pieces, so as to improve its corrosion resistance.18,21,22) In addition, electric pulse treatment can significantly accelerate the dislocation migration rate of metallic materials, promote dislocation annihilation, and reduce residual stress.23–25) However, the effect of electric pulse treatment on corrosion resistance of HAZ in welded joint has been rarely reported.

Therefore, the purpose of this study is to propose a new method to improve the corrosion resistance of the HAZ of welded joints in pipeline steel by exploring the influence of electropulsing treatment on the corrosion resistance of the HAZ mainly containing martensite. To this end, the samples of the HAZ of simulated X80 welded joints, whose microstructures were mainly martensite, were prepared by water quenching. Then, the corrosion resistance of the simu-lated HAZ samples treated by electric pulse with different parameters was studied by electrochemical detections and immersion corrosion experiments.

2. Experimental Procedures

2.1. Initial Specimen PreparationThe experimental material used in this study was API X80

high strength low alloy pipeline steel containing (wt.%): 0.058 C, 0.218 Si, 1.699 Mn, 0.007 P, 0.001 S, 0.226 Cr, 0.218 Ni, 0.247 Mo, 0.006 Cu, 0.029 Al, 0.073 Nb, 0.003 Ca, 0.014 Ti and Fe balanced. For low alloy steels with carbon content less than 0.6 wt.%, the calculation formula of austenite transformation temperature Ac1 and Ac3 is:14)

Ac C Mn Ni Si

Cr As1 723 10 7 3 9 29

16 7 290

( ) . ( ) . ( ) ( )

. ( ) ( )

� � � � ��

� � �� 66 38. ( )� W

... (1)

Ac C C Ni Si

V M3

0 5910 230 15 2 44 7

104 31 5

( ) ( ) . ( ) . ( )

( ) . (

.� � � � ��

� � �� oo W) . ( )�13 1� ... (2)

where ω refers to the percentage of the mass of elements. Through calculation, the values of Ac1 and Ac3 are 714°C and 869°C, respectively.

Cutting to a size of 1.5 × 10 × 30 mm3, all specimens were grinded by 1 500 mesh sandpapers and austenitized at 900°C for 10 min before water quenched. During water quenching, the samples underwent martensite transforma-tion, which resulted in tensile residual stress in the surface. Simultaneously, the thermal stress was produced due to the difference of cooling rate in different depth of the samples. However, the residual stress caused by phase transforma-tion was dominant in this study. Therefore, tensile stress was eventually created in the surface of the samples and compressive stress was formed in the core. Correspond-ingly, dislocations were also generated during the phase transformation initiated by water quenched.

2.2. The Electropulsing TreatmentAs shown in Fig. 1(a), the electropulsing device consisted

of several capacitors in parallel, to obtain different working current density J. The samples were clamped firmly by two large copper electrodes during the experiment. The electric pulse was detected by a Rogowski coil-based system. A digital storage oscilloscope was used to show the output waveform. According to the actual waveform recorded by the oscilloscope, the calculation formula of instantaneous current intensity is:23)

Fig. 1. (a) The schematic diagram of electric pulse test device, (b) profiles of current density and temperature rise ver-sus time for an electropulsing. (Online version in color.)


© 2020 ISIJ2017

I I e ttt �

� �

�m sin� �

�2 .......................... (3)

where It is the current intensity, Im is the maximum value of the first sinusoidal wave peak, α0 is attenuation coef-ficient, τ0 is the sine period. A typical profile of current density versus time for electropulsing of Im = 83 430 A, τ0 = 300 μs, α0 = 2 000 based on the waveform recorded by the oscilloscope are given in Fig. 1(b). The current density

JI

St

t= , in which S = 15 mm2 was the cross-sectional area

of specimens. The negative current density means the cur-rent direction opposite to the initial current direction. The temperature rise during the electric pulse treatment can be approximately expressed as:23)

� � � �T J C d dtt� t

2p

0

1( ) ........................ (4)

where ΔT is the temperature rise, ρ is the resistivity, Cp is specific heat, d is the density. Relevant studies have shown that there were good agreements between the temperature rise measured after electric pulse treatment and the tem-perature calculated based on Joule heating under adiabatic conditions.22) In the present investigation the maximum tem-perature rise was about 240°C (S = 15 mm2, the maximum current density J is 5.2 kA/mm2, ρ = 2.746×10 −5 Ω·cm, Cp = 540 J/(kg·K), d = 8 g/cm3). The temperature curve versus time is shown in Fig. 1(b). In order to eliminate the influence of Joule heat generated during the electric pulse treatment on the corrosion resistance of the simulated HAZ samples, the simulated samples were tempered at 250°C for 1 min to be used as temperature comparison specimens.

2.3. Residual Stress AnalysisWith the surface chemically polished, the surface residual

stress along the long axis in the middles of the specimens (measured area diameter: 2 mm) was measured by an X-ray diffractometer (X-350A) using the fixed ψ method, before and after the electropulsing treatment.26,27) The following diffraction conditions were adopted:23) Cr anode and Kα radiation was used in the residual stress measurement; The diffraction spot diameter was 2 mm; The family of crystal lattice planes {221} was used in this measurement; The scanning voltage was 20 kV; The scanning step size was 0.1°; Diffraction angles ranged from 151° to 162°; The count time per step was 0.5 s; The elastic constants was 316 MPa/degree; The scanning current was 5 mA. Besides, the method of fixed ψ (the angle between the normal of the specimen and the normal of the diffracting lattice planes) was applied, the ψ respectively was 0°, 8°, 16°, 24.2°, 30°, 35.3°, 40°, 45°.

2.4. Dislocation Density MeasurementThe dislocation densities of samples were measured by

X-ray diffraction,23) based on the modified Williamson-Hall method.28) The electrolytic polishing of specimens was car-ried out prior to measurement. A Bruker type D8-Advance diffractometer was used to perform the X-ray diffraction experiments. Cu was used to be the diffraction anode, Cu Kα1 radiation, λ = 0.15405 nm. Diffraction angles ranged from 40° (2θ) to 150° (2θ). The scanning interval was 0.02°.

The count time per step was 0.6 s. We measured the reflec-tions of {222}, {110}, {310}, {200}, {220}, {211} of the body-centered cubic structure.

2.5. Potentiodynamic Polarization Curves Measure-ment

The samples were cut into a square with an area of 1 cm2 along the middle part. Proper length of copper wires was welded on the back of the samples. The samples were encapsulated with epoxy resin, leaving only the 1 cm2 front, grinded by 2 000 mesh sandpaper, then carefully machined with 3 μm diamond suspension, rinsed with deionized water and anhydrous ethanol and dried by cold air to make the working electrode. The polarization curves of the samples were measured with a three-electrode system electrochemi-cal workstation, in which the saturated calomel electrode (SCE) was used as the reference electrode and a platinum plate as the opposite electrode. The potential polarization curves were measured in wt.% 3.5 NaCl solutions. The potential range was from −0.9 V (vs SCE) to −0.1 V (vs SCE). The scanning rate was 0.33 mV/s.

2.6. Immersion Corrosion ExperimentsThe samples were cut along the middle part into specifi-

cations of 13 × 10 × 1.5 mm3 sheet samples. Then, the back and sides of the samples were all encapsulated with epoxy resin, and only the front side of the rectangle with an area of about 13 × 10 mm2 was exposed to the corrosive solution. Samples were grinded by 2 000 mesh sandpapers, then care-fully polished with 3 μm diamond suspension, rinsed with deionized water and anhydrous ethanol and dried by cold air. The weight of samples was measured by an analytical balance with an accuracy of 0.0001 g. The corrosive solu-tion used in the test was a neutral salt solution, prepared according to the GB-T 19746-2005 specification, which could be used to simulate corrosion effects in the marine environment. The main composition of the etch solution was a sodium chloride neutral salt solution with a concen-tration of 35 g/L, which was prepared by deionized water and analytical reagent of sodium chloride. Samples were immersed in 25°C etch solutions, facing up, and removed after 168 hours. The etch products were removed with 20% ammonium citrate solutions (1 000 mL solutions prepared by adding 200 g ammonium citrate and deionized water). Then the samples were rinsed thoroughly with deionized water and re-weigh after drying. The calculation formula of the average corrosion rate is:

��

��

� � �W W

Stb a 1 000 365 24 .................. (5)

where ν (mm/y) is the annual thickness loss or corrosion rate, Wb (g) and Wa (g) is respectively the weight of the sample before and after corrosion, S (mm2) is the area of the sample exposed to the solution, t (h) is the soaking time of the sample in the corrosive liquid, ρ is the density of the sample, which is approximated to 7.85 g/cm3.

2.7. EIS Measurement of RustThe rust of simulated weld HAZ samples treated with

different pulse current densities was obtained during the immersion corrosion test in 3.5 wt.% NaCl solutions at


© 2020 ISIJ 2018

35°C for 58 hours. Electrochemical impedance spectroscopy (EIS) measurements were performed for the rust samples in 3.5 wt.% NaCl solutions at room temperature. Samples were covered by a thin layer of epoxy resin with an exposed area of 1 cm2 as working electrodes. The same three-electrode system electrochemical workstation was used in EIS mea-surement. Working electrodes were allowed to stabilize at open circuit conditions in the solution during 0.5 h. Then EIS was conducted at stable open-circuit potentials with a perturbation amplitude of 10 mV in the frequency ranged from 100 kHz to 10 mHz.

3. Results

The kinetic potential polarization curves measured by electrochemical workstation were displayed in Fig. 2. According to the dynamic potential polarization curves, the corrosion potential, corrosion current density and corrosion rate of each sample could be obtained according to the Tafel straight line extrapolation method29) and Eq. (6),18) as shown in Table 1.

��

��

�A I

F n

corr87 600 ........................ (6)

where A is the atomic weight, n is the metal valence, F is Faraday’s constant (F = 26.8 A·h).

As shown in Fig. 2 and Table 1, after water quenching, the corrosion potential (Ecorr) of the samples decreased

significantly, and the corrosion current density (icorr) and corrosion rate (ν) increased obviously, which meant the decreasing of the corrosion resistance. After electric pulse treatment, the corrosion resistance of simulated weld HAZ samples increased significantly, as shown in Fig. 2. With the increased pulse current density, the Ecorr of the simu-lated weld HAZ samples increased gradually, and the icorr and ν reduced accordingly. When the pulse current density reached 5.2 kA/mm2, the Ecorr of the simulated weld HAZ samples was higher than that of the X80 original samples, but the icorr and the ν of the simulated weld HAZ samples were lower than those of X80 original samples. The corro-sion resistance of simulated weld HAZ samples got fully recovered to the level of base metal. On the other hand, after the simulated weld HAZ samples were tempered, the Ecorr increased slightly, the icorr and ν decreased slightly. The effect of tempered treatment at 250°C on the corrosion resistance it not vital.

The average corrosion rate of the samples in neutral salt solutions immersion corrosion experiments was presented in Fig. 3. It could be observed in Figs. 2 and 3 and Table 1 that the results of dynamic potential polarization curves detec-tion and immersion corrosion experiments were in good agreements. After quenching, the corrosion resistance of X80 pipeline steel decreased significantly, and the corrosion rate increased by 57.3% compared with the X80 original samples. Treated by pulsed electric current, the corrosion resistance of the simulated weld HAZ samples improved with the increased current density. When the electric pulse treatment parameters reached 5.2 kA/mm2, the corrosion rate reduced by 40.1% compared with the simulated weld HAZ samples, however, the corrosion resistance was 5.8% higher than the original X80 pipeline steel. The corrosion resistance of simulated HAZ sample was greatly improved by electropulsing treatment. Besides, after 250°C tempering, the corrosion resistance of the simulated weld HAZ samples was slightly improved, and the corrosion rate was only decreased by 13.9% compared with the quenched samples, which was far from the level of the pulsed samples when the current density was 5.2 kA/mm2. Therefore, it implied that Joule heat generated during electrical pulse treatment had

Fig. 2. The dynamic potential polarization curves of the original X80 sample, simulated HAZ sample, tempered sample and the simulated weld HAZ samples processed with different pulse parameters. (Online version in color.)

Table 1. Corrosion parameters obtained by fitting the dynamic potential polarization curves.

Sample Ecorr (mV) icorr (μA·cm −2) ν (μm/y)

X80 original −651 5.41 63

Simulated weld HAZ −695 8.55 100

Tempered −686 7.29 85

With current 3.9 kA/mm2 −682 7.03 82



Fig. 3. Effects of electric pulse treatment parameters on the aver-age corrosion rate of immersion experiments. (Online ver-sion in color.)


© 2020 ISIJ2019

limited influence on the corrosion resistance of simulated weld HAZ samples, that is, temperature was not the main reason for the improvement of corrosion resistance in this investigation.

4. Discussion

4.1. Dislocation Density and Residual Stress AnalysisIn order to observe the change of dislocation structure

and distribution, the simulated weld HAZ samples and the pulsed samples were observed by transmission electron microscope (TEM), as shown in Fig. 4. A large number of dislocations in the simulated weld HAZ sample were highly interwoven and distributed in a network of tangles, as shown in Fig. 4(a). However, the dislocation density was significantly reduced after electric pulse treatment, and the previously entangled dislocations were dispersed by electric pulse, as shown in Fig. 4(b).

The dislocation density and residual stress of the simu-lated weld HAZ samples before and after electric pulse treat-ment were measured. The variation of dislocation density with pulse current density is shown in Fig. 5. The decrease of residual stress with the same initial value and change of corrosion rate with pulse current density is shown in Fig. 6. It could be found from Figs. 5 and 6 that the dislocation density and the tensile residual stress in the surface of the samples decreased significantly treated by electropulsing. Simultaneously, the corrosion rate of simulated weld HAZ

samples decreased with the increased pulse current density.As reported in the previous studies,23) electropulsing can

promote the migration of dislocations. Under the action of drift electrons, the movement of vacancies and dislocations were enhanced, which accelerated the dislocation annihila-tion. The decrease of dislocation density would improve the corrosion resistance of simulated weld HAZ samples, as high corrosion rates were obtained at sites where high concentrations of dislocations intersect the exposed elec-troactive surface.30,31) In this study, the dislocation mobility was enhanced under the action of electropulsing, resulting in plastic strain and the decrease of residual tensile stress in the surface of the samples. In general, surface tensile stress greatly deteriorates corrosion resistance.3,4,32) Once the surface tensile stress is effectively controlled, the corrosion resistance increases accordingly. Therefore, the reduction of the dislocation density and residual stress caused by electric pulse treatment may be one of the main reasons for improv-ing corrosion resistance of simulated weld HAZ samples.

4.2. Microstructural AnalysisAs is known to all, the microstructure is one of the main

factors affecting the material properties. In this study, the microstructures of the samples were observed by optical microscope and scanning electron microscope (SEM), as shown in Figs. 7 and 8 respectively.

By comparing the Figs. 7(a) and 7(b), it could be found that after quenching, the main microstructure of X80 pipe-line steel changed from ferrite to martensite, during which a large amount of dislocations and residual stress would be generated that causing the reduction of the corrosion resis-tance of the steel. Besides, there was no obvious change in the microstructure of simulated weld HAZ samples before and after the pulsed treatment, as shown in Figs. 7(b)–7(d) and 8.

However, the dislocation density and residual stress of the simulated weld HAZ samples were significantly reduced after electric pulse treatment (Figs. 5 and 6). In order to fur-ther explore the relationship among the dislocation density, the residual stress and the corrosion resistance, it is neces-sary to understand the influence of the above factors on the corrosion rust layer.

Fig. 4. TEM images of simulated weld HAZ samples (a) before electric pulse treatment, and (b) treated by electric pulse current density of 5.2 kA/mm2. (Online version in color.)

Fig. 5. Effects of electrical pulses on dislocation density. (Online version in color.)

Fig. 6. The relationship between residual stress reduction and corrosion rate under the action of electrical pulse. (Online version in color.)


© 2020 ISIJ 2020

4.3. Rust Layer AnalysisAs a barrier between the corrosive medium and the

metal matrix, rust layer plays a great role in the corrosion resistance of materials.4) The rust layer can reduce the corrosion effect of corrosive medium on the steel surface through physical resistance. The denser the rust layer was, the stronger the resistance would be, and the better the cor-rosion resistance of the material would be.3) SEM was used to observe the rust layer of simulated weld HAZ samples after immersion corrosion test, as shown in Fig. 9.

Electropulsing can effectively improve the compact of rust layer within the low corrosive condition, so far as the condition of slight corrosion depth of about 2 μm, thus improve the corrosion resistance of simulated weld HAZ samples. It can be seen from the Fig. 9(a) that there are many wide cracks in the rust layers of simulated weld HAZ samples after soaking corrosion. However, treated by elec-tropulsing of different currents, the rust layers became more uniform and denser as shown in Figs. 9(b), 9(c) and 9(d). As the current density increased to 5.2 kA/mm2, few cracks

can be seen in the rust layers as shown in Fig. 9(d). The densification of rust layer was related to the effect of electric pulse treatment on reducing the residual tensile stress and dislocation density of simulated weld HAZ samples.3,4,32) The mechanical mechanism of the cracking of rust layer under residual tensile stress is shown in Fig. 10. Because of martensitic transformation, residual tensile stress was gener-ated in the surface, and residual compressive stress was gen-erated in the core of the sample during quenching.32) Due to the expansive nature of the corrosion products, oxide growth stress (compressive stress) developed in the rust layer.33) In the initial state, the residual tensile stress in the surface and the residual compressive stress in the core of the samples are balanced. When corrosion occurs on the surface of the sample, the corrosion of matrix on the surface of the sample will lead to the release of the tensile stress in the dissolved matrix. The release of tensile stress in the surface results in the disruption of the equilibrium system of forces, that is, the compressive force in the core is greater than the tensile force in the surface. The surface matrix of the sample will

Fig. 7. Microstructure under optical microscope: (a) the X80 original sample, (b) the simulated weld HAZ sample, (c) the simulated weld HAZ sample was treated by electric pulse with current density of 3.9 kA/mm2, (d) the simu-lated weld HAZ sample was treated by electric pulse with current density of 5.2 kA/mm2. (Online version in color.)

Fig. 8. Microstructure under SEM: (a) the simulated weld HAZ sample, (b) the simulated weld HAZ sample was treated by electric pulse with current density of 5.2 kA/mm2. (Online version in color.)


© 2020 ISIJ2021

undergo elastic or plastic deformation under greater tensile force, so as to reach the force balance again. And this tensile deformation is going to increase gradually as the corrosion goes on. Generally, bending deformation can be caused when a part of residual stress is released by the progress of corrosion in the steel surface, as shown in Fig. 11. However, for sheet samples size of 13 × 10 × 1.5 mm3 immersion corrosion for 7 days, the maximum average corrosion depth is only 2 μm, resulting in a very small degree of bending. We assume that the transversal line of the sample surface is

Fig. 9. SEM diagrams of the corroded rust layer in simulated weld HAZ samples treated with different pulse current densities: (a) with current density of 0 kA/mm2, (b) with current density of 3.9 kA/mm2, (c) with current density of 4.5 kA/mm2 , (d) with current density of 5.2 kA/mm2. (Online version in color.)

Fig. 10. SEM diagrams of the cross section of corroded rust layer in simulated weld HAZ samples. (Online version in color.)

a parabola. According to the coordinates of A, B and C, we can compute that the function expression of the parabola is y = −8 × 10 −6x2 + 2. The calculation formula of curvature K is:

Ky

y�

��

� �� 1 2 3 2/ ............................. (7)

By calculation, the maximum curvature of the sample sur-face is 1.6×10–5, which is so small that it can be ignored. In addition, previous studies have shown that the depth with tensile residual stress of samples before corrosion was more than 350 μm.23) Residual tensile stress was generated on the whole surface of specimens due to quenching treatment. So the region with tensile residual stress of samples before cor-rosion was greater than 13 000 × 10 000 × 350 μm3. For samples with the tensile residual stress region of 13 000 × 10 000 × 350 μm3, when the corrosion depth was only 2 μm, the variation of residual stress distribution caused by

Fig. 11. The schematic diagram of specimen surface bending caused by the release of residual tensile stress due to cor-rosion. (Online version in color.)


© 2020 ISIJ 2022

the curvature of 1.6×10–5 could be ignored. Considering plastic deformation energy approximates to zero for iron oxides,34) the possibility of prevention of tensile stress by bending deformation in the rust layers can be ignored. In this case, the rust layer will deform with the steel matrix when the rust layer is well combined with the steel matrix. During this continuous deformation process, the compres-sive stress in the rust layer is released gradually, and finally the tensile stress is formed. Simultaneously, because of the small modulus of elasticity and poor deformability of the rust layer, cracks will finally present in the rust layer with the continuing corrosion.35) According to Fig. 10, there is no peeling of the rust layer, even though cracks are gener-ated in the rust layer, implying that the steel surface is in good bonding with the formed rust layer.35) This process of continuous deformation will be accompanied by the whole process of corrosion, resulting in a great number of cracks in the rust layer. Moreover, greater tensile residual stress in the surface matrix will lead to larger surface deformation, thus causes more or wider cracks in the rust layer. The non-dense rust layer cannot provide good protection for the matrix, thus deteriorating the corrosion resistance of the sample. In addition, lattice distortion and dislocation stress field will occur near the region where the dislocation exists, which will lead to lower chemical stability of the region.31) At the intersection of dislocation line and sample surface, the corrosion resistance is lower than other parts of the surface, resulting in corrosion at the area in priority.30) As shown above, with the increase of the pulsed current den-sity, the surface tensile residual stress and dislocation den-sity of samples decreased dramatically. The reduced tensile residual stress in the surface leads to the less degree of the surface deformation during the surface corrosion, leading to the densification of rust layer. Furthermore, the reduction of dislocation density was conducive to the generation of uniform corrosion product layer due to conversion of local-ized corrosion to the general corrosion, thus improving the corrosion resistance of the rust layers.

Generally, EIS can be applied to evaluate the corrosion resistance of materials under the nondestructive influence on the corrosion status of the sample surface.36,37) Figure 12 shows Bode plots and Nyquist plots of the rust formed on simulated weld HAZ samples treated by different cur-rent density pulses after immersion in the 3.5 wt.% NaCl solutions. It can be seen from Bode plots (Figs. 12(a) and 12(b)) that, for the four samples of different states, only one time constant exists, which indicates that the stability of electrochemical reaction is mainly determined by the state of the rust layer on the electrode surface, and the sample’s corrosion resistance is mainly affected by the change of the rust layer. As can be seen from Nyquist plots (Fig. 12(c)), the characteristics of Nyquist plots remained unchanged as the current density increased under electropulsing treat-ment, while the surface impedance modulus of the rust layer increased. Meanwhile, the Fig. 12(c) shows that with the pulse current density increases, rust layers present substan-tially larger conductive loop diameter, which is associated with charge-transfer resistance of the rust layer. The higher the charge-transfer resistance, the less the number of active ions on the surface of the rust layer, and the higher the corrosion resistance of the rust layer. As discussed above,

treated by the electropulsing, the rust layer became denser which impeded the penetration of Cl − , thus reducing the contact between matrix and Cl −, and the acidification on the steel matrix surface was inhibited, leading to the decrease of the corrosion current density and the increased surface impedance modulus. Therefore, it is clear that the densification of rust layer and the disappearance of crack play an important role in the improvement of the corrosion resistance of the samples.

Fig. 12. EIS spectra of the rusted samples of simulated weld HAZ treated with different pulse current densities: (a) Bode plots of impedance module, (b) Bode plots of phase angle, (c) Nyquist plots. (Online version in color.)


© 2020 ISIJ2023

5. Conclusions

In order to study the effect of electric pulse treatment on corrosion resistance of X80 pipeline steel weld heat affected zone, the simulated weld HAZ samples were prepared by quenching method. Then electrochemical and immersion corrosion tests were carried out on the simulated weld HAZ samples which were treated with different current density pulses. In addition, the mechanisms of improving the corro-sion resistance of simulated weld HAZ samples by electric pulse treatment were studied and discussed. The main con-clusions are as follows:

(1) The X80 samples were water quenched to simu-late weld HAZ samples, which process caused martensitic transformation and resulted in residual stresses and lattice defects such as dislocations, which significantly reduced its corrosion resistance. The corrosion rate of the original X80 samples increased from 62 μm/year (original X80 samples) to 100 μm/year (water quenched samples), after water quenching.

(2) Electropulsing can effectively improve the corrosion resistance of simulated weld HAZ samples within the low corrosive condition, so far as the condition of slight corro-sion depth of about 2 μm. Electric pulse treatment reduced the dislocation density and the tensile residual stress in the surface of the simulated weld HAZ samples. Simultane-ously, the reduction of tensile residual stress and dislocation density gave rise to the increase of the corrosion resistance of the simulated weld HAZ samples. When the current density reached 5.2 kA/mm2, the corrosion resistance of the simulated weld HAZ samples could be improved to the base metal level.

(3) Treated by the electropulsing, there was few changes in the microstructure of the simulated weld HAZ samples. However, the rust layer became more uniform and compact, and there were few cracks in it, after the elec-tropulsing treatment. The better protection of the compact rust layer on the steel surface gave reason to the improved corrosion resistance.

(4) Electric pulse treatment, with its characteristics of high efficiency, good effect, energy saving and environmen-tal protection, provides a practical and effective new process for improving corrosion resistance of pipeline steel welding heat affected zone.

AcknowledgementsThe work was financially supported by National Natu-

ral Science Foundation of China (U1860206, 51874023, 51601011), Fundamental Research Funds for the Central Universities (FRF-TP-18-003B1), and Recruitment Program of Global Experts.

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