The effect of ultrasonic nanocrystal surface modification (UNSM) on the corrosion and stress corrosion cracking behavior of low carbon steel (ASTM A139) welded joint in the simulated district heating water (internal heating water) were investigated. After UNSM treatment, the microstructures of welded joint were transformed from the grain boundary ferrite and widmanstätten ferrite to polygonal ferrite accompanied by grain refinement. In electrochemical tests, the corrosion resistance of the welded joint was increased after UNSM treatment as a result of the grain refinement and improved stability of the oxide film. The stress corrosion cracking behavior was measured by slow strain rate tests with accelerated anodic and cathodic reactions. The results indicated that the UNSM treatment had a significant effect on the corrosion condition, whereas UNSM had no effect on hydrogen embrittlement.
The serious energy and environment problems are some of the important tasks for humanity at present. With the increase of these concerns, the district energy system has been on the rise as an improvement measurement in environment and economic aspects1-4 and actively applied in various countries.5-8 In the district heating system, the heat energy is distributed by circulating hot water through the underground pipeline.9 The buried pipe is exposed to various stresses in operation as a result of various external and internal conditions. The major stresses are as follows: (a) the friction stress between the soil and buried pipe resulting from the expansion and contraction from temperature changes of pipe, (b) thermal fatigue resulting from the temperature change of circulated water in buried pipe, and (c) hoop and axial stress from internal fluid pressure.10-12 These conditions influence the failure of pipeline.
Also, the residual stress on the welded joint in pipe is an important factor for buried pipeline. The welding residual stress may make a huge contribution to the total stress field. The residual stress caused by welding can produce the risk for static fracture in piping systems containing materials with brittle fracture behavior.13 For instance, a large residual stress (tensile stress) in the heat affected zone (HAZ) is considered as one of the important factors of stress corrosion cracking.14 Thus, to reduce the residual stress in welded joints of pipe, various treatments have been applied and studied.
Various treatments such as heat treatment, surface modification, and installation of buffer have been used as improvement methods of welded joint. Especially, the heat treatment was conventionally used in circumferential butt-welded joint in the pipe.15-16 However, the heat treatment was only applied before the pipe burial and required several pieces of equipment to heat the welded joint, which required a long time. Therefore, another treatment was needed for portable use, high operability, and short preparation time. As a result, the surface mechanical attrition treatments (SMAT) have been promising alternative methods.17-18
SMAT is a surface modification technology that has been researched over the past two decades.17-19 This treatment was based on the severe plastic deformation (SPD) treatment, which improved the various mechanical properties such as hardness, yield strength, and toughness, etc., according to the Hall-Petch relationship.20-23 The SMAT method can improve the comprehensive mechanical properties of the specimen by the induction of strengthened surface layer without changing good ductility and elasticity of interior materials.24-25 Some peening methods in SMAT were reported to decrease the corrosion resistance.26-29 The peening method decreased the corrosion resistance of stainless steel because of the increase of strain-induced martensite, which gave rise to a galvanic effect between austenite and martensite.30 In the case of shot peening, the corrosion resistance is reduced because of the increase of surface roughness, which increases the surface energy.31 Thus, both corrosion and mechanical behavior after SMAT are important issues.
In this work, an ultrasonic nanocrystal surface modification (UNSM) method was applied as a SMAT treatment. UNSM, a cold process treatment which applies an ultrasonic peening source, has been widely used for various steels and aluminum alloys to enhance their mechanical properties.32-34 The specific applications of the UNSM treatment are the improvement of fatigue life35 and wear resistance36-37 for welding joints, bearings, and industrial knives, and restoration of the components operated in long service such as bearings, turbine blades, and rolling stock.38 However, the influence of UNSM on the corrosion and stress corrosion cracking behavior of welded joints in carbon steel have not yet been systematically studied. Thus, in the present work, welded joint of pipe treated by UNSM was prepared. After UNSM treatment, the evaluation of corrosion and stress corrosion cracking behavior with microstructure analysis was investigated by electrochemical and slow strain rate tests (SSRT).
The material used in this study was a welded joint of SPW 400 (ASTM A139) pipe. The welded joint was produced by multi-pass gas tungsten arc welding (GTAW). The chemical composition is as follows: 0.25 C-1.00 Mn-0.040 P-0.040 S-Fe (wt%). The specimens were machined from the welded pipe joint according to ASTM E8. The schematic sectioning procedure and specimen size are illustrated in Figure 1. The UNSM treated specimens were produced by ultrasonic equipment, which consisted of a piezoelectric transducer and a tungsten carbide tip. The frequency of the UNSM treatment is 20 kHz. The schematic process of UNSM treatment is illustrated in Figure 2. Before and after the UNSM treatment, the surface roughness was measured by the Mitutoyo Surftest SJ-200†. The maximum height roughness (Ry), ten-spot average roughness (Rz), and arithmetical average roughness (Ra) of each specimen are the average values of three measurements. To investigate the microstructure of the welded joint after UNSM treatment, the specimens were etched by 2% Nital (98% ethanol + 2% nitric acid), and then observed with an optical microscope.
To investigate the corrosion resistance of the welded pipe joint in district heating system, electrochemical measurements such as potentiodynamic polarization, galvanostatic, and electrochemical impedance spectroscopy (EIS) tests using multi-potentiostat/galvanostat model VSP-300† were conducted in deaerated district heating water environment at 60°C. The chemical composition of simulated district heating water is listed in Table 1 and pH was controlled by 0.1 M NaOH solution. The simulated district heating water is based on the actual environment and the chemical composition was the average value of district heating water in the five locations of the district heating system. Generally, the internal heating water of the district heating system was controlled for the prevention of various problems such as scale and corrosion. A three-electrode cell was constructed with the specimen (welded joint) with and without UNSM treatment as the working electrode (WE), two pure graphite rods as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). Before the electrochemical measurements, the as-received specimen was abraded with SiC paper from 220 to 600 grit sizes, and the specimens were immersed in the test solution for 6 h to attain the equilibrium condition. Potentiodynamic polarization measurements were performed at a potential sweep of 0.166 mV/s from an initial potential of −250 mV versus open-circuit potential (OCP) to the final potential of 1,000 mVSCE.
To investigate the oxide layer on the surface after UNSM treatment, the corrosion reaction of specimens was accelerated by galvanostatic tests (3 mA/cm2 for 20 h) in the test solution. The surfaces of specimens were then analyzed using an x-ray diffractometer (XRD, Bruker D8 Advanced†) using Cu-Kα radiation operated at 18 kW with a scanning speed of 2°/min.
EIS tests were performed at OCP after a 6 h immersion in the test solution to investigate the surface properties. The frequency range of the EIS tests was from 100 kHz to 10 mHz and an alternating current amplitude of ±10 mV. The impedance plots were interpreted on the basis of an equivalent circuit using a suitable fitting procedure by the ZsimpWin† software.
Vickers microhardness (HV) measurements were performed on the surface according to weld zone, HAZ, and base metal before and after UNSM treatment with a load of 100 gf (0.98 N) in 10 s.
SSRTs, which are used to study stress corrosion cracking, were performed in the district heating water. The specimens with and without UNSM treatment were tested under three conditions (OCP and ±1 mA/cm2). Positive and negative currents were applied for the acceleration of anodic (corrosion) or cathodic (hydrogen reduction) reactions, respectively. Under these conditions, the SSRTs were conducted at a constant crosshead speed of 4.8 μm/min which corresponds to a nominal strain rate of 5 × 10−6 s−1. After the SSRTs, a scanning electron microscope (SEM) from HITACHI S3000H† was used for fractographic analysis of the fractured specimen surfaces.
RESULTS AND DISCUSSION
Figure 3 shows the optical micrographs of the cross-sectional area of UNSM treated welded joint (Figure 3[a]) and their corresponding microstructures with and without UNSM treatment (Figures 3[b] and [c], respectively). The cross-sectional area was divided into UNSM treated and untreated layers as shown in Figure 3(a). The grain in the UNSM treated region was refined compared to the untreated region (welded joint). According to ASTM E112, the numbers of the grain sizes in the untreated and treated specimens were 5 and 8, respectively. This result indicates that the UNSM treatment influences the grain refinement at the metal surface. The UNSM treated layer consists of polygonal ferrite, while the untreated welded specimen consists of widmanstätten ferrite with aligned secondary phases.39 The UNSM treatment generated the deformation of the surface with a thickness of about 75 μm, which resulted in the aforementioned phase transformation.
The mechanism of grain refinement by UNSM has been discussed in previous studies.40-44 The normal stress applied to the specimen surface resulted in semi-spherical stress field gradient in the original large grains. Stacking faults were generated by the increased strain at the surface, which activated the dislocation slip systems. High-density dislocations were arranged due to the dislocation movement and cross slip. These dislocation arrays occurred in the subgrain region and were accompanied by storage of high strain energy. The stored strain energy decreased the energy for recrystallization and transformation of the materials, and finally grain refinement happened.43-44
In the first law of thermodynamics, mechanical work can increase the internal energy, which is related to the total free energy of a system. Thus, the total driving force for phase transformation and recrystallization (ΔGdriving) is expressed as the summation of the chemical driving force (ΔGchem) and the driving force caused by the stored deformation energy (ΔGdef), as expressed in the following equation:45-47
From a microstructural perspective, the stored energy of deformation is a function of the microstructural dislocation density changes.48 It means that the increase of dislocation density resulting from the UNSM treatment leads to an increase in total driving force (ΔGdriving) for transformation and recrystallization, which accelerates nucleation and growth kinetics.49-51 Therefore, after the UNSM treatment, the microstructure of the welded joint was transformed from the grain boundary and widmanstätten ferrite to the polygonal ferrite.
Figure 4(a) shows the potentiodynamic polarization curves of the untreated and UNSM treated specimens in deaerated district heating water at 60°C. Parameters such as corrosion current density (icorr), passive current density (ipp), corrosion potential (Ecorr), and pitting potential (Epit) were determined from polarization curves and are listed in Table 2. As shown in the polarization curves, both specimens were in the passive state. The UNSM treated specimen showed a lower value of passive current density than untreated specimen. The increase in the pitting potential from −489 mVSCE to −391 mVSCE indicates that the pitting resistance of UNSM treated specimen was improved. Because of the competition between dissolution and passivation of the film on the surface, an anodic current hump in the polarization curve appears at about −570 mVSCE.
After UNSM treatment, the icorr, ipp, and Ecorr decreased and Epit increased, which indicates that the UNSM treatment has a beneficial effect on the corrosion resistance of the welded joint. The decrease of surface roughness was related to the increase of corrosion resistance after UNSM treatment.52 However, the surface roughness after UNSM treatment was higher than that of the polished specimen, as listed in Table 3; thus, the surface roughness could not be related to the increase of corrosion resistance in this study. One of the main reasons for the preferential weld metal corrosion was related to microstructure. Weld metal that has largely unrefined ferrite with aligned second phase microstructures had lower corrosion resistance. However, the refined weld metal with less aligned second phase had similar corrosion resistance to the base metal.53-54 The schematic polarization curve after UNSM treatment in the welded joint is illustrated in Figure 4(b). Because of the microstructure transformation, the total reactions were decreased and passivity was increased (increase of Tafel slope). It affects the polarization curve shift and changes the electrochemical parameters in polarization curves.
Figure 5 shows the Pourbaix diagram of iron and the reaction potentials calculated for magnetite (Fe3O4) and hematite (Fe2O3) at pH 10.55 As shown in Figure 5, the potential range of the Fe3O4 formation at pH 10 is from −879 mVSCE to −561 mVSCE, and Fe2O3 is formed above the −561 mVSCE. It implies that the oxide layers of Fe2O3 and Fe3O4 acted as a protective layer formed on the surface.56-58 Also, the potential at about −570 mVSCE was equivalent to the Fe2O3 formation in Pourbaix diagram, which indicates the change of oxide (Fe3O4 → Fe2O3) at the hump of polarization curves. In XRD results of the corrosion acceleration specimens, the formation of Fe3O4 and Fe2O3 was observed, as shown in Figure 6. It indicates that the oxide layers of Fe2O3 and Fe3O4 were formed regardless of UNSM treatment, and it verifies the relation between the formation of an oxide layer and the Pourbaix diagram.
Figure 7 shows the Nyquist plots for UNSM treated and untreated welded joint specimens in deaerated district heating water at 60°C. The increase of corrosion resistance after UNSM treatment in welded joint was indicated in EIS results. Figure 8 shows an equivalent circuit containing two relaxation time constants, which is used to show the formation of oxide film on the metal surface.59-61 The equivalent circuit consists of the following elements: Rs is the solution resistance, Rfilm is the electrical film resistance from the formation of an ionic conduction path through pores in the oxide film, Cfilm (CPE1) is the film capacitance generated by the dielectric properties of the film, Rct is the charge transfer resistance from the metal dissolution, Cdl (CPE2) is the double-layer capacitance generated by the electric double-layer capacitance at the water/substrate interface. Constant phase element (CPE) has been used in the equivalent circuit instead of the pure capacitor to get a more accurate fit. The impedance CPE is expressed as:
where Y0 is the magnitude of the CPE, j is the imaginary number (j2 = −1), ω is the phase angle of the CPE, and n = α/(π/2). CPE describes an ideal capacitor when n = 1, which measures the deviation from the ideal capacitive behavior.
Table 4 summarizes the impedance parameters for UNSM treated and untreated specimens in district heating water with deaeration at 60°C. The CPE is converted into the capacitance (C) by using the following conversion equation:62
where fmax is the frequency at which the imaginary component of the impedance reaches the maximum value.
After UNSM treatment, Rfilm and Rct increased, indicating that the resistance of the oxide film to corrosion increased and the charge transfer reaction between metal surface and solution was decreased, respectively. These results are well matched with the potentiodynamic polarization results because the decrease of ipp was related to the increase of Rfilm after UNSM treatment and icorr is inversely proportional to the total resistance (Rtotal = Rfilm + Rct) according to the following equation:
where βa and βc are the anodic and cathodic Tafel slopes, respectively.
The capacitance related to the oxide film (Cfilm) and Helmholtz electrical double layer (EDL, Cdl) between the water and substrate are described by the following equation:
where ε is the dielectric constant of the oxide film or EDL, ε0 is the vacuum permittivity, A is the effective surface area of the electrode, and d is the thickness of the oxide film or EDL. Cfilm is decreased after UNSM treatment, implying that the thickness of the oxide film is increased. Also, the parameter n of the Cfilm is increased after UNSM treatment. The n indicates various surface conditions such as roughness, heterogeneity, and porous layer.63 Thus, after UNSM treatment, the oxide film not only thickened more but also became stable, which influences the corrosion resistance of oxide film. In addition, the change of oxide film increases the Rct as a result of the obstruction of water molecules adsorption on the metal surface by a more protective oxide film. In summary, UNSM treatment influences the stability of oxide film and increases the corrosion resistance of welded joint.
Hardness and Slow Strain Rate Tests
Figure 9 is the profile of the hardness of the specimens with and without UNSM treatment. The hardness at the weld zone, HAZ, and base metals were increased in all parts. This means the compressive stress and grain refinement influence the increase of surface hardness. Also, the hardness difference between the base metal and weld zone is decreased from 80 HV to 35 HV, indicating the uniformity of UNSM treatment. Figure 10 shows the SSRT stress vs. strain curves for UNSM treated and untreated specimens in the district heating water under OCP and ±1 mA/cm2. The percentage of elongation and reduction area was also obtained (Figure 11). The hardness, yield, and tensile strengths were increased after UNSM treatment because of the Hall-Petch equation relating yield stress or hardness to grain size of materials. Also, the strain which is related to the ductility was improved after UNSM treatment.
Although the UNSM was only treated on the surface (75 μm), the strength and ductility of the bulk tensile specimen which is 4 mm in diameter were increased. This means that the surface modification can affect the mechanical properties of bulk metal. In previous studies, it was indicated that the UNSM treatment on the surface has an effect on the mechanical properties (hardness, strength, toughness, wear) of bulk materials.64-65 The surface conditions such as roughness, oxide, coating, and microstructure can affect the mechanical properties of metallic materials. Especially, the microstructure differences (phase and grain size) on the surface influence the dislocation movement, and thus the SPD method, which is the mechanical treatment on the surface, is applied for the improvement of mechanical properties.66 It is widely known that refined grain by SPD method can significantly improve the hardness and strength of metallic materials. Because of the existence of high density of grain boundary, the mean free path for dislocation is decreased and therefore the alloy strength and hardness are increased.67-68 In the case of ductility, most dislocations in refined grained materials are not operative,69 and thus it is hard to induce the plastic deformation of materials. Also, dislocations at the nanoscale grain boundaries on the surface tend to disappear and serve as sinks for dislocation annihilation.70-71 However, in this study, the ductility of UNSM treated specimen was significantly improved, which indicated that it was not the refined grain size but the phase transformation and compressive residual stress that affected the improvement of ductility by providing more movable dislocations to sustain the plastic deformation.72
Under OCP and 1 mA/cm2, the ductility index of UNSM treated welded joint was improved about 78.5% and 72.2%, respectively. This indicates that the UNSM treatment not only increased the ductility under equilibrium condition but also increased the ductility in the corrosion environment. As mentioned above, the improvement in ductility after UNSM treatment was related to the transformation of microstructure in welded joint. Before the UNSM treatment, the grain boundary ferrite and widmanstätten ferrite were observed in the welded joint. However, these microstructures were transformed to polygonal ferrite after UNSM treatment. Grain boundary and widmanstätten ferrite as well as laminar ferrite increase the brittle behavior of the microstructure and generally reduce the toughness.39,73 The dislocation is more easily movable on the surface, and thus ductility of the welded joint was increased after UNSM treatment. In addition, compressive residual stress caused by UNSM treatment can increase the ductility of the welded joint.35 Under accelerated corrosion conditions, the corrosion reaction did not degrade the mechanical properties of UNSM treated welded joint because the UNSM treatment increased the corrosion properties of welded joint as mentioned previously.
On the other hand, the ductility index under −1 mA/cm2 did not significantly increase (only about 12.5%) after UNSM treatment. In an alkaline solution under deaerated conditions, the main cathodic reaction is:52
The embrittling effect of H on the mechanical properties of metals has been well reported in various fields.73-74 Hydrogen embrittlement is particularly severe in steels because of the high mobility of H in Fe. Although various mechanisms have been proposed like H-enhanced decohesion and H-enhanced local plasticity, the basic concept is related to the H diffusion on the surface and crack tip.75-76 Thus, the results indicate that UNSM treatment did not have a significant effect on the hydrogen embrittlement of welded joint. It implies that transformed microstructure by the UNSM treatment does not have a significant effect on the hydrogen diffusion in steel.
SEM morphologies of the fracture surfaces for UNSM treated and untreated welded joint are shown in Figures 12 and 13, respectively. Under OCP and 1 mA/cm2 conditions, the area reductions and the dimple structure that correspond to ductile fracture were observed regardless of UNSM treatment as shown in Figures 12(a), 12(b), 13(a), and 13(b). A dimple structure with quasi-cleavage was observed in the untreated specimen, while only a dimple structure was observed in UNSM treated specimen. This suggests that the region of ductile fracture was expanded after UNSM treatment, which in good agreement with the SSRT results. On the other hand, brittle fracture was clearly observed, such as scant decrease of reduction area and cleavage feature under −1 mA/cm2 regardless of UNSM treatment as shown in Figures 12(c) and 13(c). It indicates that the hydrogen attack was not inhibited by UNSM treatment.
The results indicated that the UNSM treatment can improve the mechanical properties of welded joint not under the hydrogen-induced condition but under the corrosion condition. It means the UNSM treatment is an effective application method for the increase of reliability and durability of the low carbon steel welded joint in corrosive environment without the replacement of the high-price materials or change of welding process. And, it can be applied for the in-service welded joint for restoration without installing a new one, which has a high economical advantage. In addition, the UNSM treatment can be applied not only to the welded joint in the district heating system but also to the welded joint in the various structures and components that need high mechanical properties and are exposed to corrosive environment.
Microstructure analysis, electrochemical tests, and slow strain rate tests were performed to investigate the influences of UNSM treatment on the corrosion and stress corrosion cracking properties of a GTAW welded ASTM A139 pipe joint. The following conclusions were drawn from the experimental results and discussion:
Grain boundary ferrite and widmanstätten ferrite microstructures in welded pipe joint were transformed to polygonal ferrite with refined microstructure after UNSM treatment as a result of an increase in dislocation density and recrystallization.
The corrosion resistance of the welded joint in deaerated district heating water was increased after UNSM treatment. Surface roughness did not affect the stability of the oxide film, but the refined and transformed microstructure did affect oxide film, which is related to the corrosion resistance.
Stress corrosion cracking properties measured by SSRT indicated that the ductility of welded joint was significantly increased after UNSM treatment under OCP and 1 mA/cm2 (corrosion-accelerated condition). This was not the case under the −1 mA/cm2 (hydrogen-generated condition). It implies that the application of UNSM treatment is appropriate for components exposed to a deaerated district heating water environment at 60°C. The UNSM treatment did not improve the hydrogen embrittlement resistance.
This research was supported by the Korea District Heating Corporation (No. 0000000014524). This research was supported by Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015H1A2A1033362).