Galvanic corrosion behavior of bainite weathering steel welded joint in simulated marine atmosphere environment based on capillary microcell

doi:10.21203/rs.3.rs-3217410/v1

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Galvanic corrosion behavior of bainite weathering steel welded joint in simulated marine atmosphere environment based on capillary microcell

https://doi.org/10.21203/rs.3.rs-3217410/v1

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The kinetical behavior and mechanism of micro-galvanic corrosion of Q690 bainitic weathering steel welded joint in marine atmosphere were investigated. The results showed that a micro-galvanic corrosion cell formed among different regions of the Q690 welded joint. The kinetical behavior of micro-galvanic corrosion could be divided into four stages: the initiation and development of micro-galvanic corrosion, the decreasing of galvanic corrosion rate, the polarity reversal of galvanic couple, and the disappearance of galvanic effect. The difference of corrosion rate between Q690 steel and its welded joint is closely related to the micro-galvanic corrosion kinetic behavior.

Physical sciences/Materials science/Structural materials/Metals and alloys

Physical sciences/Materials science/Structural materials

Micro-galvanic corrosion

Welded joint

Q690 bainitic weathering steel

Marine atmosphere

Rust layer

Bridge steel has become a widely used steel after years of development. With the rapid development of social progress and bridge technology, higher requirements have been put forward for bridge steel, i.e., high strength, good corrosion resistance and welding performance, especially in the harsh marine environment. However, the microstructure of steel welded joints is often affected by the welding thermal cycling, resulting in a significant change compared with that before welding^[1], thus becoming the weak link of corrosion^[2], especially the local corrosion dominated by galvanic corrosion^[3–5] and stress corrosion^{[6, 7]}. The galvanic corrosion is the most common, with a high local corrosion rate and hidden, and a high likelihood of early catastrophic failure.

There are many reports on galvanic corrosion of welded joints of steel structures, mainly focusing on the research of influencing factors, including electrochemical factors (potential difference and polarization ability ^{[8, 9]}), geometric factors (cathode-anode area ratio, galvanic distance and spatial position^[8–12]), and environmental factors (temperature, oxygen content, conductivity Ph, etc.^{[9, 10, 13–15]}). For the above factors, researchers have tried a variety of quantitative and qualitative testing methods for different macroscopic galvanic corrosion, such as weight loss measurement, morphology observation, traditional electrochemical measurement technology^[16–18]. However, with the continuous development of various new technologies, various micro-zone testing methods has been employed to study the micro-galvanic corrosion, which can be induced by the difference in microstructure between the micro-regions, such as base metal (BM), heat affected zone (HAZ) and weld metal (WM). Wu et al. ^[19] detected the high and low potential areas on the surface of traditional and new 3% Ni weathering steel by scanning Kelvin probe (SKP). Li et al^[20] simulated the welded joint according to the composition and area ratio by using the array electrode technology, and studied its initial corrosion behavior. N. Wint et al. measured ^[1] the current density distribution in the laser welded joints of high strength low alloy (HSLA) and hot stamping ultra high strength steel (UHSS), and comparatively studied the galvanic corrosion behavior by employing scanning vibrating electrode technique (SVET). In our previous work^[21], the Volta potential distribution of A710 steel welded joints was determined by SKP to investigate the corrosion thermodynamic behavior of each micro-region of welded joints. However, conventional micro-electrochemical methods such as SKP, SVET, etc., could only give the surface potential or current distribution through the electrical signal between the probe and the metal surface, the electrochemical polarization behavior of each micro-region still cannot be obtained through the above technologies.

The self-designed three-electrode capillary micro-cell can accurately locate the tested micro-region by adjusting the capillary port diameter, thereby obtaining the potentiodynamic polarization curves, and then study polarization behaviors of different micro-regions of the welded joint. Therefore, the technology can overcome the current technical deficiencies and provide a great help for deeply studying the electrochemical mechanism of galvanic corrosion in the micro-regions of welded joint^[3]. Recently, our group improved the structure of capillary microcell, optimized the preparation parameters of Ag / AgCl reference electrode^[22], and used it to study the mechanism of inclusion-induced pitting initiation in stainless steel^{[21, 23]}.

In the paper, the 3.5% NaCl was used to simulate the marine atmospheric environment. The surface electrochemical activity and galvanic current of the welded joint of the new generation Q690 bainitic weathering bridge steel were studied by employing SKP, Scanning Reference Electrode Technology (SRET), Zero Resistance Current Meter and self-designed capillary microelectrode technique. At the same time, combining with Raman spectroscopy, field emission electron probe microanalysis (FE-EPMA) and other characterization techniques, and the influence law and mechanism of micro-galvanic corrosion on the corrosion kinetics behavior of the whole welded joint would be clarified, which can provide theoretical basis and data support for the service safety and protection of 690 MPa high strength bainitic weathering steel in simulated marine atmospheric environment.

2.1 Material preparation

The Q690 high-strength weathering steel used in this research was supplied by Wuhan Iron and Steel Corporation, and the welded joints was obtained using a gas shielded arc process. The chemical composition of the material and the welding parameters were shown in Tables 1 and 2.

Table 1

Chemical composition of welding wire (wt. %)
welding wire	C	Si		Mn	P	S	Cr	Ni	Mo	Fe
XY-ER80QNH	≤ 0.1	≤ 0.65	≤ 2.00		≤ 0.015	≤ 0.015	≤ 0.4	≤ 1.62	≤ 0.60	Bal.

Table 2

Gas shielded welding process parameters
welding wire	tensile strength R_m(MPa)	yield strength R_p0.2 (MPa)	elongation A (%)	Protective Gas
XY-ER80QNH	≤ 280	≤ 28	≤ 30	80%Ar + 20%CO₂

Figure 1 shows the microstructure of the welded joints of 690 bainite weathering steel. It is seen that the base metal (BM) is primarily composed of lath bainite (LB) and a small amount of ferrite (F), and the microstructure is relatively uniform (Fig. 1c). The fine grain heat affected zone (FGHAZ) mainly consists of polygonal ferrite (PF) and granular bainite (GB), with different grain sizes (Fig. 1d). The coarse grain heat affected zone (CGHAZ) (Fig. 1e) has coarse grains, and a large number of primary austenite boundaries (PABs) are observed (red dotted line in Fig. 1e), the microstructure is mainly composed of bainite and ferrite. The weld metal (WM) is mainly composed of non-uniform bainite and ferrite with coarse grains. There are significant differences in the grain size and microstructure in the different regions of the welded joint, which would result in the difference in the electrochemical activity between the different regions, and then initiating the galvanic effect between different regions ^{[24, 25]}.

2.2 Wet/dry alternating corrosion testing

The wet/dry alternating corrosion test was conducted to investigate corrosion of Q690 welded joint steels in simulated marine atmospheric environments according to standard GB/T 19746 − 2018^[26]. The total duration of the test was 576 h, and sampling was performed at 24 h, 48 h, 72 h, 96 h, 192 h and 576 h. Seven specimens were used for each type of steels for reproducibility of the results, where five parallel specimens (60 mm × 40 mm × 5 mm in dimension) were used for weight-loss testing, one (20 mm × 15 mm × 5 mm) was used for Raman analysis, one (20 mm × 15 mm × 5 mm) was used for morphological and elemental analysis by FE-EPMA. The test solution was 3.5 wt.% NaCl solution. The temperature in the test chamber during the drying period was (45 ± 2) ^oC, and the relative humidity was (70 ± 2) %. Each dry/wet cycle was 60 min, with a drying time of 48 min and an infiltration time of 12 min.

After wet/dry alternating corrosion test, the specimens used for weight-loss testing were ultrasonically cleaned in a descaling solution (500 mL hydrochloric acid + 500 mL distilled water + 3.5 g hexamethylenetetramine), and then washed with absolute ethanol, dried in high-purity nitrogen (99.999%) and weighed by a balance (m_t) with an accuracy of 0.1 mg. To determine the error caused by dissolution of the steel specimen in the descaling solution, a blank specimen (without corrosion test) of the same size was used for weight-loss correction in the descaling process. The blank specimen was placed in the etching solution with the same time, and the weight loss was recorded as m₁. The corrosion rate was calculated by:

$$\begin{array}{c}{\text{V}}_{\text{a}}\text{=}\frac{{\text{m}}_{\text{0}}\text{-}{\text{m}}_{\text{t}}\text{-}{\text{m}}_{\text{1}}}{\text{St}}\left(\text{1}\right)\end{array}$$

where V_a is the average corrosion rate (g/m²h). S is the sample area (m²). t is time(h). m₀ is the initial weight of the specimen. m_t is the weight loss of the specimen. m_l is the weight loss of blank specimen.

2.3 SKP testing

The Volta potential distributions for different regions of the Q690 weathering steel welded joints were measured using a VersaSCAN electrochemical workstation with the SKP technique with a size of 50 mm × 10 mm scanned area, a 100 µm/s scanning step at room temperature (~ 25 ^oC) and a relative humidity of 40%. A tungsten wire probe with a diameter of 100 µm was invoked as the needle, whose amplitude and frequency were 30 µm and 1 kHz, respectively. And the average distance between the sample surface and the Kelvin probe was approximately 100 µm during the testing. The welded joint specimens were wet ground with silicon carbide papers up to 2000 grit, and then polished with diamond suspensions (size of 3 µm). After that, the specimen was etched slightly with 4% nital (4 mL nitric acid + 96 mL ethyl alcohol) to distinguish the BM, HAZ and WM regions of the welded joint.

2.4 SRET testing

The surface current distribution of the welded joint during corrosion was measured by XMU-BY-LG scanning electrochemical workstation with a scanning size of 20 mm × 10 mm. The sample was first etched to show the three regions according to the experimental steps described in 2.3, and the boundary of each region was marked and then the sample was re-polished. According to the previous marker, the scanning area was selected to include BM, HAZ and WM.

2.5 Micro-electrochemical measurements

The electrochemical polarization behavior of different regions of Q690 steel welded joint was studied using a self-designed capillary microcell, as shown in Fig. 2. The traditional three-electrode system was fabricated using a capillary with a diameter of about 120 µm with the steel sample as the working electrode (WE), the Ag / AgCl wire (200 µm) as the reference electrode (RE), and the platinum wire wrapped outside the capillary as the counter electrode (CE). The linear polarization scanning range was ± 10 mV (vs OCP) and the scanning rate was 0.25 mV / s. The frequency range of electrochemical impedance was 10^− 2 ~ 10⁵ Hz, and the amplitude of AC excitation signal was 10 mV. The scanning range of potentiodynamic polarization was ± 250 mV (vs. OCP), the scanning rate was 1 mV / s, and the temperature was 25 ± 2 ^oC.

2.6 Galvanic current measurement in different regions of welded joints

Figure 3 shows the schematic diagram of the galvanic couple current measurement between different regions of Q690 steel welded joint. In the dry / wet cycle accelerated corrosion testing, the CST500(ZRA) was used to continuously measure the galvanic current in different regions of the Q690 steel welded joint. If the current measured by CST500 is positive, the working electrode I is the anode; on the contrary, the working electrode II is the anode. In addition, the area ratio of BM, HAZ and WM is the same as that in the dry/wet accelerated corrosion cycle testing, and the measurement were stop until the galvanic current approached zero.

2.7 Morphological and element composition characterization

The morphologies of the steel specimens after dry/wet alternating corrosion testing for various times were observed using a digital camera (LEICAVARIO-SUMMILUX-H). After the test was completed, the specimens were sealed in epoxy resin. The cross-sectional morphology of the corroded specimen was characterized by FE-EPMA-8050G with an acceleration voltage of 15 kV, irradiation current of 100 nA, and beam diameter of 1 µm.

Raman spectroscopy (Renishaw, inVia Qontor, England) was used to determine the composition of the corrosion products, with the laser wavelength of 532 nm, the power of 50 mW and the exposure time of 10 s. The scanning range was within 0∼1800 cm^− 1, with a linear scan range of 200 µm and the sampling dimension of 1 point per 90 µm. The results were analyzed using NanoScope Analysis 2.0 software.

3.1 Volta potential distribution of Q690 steel welded joint

The Volta potential distribution of the Q690 welded joint measured via Kelvin probe is shown in Fig. 4. From Fig. 4(b), it is seen that the potential for different regions of the Q690 welded joint progressively decreases in the order of BM, FGHAZ, WM and CGHAZ, and the maximum potential difference between these regions exceeds 100mV, reaching the electrochemical conditions for galvanic corrosion ^[27].

M · Stratmann^[28] proved that the self-corrosion potential E_corr of the metal had the following relationship with the volta potential ∆ Ф measured by the scanning Kelvin probe test :

$$\begin{array}{c}{\text{E}}_{\text{corr}}\text{=}\text{∆}\text{Ф}\text{+}\text{C}\text{o}\text{n}\text{s}\text{t}\left(\text{2}\right)\end{array}$$

where Const is the constant determined by the experiment.

Therefore, according to the measured volta potentials, the order of the Volta potentials of the different regions in the welded joint could be judged as follows: BM ( 482mV ) > FGHAZ ( 435mV ) > WM ( 417mV ) > CGHAZ ( 379mV ). The large potential difference in the different regions indicates that there may be galvanic corrosion for the Q690 steel welded joint in the harsh marine atmospheric corrosion environment.

3.2 Surface current distribution of Q690 steel welded joint

Figure 5 give the surface current distribution of Q690 steel welded joint in 0.01 wt % NaCl solution. An anodic current about 2.944 µA is observed on CGHAZ, which is higher than those of other regions, while a cathodic current about − 5.296 µA is observed on BM. The higher anodic current means that the CGHAZ had higher electrochemical activity, which acts as the anode of the micro-region galvanic corrosion cell. This is consistent with the testing results of SKP.

3.3 Corrosion kinetics of Q690 steel and its welded joint under cyclic immersion test

Figure 6 shows the average corrosion weight loss rate of Q690 steel base metal and welded joint after the wet/dry alternating test. It is seen that both the corrosion rates of Q690 base metal and welded joint increase firstly (24h-72h), and then reaches the maximum at 72h, and then gradually decreases. Comparing these two corrosion rates, it can be found that the corrosion rate of welded joint is larger than that of base metal, indicating that the corrosion resistance of welded joint is worse than that of base metal. The dotted line in Fig. 6 indicates the corrosion rate difference between the base metal and welded joint. It could be seen that with the increase of immersion time, the corrosion rate difference increases first and then decreases, reaching the maximum on the 6th day (0.2874 gm^− 2h^− 1).

3.4 The surface macro morphology of Q690 base metal and welded joint before and after removal of corrosion products

Figure 7 shows the macroscopic morphology of Q690 base metal and welded joint before and after rust removal in different corrosion cycles. With the increase of corrosion time, the rust layers on base metal and welded joint are composed of two layers of corrosion products: reddish brown outer layer and black inner layer, and gradually becomes dense (Fig. 7a). After removing the rust layer, the surface morphology of base metal after different corrosion cycles is almost the same, showing uniform corrosion characteristics (Fig. 7b). While for the welded joint, the samples emerge different degrees of groove-like corrosion morphology in the weld area after removing the rust layer, and the corrosion groove gradually becomes obvious (Fig. 7b).

3.5 Cross-section morphology and element distribution of the rust layer

The cross-sectional morphology and the element distribution of the rust layers of Q690 base metal and welded joint after corrosion for 24 days are shown in Fig. 8. It could be seen that both the rust layers of BM and WM are composed of inner and outer layers. The overall thicknesses of the rust layers of BM and WM are about 530 µm and 580 µm, respectively. There is not much different in the thickness of the inner rust layer, but the cavities and cracks in the BM rust layer are less than those in the WM rust layer, and the rust layer of BM is denser. In addition, Cr, Si and Cu elements are enriched in the inner rust layer, and Cu and Cr are enriched in the inner rust layer near the substrate side. Furthermore, the enrichment degree of Si, Cr and Cu elements in BM rust layer is higher than that in WM rust layer.

3.6 Raman spectrometer analysis of the rust layers of Q690 steel base metal and welded joint

The Raman spectra of the rust layers formed on the Q690 steel and welded joint are shown in Fig. 9. It can be seen from Fig. 9 that there is no difference in the chemical composition of the inner and outer rust layers between BM and WM, which are composed of α-FeOOH, γ-FeOOH and Fe₃O₄. It is worth noting that the intensity of α-FeOOH peak in BM inner rust layer is higher, which indicates that the content of α-FeOOH in BM inner rust layer is higher.

3.7 Electrochemical corrosion behavior of different micro-regions in Q690 steel welded joint

From the above experimental results, it is seen that macroscopic galvanic corrosion could occur on the Q690 steel welded joint in the marine atmospheric environment. In order to further clarify its galvanic corrosion mechanism, the electrochemical corrosion behavior of different regions of Q690 steel welded joint was measured by self-designed capillary microcell.

3.7.1 Micro-regions corrosion potential measurement

Figure 10 shows the variation curve of open circuit potential with time in three different regions (base metal, heat affected zone and weld metal) of Q690 steel welded joint. It could be seen from Fig. 10 that the open circuit potentials of the three different regions of the Q690 steel welded joint gradually decline until stable. The order of stable open circuit potentials in the three different regions is: BM (-0.474V) > FGHAZ (-0.515V) > WM (-0.526) > CGHAZ (-0.538V). The large potential difference between BM and CGHAZ indicates that the CGHAZ has a greater corrosion tendency than the other regions^[29], and the overall welded joint is more susceptible to galvanic corrosion.

3.7.2 Micro-regions linear polarization (LPR) measurement

The polarization resistance R_p could reflect the corrosion rate of metal to a certain extent^[30]. A large R_p value indicates the high corrosion resistance of the steel. The linear polarization curves of the different regions in Q690 steel welded joint are shown in Fig. 11. The fitted polarization resistance R_p values are: BM (2163.55Ω / cm²) > FGHAZ (1705.51Ω / cm²) > WM (1284.9Ω / cm²) > CGHAZ (960.38Ω / cm²), which indicates that BM has the best corrosion resistance, while CGHAZ has the worst corrosion resistance.

3.7.3 Micro-regions electrochemical impedance spectroscopy (EIS) measurement

EIS is one of the effective methods to study the corrosion resistance of steel ^{[31, 32]}. Figure 12 shows the Nyquist and Bode diagrams of the four regions (BM, FGHAZ, CGHAZ and WM). It is seen that the impedance spectra of the four regions emerge an extended semicircle (Fig. 14a).

In order to quantitatively analyze the corrosion resistance of the four regions of Q690 steel welded joint, the EIS were fitted with the equivalent circuit in Fig. 14a. The fitting results are shown in Table 3. The CPE-Y and CPE-n are a parameter related to capacitance and exponential term related to roughness of electrode surface, respectively. The electrode/solution transfer resistance R_ct, which reflects the difficulty of charge transfer at the electrode / solution interface during corrosion, is in the order: R_{ct (BM)} > R_{ct (FGHAZ)} > R_{ct (WM)} > R_{ct (CGHAZ)}. Therefore, the corrosion resistance of BM is the best, and the surface activity of the CGHAZ is highest (worst corrosion resistance), which is consistent with the linear polarization results.

Table 3

Q690 welded joint fitting results
	R_s/(Ω·cm²)	R_ct/(Ω·cm²)	CPE-Y(10^− 4Ω⁻¹cm⁻²s^CPE−n)	CPE-n
BM	45.02	1922.91	3.456	0.6079
FGHAZ	44.41	1212.81	4.801	0.6959
WM	44.89	1191.33	5.387	0.6795
CGHAZ	44.78	787.76	7.294	0.6825

3.8 Galvanic corrosion between different regions in Q690 welded joint

From the electrochemical measurement of each region in the Q690 welded joint ( Fig. 10–12 ), it can be seen that the surface electrochemical activity of BM is quite different from the other three regions. Combined with the surface volta potential ( Fig. 4 ) and current distribution ( Fig. 5 ), it could be preliminarily inferred that in the early stage of corrosion, BM is inclined to be protected in galvanic corrosion, while the corrosion of HAZ ( FGHAZ and CGHAZ ) is inclined to be accelerated, especially for CGHAZ. Therefore, there may be galvanic corrosion between the local regions of Q690 steel welded joint.

To intuitively and quantitatively analyze the galvanic corrosion behavior of Q690 steel welded joint, the galvanic current between the BM and HAZ + WM couples was measured, as shown in Fig. 13. In each dry/wet cycle, there are current fluctuations during alternate dry/wet cycles, and no current is displayed during the 48 minutes of drying period of each cycle. In addition, if the measured current is positive, the working electrode I ( WM + HAZ ) is the anode. It can be seen from Fig. 13 that a positive galvanic current is present in the initial stage of corrosion, which indicates that the WM + HAZ acts as anode and BM acts as cathode. By calculating the average value of the galvanic current in the immersion stage of each cycle, the variation of the galvanic current with time can be quantitatively analyzed. As shown in in Fig. 14, the change of galvanic current of Q690 welded joint under dry/wet cycle conditions can be obviously divided into four stages: (I) initiation stage; (II) development stage; (III) polarity reversal inversion stage; (IV) galvanic effect disappearance stage. Correspondingly, the galvanic current first rises, and reaches the highest value of 0.00256 A / cm², and then gradually decreases to zero, and finally the polarity is reversed. The negative galvanic current also increases first and then gradually decreases to zero.

4.1 Corrosion kinetics behavior of Q690 steel and its welded joint in simulated marine atmospheric environment

The double logarithmic model is usually used to study the atmospheric corrosion kinetics of metal materials^[33]:

$$\begin{array}{c}\text{lg}\left(\text{C}\right)\text{=}\text{n}\text{l}\text{g}\left(\text{t}\right)\text{+}\text{lg}\left(\text{A}\right)\left(\text{1}\right)\end{array}$$

where C is the reduction in thickness (µm) of metal material; t is the corrosion time (h). A and n are constants.

The double logarithmic model was employed to explore the corrosion kinetics of Q690 steel base metal and its welded joint with the measured weight loss corrosion rate, as shown in Fig. 15. The n value in Eq. (1) could reflect the characteristic of the corrosion kinetics. n < 1 means a declined corrosion process, and n > 1 means an accelerated corrosion process, while n = 1 means a constant corrosion process. From Fig. 15, the n value transfers from n > 1 to n < 1 between 72h-96h, which is consistent with the change of corrosion rate with time in Fig. 5. In general, the greater the n value, the greater the corrosion trend^[34]. In the initial stage of corrosion, the n value of the base metal and the welded joint is quite different ( Δn = 0.12 ) ( Fig. 15 ), but Δn decreases to 0.01 after 72 h. Therefore, the difference of corrosion rate between Q690 steel base metal and its welded joint decreases after forming a stable rust layer in the later stage of corrosion.

4.2 Evolution mechanism of the galvanic corrosion of Q690 steel welded joint

Generally speaking, the microstructure has a great influence on the corrosion resistance of weathering steel before the formation of rust layer^[35]. The Q690 bainitic weathering steel welded joint in this study has different electrochemical activities in different regions ( BM, HAZ, WM ) due to the difference in the microstructure in these regions ( Fig. 1 ) ( Fig. 4–5, Fig. 10 ). Then the corrosion behavior in different regions is significantly different ( Fig. 10–12 ), which leads to the occurrence of galvanic corrosion between different regions ( Fig. 13 ).

Figure 16 shows the four-stage diagram of the evolution mechanism of the galvanic corrosion of the Q690 steel welded joint in the marine atmosphere. In the early stage of galvanic corrosion ( before 72 h ) ( Fig. 16.I ), the HAZ + WM region with high electrochemical activity acts as the anode of the galvanic corrosion cell to dissolve preferentially, while the BM acts as the cathode to be protected. This forms a small anode/large cathode galvanic corrosion cell. ( the area of BM in the welded joint is larger than that of WM + HAZ ). With the formation of rust layer in the area of WM + HAZ, the area of small anode decreases and the corrosion rate increases. The corrosion of welded joint enters the first stage of galvanic corrosion evolution ( Fig. 13–14 ), and corrosion grooves begin to appear in the weld zone accordingly, and gradually deepens ( Fig. 7b ).

In the middle stage of corrosion ( 72 h − 144 h ), the larger area of WM + HAZ is covered by the rust layer ( Fig. 16.Ⅱ ). Although the area of the small anode is further reduced, some areas in the BM are also covered by the rust layer, resulting in a decrease in the overall galvanic corrosion rate ( Fig. 6. 72 h − 144 h ). At this time, the corrosion of welded joint enters the second stage with a declined galvanic current ( Fig. 14 ). Due to the existence of galvanic corrosion, the corrosion of the welded joint is accelerated, resulting in the deeper grooves on the surface of WM, which is most obvious on the 6th day ( Fig. 7b ).

With the further corrosion, the surface of WM + HAZ is covered by corrosion products in the middle and late stages of corrosion ( 144 h − 288 h ), and the self-corrosion potential of WM + HAZ shifts positively, which is higher than that of BM, so that the polarity of the galvanic corrosion cell is reversed^[36]. At this time, WM + HAZ is reversed to the cathode of the galvanic corrosion cell. BM reverse to anode, corrosion accelerated and the corrosion galvanic current changes from positive to negative, entering the third polarity reversal stage of galvanic corrosion ( Fig. 16.III ).

In the later stage of corrosion (288 h − 576 h ), with the extension of dry / wet cycle accelerated corrosion time, all the regions of the galvanic corrosion cell of welded joint are covered by a stable rust layer. The corrosion enters the fourth stage of galvanic corrosion ( Fig. 16.IV ). Then the galvanic effect is weakened until it disappears, and the galvanic current approaches to zero ( Fig. 14 ). At this time, the difference in the corrosion rate between the base metal and the welded joint is reduced, the difference in corrosion depth between the different regions is also gradually decreasing, and the corrosion grooves disappear with the time ( Fig. 7. 288 h − 576 h ). The galvanic effect ^[37], and the corrosion resistance of the welded joint is mainly determined by the protective performance of the rust layer ( Fig. 8 ). In addition, due to the enrichment of Cr, Cu, Si elements and α-FeOOH which endues the rust layer with good protection, the synergistic effect of these factors could greatly improve the protection ability of the rust layer, so that the corrosion rate difference between the base metal and the welded joint tends to be stable ( Fig. 6 ).

(1) In the simulated marine atmospheric environment, the whole corrosion process of Q690 weathering steel welded joint could be divided into two stages: accelerated corrosion and decelerated corrosion. In the same corrosion cycle, the corrosion rate of Q690 welded joint is larger than that of its base metal, and their corrosion rate difference increased first and then decreased until stable with the corrosion time goes by.

(2) The micro-galvanic corrosion can occur among the micro-regions of BM, FGHAZ, CGHAZ and WM of Q690 weathering steel welded joint due to the different surface electrochemical activities caused by its difference microstructure, and the kinetical behavior of the micro galvanic corrosion can be divided into four stages: firstly galvanic corrosion initiates, and HAZ and WM act as anodes with an accelerated corrosion; secondly, with the increasing area covered by rust layer in WM + HAZ, the overall galvanic corrosion rate decreases; thirdly, the polarity of the micro galvanic corrosion cell reversal occurs with the rust layer covering the surface of HAZ and WM, BM begins to act as an anode and the micro-galvanic corrosion is accelerated; Finally, the micro-galvanic effect of Q690 weathering steel welded joint will be weakened until it disappears due to the protection of the stable rust layer.

(3) The corrosion rates of the base metal and welded joint increase first with the galvanic effect, and tend to be stable after the reversal. The more significant the galvanic effect, the greater the difference in the corrosion rate between the base metal and welded joint.

Conflict of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NO. U21A20113) and Natural Science Foundation of Hubei Province of China (NO. 2021CFA023)

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

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You are reading this latest preprint version

Galvanic corrosion behavior of bainite weathering steel welded joint in simulated marine atmosphere environment based on capillary microcell

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Abstract

Figures

1. Introduction

2. Material and methods

2.1 Material preparation

2.2 Wet/dry alternating corrosion testing

2.3 SKP testing

2.4 SRET testing

2.5 Micro-electrochemical measurements

2.6 Galvanic current measurement in different regions of welded joints

2.7 Morphological and element composition characterization

3. Results

3.1 Volta potential distribution of Q690 steel welded joint

3.2 Surface current distribution of Q690 steel welded joint

3.3 Corrosion kinetics of Q690 steel and its welded joint under cyclic immersion test

3.4 The surface macro morphology of Q690 base metal and welded joint before and after removal of corrosion products

3.5 Cross-section morphology and element distribution of the rust layer

3.6 Raman spectrometer analysis of the rust layers of Q690 steel base metal and welded joint

3.7 Electrochemical corrosion behavior of different micro-regions in Q690 steel welded joint

3.7.1 Micro-regions corrosion potential measurement

3.7.2 Micro-regions linear polarization (LPR) measurement

3.7.3 Micro-regions electrochemical impedance spectroscopy (EIS) measurement

3.8 Galvanic corrosion between different regions in Q690 welded joint

4. Discussion

4.1 Corrosion kinetics behavior of Q690 steel and its welded joint in simulated marine atmospheric environment

4.2 Evolution mechanism of the galvanic corrosion of Q690 steel welded joint

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1