Corrosion Crack Nucleation Mechanism in the Welded Structures of X 65 Steel in Natural Seawater

+e nucleation mechanism of corrosion cracks in welded structures in natural seawater has been revealed through investigation on the corrosion behavior of X65-welded structures in natural seawater. +e evolution laws of surface corrosion morphology, residual stress, and areas of welded structures with the most serious corrosion damage were analyzed by microstructure observation, corrosion morphology observation, residual stress detection, and magnetic field detection. +e results show that the main factors of determining surface corrosion morphology evolution law of welded structures are microstructures of the weld joint, heat-affected zone (HAZ), and the fusion line. Due to the difference of corrosion rates between the weld joint, HAZ, and the coarse Widmänstatten structure in fusion line, the fusion line of welded structures receives the most serious damage while being corroded. Meanwhile, mutual reinforcement of residual stress and corrosion damage on the surface of welded structures further accelerate the nucleation of corrosion cracks. Under the influences of microstructure, residual stress, and corrosion, corrosion cracks that are parallel to weld joints generate first in the fusion line of welded structures.+erefore, it makes welded structures the source of marine steel structures’ corrosion failure.


Introduction
Due to the good connection performance and excellent air tightness, welding fabrication has been applied in the manufacturing of marine steel structures such as drilling platforms and seabed pipelines [1][2][3][4].However, welded structures will always be high-risk areas for the occurrence of corrosion crack in the sea and source of corrosion failure of marine steel structures [5][6][7][8].With international investment increasing in marine projects, corrosion protection of welded structures in the marine environment has been a focus of research [9][10][11][12].
In marine environments, embalmment of welded structures is usually done by smearing marine anticorrosive coating [11,[13][14][15].However, absorption and osmosis of corrosion factor (Cl − ) still enable it to penetrate protective coating, leading to the generation of corrosion couple on metal surface and coating failure [16,17].erefore, to prevent the occurrence of corrosion failure accidents of welded structures in the marine environment, research on the corrosion crack nucleation mechanism of welded structures in the marine environment needs to be carried out.
With studies and reports on corroded welded structures increasing in recent years, researchers have deeper understandings of corroded welded structures [18][19][20][21].However, the research emphasis is always on the corrosion characteristics of different weld joints [7,8,20], extended behavior of corrosion fatigue crack [22][23][24], and specific corrosion environment's impacts on corrosion characteristics of welded structures [25][26][27].Relatively little research is dedicated to the nucleation mechanism of corrosion cracks in welded structures.Moreover, although theories such as hydrogen-induced cracking and anodic dissolution can reveal the corrosion crack nucleation mechanism of nonwelded structures to some extent [28][29][30][31], such traditional theories are not sufficient to reveal the nucleation mechanism of corrosion cracks in welded structures because of the particularities in terms of crystal structure and residual stress distribution of welded structures.
As an important factor in influencing the corrosion behavior of welded structures, residual stress has obvious impacts on the corroded damaging rate of welded structures [32][33][34][35].However, most reports focus on the impact of residual stress on the corrosion behavior in the research of corroded welded structures, but mutual effects between residual stress and corrosion damages are ignored.e overlook of influence on the surface residual stress distribution of welded structures arising from corrosion morphology changes makes research conclusions inaccurate.us, interaction between residual stress and corrosion damage needs to be studied to reveal the nucleation mechanism of corrosion cracks in welded structures.
In this paper, welded structures of X65 steel were studied, and the microstructures of the weld joint, HAZ, and fusion line were observed with an optical microscope.Moreover, residual stress inside the part of the specimens was lowered with vibratory stress relief (VSR) method.Evolution of corrosion morphologies of welded specimens that have different residual stress in natural seawater was observed with scanning electron microscope (SEM), aiming to show the influence of residual stress on the corroded damaging rate of different parts of the welded structure.Blind-hole method was used to detect the evolution law of surface residual stress of the corroded welded specimen.
en, influence of change of surface corrosion morphology on residual stress distribution was obtained, and the interaction between residual stress and corrosion damage was summarized.Additionally, a magnetometer was used to detect magnetic field distribution of welded specimens' surface in the process of corrosion, so as to study evolution law of distribution of areas that received the most serious corrosion damage on the surface of the welded structure.Nucleation mechanism of corrosion cracks in the welded structures in natural seawater was analyzed by synthesis with experimental data.

Materials and Welding Process.
e dimensions of X65 steel-welded specimens were 350 mm × 150 mm × 10 mm after welding, and the direction of weld joint was parallel to the short edge with length 150 mm. e chemical components of X65 steel are listed in Table 1.
e groove type was V-groove, groove angle was 60 °, counterpart gap was 2.5 mm, and truncated edge was 1.5 mm. e weld reinforcement was 1.6 mm, and the cap weld was 1.6 mm wider than both sides of the groove after welding.e schematic of groove is shown in Figure 1.
All specimens were processed with root welding of cellulose electrode.Furthermore, semiautomatic selfprotection welding with flux-cored wire was utilized in weld beads of filling and covering.Electrode of cellulose type was E6010 and flux-cored wire was E71T8-K6.Welding parameters for each layer of weld are shown in Table 2.  3.While the specimens were immersed in the seawater, distances of 2 m and 0.8 m were maintained from the upper surface of seawater and seafloor, respectively.Test specimens were connected with graphite flake through wires to constitute a galvanic cell structure in the experiment, aiming to increase the corrosion rate of the welded specimen.rough comparison of the corrosion rates of specimens with and without graphite flake in 60 days, the specimen-graphite galvanic cell structure was proved to increase the corrosion rate of the specimen by 69.1%.Welded specimens were divided into 8 groups before the test, numbering A1-A8.

Seawater Corrosion
ere were three specimens in each group, numbering A1-1, A1-2, . . ., A8-2, A8-3.To exclude the effect of difference among specimens' initial surface morphologies on evolution of corrosion morphology of specimens, the surface of each specimen was burnished before corrosion test.
To eliminate the residual stress in specimens of group A1-A4, vibration was imposed by high-speed motor on specimens of group A1-A4 with the VSR method before the corrosion test.e effect of surface residual stress of the welded structure on corroded damaging rates of the weld joint, HAZ, and fusion line was studied through the comparison of differences between surface corrosion morphologies of specimens processed and unprocessed by VSR [36,37].In order to guarantee the residual stress elimination effect of the VSR method, a three-dimensional stress magnetometer of JH-80 was used to detect the surface residual stress of group A1 before and after VSR. e sensitivity coefficient of X65 steel needed to be measured before detection to improve the accuracy of detection results, and the value of X65 steel's sensitivity coefficient was 0.0153 mA/MPa, which was calculated from measurement results.Limited by the size of specimens, only transversal residual stress whose direction was perpendicular to the weld joint was detected.Detection results showed that the After the seawater corrosion test starts, specimens from group A1-A8 were taken out, respectively, on day 60, 120, 180, and 360, and corrosion products on the surface were removed by H 2 SO 4 solution (GB 6384-86, UDC 669: 620.193.1).Additionally, corrosion inhibitor of thiourea was added to protect the base material while pickling.After wiping out corrosion products, corrosion morphology, residual stress distribution, and magnetic field distribution on the surface of specimens were detected and experimental data were collected.e specimens taken at each time node and data needed to be collected in the seawater corrosion test are shown in Table 4.

Collection of Test Data.
e weld joint, HAZ, and fusion line of the welded specimen were each intercepted from an uncorroded welded specimen whose welding process was identical to specimens of group A1-A8, used for the manufacturing of metallographic specimens.After interception, inspected surface size of metallographic specimens was 30 mm × 10 mm and the height of specimens was 10 mm.Surface treatment of metallographic specimens followed the Chinese national technique standard GB/T13298-2015.After wearing off the specimen's surface, the surface was burnished by a mechanical prototype and polished by a polishing machine.en, the metal surface was soaked in a mixed solution made from HNO 3 and alcohol to show the microstructure.HNO 3 accounted for 5% of the eroded solution and the eroding time was 45 s.After finishing making metallographic specimens, the microstructure on the surface was observed with an optical microscope.Observation of corrosion morphology was finished with SEM.When observed, corrosion morphologies of the corroded specimen's weld joint, HAZ, and fusion line's surface were recorded, respectively.Furthermore, magnetic field distribution on the corroded surface of the welded specimen was detected with stress concentration magnetic detector TSC-2M-8 [38].e magnetic field in the middle area of the corroded specimen's surface, which covers 240 mm × 60 mm, was detected, and the collected data were processed with software MMM-System 3.0.
e external magnetic field compensation was open while being measured, and step size S and measured spacing b were 2 mm and 4 mm, respectively.
Before the detection of residual stress, stress-releasing coefficients of X65 steel in the blind-hole method needed to be measured according to the Chinese national technique standard CB 3395-92, used for the calculation of residual stress.A special drilling device of ZS21B and static strain gauge of DH3820 were used to measure the stress-releasing coefficients, and values of borehole depth and borehole aperture were 1.5 mm and 2 mm, respectively.e measurement results of stress-releasing coefficients were A � −0.04882 and B � −0.05533.Instruments and borehole parameters used in the detection of residual stress were identical to that used in the measurement of stress-releasing coefficients, and the detection and calculation of residual stress in the blind-hole method followed the Chinese national technique standard CB 3395-92 as well.
As the corrosion surface of the welded specimen was not flat, the surface of the measured point was polished up with a handheld high-speed grinding head to paste strain flower effectively.Considering that surface polish might have some impacts on the detection results of residual stress, additional strain arising from the grinding operation needed to be measured with tests so that it could be applied to revise the results of residual stress detection.To determine the additional strain, first the high-speed grinder head was used to polish up the surface of a X65 steel plate without internal stress.en the steel plate's surface, including the polished and unpolished area, was drilled with drilled holes' parameter, which was the same as the one detected from residual stress.Finally, the difference between average value of released strain that passed the grinding position and average

Results and Discussion
3.1.Microstructure.Microstructures of the weld joint, HAZ, and fusion line are shown in Figure 2.While being welded, weld joint, whose microstructure is made of granular ferrite, acicular ferrite, and pearlite, is formed through crystal nucleation growth and solidification of welding pool.Large amounts of granular pearlite are distributed in the space between granular ferrite and acicular ferrite, making the microstructure of the weld joint finer (Figure 2(a)).Due to the heat input in the process of the welded structure, crystal expansion and bulk ferrite with large size appear in HAZ.Meanwhile, a small amount of bainite is deposited.us, the microstructure on the surface of HAZ is mainly made of granular ferrite, with a small amount of bainite (Figure 2(b)).e volume of metal crystal between base metal and weld joint increases with more heat input absorbed by the base metal besides the fusion line; moreover, temperature gradient changes of metal in the fusion line are greater while cooling, leading to the creation of coarse Widmanstätten at last (Figure 2(c)).

Corrosion Morphology Evolution.
e corrosion morphology observation results of the weld joint, HAZ, and fusion line's surface of specimens in groups A1-A4 and groups A5-A8 were summarized and used for analyzing evolution laws of corrosion morphologies in different areas of welded structures with different residual stress.Different evolution laws of corrosion morphologies on weld joint of welded structures processed and unprocessed by VSR are shown in Figure 3.It is seen from Figure 3 that corrosion damages on the surface of the corroded weld joint processed and unprocessed by VSR worsen as the corrosion time increases.Additionally, corrosion damage on the surface of the weld joint unprocessed by VSR are more severe than the damage of the weld joint processed by VSR, which means that stronger residual stress could make the corrosion damage on the weld joint's surface worse.In the early days of corrosion (60 d), corrosion morphology on the surface of the weld joint shows characteristics of pitting.As the corrosion time increases, density of corrosion pitting on the surface becomes greater, as does the depth and diameter of the pitting.When the corrosion time reaches 180 d, pitting becomes united and forms continuous corrosion damages.When the corrosion time is 360 d, corrosion damage on the surface of the weld joint enlarges again and becomes a continuous corrosion pitting with a large diameter.e explanation is that granular ferrite on the surface of the weld joint will be first damaged and forms corrosion pitting while being corroded, but acicular ferrite and granular pearlite regions enjoy strong corrosion resistance.us, there is relatively little corrosion damage.Although stronger residual stress on the surface of the weld joint unprocessed by VSR could increase the depth and diameter of pitting, corrosion morphologies of the weld joint's surface are still mainly corrosive pitting in the evolution.
Evolution of corrosion morphology on the surface of HAZ is shown in Figure 4.After being processed by VSR, corrosion morphology on the surface of HAZ changes from pitting to uniform corrosion as the time increases.However, it could not fully turn itself into uniform corrosion (Figure 4(a)).Furthermore, corrosion morphology on the surface of HAZ unprocessed by VSR has been turned into uniform corrosion when the corrosion time is 180 d.When the corrosion time reaches 360 d, both corrosion pits with large aperture and continuous corrosion damage appear on HAZ's surface again (Figure 4(b)).It is seen that corrosion morphologies on the surface of HAZ are interchangeable between pitting and uniform corrosion.Additionally, the stronger the residual stress on the HAZ's surface is, the faster the reciprocal conversion rate between pitting and uniform corrosion will be.Corrosion damaging rate of HAZ's surface will be faster too.
While analyzing the microstructure of HAZ, bulk granular ferrite on HAZ's surface will be destroyed at first while being corroded and a large-aperture pit appears.en, further corrosion occurs in other regions with bainite shedding and leads to the uniform corrosion morphology on the surface of HAZ.When corrosion separation appears on surface metal, the procedure mentioned above is repeated on the surface of HAZ and interchangeable pitting and uniform corrosion occurs.
Evolution of corrosion morphology in the fusion line is shown in Figure 5.After the welded structure is corroded, apparent corrosion grooves appear in the fusion line.On one hand, the generation of corrosion grooves results from the difference of corrosion rates between the weld joint and HAZ near the fusion line.On the other hand, the coarse Widmänstatten (Figure 2(c)) structure weakens the corrosion resistance of metal in the fusion line and further accelerates propagation rate in depth of corrosion grooves.Meanwhile, the longer the corrosion time is, the deeper the corrosion grooves in the fusion line are.
To study the influence of residual stress on the rising rate of corrosion groove depth, four parts of corrosion grooves in each specimen were cut randomly (cutting direction is vertical to weld joint) and the depth of each cutting groove was measured.us, evolution law of corrosion groove depth in the welded structure with corrosion time increases was obtained by calculating the average depth value of measured corrosion grooves in each specimen group.Both the welded structures processed and unprocessed by VSR are studied, and average depth evolution laws of grooves are shown in Figure 6.
e findings show deeper corrosion grooves that are formed in the fusion line of specimens unprocessed by VSR.However, as corrosion products continue to gather around corrosion grooves, an increasing rate of corrosion depth goes down with corrosion time increases.Depth difference between corrosion grooves in the fusion line unprocessed and processed by VSR is widened.By the contrast between Figures 7 and 8, transverse and longitudinal residual stress on the surface of the welded structure processed by VSR is obviously weaker than that on the surface of the welded structure unprocessed by VSR.However, transverse and longitudinal residual stress in the fusion line is relatively strong when corrosion time is 360 d.Meanwhile, evolution laws of transverse and longitudinal residual stress in the fusion line are positively related to the 6 Advances in Materials Science and Engineering evolution law of corrosion grooves' depth in the fusion line, whose increasing rate decreases with corrosion time increases.It can be inferred that corrosion damages in the fusion line of the welded structure can e ectively increase residual stress in the fusion line and serve as an important factor in in uencing residual stress in the fusion line.e evolution law of comprehensive residual stress distribution on the surface of the welded structure is shown in Figure 9.
e comprehensive residual stress is tensile stress on the surface of the welded structure, and the peak point of comprehensive residual stress is located in the fusion line.As the distance from the fusion line grows, comprehensive residual stress on the surface of the welded structure gets weaker.When corrosion time grows, comprehensive residual stress on the surface of the welded structure gets stronger.Furthermore, comprehensive residual stress in the fusion line rises most stably.is shows that although energies such as bond energy in structures dissipate into the environment through electrochemical reactions in corrosion, part of the surface energy can be converted into internal energy due to changes of corrosion morphology on the surface of the structure.us, comprehensive residual stress on the surface of the welded structure is increased at last.When the welded structure is being corroded, residual stress and corrosion both not only contribute to the initiation of corrosion cracking in a welded structure but also contribute to each other's progress.On one hand, residual stress accelerates corroded damaging of the welded structure, and on the other hand, more corrosion damage leads to greater residual stress on the surface of the structure.erefore, initiation of corrosion cracking of the welded structure is further accelerated.

Detection of Magnetic Field Distribution.
To ensure the initial surface of corrosion specimens is intact, magnetic eld distribution needed to be carried out before residual stress was detected.Detection results of magnetic eld distribution on the corroded surface of welded structures processed and unprocessed by VSR are shown in Figure 10.Among them, Hp-1, Hp-3, Hp-5, and Hp-7 are tangential components of magnetic eld leakage in each channel and Hp-2, Hp-4, Hp-6, and Hp-8 are normal components of magnetic eld leakage in each channel.Furthermore, weld center position is x 120 mm. e corroded damaging area on the surface of structure is decided by magnetic eld leakage characteristic curve: the peak point of the tangential component in leakage magnetic eld is located in the corrosion damage center, and magnetic eld curve of the normal component in the corrosion damage center passes the zero point (as shown in Figure 11).For the welded structure processed by VSR, corroded magnetic eld leakage characteristic curve on the surface always appears within the area 20 mm from the weld center (Figure 10(a)).us, although welded structures have been processed by VSR, generation of severe corrosion damage still occurs, resulting from a coarse microstructure near the weld joint.However, in the areas whose distance is more than 20 mm away from the weld center, magnetic eld leakage characteristic curve never occurs.is means that serious corrosion damage does not appear in areas far away from the weld center after the welded structure is processed by VSR.However, obvious magnetic ux leakage is detected in all parts of the corroded surface of the welded structure unprocessed by VSR (Figure 10(b)).erefore, under the in uences of residual stress and corrosion, serious corrosion damage not only generates in areas near the weld joint but also appears in areas far away from the weld center due to galvanic corrosion accelerated by residual stress.
While analyzing the magnetic memory signal of a metal structure, the extent of corrosion damage on the metal surface can be e ectively measured with changes of magnetic eld gradient: the greater the changes of magnetic eld gradient are, the more serious the corrosion damage is [39,40].Evolution laws of three-dimensional surface magnetic eld gradient distribution of both welded structures processed and unprocessed by VSR are shown in Figure 12. e darker the color is, the greater the changes in magnetic eld gradient are.It is found that corrosion damages in the areas near the weld joint of the welded structure processed by VSR are heavier than damages in other regions (Figure 12(a)).For the welded structure unprocessed by VSR, corrosion damage on di erent parts of the surface sees no obvious laws in the early days of corrosion (60 d).However, as corrosion time increases, the most seriously damaged areas gradually transfer to the weld joint.When corrosion time is 360 d, the most seriously damaged areas are fully along the weld joint and parallel to the weld joint direction (Figure 12(b)).
us, under the in uences of microstructure, residual stress, and corrosion, corrosion damage near the weld joint of the welded structure will be more serious than the damage in other regions as the corrosion time increases.en, continuous corrosion damages that are parallel to the direction of the weld joint will converge together, leading to the corrosion cracks that are parallel to the welding direction

Conclusions
In this paper, evolution laws of surface corrosion morphology, residual stress, and areas with the most serious corrosion damage in corroded welded structures in natural seawater are studied.Furthermore, corrosion crack nucleation mechanism of welded structures in natural seawater is revealed by microstructure observation and the conclusions are as follows: (1) While serving in natural seawater, microstructures of the weld joint, HAZ, and the fusion line are the main factors in determining surface corrosion morphologies and evolution process in all areas.(2) Compared to the weld joint and HAZ, areas with the most serious corrosion damage generate in the fusion line at last.Additionally, continuous corrosion grooves are formed, which are caused by two aspects: one is the di erence of corrosion rates between the weld joint and HAZ near the fusion line, the other is the coarse Widmanstätten structure in the fusion line.Data Availability e data used to support the ndings of this study are included within the article.

Figure 3 :
Figure 3: Evolution of corrosion morphology on weld joint's surface.(a) Weld joint processed by VSR.(b) Weld joint unprocessed by VSR.

3. 3 .
Residual Stress Evolution.After detecting residual stress, the average value of surface residual stress detection results of three corroded specimens in each group is referred to as the analysis data of residual stress.us, evolution laws of surface transverse residual stress distribution of welded structures are shown in Figure7.In the early days of corrosion (60 d), transverse residual stress on the surface of the welded structure rst increases as it gets farther from the fusion line, then the stress decreases.e peak point of stress is 20 mm from the fusion line.However, transverse residual stress in the fusion line becomes greater as the corrosion time increases.When the corrosion time is 360 d, the value of transverse residual stress in the fusion line of the welded structure unprocessed by VSR is greater than the value of transverse stress that is 20 mm from the fusion line, and the value of transverse residual stress in the fusion line becomes the peak point of residual stress.On the other hand, whether the welded structure enjoys VSR or not, an increasing rate of transverse residual stress in the fusion line decreases as corrosion time increases.e reason is that the corroded damaging rate in the fusion line decreases as corrosion time increases while the welded structure is being corroded.eevolution law of longitudinal residual stress whose direction is parallel to the weld joint is shown in Figure8.While being corroded, evolution laws of longitudinal residual stress distribution of welded structures processed and unprocessed by VSR are generally the same, and peak values of longitudinal residual stress on welded structures' surface are both in the fusion line.Furthermore, as it becomes farther from the fusion line, longitudinal residual tensile stress on the surface of the welded structure turns to residual compressive stresses swiftly.Finally, it gradually approaches the zero line.While being corroded, longitudinal residual stress in the fusion line increases with corrosion time growing.Furthermore, increasing rate of longitudinal residual stress in the fusion line slows down.However, longitudinal residual stress shows no obvious laws as corrosion time increases in the area 20 mm away from the fusion line.

Figure 5 :Figure 6 :
Figure 5: Evolution of corrosion morphology in the fusion line.(a) Fusion line processed by VSR.(b) Fusion line unprocessed by VSR.

Figure 7 :Figure 8 :
Figure 7: Evolution law of transverse residual stress distribution.(a) Welded structure processed by VSR.(b) Welded structure unprocessed by VSR.

( 3 )
On one hand, residual stress in the welded structure speeds up the generation of corrosion damage on the surface of the structure; on the other hand, the development of corrosion damage in turn increases the level of residual stress.us, nucleation of corrosion cracks is further accelerated by the mutual promotion between residual stress and corrosion damages.(4) Under the in uences of microstructure, residual stress, and corrosion, areas with the most serious corrosion damage will generate in the fusion line as corrosion time increases.Finally, as several serious corrosion damages get together, corrosion cracks parallel to the welding direction rst generate in the fusion line, making welded structures the origin of corrosion failure in marine steel structures.

Figure 12 :
Figure 12: Evolution law of magnetic eld gradient on surface of welded structure.(a) Welded structure processed by VSR.(b) Welded structure unprocessed by VSR.
Test.Seawater corrosion test was finished according to the Chinese national technique standard GB/T 6384-2008, with natural seawater from the East China Sea corroding the test specimens.Major chemical components of seawater from the East China Sea are listed in Table

Table 1 :
Chemical components of X65 steel.

Table 3 :
Major chemical components of seawater from the East China Sea.

Table 4 :
Date when specimens go out of water and data collection.Advances in Materials Science and Engineering value of the strain that missed the grinding position was the additional strain caused by grinding.Detection results of grinding additional strain in the direction of 0 °, 45 °, and 90 °were ε 0 ∘ � 0.076 μm, ε 45 ∘ � 0.003 μm, ε 90 ∘ � −0.036 μm.