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Stress Corrosion Cracking ( SCC ) has been considered as a critical menace to the safe in operation of gas or oil transmittal grapevines. Because it may do important failures in grapevine, and these failures are normally low-probability, high-consequence events. To day of the month, two signifiers of SCC incursion from the external surface of the grapevine have been recognized: one is the intergranular SCC, the other is a sort of transgranular SCC. The former 1 is besides called ‘ high pH SCC ‘ and has been studied extensively. There are some well-accepted mechanisms developed for the high pH SCC, such as anodal disintegration and movie rupture at cleft tip. However, there are comparatively limited apprehension of the transgranular SCC, besides known as ‘ near-neutral SCC ‘ . In this work, the corrosion behavior of grapevine steels in near-neutral solution has been studied utilizing cyclic voltammetry based proving technique. The consequences showed strong grounds that the function of H was important in near-neutral pH SCC. Furthermore, Slow Strain Rate Testing ( SSRT ) technique were besides applied to depict the microsttuctural consequence in SCC.

1. Introduction

1.1 Background

Stress Corrosion Cracking ( SCC ) is a signifier of environmental aided snap. The environmental aided snap is known as the snap in stuff that can be affected by the environment and the emphasis. Concretely, SCC can be defined as the interaction of a tensile emphasis and an aqueous environment moving on a susceptible metallic surface to originate and propagate clefts [ 1 ] . The SCC is a delayed failure, it means the cleft initials and propagates at a slow strain rate until the emphasis reaches the break strength. The sequence of events involved in the SCC procedure can be separated into three phases ( shown in figure 1 ) [ 2 ] :

Crack induction and 1 phase extension

Phase 2 or steady-state cleft extension

Phase 3 cleft extension or concluding failure

Many researches have been conducted to develop mechanisms seeking to explicate the interactive stress-environment interactions at the cleft tip. In general, two basic mechanisms have been identified: anodal and cathodic mechanisms [ 2 ] .

Fig.1 Schematic diagram of typical crack-propagation rate as a map of crack-tip stress-intensity behavior exemplifying the parts of phase 1, 2, and 3 cleft extension every bit good as placing the tableland speed and the threshold emphasis strength [ 2 ] .

Since the first observation of SCC on the external surface of a inhumed natural gas transmittal grapevine in 1965, SCC of grapevine has occurred in several states around the universe and has been regarded as a major failure in some gas or oil transmittal grapevines [ 3 ] . For illustration, a sedate accident happened in Canada on 15 April 1996, which was caused by a SCC-induced rupture of the Trans Canada Pipeline placement at 10km south West of Winnipeg, a nearby house was wholly damaged by the detonation and immense bolide [ 4 ] . Generally, buried grapevines are located within of all time altering environmental conditions which may take to a caustic environment. Pipe coatings and cathodic protection are normally used to contend against corrosion. However, SCC is still bing.

1.2 Types of emphasis corrosion checking in grapevine steels

Due to the environment-dependence of SCC, the environmental conditions that are present during the cleft extension determines the type of SCC that can happen. So far, two types of SCC have been characterized, they are intergranular and transgranular SCC. Approximately, the pH value of the electrolyte will make up one’s mind whether the snap is intergranular or transgranular. Normally, high pH conditions cause intergranular SCC, besides called classical SCC, while low pH conditions result in transgranular SCC with assorted manners around the cleft tip. To day of the month, a great trade of surveies have been conducted concentrating on the classical high pH intergranular SCC, and some possible mechanisms have been put frontward. However, there is comparatively limited researches on low pH SCC.

Intergranular emphasis corrosion snap ( IGSCC ) of grapevine steels normally occurs in extremely concentrated bicarbonate-carbonate ( ) solution with high pH between 9-11. Fang et al [ 5 ] developed a well-accepted mechanism for the formation of high pH status in grapevine steels, it can be described as: the inhumed grapevines make usage of cathodic protection to forestall corrosion but, unluckily, the cathodic current can be big plenty to interrupt the H2O into hydroxyl ions, therefore increase the pH value. Then the high pH solution can absorb from the environment to organize a complex bicarbonate-carbonate solution. Based on this high pH solution, more attempts have been spent concentrating on the mechanisms of IGSCC. After the probe of corrosion behavior of X65 grapevine steel, Wang et al [ 4 ] provided a possible account, if the concentration of carbonate is high plenty in the high pH solution to passivate the steel pipe surface, IGSCC can happen by anodal disintegration mechanism along the grain boundaries. Parkins [ 6 ] besides developed a mechanism related to the rupture of oxide movie at cleft tip, as plastic strain in front of the cleft tip can halt the formation of protective oxide movie around the cleft tip, therefore the extension of cleft can go on. This IGSCC can merely happen in a really restricted scope of electrochemical potencies ( -550 millivolt V a concentrated mercurous chloride electrode [ SCE ] to -650 m as reported by Gonzalez-Rodriguez et Al [ 7 ] ) . Therefor, IGSCC may be reduced or avoided by commanding the electrochemical potencies and operating in lower temperature.

In contrast, transgranular emphasis corrosion snap ( TGSCC ) can happen in grapevines under normal operating conditions, where the pH value of the electrolyte is much lower, normally about 5.5-7.5, so it is besides called near-neutral pH SCC. This near-neutral electrolyte is by and large characterized with anaerobiotic, dilute solutions. The formation of this near-neutral environment is related to shriek surfacing that is broken or disbonded from the external surface of the pipe, therefore groundwater comes into contact with the surface, and ensuing in the loss of cathodic protection. Roughly, the TGSCC has a a transgranular, quasi-cleavage cleft morphology with really small ramifying [ 5 ] . Compared to classical IGSCC, there are comparatively limited understanding on TGSCC.

In add-on, Liu et al [ 8 ] late published a paper about the probe of corrosion behavior of grapevine steel in acidic solution. In his study, an acidic dirt with an mean pH of 3.5-6.0, named ”Red Soil ” , was mentioned in several states in southeast China where some natural gas transmittal grapevines were runing in those countries. The consequences of their experiment showed the procedure and mechanisms of SCC in acidic solution was mix-controlled by the anodal metal disintegration and immersion of H. In item, anodal metal disintegration mechanism dominated the SCC behavior at a comparatively less negative electrochemical potency, and H began to take consequence when the applied potency decreased more negatively. With the farther negative displacement of applied potency, the SCC of the steel follows wholly a hydrogen-based mechanism, with a river-bed molded brickle characteristic of the break surface.

1.3 The behavior of near-neutral pH emphasis corrosion checking

The underlying mechanism of near-neutral pH SCC, besides called TGSCC, is still equivocal. Recently, positive advancement has been achieved. Cheng et al [ 9 ] showed strong grounds that H did affect in procedure of TGSCC. In their survey, the near-neutral electrolyte, which has been described above in subdivision 1.2, were ever associated with high H pervasion currents and sub-surface H concentrations. This observation indicates hydrogen development reaction does happen in TGSCC. Therefore, the function of H should be portion of the TGSCC mechanism. Since the happening of H, the mechanisms for TGSCC will be much different from that of IGSCC, where H development reaction is impossible due to the high pH status. Besides, Beavers ‘ experimental consequence [ 10 ] mentioned the phenomenon that the cleft growing rates in the heat-affected zone following to the dyer’s rocket line were some 30 % higher than other parts. This indicates the microstructure or the residuary emphasis in steel may act upon the SCC procedure, the farther apprehension of this phenomenon may assist bettering the SCC opposition in the grapevine industry.

The intent of present paper is to give a better apprehension of the near-neutral pH SCC. The happening of H development reaction in TGSCC will be demonstrated and analysed utilizing cyclic voltammetry measuring. Then slow strain rate testing ( SSRT ) technique will be applied to demo the possible microstructural consequence. Finally, A thermodynamic theoretical account will be introduced to cipher the cleft growing rate, which can be contributed to foretell the life-time of the grapevine.

2. Experimental processs

2.1 Probe of the function of H: Cyclic voltammetry measuring

The working electrode used in this electrochemical trial was made of a sheet of API ( American Petroleum Institute ) X-70 grapevine steel with chemical composing ( wt % ) : C 0.06, Mn 1.44, Si 0.31, S 0.004, P 0.01, Ni 0.034, Cr 0.16, Mo 0.25, V 0.005, Cu 0.015, Ti 0.01, B 0.002 and Al 0.029. The metallurgical observation indicated the primary microstucture of X-70 steel was bainitic ferrite. Each specimen was land with 600 grit emery paper on all faces. The borders were coated utilizing a cover pigment to forestall cranny corrosion between the epoxy saddle horse and the electrode. Then all specimens were embedded in epoxy rosin

manufactured by LECO go forthing a on the job country of 0.155. The working surface was later polished with 3I?m and 1I?m diamond pastes, cleaned by distilled H2O and methyl alcohol. The trial solution used was 0.01 M Na hydrogen carbonate solution that was typically used to imitate the dilute hydrogen carbonate electrolyte trapped between the coating and the grapevine. The solution was made from analytic class reagents ( Fisher Scientific ) and ultra-pure H2O ( 18 MI© ) . All the trials were performed at ambient temperature ( 22 ) . To reproduce the near-neutral pH environmental status, the solution was purged with high pureness ( category 2.2 ) 5 % balanced with gas for 2 H prior to and throughout the trials [ 11, 12 ] .

The cyclic voltammogram ( CV ) measurings were performed on a three-electrode system through PINE bipotentiostat. X-70 steel was used as working electrode, saturated calomel electrode ( SCE ) was used as mention electrode and a coiled Pt wire was used as the counter electrode. All potencies informations from the CV measuring were converted to values relative to the standard H electrode ( SHE ) . Before the CV measurings, the specimen was cathodically polarized at -1.5 V for 3 min to take the air-formed oxide movie. The potency was scanned in the positive or negative way between preset lower and upper shift potencies.

2.2 Probe of microstructural consequence: Polarization opposition measuring and Slow strain rate testing technique

The same API X-70 grapevine steel was besides the proving stuff here. In order to obtain assorted microstructures and a scope of hardness, assorted heat intervention were applied and listed below [ 13 ] :

The samples were cut from the X-70 steel skelp with mean grain size about 5.8 I?m. The microstructures were illustrated utilizing metallographic microscope and listed in figure 2. The polarisation opposition measuring was performed in a NS4 solution with chemical composing of ( g/L ) 0.11. 0.483, 0.137, 0.131 to imitate the near-neutral groundwater in Canada. During the trial, the solution was bubbled with to guarantee an anaerobic

Fig. 2 Typical microstructure of the

X-70 grapevine steel, ( a ) annealed, ( B )

normalized, ( degree Celsius ) quenched, ( vitamin D ) quenched

and tempered, ( vitamin E ) as-rolled [ 13 ]

status and a pH value of 6.7. Before the trial, the solution should be deaerated for at least 4 hours to take the O. The ground of the remotion of O would be discussed subsequently. Polarization opposition measuring was so conducted utilizing scanning possible scope from -10mVto +10mV relation to open circuit possible status, and the scanning rate was 0.125 mV/S. In add-on, the trial consequences were confirmed utilizing electrochemical electric resistances spectrometry ( EIS ) .

Slow strain rate testiing ( SSRT ) technique was carried out to specify the SCC opposition. The samples had a gauge length of 25 millimeters and a diameter of 3mm. The applied strain rate was. The SCC opposition was defined as the ratio of decrease in country measured in caustic medium and air ( ) [ 14 ] .

3. Consequences and treatment

3.1 The function of H

The consequences of the CV measuring is plotted in figure 3 with different upper shift potencies at a possible expanse rate of 50mV/s. It is shown in the curves recorded, that the positive current supports increasing with increased potency after a ‘ possible shoulder ‘ , while a negative current extremum is identified in the cathodic polarisation curve. Approximately, the value of this negative possible current extremum additions with increased upper bound potency.

Fig. 3 Cyclic voltammograms measured on X-70 steel with the different upper potency bounds at a possible expanse rate of 50 mV/s in 0.01 M Na hydrogen carbonate solution. [ 11 ]

The consequence of CV measuring indicates that the mechanism of TGSCC should be much different to high pH IGSCC. In detaill, the anodal polarisation behaviors of them are non the same. The polarisation behavior of steel in IGSCC was reported by Gu et al [ 15 ] , which involved two-step oxidization: the pre-oxidation measure about the formation of sedimentation bed and followed by the formation of. A stable oxide Fe movie would organize by farther polarisation. The extension of cleft could happen due to the rupture of this oxide movie in front of cleft tip. In contrast, the polarisation behavior of TGSCC described by the CV measuring indicates that there is no formation of the stable oxide movie on the surface of the steel in the near-neutral anaerobiotic environment. As shown in figure 3, the observation of the ‘ possible shoulder ‘ is related to the pre-oxidation measure of the steel to organize sedimentation bed, the electrochemical reaction is:

Then, the current denseness continues to increase with positively increased potency, i.e, the steel is still in active disintegration position, no passivation occurrs due to no stable oxide movie signifiers. Furthermore, the cathodic extremums marked in figure 3 can be attributed to the decrease of oxidization merchandise formed before, and it could be summarized that the cathodic extremum is more negative when the upper possible bound is more positive, as more oxidization merchandise signifiers. So far, the analysis of CV measuring demonstrates that no oxide movie formed on the surface of grapevine steel in TGSCC. Therefore, the movie rupture mechanism becomes unsuitable for near-neutral pH SCC, and some mechanical factors, e.g. immersion of H might take consequence in the procedure of TGSCC. Further CV measuring is conducted by plotting the cathodic Tafel slopes as a map of upper possible bounds, as shown in figure 4. Typically, dispatching of in a low pH solution has a value about -100 mV/decade, which is much higher than that marked in the figure ( -450 – -500mV/decade ) . Possible account to this happening is that the H development reaction is dominated by decrease of H2O molecules in near-neutral pH status, alternatively of decrease, because more cathodic over-potential is required in H2O decrease therefore leads to a more negative Tafel incline [ 11 ] .

Fig. 4 The cathodic Tafel incline as a map of upper possible bounds [ 11 ]

To sum up, H Acts of the Apostless as a critical function in near-neutral pH SCC, the decrease of H2O molecules plays a decisive function in the H development reaction. And the pre-oxidation measure generates an unstable sedimentation bed and followed by either an electrochemical H recombination reaction or a hydrogen soaking up reaction [ 12 ] . However, it should be emphasized that all these experiments were moving at anaerobiotic conditions, as the debut of O would bring forth a oxide movie which is stable and can acte as inhibitor of the H development reaction.

3.2 Microstructural consequence

The consequence of SSRT is shown in figure 5, it is obvious that the difference in microstructure could take to different SCC opposition. Three groups with typical microstructure are highlighted in the figure. By and large talking, the SCC opposition of specimens with same microstructure decreases with increasing output strength.

Fig. 5 Relationship between SCC opposition and output strength of X-70 steel [ 13 ]

The SCC information shows strong microstructure- dependant behavior, it means that even the specimens have the same output strength, the SCC opposition varies with different microstructure. Further treatment is developed based on the comparing between X70Q and the bluish group. The SCC opposition of water-quenched specimen ( X70 Q ) is lower than as-rolled status ( bluish group ) , although they all have a bainitic construction and water-quenching additions the output strength. In add-on, pique after water-quenching gives rise to great addition in SCC opposition with little bead in output strength ( See the informations of X70 QT ) . Possible account to this phenomenon is concentrating on the high micro-stresses induced by slaking due to extra C trapped interstitially [ 13 ] . In general, little clefts were reported to be more likely to originate and propagate along the pearlite/ferrite boundaries [ 13 ] , so the presence of pearlite-ferrite construction will connote hapless SCC opposition. To sum up, grapevine industry prefers grapevine steel which has a powdered bainite+ferrite microstructure to that has a pearlite+ferrite microstructure due to better combination of SCC opposition and mechanical belongingss.

The consequence of polarisation opposition measuring is besides analysed in this paper, as shown in figure 6. The additive relationship of polarisation opposition and SCC opposition indicates that, besides the immersion of H, anodal disintegration besides contributes to the near-neutral pH SCC. Therefore, the SCC behavior of grapevine steels in near-neutral solution can be identified as the consequence of combined action of emphasis, H, and anodal disintegration at the cleft tip in near-neutral pH SCC, i.e. the TGSCC.

Fig. 6 Relationship between SCC opposition and polarisation opposition. [ 13 ]

3.3 Thermodynamic theoretical account of TGSCC

In order to gauge the cleft growing rate and to about foretell the life-time of the grapevine steels in near-neutral pH environment, a thermodynamic theoretical account of near-neutral pH SCC has been developed by Cheng [ 16 ] . The interactions of emphasis, H, and anodal disintegration at cleft tip were to the full illustrated in Cheng ‘s theoretical account. The modeling starts from the electrochemical reactions, that occurs in TGSCC:

Anodic reaction: ( 1 ) , and

Cathodic reaction: ( 2 )

bases for the H atoms adsorbed on the surface, and every two can unite into a: ( 3 ) . Then these H can perforate into the surface and reassign into captive H atoms: ( 4 ) . Therefore, the whole electrode reaction can be described as:

( 5 )

In equation ( 5 ) , ten bases for the figure of H atoms pervading into the steel. Now, the consequence of emphasis will be taken into history. Yokobori et al [ 17 ] has published a theoretical account for the H motion at the cleft tip as shown in figure 7, which indicated that the H atoms generated through cathodic reaction would spread toward the cleft tip and the accumulate in the plastic zone, due to the emphasis concentration at the cleft tip. Hence, the equation ( 5 ) should be sightly modified in the presence of emphasis, as followed:

, where Y is besides the figure of H atoms but should be larger than ten.

Fig. 7 Model for H diffusion and disintegration reaction at the crack-tip for a stressed steel specimen in deoxygenated, nearneutral pH solution [ 17 ]

Further mold is to depict the free-energy alteration between equation ( 5 ) and ( 6 ) , i.e. between unstressed and stressed steel. and, and, severally represent the free-energy alterations and electrochemical potencies of equation ( 5 ) and ( 6 ) .

( 7 )

( 8 )

where G stands for the formation free-energy, n the figure of negatrons exchanged, F represents the Faraday invariable. The free-energy alteration is used to cipher the anodal disintegration current denseness in reaction ( 5 ) : ( 9 )

The same as reaction ( 6 ) : ( 10 ) . Assuming the exchange current denseness remains changeless, and the difference in free-energy alteration chiefly consequences from the presence of emphasis, therefore: ( 11 ) , is the charge transportation coefficient. In general, where bases for the internal energy alterations, S is the information, is strain energy denseness, and is the interaction energy between the lattice strain induced by H atoms and external emphasis field. As H=0 and the presence of emphasis, can be simplified as: ( 12 ) where W stands for molar weight, is chief emphasis, is denseness, E is Young ‘s modulus. So the equation ( 11 ) can be re-written as:

( 13 ) ,

where is the stress factor. Similar derivation is applied to all the factors and unite these factors together giving:

( 14 )

where is the consequence of concentration difference of H atoms between unstressed and stressed samples. is the consequence of H on the disintegration rate without emphasis. is the consequence of emphasis on disintegration rate without H. is the consequence of combined action of emphasis and H on the anodal disintegration. Finally, harmonizing to the cleft growing rate in anodal disintegration mechanism: , the cleft growing rate under the influence of emphasis and H can be described as In sum-up, this thermodynamic theoretical account is unsmooth and more surveies are required to give better apprehension of this near-neutral pH SCC. So far, it is still hard to exactly foretelling the life-time of these grapevines, so periodic sensing is necessary to forestall SCC-induced accident.

4. Decision

The emphasis corrosion snap of grapevine steels is introduced in this work, and more attempt is spent to look intoing the near-neutral pH SCC. Although the implicit in mechanism is still uncomplete, it is demonstrated that H plays a critical function in near-neutral SCC and attach toing with the anodal disintegration mechanism. However, the anticipation of the life-time of the grapevines is still hard.

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