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Corrosion Science

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In situ monitoring of crevice corrosion morphology of Type 316L stainlesssteel and repassivation behavior induced by sulfate ions

Takahito Aoyama⁎, Yu Sugawara⁎, Izumi Muto, Nobuyoshi HaraDepartment of Materials Science, Tohoku University, Aoba-ku, Sendai 980-8579, Japan

A R T I C L E I N F O

Keywords:Stainless steelPotentiostaticCrevice corrosionRepassivation

A B S T R A C T

To analyze the repassivation mechanism induced by SO42− ions, in situ monitoring of the crevice corrosion

morphology on Type 316L stainless steel was performed. The repassivation of the crevice corrosion was observedwhen the solution was changed from 1 M NaCl to 0.88 M Na2SO4 or to 1 M NaCl-0.88 M Na2SO4. The activedissolution of Type 316L stainless steel was suppressed by SO4

2− ions at pH 0.4 in anodic polarization mea-surements. The transition from active to passive states inside the crevice was likely promoted by SO4

2− ions.

1. Introduction

In chloride environments, stainless steel structures sometimes sufferfrom crevice corrosion. The initiation process of the crevice corrosion ofstainless steels was successfully described by the concept of combina-tion of the IR drop theory [1,2] and the critical crevice solution theory[3]. This concept has also been supported by evidence obtained from insitu observations [4–6] and mathematical models [7–9].

In the IR drop theory [10–16], crevice corrosion is initiated whenthe IR drop (potential drop) from outside to inside the crevice becomeslarge enough to cause transition from a passive to an active state insidethe crevice. The magnitude of the potential drop is affected by manyfactors, including crevice geometry and solution composition/con-centration. In the critical crevice solution theory, acidification insidethe crevice is caused by hydrolysis of metal ions which dissolve in apassive state [17]. The electromigration of Cl− ions toward the creviceincreases the corrosivity of the crevice solution [18–21]. Once the pHand Cl− concentration of the crevice solution reach the critical value totrigger the transition from passive to active state, crevice corrosion isinitiated.

Clarifying the initiation mechanism of crevice corrosion can reducethe risk of corrosion [22]. It is known that cathodic protection is one ofthe simplest and most powerful methods of reducing that risk [23,24].Li et al. reported that cathodic protection decreased the potential insidecrevice and generated hydroxide ions, which provided an increase inpH inside the crevice and inhibited the initiation of crevice corrosion[25]. In other techniques, inhibitive anions are added to solutions forcorrosion protection [26]. Brossia et al. demonstrated that the additionof SO4

2− or NO3− ions to the bulk solution inhibited the initiation of

crevice corrosion effectively even in chloride environments [27].McCafferty et al. found that competitive adsorption of CrO4

2− ions onmetal surfaces inhibited the initiation of crevice corrosion, and therewas a critical ratio of inhibitive anions to Cl− ions required to preventthe initiation of crevice corrosion [28]. However, because crevice cor-rosion is readily initiated in low Cl−-concentration solutions at lowpotentials, it is difficult to ensure that the initiation of crevice corrosionhas been completely prevented.

Repassivation techniques are expected to be more effective at re-ducing crevice corrosion damage. The addition of SO4

2− ions is knownto suppress the growth of stable and metastable pits on stainless steels[29,30]. Pistorius and Burstein revealed that the SO4

2− ions inhibitedboth the initiation and the propagation of pits on Type 304 stainlesssteel in 1 M Cl− solution. The growth mechanism of pitting is similar tothat of crevice corrosion [31,32]. In the growth stages of the localizedcorrosion, the hydrolysis reaction of dissolved metal ions and the ac-cumulation of Cl− ions are predominant factors [33–35]. As such, itwas expected that the addition of SO4

2− ions to the bulk solution wouldinhibit the growth of crevice corrosion and promote repassivation in-side the crevice. Ilevbare reported that the presence of sulfate increasedthe resistance of Alloy 22 to the initiation of the crevice corrosion in4 M NaCl [36]. The mechanism of action for SO4

2− ions were assumedto include the prevention or delay of the formation of the critical cre-vice solution required for self-sustaining growth. However, insufficientinvestigations have been carried out on the inhibition of crevice cor-rosion by SO4

2− ions, and the objective of this study was to analyze theeffect of SO4

2− ions on the repassivation behavior of crevice corrosionon Type 316L stainless steel. The investigation of the repassivation ofcrevice corrosion is not only expected to develop the technique to

http://dx.doi.org/10.1016/j.corsci.2017.08.005Received 1 May 2017; Received in revised form 24 July 2017; Accepted 9 August 2017

⁎ Corresponding authors.E-mail addresses: takahito.aoyama.t7@dc.tohoku.ac.jp (T. Aoyama), sugawaray@material.tohoku.ac.jp (Y. Sugawara).

Corrosion Science 127 (2017) 131–140

Available online 12 August 20170010-938X/ © 2017 Elsevier Ltd. All rights reserved.

MARK

reduce the maintenance and operation costs of stainless steel structuresbut also provide an alloy design for high corrosion-resistant stainlesssteels.

In situ observation on the corrosion morphology provides usefulinformation on corrosion mechanism. Brigham [37] and Yang et al.[38] used a glass lens to form a crevice on stainless steels in acidchloride solutions, and the electrochemical reactions associated with anaturally growing crevice including hydrogen production and metaldissolution were observed directly. In situ observation techniques aresuccessfully applied for the investigation on the initiation of crevicecorrosion [39]. On the other hand, the repassivation behavior of crevicecorrosion is usually evaluated by the change in current with time.However, to clarify the repassivation behavior of crevice corrosionmore precisely, in situ observation was used in this study. This ob-servation allowed the quantitative relationship between current andcorrosion morphology to be analyzed.

In this study, a flow cell for crevice corrosion tests was fabricated tomake clear the effect of the solution change on the repassivation be-havior of crevice corrosion. After the initiation of crevice corrosion, thecomposition of the bulk solution was changed from a chloride solutionto an SO4

2−-containing solution. Potentiostatic crevice corrosion testswere performed for a commercial Type 316L stainless steel, and in situobservation inside the crevice surface was performed during the in-itiation and repassivation processes. A glass plate was used as the cre-vice former, and time lapse images inside the crevice were taken. Themorphological characteristics of the changes in crevice corrosion wereextracted by digital image processing, and the repassivation processesintroduced by SO4

2− ions were discussed in light of corrosion mor-phology and current signals.

2. Experimental method

2.1. Specimens

A commercial Type 316L stainless steel sheet was used. The che-mical composition of the steel is shown in Table 1. The stainless steelwas heat-treated at 1373 K for 1.8 ks and then water-quenched.

2.2. Potentiodynamic polarization

Pitting potential was measured in deaerated 1 M NaCl at 298 K. ThepH of the 1 M NaCl was 5.3. In addition, to estimate the electrochemicalproperties of the steel in crevice solutions, the anodic polarizationcurves in acidic solutions (pH 0.4) were measured in deaerated 1 MNaCl, 0.88 M Na2SO4, and 1 M NaCl-0.88 M Na2SO4 at 298 K. In thecase of 1 M NaCl and 1 M NaCl-0.88 M Na2SO4, the pH values of thesesolutions were adjusted with HCl. The pH of 0.88 M Na2SO4 was ad-justed with H2SO4. The surface of the specimen was ground in-crementally with SiC paper, starting from #320 down to #1500 andwas finished with a 1 μm diamond paste. After that, ultrasonic cleaningwas carried out in ethanol. The surface of the specimen was coveredwith an epoxy resin except for the electrode area (ca. 10mm × 10 mm). The potential scan rate was 3.8 × 10−4 V s−1

(23 mV min−1). All the potentials cited in this paper are in reference tothe Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogenelectrode at 298 K).

2.3. Crevice corrosion tests

The initiation potential of crevice corrosion was evaluated innaturally aerated 1 M NaCl at 298 K. The schematic illustration of thespecimen used in the evaluation of the initiation potential of crevicecorrosion is shown in Fig. 1. After heat-treatment, the stainless steelwas cut into ca. 20 mm× 20 mm× 1 mm coupons. Then, a through-hole with a diameter of 5 mm was drilled at the center of the electrodearea. The specimen surface was polished down to 1 μm with a diamondpaste. A lead wire was soldered to the specimen. Ultrasonic cleaningwas then carried out in ethanol. With the exception of the electrodearea, the surfaces of the specimen were covered with a silicone sealant(Shin-Etsu Silicone, KE45W). The size of the electrode area was 18mm× 18 mm. A crevice was created between the electrode surface anda polycarbonate washer (inner diameter 4.5 mm, outer diameter 10mm). The specimen and the washer were fixed by a polycarbonate boltand nut in 1 M NaCl. Potentiostatic polarization was carried out at 0.20,0.25, 0.30, 0.35, and 0.40 V. The initiation time of crevice corrosionwas measured at each potential. The initiation time of crevice corrosionwas determined by the time required for the large increase in current.

To assess the effect of the solution composition on the repassivationbehavior of crevice corrosion, a flow cell was fabricated. Fig. 2 showsthe schematic illustration of the cell. The specimen was placed at thecenter of the cell. A glass plate (8 mm× 8 mm× 1.1 mm) was pressedto the electrode surface to make the crevice. The average thickness ofthe crevice was estimated ca. 6 μm, which was calculated from the masschange when ethanol was impregnated only into the crevice. The so-lution flowed through the inlet to the outlet. In this cell, the solution ofthe crevice corrosion environments can be changed by valve operation,so that it is possible to evaluate the effect of solution composition on therepassivation behavior inside the crevice without any specimentransfer. In addition, in situ observation was available through the glasswindow using an optical microscope. An Olympus BX51 M equippedwith an Olympus PlanApo N 2 X objective was used for the observation,and optical micrographs were taken every 10 s. This type of flow cell issuitable for in situ monitoring of the change in corrosion morphologyduring the solution change. Moreover, the flow cell has some ad-vantages such as the simple operation and the reproducibility for thesolution change.

For the crevice corrosion test apparatus shown in Fig. 2, the

Table 1Chemical composition of steel (mass%).

C Si Mn P S Ni Cr Mo Cu Al Ti N o

0.012 0.64 1.20 0.03 0.0012 12.0 17.4 2.07 0.29 0.002 0.002 0.019 0.003

Fig. 1. Schematic illustration of polycarbonate/metal crevice specimen. (a) Side and (b)front views.

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stainless steel was cut into 25 mm× 25 mm× 1 mm coupons afterheat-treatment. All the surfaces were polished down to 1 μm with adiamond paste. Ultrasonic cleaning was carried out in ethanol, and thenthe specimen was passivated in 30 mass% HNO3 at 323 K for 1.8 ksbased on ISO 16048 to prevent the crevice corrosion between the O-ringand the backside of the specimen. Just before the crevice corrosiontests, the upper surface of the specimen was polished down to 1 μmwith the diamond paste again. To create the crevice between the spe-cimen and the glass plate, the surface of the specimen was perfusedwith 1 M NaCl, and then the glass plate was pressed. In the crevicecorrosion tests, the specimen was polarized at 0.25 V in 1 M NaCl at298 K. Crevice corrosion was initiated approximately 400 s, and thenthe inlet solution was changed when the current increased to a value of100 μA. The flow rate of the solution was 24 mL h−1 during the crevicecorrosion tests. Because the volume of the cell was ca. 24 mL, it tookapproximately 1 h to change the solutions completely. In the solutionchange experiments, 1 M NaCl was changed to 0.88 M Na2SO4 or 1 MNaCl-0.88 M Na2SO4. Because the electrical conductivity of both 1 MNaCl and 0.88 M Na2SO4 was the same value, the electrical con-ductivity of the test solutions was constant during the solution changeexperiments from 1 M NaCl to 0.88 M Na2SO4. Additional experimentswere performed, in which the solution was changed from 1 M NaCl to1 M NaCl-0.88 M Na2SO4 to make clear the effect of SO4

2− ions onrepassivation behavior in Cl−-containing solutions. In this case, theelectrical conductivity of the 1 M NaCl-0.88 M Na2SO4 was not thesame with that of 1 M NaCl or 0.88 M Na2SO4.

3. Results and discussion

3.1. Initiation potential of crevice corrosion

To elucidate the initiation potential of the crevice corrosion for Type316L, potentiostatic polarization was performed using the crevice spe-cimen shown in Fig. 1. The effect of electrode potential on the incubationtime to the crevice corrosion of the polycarbonate/steel crevice specimenswas studied using potentiostatic polarization at 0.20, 0.25, 0.30, 0.35, and0.40 V. The time variations of the total currents of the polycarbonate/steelcrevice specimens in the naturally aerated 1 M NaCl solution at 298 K areshown in Fig. 3. The currents decreased with time at first, and then, the

large current increases were observed due to the initiation of crevicecorrosion. The incubation time for crevice corrosion was defined as thattime required for the large increase in current. The black open circlesindicate the initiation time of the crevice corrosion. The incubation timeincreased with decreasing potential. The effect of electrode potential onthe incubation time to the crevice corrosion of the polycarbonate/steelcrevice specimens is summarized in Fig. 4. In this figure, the pitting po-tential of the specimen in 1 M NaCl is also given. The pitting potential onthe crevice-free specimen was ca. 0.5 V, and it was confirmed that thepolarization potentials in the potentiostatic crevice corrosion tests wereconsiderably lower than the pitting potential. It was clear that the in-cubation time increased with decreasing potential. At 0.20 V, no initiationof crevice corrosion was observed within 574 ks. Therefore, the criticalpotential for crevice corrosion initiation on the polycarbonate/steel cre-vice specimens was determined to be 0.25 V.

Fig. 2. Schematic illustration of electrochemical flow cell for crevice corrosion tests. (a) Side and (b) top views.

Fig. 3. The time variations of the total currents of polycarbonate/steel crevice specimensin the naturally aerated 1 M NaCl solution at 298 K. The black open circles indicate theinitiation of crevice corrosion.

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3.2. Current changes of crevice corrosion in solution change experiments

On the basis of the data shown in Fig. 4, the solution change ex-periments were conducted at 0.25 V in the flow cell to analyze the effectof solution composition on the repassivation behavior of crevice cor-rosion. Because it took approximately 1 h (3.6 ks) to completely changethe solutions in the flow cell, a lower growth rate of the crevice cor-rosion was suitable for the solution change experiments. Thus, the so-lution change experiments were performed at 0.25 V. Fig. 5a shows thetime variation of the total currents of the specimen in the flow cell in1 M NaCl. In this experiment, no solution change was performed. At thebeginning of the crevice corrosion test, the total current decreased toca. 4 μA with time. Around ca. 260 s, the current increased rapidly. Thiscurrent increase was due to the initiation of crevice corrosion. Oncecorrosion had been initiated, the current increased up to ca. 5 mA. Thecurrent of crevice corrosion was saturated at ca. 10 ks. At this moment,almost the entire surface inside the crevice was corroded, which wedescribed later. The polarization was then stopped at ca. 10 ks. Theresult shown in Fig. 5a indicates that crevice corrosion was initiated in1 M NaCl, and no spontaneous repassivation was generated in this so-lution.

The effect of solution change on the growth and repassivation be-havior of crevice corrosion is shown in Fig. 5b. In this case, the inletsolution was changed from 1 M NaCl to 0.88 M Na2SO4. The verticaldashed line in Fig. 5b indicates the time of the solution change. Also inthis case, the current gradually decreased with time in 1 M NaCl. Ap-proximately 400 s, the current increased rapidly due to the initiation ofcrevice corrosion. At 631 s, the inlet solution was changed to 0.88 MNa2SO4. From 631 s to 2.62 ks (2.2 ks from crevice corrosion initiation),the current increased to 2.9 mA. A comparison of Fig. 5a and b revealsthat the current-time curve shown in Fig. 5b was almost the same withthe current measured in 1 M NaCl (no solution-change experiment).These results indicate no effect of the solution change within 2 ks of thesolution change because it took approximately 3.6 ks to change thesolution completely. At 2.62 ks, the current started decreasing due tothe solution change, and eventually fell below 1.3 μA. This findingsuggests that the solution change from 1 M NaCl to 0.88 M Na2SO4

caused repassivation inside the crevice. After the solution change,SO4

2− ions were thought to move toward the inside of the crevice byelectromigration. It was expected that SO4

2− ions were likely to act asan inhibitor against crevice corrosion. In this case, the concentration ofCl− ions in the bulk solution also decreased due to the solution change.The concentration of Cl− ions in the bulk solution decreased from 1 to 0

M. Therefore, it was not clear whether the electromigration of SO42−

ions or decrease in concentration of Cl− ions were critical for the re-passivation of crevice corrosion. To solve this problem, the solutionchange experiment using 1 M NaCl-0.88 M Na2SO4 was conducted. In

Fig. 4. Effect of potential on incubation time for crevice corrosion of polycarbonate/metal crevice specimen in 1 M NaCl solution at 298 K.

Fig. 5. Effect of solution change on time variation of total current of the specimen in theflow cell. (a) 1 M NaCl (no solution-change), (b) solution change from 1 M NaCl to 0.88 MNa2SO4, and (c) solution change from 1 M NaCl to 1 M NaCl-0.88 M Na2SO4. The verticaldashed lines indicate the time of the solution change.

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Fig. 5b, some fluctuations in the current after ca. 30 ks were electricalnoise, which can be ignored.

Fig. 5c shows the effect of solution change from 1 M NaCl to 1 MNaCl-0.88 M Na2SO4. The vertical dashed line indicates the time of thesolution change. The increase in current was also observed at ap-proximately 60 s. At 160 s, the inlet solution was changed from 1 MNaCl to 1 M NaCl-0.88 M Na2SO4. From 160 s to 2.53 ks (2.2 ks fromcrevice corrosion initiation), current increased to ca. 2.9 mA. Because ittook approximately 3.6 ks to change the solution completely, thiscurrent value is almost the same as that in the two previous cases(Fig. 5a and b). As with the case shown in Fig. 5b, the current starteddecreasing at ca. 2.5 ks. At ca. 4.5 ks, the current decreased to ca. 160μA. This current change from ca. 2.5 ks to ca. 4.5 ks was quite similar tothe case of the solution change from 1 M NaCl to 0.88 M Na2SO4. Theresults thus far clearly indicate that the current decrease after the so-lution change was caused by SO4

2− ions, not by decrease in con-centration of Cl− ions.

In the solution change from 1 M NaCl to 0.88 M Na2SO4, the largedecrease in current was observed after the solution change (Fig. 5b).The decrease in the concentration of Cl− ions is the predominant factoraffecting the current decrease. Although the degree was far less thanthat in 0.88 M Na2SO4 after changing the solution, the decrease incurrent was observed in 1 M NaCl-0.88 M Na2SO4. It is likely to be thefact that SO4

2− ions act as a weak inhibitor in the chloride solution.In Fig. 5c, the electromigration of SO4

2− ions from the bulk solutionto inside the crevice suppressed the growth of the crevice corrosion, butthe current increased again after ca. 7 ks. Finally, the current increasedto ca. 1.4 mA. This is thought to be the re-activation inside the creviceby Cl− ions. In this case, in contrast to the case of Fig. 5b, the con-centration of Cl− ions in the bulk solution did not change even after thesolution change. Thus, the accumulation of Cl− ions continued evenafter the solution change. The concentration of SO4

2− ions inside thecrevice was likely insufficient to completely prevent the initiation ofcrevice corrosion above 1 M Cl− inside the crevice. The increasing rateof the current after 7 ks was much lower than that before 2.3 ks. Thisfinding suggests that SO4

2− ions suppressed the dissolution rate even inthe Cl−-containing solution. In this study, we have confirmed that thereproducibility of the crevice corrosion tests in 1 M NaCl and the so-lution change experiments.

3.3. In situ observation of growth and repassivation behavior of crevicecorrosion

An in situ observation inside the crevice was conducted to analyzethe repassivation behavior of crevice corrosion in terms of corrosionmorphology. Fig. 6 shows the optical micrographs inside the creviceduring the crevice corrosion test shown in Fig. 5a. In this crevice cor-rosion test, the solution was 1 M NaCl, and the solution was notchanged. By placing a square glass plate at the center of the specimen, acrevice was formed between the specimen and the glass plate. At 301 s,two small dark spots were observed inside the crevice. These spots werethe initiation sites of crevice corrosion. The dark areas grew to thecrevice mouth from 301 to 408 s. As shown in Fig. 6a2–8, the corrodedareas proceeded along the crevice mouth once they reached the crevicemouth. Crevice corrosion initiation was thought to be mainly attributedto the decrease in pH inside the crevice due to the hydrolysis of dis-solved metal ions. Therefore, crevice corrosion tends to be initiated atan inside region of the crevice. On the other hand, because the growthof crevice corrosion is determined by the electromigration of anions andcations between the inside and outside of the crevice, the crevice mouthis thought to be suitable for the growth of crevice corrosion. This factexplains the growth of crevice corrosion from inside the crevice to thecrevice mouth immediately after initiation, as shown in Fig. 6a2–4.After the initiation of crevice corrosion, the growth of corroded areatoward the inside of the crevice was also observed. This type of cor-rosion growth is thought to be caused by active dissolution produced by

the acidification and the IR drop. During the corrosion propagationtoward the inside of the crevice, the generation of gas bubbles wasconfirmed. Gas bubbles were observed slightly inside the corroded areaas indicated by the arrows in Fig. 6. The electrode potential on thecorrosion front inside the crevice was thought to be more negative thanthat of the hydrogen evolution reaction due to the IR drop. A

comparison of Fig. 6a8-9 reveals that the crevice corrosion propa-gated toward the inside of the crevice once the entire crevice mouthwas corroded. Because of the existence of the gas bubbles, the centerpart of the crevice was not corroded completely even after 10 ks, butalmost the entire surface inside the crevice was corroded. At this mo-ment, the current seemed to reach the saturation value. As shown inFig. 5a, 5 mA was likely to be the saturation value for the growthcurrent of the crevice geometry used in this study. The corrosion be-havior observed in this study correlates well with the work of Matsu-hashi et al. [40]. They also observed that crevice corrosion areas grewalong the crevice mouth, and after that, the crevice corrosion propa-gated towards the inside of the crevice. In addition, the saturationcurrent value of the crevice corrosion growth for Type 316L stainlesssteel was estimated to be 5 mA in 1 M NaCl at 298 K. This is consistentwith the change in current with time in Fig. 5a.

Fig. 7 shows the optical micrographs inside the crevice during thecrevice corrosion test shown in Fig. 5b. As shown in Fig. 7b2-3, crevicecorrosion was initiated at the position close to the crevice mouth andgrew to the crevice mouth. This initiation behavior of crevice corrosionwas similar to that shown in Fig. 6a2–4. At 631 s, the corroded areareached the crevice mouth and grew along the crevice mouth. At 631 s,the inlet solution was changed from 1 M NaCl to 0.88 M Na2SO4.However, the corroded area continued to propagate along the crevicemouth as shown in Fig. 7b4–6. It took approximately 3.6 ks to changethe solution completely. At 4.47 ks, the propagation of crevice corro-sion along the crevice mouth stopped. Once this process ceased, thecorroded area began propagating toward the inside of the crevice. Fromthe comparison between Figs. 5b and 7, it was clear that the corrodedarea increased from 2.62 ks (Fig. 7b6) to 20.0 ks (Fig. 7b12), but thecurrent deceased as shown in Fig. 5b. It was suggested that the dis-solution rate in the corroded area decreased after 2.62 ks. The reasonfor this is not clear but is probably due to the formation of corrosionproducts inside the crevice [9,41,42]. It would appear that the corro-sion products formed in the sulfate-containing solution prevent theactive dissolution inside the crevice. On the other hand, in the case of1 M NaCl (see Figs. 5a and 6), the current increased with the corrodedarea, suggesting that the protective ability of the corrosion productsformed in the NaCl solution is low compared to those in the Na2SO4

solution. The corrosion products play important roles in the re-passivation of the crevice corrosion, and the solution compositions canaffect the type and protective property of these.

In addition to the above corrosion behavior, as indicated by thearrows in Fig. 7, gas generation was observed. As shown in Fig. 7b7–12,the gas bubbles accumulated at the center of the crevice, and the gasaccumulation area act as a physical barrier against the growth of cre-vice corrosion. From 4.47–20.0 ks, the crevice corrosion grew, but thegas accumulation area was surrounded by the corroded area. After-wards, the center of the crevice, where was the gas accumulation area,was corroded partly. After 91.8 ks, there was no further change in thesize and shape of the corroded area, suggesting that the surface of thecorroded area was completely repassivated.

Fig. 8 shows the optical micrographs inside the crevice during thecrevice corrosion test shown in Fig. 5c. In a similar manner as the othertwo cases, it was observed that crevice corrosion was initiated at theposition close to the crevice mouth and grew to the crevice mouth.Next, the corroded area propagated along the crevice mouth initially asshown in Fig. 8c2-3. At 160 s, the inlet solution was changed from 1 MNaCl to 1 M NaCl-0.88 M Na2SO4. After 1.72 ks, as indicated by thearrow in Fig. 8c4–7, accumulation of gas bubbles was also observed. At3.55 ks, the corrosion propagation along the crevice mouth almost

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stopped. It was expected that it took approximately 3.6 ks to change thesolution completely. Afterwards, as shown in Fig. 8c6-7, the corrodedarea propagated toward the inside of the crevice similarly to the case ofFig. 7. This result clearly indicates that SO4

2− ions inhibited corrosionpropagation along the crevice mouth even in the Cl−-containing solu-tion. However, in the case of 1 M NaCl-0.88 M Na2SO4, the currentincreased again due to the initiation of another crevice corrosion after7.05 ks (Fig. 5c). As shown in Fig. 8c7–9, the crevice corrosion grewtoward the right side of the crevice shown in Fig. 8. Additionally, smallcorrosion areas were newly observed. In addition to these, it seems thatthe crevice corrosion grew along the crevice mouth again. The con-centration of SO4

2− ions is likely insufficient to completely prevent theinitiation of crevice corrosion.

3.4. Effect of SO42− ions on growth rate of crevice corrosion

To confirm the inhibition of the growth rate of crevice corrosion bySO4

2− ions, the length of the corroded areas at the crevice mouth andthe corroded areas inside the crevice were evaluated from the time-lapse images. A corroded volume should be more appropriate to de-termine the corrosion inside crevice; however, a long time was neces-sary to obtain the depth profile inside the crevice because the size of thecrevice was 8 mm× 8 mm. In this study, the corroded area and thelength of the corroded areas were used as representative values of themorphological change in crevice corrosion.

The length of the corroded areas was visually defined by the lengthof dark area at the crevice mouth and measured using the scale of op-tical micrographs. Fig. 9 shows the time variation of the length of the

corroded areas at the crevice mouth during the crevice corrosion testsshown in Fig. 5. The black solid circles in Fig. 9 indicate the time whenthe corrosion growth along the crevice mouth stopped. The times of thesolution change are indicated by the vertical broken lines. In 1 M NaCl(Fig. 9a), at first, the length of the corroded area at the crevice mouthincreased continuously. The increase in length of the corroded areas atthe crevice mouth almost stopped at ca. 4 ks. As shown in Fig. 6, thecrevice corrosion mainly grew along the crevice mouth until 4 ks. Afterthat, the crevice corrosion grew toward the inside of the crevice. It isclearly seen that there is a change in the growth morphology of thecrevice corrosion from the slope of the plots shown in Fig. 9.

In the solution change experiment from 1 M NaCl to 0.88 M Na2SO4

(Fig. 9b), the length of the corroded area at the crevice mouth increasedconstantly until 4.47 ks, and then the growth of the length of the cor-roded area at the crevice mouth stopped at 4.47 ks (Fig. 7b7). From thistime onward, the corroded area grew toward the inside of the crevice.The length of the corroded area at the crevice mouth was smaller thanthat in the 1 M NaCl solution. It was expected that SO4

2− ions werelikely to act as an inhibitor against the growth of the crevice corrosionat the crevice mouth. In the solution change experiment from 1 M NaClto1 M NaCl-0.88 M Na2SO4 (Fig. 9c), the increase in the length almoststopped at 2.53 ks. Afterwards, the corroded area grew toward the in-side of the crevice. After 10 ks, the length of the corroded area at thecrevice mouth slightly increased again. Additionally, the length of thecorroded area at the crevice mouth was smaller than that in 1 M NaCl.In the early stage of the repassivation process, SO4

2− ions inhibited thegrowth of crevice corrosion at the crevice mouth. To assess the growthof the crevice corrosion toward the inside of the crevice, the corroded

Fig. 6. Optical micrographs of inside crevice during crevice corrosion test shown in Fig. 5a. The specimen was polarized at 0.25 V in 1 M NaCl.

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areas were also evaluated from the time-lapse images. The corrodedareas were calculated by binarized images using digital image proces-sing. The threshold level was adjusted individually for each image to

match the size of the binarized and corroded areas. The areas coveredwith gas bubbles were not included in the corroded areas because thegas accumulation areas were not corroded. Fig. 10 shows the time

Fig. 7. Optical micrographs of inside crevice during crevice corrosion test shown in Fig. 5b. The specimen was polarized at 0.25 V. The solution was changed from 1 M NaCl to 0.88 MNa2SO4 at 631 s.

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Fig. 8. Optical micrographs of inside crevice during crevice corrosion test shown in Fig. 5c. The specimen was polarized at 0.25 V. The solution was changed from 1 M NaCl to 1 M NaCl-0.88 M Na2SO4 at 160 s.

Fig. 9. Effect of solution change on length of corroded area at crevice mouths. (a) 1 MNaCl (no solution change), (b) from 1 M NaCl to 0.88 M Na2SO4, and (c) from 1 M NaCl to1 M NaCl-0.88 M Na2SO4.

Fig. 10. Effect of solution change on time variation of corroded area. (a) 1 M NaCl (nosolution change), (b) from 1 M NaCl to 0.88 M Na2SO4, and (c) from 1 M NaCl to 1 MNaCl-0.88 M Na2SO4.

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variations of the corroded areas during the crevice corrosion testsshown in Fig. 5. The time of the solution change is indicated by verticalbroken lines. The black solid circles in Fig. 10 indicate the time whenthe corrosion growth along the crevice mouth stopped. In 1 M NaCl(Fig. 10a), the corroded area increased constantly with time until theend of the corrosion test. As shown in Fig. 6a9, almost all the surfaceinside the crevice was corroded. The crevice corrosion did not stopspontaneously in 1 M NaCl.

In the case of the solution change experiment from 1 M NaCl to 0.88 MNa2SO4 (Fig. 10b), the corroded area increased constantly with time untilca. 10 ks. From ca. 10 ks to ca. 70 ks, the corroded area grew slowly. Thisslow propagation inside the crevice can be confirmed in Fig. 7b11–13. At70.0 ks, the increase in the corroded area stopped completely (Fig. 7b14-15). The crevice corrosion was repassivated by the solution change from1 M NaCl to 0.88 M Na2SO4. In the solution change experiment from 1 MNaCl to 1 M NaCl-0.88 M Na2SO4 (Fig. 10c), the growth rate of the crevicecorrosion was almost equal to that in 1 M NaCl at the early stage of crevicecorrosion. The decrease in the growth rate was observed ca. 3 ks. As shownin Fig. 8c7–9, after ca. 10 ks, a large decrease in the growth rate wasobserved. In this case, the corrosion propagation toward the inside of thecrevice was inhibited. Therefore, it was found that the growth rate of thecorroded area was definitely decreased due to the addition of SO4

2− ionseven in the Cl−-containing solution.

From the comparison of Figs. 9 and 10, the repassivation at the crevicemouth was observed ca. 4 ks as shown in Fig. 9b and c. On the other hand,the repassivation inside the crevice was observed approximately 10 ks asillustrated in Fig. 10b and c. It was confirmed that crevice corrosion wasrepassivated at the crevice mouth first, and then inside the crevice.

3.5. Effect of SO42− ions on active dissolution in acidic solutions

The growth of the crevice corrosion was due to active dissolutioninside the crevice. In the propagation period of crevice corrosion, thepH inside the crevice was low. Turnbull reviewed the research on so-lution composition inside crevices [43]. The solution pH inside thecrevice after crevice corrosion initiation was estimated to be between 1and 2 for stainless steels. Kaji et al. reported that the pH inside thecrevice of Fe-18Cr-10Ni–5 Mn in 0.01 M NaCl decreased to 0.4 aftercrevice corrosion initiation [4]. To confirm the inhibition effect ofSO4

2− ions on active dissolution behavior in acidic solutions,　anodicpolarization was performed at pH 0.4. Fig. 11 shows the anodic po-larization curves of Type 316L stainless steel in the deaerated 1 M NaCl,0.88 M Na2SO4, and 1 M NaCl-0.88 M Na2SO4 solutions at pH 0.4. In allthe solutions, active and passive regions were observed. In 1 M NaCl atpH 0.4 (Fig. 11a), the active region was clearly observed from ca. −

0.32 V to −0.04 V. In 0.88 M Na2SO4 at pH 0.4 (Fig. 11b), the currentdensities in the active region (−0.27 V to −0.04 V) were lower thanthose in the 1 M NaCl solution, which suggests that SO4

2− ions sup-pressed active dissolution inside the crevice. The suppression of theactive dissolution rate was likely to prevent the hydrolysis reaction ofmetal ions inside the crevice, resulting in neutralization of the crevicesolution. This neutralization promoted repassivation inside the crevice.Additionally, in the case of 1 M NaCl-0.88 M Na2SO4 at pH 0.4(Fig. 11c), the current densities were slightly lower than those in the1 M NaCl solution around −0.25 V, and the active region was nar-rowed. Because the potential inside the crevice seemed to be an activeregion in the propagation period, the transition from the active to thepassive state is necessary to produce repassivation inside the crevice.Therefore, the active region narrowed by SO4

2− ions was suitable toinduce the transition from the active to the passive state. As mentionedabove, SO4

2− ions promoted repassivation when they migrated insidethe crevice. El-Naggar reported that SO4

2− ions facilitate passivation inthe active-passive region on carbon steel in deaerated 0.50 M NaHCO3

solutions [44]. Pickering [1,2] proposed that the acidification andchloride accumulation inside the crevice enlarge the active dissolutioncurrent and shift the passive/active transition potential to the nobledirection in potentiodynamic polarization curves, so that the IR-dropcondition for stable crevice corrosion is established readily. As shown inFig. 11, SO4

2− ions shifted the critical potential for passive/activetransition to the less noble direction, thereby decreasing the activedissolution rate and the width of the dissolution potential region, whichmade it more difficult to achieve the IR-drop condition required forcrevice corrosion to grow.

It was reported that the competitive adsorption of SO42− and Cl−

ions on metal surfaces occurred in Cl−-containing environments [45].The amount of adsorbed Cl− ions decreased in the presence of SO4

2−

ions. At high concentrations of SO42− ions, the Cl− ions were not ad-

sorbed. Li et al. demonstrated that SO42− ions inhibited the corrosion

on Type 316 stainless steel in a dilute HCl solution [46]. In 0.01 MNaCl-0.01 M Na2SO4 solution, the charge transfer resistance of Type316L was larger than that in 0.01 M HCl solution. From the abovediscussion, the repassivation of the crevice corrosion shown in Fig. 5cwas attributed to the effect of SO4

2− ions, which accumulated on thesurface of the stainless steel and increased the corrosion resistance.Molar conductivities at infinite dilution of Cl− and SO4

2− ions were76.3 and 79.8 S cm−2 mol−1, respectively, at 298 K [47,48]. This sug-gests that the molar ratio of SO4

2−/Cl− gradually increased duringcrevice corrosion in 1 M NaCl-0.88 M Na2SO4. The accumulation ofSO4

2− ions seemed to inhibit the active dissolution inside the crevice.Consequently, the pH inside the crevice was neutralized sufficiently toinduce repassivation.

In the crevice corrosion tests in this study, the composition changein the crevice solution is the critical factor for the dissolution rate andthe repassivation behavior. Pistorius and Burstein [29] studied that theeffects of sulfate on metastable and stable pitting of Type 304 stainlesssteel in 1 M NaCl. Pit propagation, in both the metastable and stablestates, is also inhibited by sulfate ions. The reduced pit propagationcurrent densities are described quantitatively with respect to the effectof sulfate on the solubility of the metal cation in the pit anolyte. Theyproposed that the solubility of metal cations in mixed sulfate andchloride solution decrease with increasing [SO4

2−]/[Cl−] molar ratio.In addition to above discussion, there is a possibility that corrosion

resistance of passive films and/or corrosion products inside crevice areimproved by SO4

2− ions. Further research is necessary to make clearthis point.

4. Conclusions

1. The repassivation of crevice corrosion on Type 316L stainless steelwas observed in the solution change from 1 M NaCl to 0.88 MNa2SO4. After crevice corrosion initiation, the solution was changed.

Fig. 11. Anodic polarization curves of Type 316L stainless steel in deaerated solutions atpH 0.4. (a) 1 M NaCl, (b) 0.88 M Na2SO4, and (c) 1 M NaCl-0.88 M Na2SO4.

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The growth of the crevice corrosion along the crevice mouthstopped at first, and repassivation inside the crevice was observedthereafter.

2. In the case of the solution change from 1 M NaCl to 1 M NaCl-0.88 M Na2SO4, the decrease in current was once measured duringthe growth stage of crevice corrosion. The accumulation of SO4

2−

ions brought about the repassivation. However, the concentration ofSO4

2− ions inside the crevice was thought to be insufficient toprevent the initiation of crevice corrosion inside the crevice com-pletely. After ca. 7 ks, crevice corrosion grew again due to furtheraccumulation of Cl− ions inside the crevice.

3. In the anodic polarizarion measurement at pH 0.4 on crevice freespecimens, the active dissolution of Type 316L stainless steel wassuppressed by SO4

2− ions, and the active region was narrowed in1 M NaCl-0.88 M Na2SO4. Because the transition from the active tothe passive state inside the crevice was thought to be promoted bySO4

2− ions, the suppression of active dissolution inside the crevicewas the predominant role of SO4

2− ions in the repassivation of thecrevice corrosion. In the crevice solution, SO4

2− ions caused sup-pression of the steel dissolution rate, resulting in prevention of thehydrolysis reaction of metal ions, which reduced the pH decreaseand enhanced repassivation of crevice corrosion.

Acknowledgements

This work was partially supported by the Center of WorldIntelligence Project for Nuclear S & T and Human ResourceDevelopment by the Ministry of Education, Culture, Sports, Science andTechnology of Japan. Y. Sugawara is grateful for the funding of hisresearch by JSPS KAKENHI Grant Number JP15H05550.

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