Monitoring the Diffusion Layer During Passive Film Breakdown on Alloy 800 with Digital Holography

Transcription

1 Acta Metall. Sn. (Engl. Lett.), 2015, 28(9), DOI /s Montorng the Dffuson Layer Durng Passve Flm Breakdown on Alloy 800 wth Dgtal Holography Da-Ha Xa 1,2,3 Jng-L Luo 3 Zh-Mng Gao 1,2 Bo-Yu Yuan 4 Lang L 5 Chao Wang 5 Receved: 4 May 2015 / Revsed: 29 June 2015 / Publshed onlne: 8 September 2015 The Chnese Socety for Metals and Sprnger-Verlag Berln Hedelberg 2015 Abstract The effects of chlorde and thosulfate ons on localzed corroson of alloy 800 are nvestgated through dynamcal observaton of the change n phase mage of the dffuson layer durng passve flm breakdown usng dgtal holography. The results ndcate that soluton chemstry has a sgnfcant effect on flm breakdown and dffuson layer. The phase dstrbuton changes at dfferent appled potentals show that n the process of flm breakdown, dssoluton of metal ons from pttng s not remarkable n chlorde-only soluton, whereas dssoluton of metal ons s sgnfcantly hgh n thosulfate and chlorde soluton. Thosulfate has a combned effect wth chlorde ons n passve flm degradaton. KEY WORDS: Passve flm; Breakdown; Dffuson layer; Dgtal holography; Alloy Introducton Passve flm breakdown s a very mportant electrochemcal process whch occurs on the metal and alloy surface, because t s usually the ntal stage of pttng corroson and stress corroson crackng (SCC) for self-passve pure Avalable onlne at & Da-Ha Xa dahaxa@tju.edu.cn & Jng-L Luo luoj@ualberta.ca Tanjn Key Laboratory of Composte and Functonal Materals, Tanjn Unversty, Tanjn , Chna School of Materals Scence and Engneerng, Tanjn Unversty, Tanjn , Chna Department of Chemcal and Materals Engneerng, Unversty of Alberta, Edmonton, AB T6G 2V4, Canada School of Physcs and Electronc Engneerng, Jangsu Normal Unversty, Xuzhou , Chna School of Chemstry and Chemcal Engneerng, Jangsu Normal Unversty, Xuzhou , Chna metals and alloys [1, 2]. Because passve flm breakdown s an electrochemcal process at nanoscale or mcro-nanoscale and s usually very localzed, tradtonal electrochemcal methods, ncludng polarzaton curve [3, 4], electrochemcal mpedance spectroscopy (EIS) [5 7] and electrochemcal nose (EN) [8], are not able to capture some localzed actvtes of ths process. Therefore, researchers have appled localzed electrochemcal methods to explore ths process. For example, Cu et al. [9] used scannng reference electrode to measure the surface potental dstrbuton. Zhu et al. [10, 11] and Xa et al. [12, 13] used scannng electrochemcal mcroscope (SECM) to obtan the surface reactvty of alloy 800 n chlorde and thosulfate solutons. Zhao et al. [14] used Scannng Kelvn Probe (SKP) to study the potental dstrbuton of a crack on 304 stanless steel. However, passve flm breakdown s a complex process ncludng dssoluton of metal and dffuson of reactant and corroson product. Thus, dynamcally trackng or montorng the actvtes n dffuson layer durng passve flm breakdown can provde new nsghts nto ths process and may help us understand the detaled mechansm of the breakdown process. Dgtal holography s a new optcal method whch offers an easy, useful, nondestructve way to n stu observe the

2 D.-H. Xa et al.: Acta Metall. Sn. (Engl. Lett.), 2015, 28(9), dynamc changes on an electrode/electrolyte nterface. L et al. [15] nvestgated current oscllatons durng pttng corroson of Fe usng dgtal holography. Yuan et al. [16] observed the anodc dssoluton of copper n NaCl soluton wth the dgtal holography and proposed anodc dssoluton mechansm. Though the anodc dssoluton of metal and alloys has been studed usng dgtal holography, the technque has not been used to nvestgate the passve flm breakdown. Therefore, the am of ths work s to nvestgate the passve flm breakdown process usng dgtal holography. 2 Expermental The specmens were prepared usng alloy 800 SG tubng (Sandvk, heat number , outer dameter of mm, wall thckness of 1.13 mm) [17]. The composton of the alloy 800 s lsted n Table 1. The tube was cut nto a long strp, the front-end was ground to very flat surfaces, and the exposed area was about 0.2 cm 2. Cyclc polarzaton was conducted at 21 C at 10 mv/s by usng a CHI660B electrochemcal staton. A three-electrode cell was used wth the alloy 800 as the workng electrode, a saturated calomel electrode (SCE) as the reference electrode and a platnum electrode as the counter electrode. For the holographc recordng system, a well-establshed dgtal camera (Sony DSRPD150P) was used to record the dynamcal process. Holograms that correspond to varous states of the alloy 800/soluton nterface were processed by numercal methods for measurement. The dynamc processes of electrochemcal reactons can be studed from the consecutve nterferograms whch were stored as AVI (audo vsual nterleaved) fles at up to 25 frames per second n a personal computer. In processng, software was developed wth MATLAB R2007b for the reconstructon of the phase maps from the AVI vdeo sgnals [18]. The ntal nterferogram at the tme when the reacton began was selected, and the dstrbuton of phase dfference was contnually dsplayed on the PC montor. Durng the holography experment, the change n the phase of the object beam s determned by the change n the refractve ndex nsde the cell, whch s assocated wth the changes n concentraton caused by the electrochemcal reacton. The prncple of the experment s based on the relatonshps between the phase dfference (D/), the soluton refractve ndex (Dn) and the soluton concentraton (DC), whch was expressed as [18]: DC ¼ kdn ¼ kk 0D/ 2pd : ð1þ where k s the concentratve refractvty, k 0 s the wavelength of the laser lght, d stands for the geometrcal path length where the refractve ndex varaton exsts. Generally, several components are present n the soluton. Thus, the net refractve ndex s the summaton of the effect of each speces concentraton: X ðdc Þ ¼ X ðdk Dn Þ ¼ k 0 2pd 3 Results and Dscusson X ðk D/ Þ: ð2þ The cyclc polarzaton curve of alloy 800 n 0.6 mol/l NaCl soluton s shown n Fg. 1. Self-passve behavor of alloy 800 was montored n NaCl soluton, and the passve flm was stable and was not broken down when the potental was lower than 0.4 V SCE. However, above 0.4 V SCE, the current ncreased due to flm breakdown and many transent peaks could be found n the curve, whch s attrbuted to the metastable pt ntaton and repassvaton [19]. Prevous results ndcated that passve flm formed on alloy 800 n chlorde-only soluton s not easy to broken down unless the anodc polarzaton potental value s hgh enough, and the pttng propagaton rate s also very low because the pts sze n ths soluton s very tny and sometmes even cannot be dentfed [17, 19]. A selecton of quanttatve phase mages, reconstructed at varous potentals whch are marked n the cyclc polarzaton curve (Fg. 1), s shown n Fg. 2, whch depcts the dstrbuton of phase dfferences at varous potentals. As shown n the dstrbuton of phase dfference, the left sdes are the alloy 800 electrode, whereas the rght sdes are the electrolyte. The nterfaces can be dentfed by the boundary between the electrode and electrolyte. It s evdent that the color varaton appears only at the nterface where color changng to yellow or red means the ncrease n the phase dfference, whle changng to blue means the decrease [20]. The green area n Fg. 2 ndcates that the phase dfference was zero or almost zero. When the passve flm was ntact and undamaged under certan potentals, such as pont A, the phase color was green, whch means there was no detected Table 1 Chemcal compostons of alloy 800 (n wt%) C S Mn P S Cr N Co T Cu Al N Fe

3 1172 D.-H. Xa et al.: Acta Metall. Sn. (Engl. Lett.), 2015, 28(9), Fg. 1 Cyclc voltammogram of the alloy 800 n 0.6 mol/l NaCl soluton wth the scan rate of 10 mv/s phase dfference n ths stage, as shown n Fg. 2a. As the potental ncreased to ponts B and C, a regon wth yellow color could be seen n the phase dstrbuton (Fg. 2b and c); therefore, the correspondng regon on the alloy 800 surface was exactly the ste where passve flm was broken down, as a result of Fe 2?,Cr 3? and N 2? generated from the dssoluton reactons (3) to(5). The release of Fe 2?,Cr 3? and N 2? ncreased the concentratons of these ons at the nterface and resulted n an ncrease n the refractve ndex. Subsequently, the phase dfference became postve. For ponts D, E, F and G n Fg. 1, the dssoluton rate ncreased due to an ncrease n anodc current; therefore, the refractve ndex contnued to ncrease, as shown n Fg. 2d g. As the potental decreased to the pont H (about 0.2 V SCE ), pts were repassvated and the generated catons dffused nto the bulk soluton, so the phase dfference decreased accordngly, as shown n Fg. 2h. Fe! Fe 2þ þ 2e : ð3þ Cr! Cr 3þ þ 3e : N! N 2þ þ 2e : ð4þ ð5þ The cyclc polarzaton curve of alloy 800 n 0.6 mol/l NaCl? mol/l Na 2 S 2 O 3 soluton s shown n Fg. 3, Fg. 3 Cyclc voltammogram of the alloy 800 n 0.6 mol/l NaCl? mol/l Na 2 S 2 O 3 soluton wth the scan rate of 10 mv/s and the correspondng phase dstrbutons at varous potentals are shown n Fg. 4. Smlar to the analyses for Fg. 2a, for pont A 0 at about 0.12 V SCE, the phase color of the electrolyte was green, ndcatng that there was no detected phase dfference n ths stage, and the passve flm was ntact under 0.12 V SCE (Fg. 4a). As the potental ncreased to 0.15 V SCE (pont B 0 ), the current began to ncrease, resultng n breakdown of the passve flm (Fg. 4b). Obvously, the pttng potental n soluton contanng thosulfate was about 200 mv lower than that n thosulfate-free soluton. As t was explaned prevously, the reason s that thosulfate reduces the pttng potental by adsorpton wthn the metastable pts and stablzes the metastable pttng nduced by chlorde ons [21]; therefore, thosulfate could block repassvaton n chlorde solutons. Newman et al. [22] found the same phenomenon for 304 SS n 0.25 mol/l NaCl soluton contanng varous concentratons of thosulfate. They clamed that the combned effect of these two ons s due to the breakdown of passve flm by chlorde ons and accumulaton of thosulfate at rapdly dssolvng pt Fg. 2 Correspondng phase maps at varous polarzaton potentals that marked by letters A a, B b, C c, D d, E e, F f, G g and H h n Fg. 1

4 D.-H. Xa et al.: Acta Metall. Sn. (Engl. Lett.), 2015, 28(9), Fg. 4 Correspondng phase maps at varous polarzaton potentals that marked by letters A 0 a,b 0 b,c 0 c,d 0 d,e 0 e,f 0 f,g 0 g n Fg. 3 precursors to form S 0 whch would stablze metastable pts and accelerate anodc dssoluton. As the potental contnued to ncrease, the dssoluton rate also ncreased at the pttng stes, resulted n that the concentraton of these ons close to the nterface became hgh, so the phase color became red, as shown n Fg. 4c f. Comparng Fgs. 1 and 2 wth Fgs. 3 and 4, t s concluded that the dssoluton of metal ons s not remarkable n thosulfate-free soluton, whereas the dssoluton of metal ons s sgnfcant n thosulfate-contanng soluton, whch s due to thosulfate-enhanced anodc dssoluton wthn the pts, and ths s consstent wth the prevous expermental observatons [19, 23]. Moreover, the current n thosulfate-contanng soluton s sgnfcantly hgher than that n thosulfate-free soluton. Therefore, the thosulfate played an mportant role n the passve flm breakdown by adsorpton on the metastable pts to catalyze anodc dssoluton. Thosulfate has a sulfur chan that s uncapped. The uncapped ends are free radcals contanng lone, unpared electrons, whch render them very reactve, especally toward bare metal surfaces, such as copper, nckel and ron, provded that the confguraton of the adsorbed speces allows access of the uncapped end to the metal atoms [20]. In addton, the pttng propagaton rate was much hgher n thosulfate-contanng soluton than that n thosulfate-free soluton, whch was attrbuted to the catalytc anodc dssoluton caused by thosulfate. Fgure 5 shows the surface morphologes of the samples after the cyclc polarzaton test. The pt sze n thosulfatefree soluton s small and n the range of several mcrometers, but the pt sze n thosulfate-contanng soluton was about 100 lm, ndcatng that the pttng propagaton rate n thosulfate and chlorde soluton was sgnfcantly hgher than that n thosulfate-free soluton. Apart from the large pttng damage shown n Fg. 5b, there are also many tny pts over the sample surfaces (Fg. 5a, b), but these pts were not recorded n the holographc photographs because the dssoluton of metal ons wthn these small pts s very lmted, so they were beyond the detectng lmt. In fact, the effects of the thosulfate and chlorde ons on flm degradaton depended on the concentraton rato of these two ons, and the combned effect can be observed at hgh concentraton ratos of chlorde to thosulfate; at low concentraton ratos, the adsorpton of thosulfate s domnant so that there s no opportunty for chlorde ons to adsorb onto the surface to damage the passve flm. Therefore, no pts are ntated at a lower chlorde-tothosulfate concentraton rato [12, 21]. The concentraton of these two ons could affect the semconductvty of passve flm formed on alloy 800, and the semconductvty type changes from n-type to p-type as the concentraton rato decreased, whch was due to sulfur ncorporaton nto oxygen vacances [21]. In summary, the soluton chemstry plays an mportant role on flm breakdown, pttng propagaton rate and dffuson layer. Dgtal holography s a new method that helps us to better understand ths process and the related mechansm. Through observng the dffuson layer changes durng flm breakdown, t was found that dssoluton of metal ons at the pts s not remarkable n chlorde-only soluton, whereas dssoluton of metal ons s sgnfcant n thosulfate and chlorde soluton.

5 1174 D.-H. Xa et al.: Acta Metall. Sn. (Engl. Lett.), 2015, 28(9), Fg. 5 SEM mages of alloy 800 after testng n 0.6 mol/l NaCl soluton a and 0.6 mol/l NaCl? mol/l Na 2 S 2 O 3 soluton b 4 Conclusons The change n phase mage of the dffuson layer durng passve flm breakdown was dynamcally observed usng dgtal holography. The results showed that the addton of a small amount of thosulfate nto chlorde solutons could lower the pttng potental and ncrease the dssoluton rate of alloy 800. At pttng potental, the dssoluton of metal ons from pttng s not remarkable n chlorde-only soluton, whereas dssoluton of metal ons s sgnfcant n thosulfate and chlorde soluton. Acknowledgments Ths work was supported by Natural Scences and Engneerng Research Councl of Canada Dscovery Grant and Natonal Natural Scence Foundaton of Chna (No ). The authors also gratefully acknowledge R.L. Tappng, P. Angell, Yucheng Lu and Stan J. Klmas of AECL for supportng ths work. References [1] L. Fan, Z. Y. Lu, W. M. Guo, J. Hou, C.W. Du, X.G. L, Acta Metall. Sn. (Engl. Lett.) 28, 866 (2015) [2] A. Parsapour, S. Khorasan, M. Fath, Acta Metall. Sn. (Engl. Lett.) 26, 409 [3] Y. Zhang, B. Chen, Z. Guo, Acta Metall. Sn. (Engl. Lett.) 27, 331 (2014) [4] D.H. Xa, L.X. Yang, Acta Phys-Chm Sn. 30, 1465 (2014) [5] D.H. Xa, S.Z. Song, W.Q. Gong, Y.X. Jang, Z.M. Gao, J.H. Wang, J. Food Eng. 113, 11 (2012) [6] D.H. Xa, S.Z. Song, J.H. Wang, H.C. B, Z.W. Han, Trans. Tanjn Unv. 18, 15 (2012) [7] C. Thee, L. Hao, J.H. Dong, X. Mu, W. Ke, Acta Metall. Sn. (Engl. Lett.) 28, 261 (2015) [8] J. L, H.J. Zheng, Y. Zheng, W.K. Kong, K. Wang, Acta Metall. Sn. (Engl. Lett.) 28, 449 (2015) [9] N. Cu, H.Y. Ma, J.L. Luo, S. Chovell, Electrochem. Commun. 3, 716 (2001) [10] R.K. Zhu, J.L. Luo, Electrochem. Commun. 12, 1752 (2010) [11] R.K. Zhu, B.T. Lu, J.L. Luo, Y.C. Lu, Appl. Surf. Sc. 270, 755 [12] D.H. Xa, H.Q. Fan, L.X. Yang, Y. Behnaman, J.L. Luo, Y.C. Lu, S. Klmas, J. Electrochem. Soc. 162, C482 (2015) [13] D.H. Xa, R.K. Zhu, Y. Behnaman, J.L. Luo, C.J. Ln, S. Klmas, J. Electroanal. Chem. 744, 77 (2015) [14] R. Zhao, Z. Zhang, J.B. Sh, L. Tao, S.Z. Song, J. Cent, South Unv. 17, 13 (2010) [15] L. L, C. Wang, S.H. Chen, X.G. Yang, B.Y. Yuan, H.L. Ja, Electrochm. Acta 53, 3109 (2008) [16] B. Yuan, C. Wang, L. L, S. Chen, Electrochem. Commun. 11, 1373 (2009) [17] D.H. Xa, J.L. Luo, Trans. Tanjn Unv. 21, 234 (2015) [18] L. L, C. Wang, B. Yuan, S. Chen, Electrochem. Commun. 10, 103 (2008) [19] D.H. Xa, S.Z. Song, R.K. Zhu, Y. Behnaman, C. Shen, J.H. Wang, J.L. Luo, Y.C. Lu, S. Klmas, Electrochm. Acta 111, 510 [20] B. Yuan, W. L, C. Wang, L. L, Sensors Actuators B 176, 509 [21] D.H. Xa, Y. Behnaman, H.N. Feng, H.Q. Fan, L.X. Yang, C. Shen, J.L. Luo, Y.C. Lu, S. Klmas, Corros. Sc. 87, 265 (2014) [22] R.C. Newman, H.S. Isaacs, B. Alman, Corroson 38, 261 (1982) [23] D.H. Xa, R.K. Zhu, Y. Behnaman, C. Shen, J.L. Luo, Y.C. Lu, S. Klmas, J. Electrochem. Soc. 161, C201 (2014)