Effect of pH on pitting corrosion of stainless steel welds in alkaline salt water

Effect of pH on pitting corrosion of stainless steel welds in alkaline salt water

Construction and Building Materials 68 (2014) 709–715 Contents lists available at ScienceDirect Construction and Building Materials journal homepage...

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Construction and Building Materials 68 (2014) 709–715

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of pH on pitting corrosion of stainless steel welds in alkaline salt water L. Li a, C.F. Dong a,b,⇑, K. Xiao a,b, J.Z. Yao a, X.G. Li a,b a b

Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Key Laboratory for Corrosion and Protection (MOE), Beijing 100083, China

h i g h l i g h t s  The pH play an important role in corrosion behavior of the weldment that consist of 304L stainless steel joint with 308L.  As the pH increase, there is a gradual enrichment in corrosion resistance of the three zones (BM/HAZ/WM) of the weldment.  WM showed a better corrosion resistance in the alkaline solution with 3.5% NaCl, followed by HAZ and BM.  The passive film of the weldment in alkaline solution, presenting a bilayer structure, composed of an outer and inner layer.

a r t i c l e

i n f o

Article history: Received 25 July 2013 Received in revised form 28 May 2014 Accepted 30 June 2014

Keywords: Stainless steel Polarization EIS Welding Pitting corrosion

a b s t r a c t Pitting corrosion was studied in the weldment of 304L austenitic stainless steels joint with 308L austenitic stainless steels. The corrosion behavior of three different weldment zones (weld metal (WM), base metal (BM) and heat affected zone (HAZ)) in alkaline solutions with NaCl was characterized by a series of electrochemical tests. The results indicated that pH values played an important role in the corrosion behavior of the weldment. With the pH increasing, the film resistance, polarization resistance and Ecorr increases, the charge carrier density N and the site of the pitting on the weldment decreases. In addition, WM showed a better corrosion resistance, followed by HAZ and BM. The pittings in BM were much bigger than that in WM and HAZ. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, stainless steels have been widely used in metal working industry, especially in the construction sector [1]. Austenitic stainless steels relied on its high corrosion resistance and excellent properties receive significant attention and play a vital role in many industrial and domestic applications [2]. During the construction and application, the properties and performance of stainless steel are inevitably relate to the welding process, which can cause the microstructure and the mechanical properties of the welded zone significantly different from those of the base metal. Thus, the corrosion behavior of the welded zone and the heat affected zone are prospective to be different from the base metal in corrosive media. Safe welding process should result in

⇑ Corresponding author at: Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 62333931 518; fax: +86 10 62334005. E-mail address: [email protected] (C.F. Dong). http://dx.doi.org/10.1016/j.conbuildmat.2014.06.090 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

welds with corrosion properties meeting comparable demand equal to the parent metal [3]. Corrosion processes of the three zones, i.e., weld metal, heat affected zone and base metal have been widely studied in recent years. It has been known that the corrosion resistance of these three zones may be different due to the weld metal, welding method and service environment [4–6]. Garcia et al. [7] studied the pitting corrosion resistance of different zones of the welded joints of austentic stainless steels (AISI 304 and 316L) in acid solution containing chlorides by potentiodynamic anodic polarization and cyclic potentiodynamic polarization, and it was shown that the pitting corrosion resistance of weld metal was higher than that of base metal. Bilmes et al. [3] studied the pitting corrosion behavior of super martensitic welds in chloride-containing media via potentiodynamic and potentiostatic techniques and pointed out that HAZ was the most critical zone for pitting corrosion and the base metal was the noble zone. There are still some problems that need to be solved despite numerous studies having been conducted. One of the problems is that the three zones in previous experiments were obtained via

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different heat treatment methods to simulate the real situations [8–10]. Although these methods provide general results that are considerably acceptable in terms of comparative studies, it still could not reflect the real behavior of the welded joints. By far, a large number of studies have focused on the corrosion behavior of welded joints in the acid solutions, but few report on alkaline environment. The materials in concrete reinforced structures generally operate in alkaline environments [11–13]. Although concrete generally shows high corrosion resistance in alkaline environment, under certain conditions, reinforced concrete can also deteriorate, for instance, carbonation of the concrete favored by unsuited dosing, the presence of chloride levels in excess of a critical concentration or the presence of mechanical stresses, reinforced concrete can also deteriorat [14,15]. Therefore, it is necessary to study the corrosion behavior of stainless steels in alkaline condition. The present work aims at studying the corrosion behavior of the real weld joints through the welding procedures. The corrosive media is alkaline solutions containing 3.5 wt% NaCl with different pH values (pH from 10.5 to 13.5), which simulates the interstitial concrete electrolytes, and the objective of this paper is to probe the different electrochemical characteristic of weld metal, heat affected zone and base metal in the simulated solutions. Moreover, imitation the practical application weldment, focus on the corrosion behavior combined effect of the three zones, which could truer exhibit the corrosion characteristic of the weldment. The electrochemical behaviors are investigated by potentiodynamic measurements, electrochemical impedance spectroscopy (EIS) and Mott–Schottky approach. The corrosion surface morphology is observed by scanning electron microscopy (SEM) together with EDS.

The capacitance measurements were performed on the films at a fixed frequency of 1 kHz a 25 mV/Step in the potential ranging from 0.5 to 1.0 V (vs. SCE). It ensured that the defect concentrations and the film thickness remained ‘frozen’ at the formation values while the capacitance was measured as a function of potential.

3. Result and discussion 3.1. Microstructures Fig. 1 shows the microstructure of the three zones (BM, HAZ and WM) of the welding. Aqua regia is the main etching technique used in these results. Regarding the base metal it can be concluded that it exhibit typical microstructure characteristic of the austenitic structure and also present some scarce chromium carbides. Weld metal shows the mesh-cast microstructure, and the WM microstructure contains continuous austenitic matrix with some remnant ferrite. The microstructure of HAZ shows a great difference with that of WM and BM. There are nearly no remnant ferrite in the HAZ, while the typical recrystallization and grain growth can be observed. In general, P, S and non-metallic inclusions have a negative effect on the pitting resistance, while Cr and Ni are beneficial alloying elements against pitting corrosion. According to the chemical composition of BM and WM, it is easy to observe that WM has a better corrosion resistance than BM. It is well known that, the joining of dissimilar metals is generally more difficult than that of similar metals because of the differences in the physical, mechanical and metallurgical properties of the base metals, but dissimilar welds could enforce the corrosion resistance [3]. With safe welding process and suitable weld metal, perfect weldment can be obtained which the corrosion properties of weld metal in keeping with/or better than that of base metal.

2. Experimental

3.2. Potentiodynamic polarization studies

In this study, 304L austenitic stainless steel was used as base metal, and 308L austenitic stainless steel as weld metal, whose chemical compositions are shown in Table 1. The test specimens were cut into 1 cm  1.5 cm from the welded workpiece containing weld metal, heat affected zone (HAZ) and base metal, ensuring the length of weld metal and base metal was approximately equal, and then sealed by epoxy resin as working electrodes. The working electrode was grounded sequentially to 2000grit SiC paper, polished with 0.1 lm alumina polishing powder, degreased in alcohol, cleaned with deionized water, and then dried. Aqueous sodium–potassium hydroxide solutions were prepared to simulate the electrolytes in the pores of concrete. The solutions were prepared by dilution of 1M NaOH and 1M KOH with double-distilled water to obtain different pH ranging from 10.5 up to 13.5. The solutions were used immediately after preparation to avoid carbonation effects and the pH value was regularly checked using a Metter pH meter. 3.5 wt% NaCl was added into the solutions to accelerate the pitting process, and simultaneously simulate the aggressive corrosive environment. All reagents were of at least ACS grade. A VMP3 was used to perform the electrochemical experiments. The experiments were conducted in a conventional three-electrode cell at ambient temperature (25 °C) with the sample as the working electrode, a saturated calomel electrode (SCE) as reference electrode, and a platinum mesh as the counter electrode. The polarization curves were recorded at a scan rate of 0.5 mV s1 starting from 0.5 V vs. OCP to transpassive potential. The corrosion potential (Ecorr) and breakdown potential (Eb) were obtained from the polarization curves. Eb represents the end of the passive potential region and the transition from passive to transpassive behavior, and sometimes it is determined when the current density reaches value of 100 lA cm2. EIS measurements started after stabilization for about 1 h at open-circuit potential (OCP) .The frequency was swept from 100 kHz down to 10 mHz, at 10 data cycles/decade, with the applied AC amplitude of 10 mV. The impedance data were interpreted on the basis of equivalent electrical circuits and fitted by ZSimpWin software.

The potentiodynamic curves can provide some significant features about the electrochemical behavior of the weldment at different pH. Fig. 2 shows the potentiodynamic polarization curves for the weldment in solutions of pH 10.5, 11.5, 12.5 and 13.5, containing 3.5% NaCl. The results show that Ecorr increased with the pH value increasing, indicating that the corrosion resistance increase is due to the increase of the pH. Olson and Landolt [16] considered this phenomenon related to the decrease in the dissolution rate and the thickness of the passive film in alkaline environment. Malik et al. [17] believes that OH- is priority attracted to the metallic surface and thus conducive to the stability of the passive film. The breakdown potential, Eb, does not meet the similar rule with Ecorr. For the same zone, Eb hardly changes in the solutions with pH 10.5, 11.5, 12.5 solutions, but sharply increases in pH 13.5 solution. Fig. 3 shows different corrosion resistance of the three zones in the same pH value solution. It is apparent to find that WM has the best corrosion resistance, while BM the worst. The results can be easily detected in all the test solutions with pH ranging from 10.5 to 13.5. From the chemical composition and microstructure of weldment after welding, it is confirmed that WM has a better corrosion resistance than HAZ and BM mostly due to the lower C, P and S.

Table 1 Chemical composition (wt.%) of the base mental (BM) and the weld metal (WM).

304L 308L

C

Si

Mn

P

S

Cr

Ni

0.01 0.03

0.27 0.57

1.95 1.02

0.018 –

0.0045 –

17.76 19.94

9.20 10.47

3.3. EIS studies Fig. 4 shows the EIS spectra of three zones in the austenite stainless steel weldment obtained in different pH solutions containing 3.5% NaCl. It can be seen in all three zones that the Nyquist plots display a capacitive arc and the radius of the arc increases with pH. In the impedance measurement, the radius of the

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Fig. 1. Optical micrographs of three zones of the weldment.

pH 10.5 pH 11.5 pH 12.5 pH 13.5

0.8

0.8

HAZ

0.4

E / VSCE

0.4

E / VSCE

pH 10.5 pH 11.5 pH 12.5 pH 13.5

0.0 -0.4

WM

0.0 -0.4

-0.8

-0.8

-8

-6

-4

-2

0

-6

2

Log I / (mA /cm2)

-4

0

2

(b)

(a) pH 10.5 pH 11.5 pH 12.5 pH 13.5

0.8 0.4

E / VSCE

-2

Log I / (m A / cm2)

BM

0.0 -0.4 -0.8 -8

-6

-4

-2

0

2

Log I / ( mA / cm2)

(c) Fig. 2. Potentiodynamic polarization curve of three zones in the alkaline solutions with 3.5% NaCl at different pH. (a) HAZ. (B) WM. (c) BM.

-0.1

WM PM HAZ

E / VSCE

-0.2

-0.3

-0.4

-0.5 10.5

12.5

11.5

13.5

pH Fig. 3. The potential of the three zones in the alkaline solutions with 3.5% NaCl at different pH.

semi-circular arc is related to the polarization resistance of the passive film. It shows an increase of the overall impedance values in pH 13.5, indicating an improvement of the corrosion resistance, which is in good agreement with the polarization curves.

Different models are proposed for explaining impedance spectra on a passive metal surface. On the basis of some references [18,19], two R–Q elements equivalent circuit in Fig. 5 are used to fit the experimental data. This model expects that the passive film does not entirely cover the metal and cannot be considered as a homogeneous layer, but as a defective one. As a matter of fact, neither real surfaces of solids in the active range nor passive films on metallic substrates might be considered ideally homogeneous. As is shown in Table 2, there is a good agreement between the equivalent circuit predictions and the experimental data. The equivalent circuit consists of the solution resistance Rs connected in series with two time constants R1[Q1(R2Q2)]. It is the pore electrical resistance caused by the formation of the ionically conducting paths across the passive layer. Polarization resistance, Rp (Rp = R1 + R2), where R1 and R2 are parameters from the fitting procedure, is commonly used as a measurement of the resistance of a metal to the corrosion damage. The Rp value and three zones in different pH solutions are shown in Fig. 6. Rp increases with the increase of the pH. Higher the Rp value, higher corrosion prevention capability [20]. For the three zones, the Rp of the WM was the highest, followed by HAZ, while BM was the lowest.

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250000

pH 10.5 pH 11.5 pH 12.5 pH 13.5

BM

150000

-Z'' / Ω .cm2

-Z'' / Ω.cm2

200000 150000 100000

pH 10.5 pH 11.5 pH 12.5 pH 13.5

HAZ

100000

50000 50000 0

0 0

50000 100000 150000 200000 250000

0

50000

100000 150000 200000

Z' / Ω.cm2

Z' / Ω.cm2

(a)

(b)

600000

pH 10.5 pH 11.5 pH 12.5 pH 13.5

WM

-Z'' / Ω .cm2

450000

300000

150000

0 0

150000

300000

450000

600000

Z' / Ω.cm2

(c) Fig. 4. Nyquist plots of three zones in the alkaline solutions with 3.5% NaCl at different pH. (a) HAZ. (B) WM. (c) BM.

BM HAZ WM

Rp / Ω .cm2

3000

Fig. 5. Equivalent circuit for fitting the experimental data.

Table 2 Best fitting parameters for the impedance spectra obtained in the alkaline solutions with 3.5% NaCl. pH 10.5 2

Rs (X cm ) Q1 (X1 cm2 sn) n1 R1 (X cm2) Q2 (X1 cm2 sn) n2 R2 (X cm2)

10.34 0.0145 0.89 194.3 0.0076 0.76 371.6

pH 11.5 8.21 0.0214 0.81 368.7 0.0189 0.79 529.8

pH 12.5 4.89 0.0472 0.77 794.5 0.0284 0.85 883.2

pH 13.5 1.503 0.0674 0.72 1191.8 0.0591 0.91 1306.9

3.4. The passive film in the alkaline solution The effect of pH value on the semiconducting properties of the passive film on the three zones was investigated by capacitance measurements. The electrochemical double layer capacitance developed in the passive oxide near the film/electrolyte interface. The relationship between capacitance and the applied potential is given by the well-known Mott–Schottky equation, which describes the potential dependence of the space charge capacity, C, of a semiconductor electrode under depletion condition [21,22]:

1 C

2

¼

  2 kT E  EFB  e  N  e  eo e

ð1Þ

2000

1000

0

10.5

11.5

pH

12.5

13.5

Fig. 6. Rp resistance of the three zones from the fitting procedure in the alkaline solutions with 3.5% NaCl at different pH.

where the negative sign is for p-type and the positive sign for n-type conductivity, e is the electron charge, N is charge carrier density, the donor density for n-type or the acceptor density for p-type semiconductors, eo is the vacuum permittivity, e is the relative dieléctric constant of the semiconductor, k is Boltzmann’s constant, T is absolute temperature, E is the applied electrode potential and EFB is the flat band potential. Fig. 7 displays the C2 vs. E for three zones of the weldment formed in different pH solutions with 3.5% NaCl. As expected, all Mott–Schottky plots contain two regions where the space charge behavior can be observed (straight lines above and below a potential of approximately, 0.2 VSCE) weldment, which assumes that the semiconducting behavior reflects the characteristic of their surface films. In the more anodic region (E > 0.2 VSCE), the negative slope results can be interpreted as representative of p-type semiconductor behavior of Cr2O3. In the region (E < 0.2 VSCE),

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2 m  e  e  eo

ð2Þ

where m is the slope of the Mott–Schottky plot in the linear-region of interest, e is the electron charge, e is the relative dielectric constant of the semiconductor, and eo is the vacuum permittivity. Table 3 reveals the calculated donor densities and flat band potential (EFB) values of the film of three zones of weldment formed at different pH in the test solutions, respectively. According to PDM [26], the donor density ND characterizes the affinity of chloride ions for the passive film and also features the pit nucleation ability. So with pH increasing, the doping densities decrease, which relates to the decrease of the number of point

C -2 / uF -2cm 4

pH 10.5 pH 11.5 pH 12.5 pH 13.5

WM

0.15 0.12 0.09 0.06 0.03

pH (cm3)

11.5

12.5

13.5

WM

ND/1020 NA/1020

10.5 6.79 3.87

4.81 3.68

2.84 1.45

1.42 1.01

HAZ

ND/1020 NA/1020

7.93 4.35

5.01 3.23

3.14 2.96

1.66 1.24

BW

ND/1020 NA/1020

10.9 7.98

5.04 4.48

3.32 3.27

1.97 2.77

defects on the film. Moreover, it is suggested that the space charge layer thickens with the chromium content, leading to higher resistance since chromium acts as a barrier to the flow of electrons and holes [27]. This evolution ensures a more effective corrosion protection at pH 13.5 in agreement with the polarization curves. 3.5. The corrosion morphology in the alkaline solution It is well known that the weldment consists of three zones with different microstructures, which leads to the differences in the corrosion resistance from each other. The corrosion starts in the weakest zone of the weldment and favorable corrosion of one of the zones can probably take place. In order to identify the preferential initial sites of pitting, metastable pits with small dimension were observed after the potentiostatic measurements in the test solutions at different pH values. To simulate the real utilization environment of the weldment, the potentiostatic measurements were carried out with integrated weldment instead of on the single zone. Fig. 8 displays the morphology of the integrated weldment after potentiostatic measurement in different pH solutions. It reveals that the pittings in the BM are much bigger and deeper than that in WM and HAZ in any pH values solutions. Besides, it is obviously that there are nearly no pittings in HAZ, due to the fact that WM and BM form galvanic carrying on galvanic corrosion and HAZ was protected. And it is observable that the number and size pH 10.5 pH 11.5 pH 12.5 pH 13.5

HAZ

0.15 0.12 0.09 0.06 0.03 0.00

0.00 -0.6

Table 3 Effect of pH on semiconducting properties of passive film formed on the three zones of the weldment in alkaline solution with 3.5% NaCl.

C-2 / uF-2cm4

the capacitance reflects that oxide films behave as a n-type semiconductor with characteristic of Fe2O3. It is similar to previous reports for anodic passive films formed on iron, stainless steels and some alloys. The C2 vs. E curves also show a second region with a lower, more negative slope at potentials than the first p-type region. This is attributed to a second donor level in the oxide layer. A second negative slope of the plot has been explained by a change in donor type and/or donor density with potential or the presence of a second donor level in the band gap, which is corresponding to the ionization of the deep donor. Simões et al. [23] proposes that the deep level, which is due to ions in octahedral sites, is partially occupied by Fe3+ ions, whereas Fe2+ and Fe+ occupy both octahedral and tetrahedral sites. And due to simultaneous hydrogen uptake from the opposite side of a steel membrane, the atomic hydrogen interstitially dissolve in the steel and its oxide [24]. In the present study the nature of a second deeper donor level will not be further analyzed. Therefore, all donor densities refer to the first level. The values of ND and NA can be obtained from the slope of the experimental C2 vs. E assuming the dielectric constant of the passive film on the stainless steels [25]. According to Eq. (2), the slopes of the linear portion of the C2 vs. E give the charge carrier density N, from the relation:

-0.4

-0.2

0.0

0.2

0.4

0.6

-0.6

0.8

-0.4

-0.2

0.10

BM

0.08

C-2 / uF-2cm4

0.0

0.2

0.4

0.6

0.8

E vs. SCE / V

E vs. SCE / V pH 10.5 pH 11.5 pH 12.5 pH 13.5

0.06 0.04 0.02 0.00 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

E vs. SCE / V Fig. 7. Mott–Schottky approach for film formed on the three zones of the weldment in the alkaline solution with 3.5% NaCl at different pH.

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Fig. 8. Surface images of three zones of the weldment in the alkaline solution with 3.5% NaCl at different pH (a) pH 10.5, (b) pH 11.5, (c) pH 12.5, and (d) pH 13.5.

of the pits increase with the decrease of the pH values. All these results echo with the consequence of the polarization curves. 4. Conclusion In this paper, we have analyzed electrochemical data of austenite stainless steel weldment for pitting behavior in alkaline solutions containing 3.5 wt% NaCl with different pH values (10.5, 11.5, 12.5 and 13.5). The principal findings of this study are as follows: (1) The pitting resistance of austenite stainless steel weldment is improved as the pH values increase from 10.5 to 13.5. The Ecorr of WM/HAZ/BM exhibit a tendency towards more noble values, while the values of Rp become larger as the pH values increases. (2) Capacitance studies reveal that passive film becomes more stable as pH values increasing. With the solution pH values increasing from 10.5 to 13.5, the donor density values decreased from 1021 to 1020 cm3.

(3) In the same solution, WM (308L stainless steel) has the best corrosion resistance for its high Ecorr, high Rp and low charge carrier density, followed by HAZ and BM (304L stainless steel). The SEM observation also agrees with the results from electrochemical analysis.

Acknowledgements The authors acknowledge the supports of the National Natural Science Foundation of China (No. 51222106) and the Research Funds from Beijing Municipal Commission of Education (No. 2013150301601). References [1] Chandra K, Vivekanand Kain, Raja VS, Tewari R, Dey GK. Corros Sci 2012;54:278–90. [2] Lai CL, Tsay LW, Kai W, Chen C. Corros Sci 2012;52:1187–93. [3] Bilmes PD, Llorente CL, Méndez CM, Gervasi CA. Corros Sci 2009;51:876–81.

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