Effect of alloy element on corrosion behavior of the huge crude oil storage tank steel in seawater

Journal of Alloys and Compounds 598 (2014) 198–204

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Effect of alloy element on corrosion behavior of the huge crude oil storage tank steel in seawater Weiming Liu ⇑, Qingjun Zhou, Liaosha Li, Zhaojin Wu, Fabin Cao, Zhifang Gao Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education Anhui University of Technology, Maanshan 200240, PR China

a r t i c l e

i n f o

Article history: Received 20 November 2013 Accepted 23 January 2014 Available online 1 February 2014 Keywords: Huge storage tank steel Alloying element Corrosion characteristic EIS

a b s t r a c t Corrosion behaviors of the high strength low alloy (HSLA) steel and the carbon steel for a huge crude oil storage tank in seawater were studied by electrochemical methods and microanalysis techniques. The research found the alloy element had decreased corrosion rates for a long time but slightly increased corrosion rate at the beginning of experiments. Potentiodynamic tests results showed that all specimens exhibited active corrosion behavior, and corrosion rate tended to increase as a result of adding alloying elements. However, the loss weight results showed the corrosion rates of the HSLA steel were much smaller than those of carbon steel. Furthermore, more alloying elements led to a remarkable decrease in the corrosion rate. The reasons for these by electrochemical impedance spectroscopy (EIS) showed that the corrosion process of all samples showed two stages. At the first 144 h, EIS of all samples showed one increasing capacitance arc. The carbon steel showed the least corrosion rate, which was due to that the corrosion rates depended on the active site on the sample surface and the alloy effects were minor. After 144 h of immersion, EIS of the HSLA steels showed two capacitance arcs and the EIS of the carbon steel always showed one capacitance arc, which indicated the HSLA steel can form a compact corrosion scale. The HSLA steel showed the smaller corrosion rate than carbon steel, which was due to that the corrosion rates depended on the protective ability of corrosion scales and the alloy elements were help to form a compact corrosion scale on sample surface. The EDS results indicated that Cr, Mo and Al were distributed densely at the interface between the rust layer and the steel surface which helped to improve corrosion resistance for the HSLA steel. Corrosion of the HSLA steels was suppressed by insoluble compound formed near the surface. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The high strength low alloy (HSLA) steel is the commonly construction material for a huge crude oil storage tank (HCOST) in oil and gas industry, which is due to its predominant weld property and excellent mechanical property. With the increase in the imported crude oil and the domestic crude oil from the highly corrosive zone in recent year, the HCOST has been suffered more and more serious chloride corrosion at its bottom, which shortened its service life and led to a great deal of economic loss. In addition, seawater has been hoped to substitute to freshwater in the hydraulic experiment that checks the whole strength and the seal performance of the HCOST before it is in use. Seawater hydraulic experiment can be thought to economize lots of freshwater resources and construction costs. However, seawater contains high density of chloride and produces serious corrosion to the HCOST ⇑ Corresponding author. Tel.: +86 18855521906. E-mail address: [email protected] (W. Liu). http://dx.doi.org/10.1016/j.jallcom.2014.01.181 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

steel. The studies on the HCOST steel were mainly paid on the welding property and mechanical ability at present, however, the researches on its corrosion property were lack [1]. Thus, it is necessary to study the corrosion behavior of the HCOST steel in seawater. It is well known that weathering steel, containing small amounts of alloying elements such as Cr, Cu, Ni, Si and P, has been widely used because of its excellent resistance to atmospheric corrosion. This is due to the development of an adherent protective layer formed on the steel [2–5]. In our previous work, the Cr and Cu compounds promote more or less protective rust layers on the weathering steel in an aqueous condition, as they do with an atmospheric condition [6,7]. Accordingly, it can be expected that the HSLA steel with a small amount of alloy elements such as Cr, Mo, Al and Ni provide a possibility for improving the corrosion resistance in the HCOST. The aim of this investigation was to determine the influence of the alloy element on the corrosion behavior of the HCOST steel in seawater under ensuring its welding ability and mechanical

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property. In this investigation, parallel studies of coupons had been carried out to achieve a better understanding of the rules of the alloy element in effect on the corrosion behavior of the HCOST steel in seawater. 2. Experimental 2.1. Specimen preparation The materials used for experiments were two kinds of typical HCOST steel: the HSLA steel (contained Steel A, Steel B(based on different alloy contents)) and the carbon steel in which compositions were given in Table 1 and the microstructures were given in Fig. 1. The crystal defect ratios of the samples were shown in Table 2 by Image Plus 6.0 software. Every electrode specimen was a disc with a surface area of 1 cm2, attached a copper wire to the rear face. Every surface was embedded in epoxy resins (EP), leaving a working surface of 1 cm2. Every weight loss sample was a rectangle with a size of 30  50  5 mm, the polishing methods used to prepare all samples surface were as follows: First, the electrode surface was polished with emery paper (grade 400) and distilled water; and then, the surface was then polished with emery paper (grade 600) until a homogeneous surface was obtained, and then rinsed with acetone; Finally, a specimen was subjected for 5 min to ultrasonic washing with acetone and dried in warm flowing air prior to every experiment. 2.2. Electrochemical experiment All electrochemical measurements were performed in a standard three-electrochemical cell, with a saturated calomel electrode as the reference electrode (SCE), a graphite electrode as the auxiliary electrode (AE) and the samples as the working electrode (WE). The polarization curve and the electrochemical impedance spectroscopy (EIS) test were carried out with a VMP3 electrochemical corrosion testing apparatus under room temperature in seawater. For the potentiodynamic polarization, the sweeping potential was from 0.25 V to 0.25 V with a scanning rate of 0.1667 mV/s by the typical value [8]. EIS measurement was performed at the open circuit potential with AC amplitude of 5 mV. The applied frequencies ranged from 0.01 Hz to 10 kHz. At least three tests were conducted for each condition to confirm the validity of the measurement results. Electrochemical impedance spectroscopy (EIS) and potentiodynamic (DP) polarization experiments began after an initial delay of 2 h for samples to reach a steady state condition. 2.3. Surface analysis To investigate the relationship between the alloying element and the surface composition of the corrosion scales, the sample surface was examined by SEM and EDS. The components of corrosion scales were examined by XRD after EIS measurements. For the examination of the microstructure and the thickness measurements, cross sections from each coupon was taken out from the electrochemical cell, dried with N2 gas flow and leaved it in a dryer. The examination of the coupon was accomplished using a 20 kV JEOL 840A SEM, equipped with an OXFORD ISIS 300 EDS analyzer and the necessary software in order to perform point/linear microanalysis of the surface under examination. The microstructures of the corrosion scales were analyzed using X-ray diffractometer conducted by using a Rigaku diffractometer with Cu Ka radiation.

3. Results and discussions 3.1. Surface analysis Figs. 2 and 3 show the results of SEM analyses on the corrosion scales of all samples after 720 h of immersion. They show that the corrosion scales of all steels present two layers. An inner rust layer is adherent characteristics and the outer rust layer is loose characteristic. It can be seen that the protective ability of the

Fig. 1. Microstructures of all samples.

Table 1 Compositions of all samples (wt%). Sample

C

Mn

Si

Al

Mo

Cr

Cu

Nb

Ni

P

Steel A

0.091

1.45

0.21

0.037

0.1

0.24

0.01

0.006

0.22

0.0056

Steel B

0.090

1.44

0.185

0.029

0.008

0.12

0.01

0.014

0.11

0.0043

Carbon steel

0.139

1.37

0.299

0.004

0.017

0.02

0.0118

0.001

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Table 2 Analytic results of the defects of the metal matrix by Image Plus 6.0. Measure type

Steel A

Steel B

Carbon steel

Grain size/lm Crystal defect ratio

19 47.91

16 48.45

25 38.17

Fig. 3. Section image of corrosion scales after 960 h immersion in seawater.

Fig. 2. Surface images of corrosion scales after 960 h immersion in seawater.

corrosion scale depends on the performance of the inner rust layer. It can be shown in Figs. 2 and 3 for the morphology of corrosion

scale for the HSLA steel that the outer rust layer is intact and the compact inner rust layer is tightly combined with the metal matrix. However, the corrosion scales for the carbon steel shows that the outer layer is cracked and the inner layer with some flaws is loosely links with the steel matrix. This show the corrosion scale of the HSLA steel can supply an effective protection in seawater. The compositions of the corrosion scales were shown in Fig. 4. It can be showed that the corrosion scales of all samples are composed of the same compositions in seawater, including corrosion

W. Liu et al. / Journal of Alloys and Compounds 598 (2014) 198–204

201

Fig. 4. EDS micro-analysis to the rust layers of sample after 960 h immersion in seawater.

products (a-FeOOH, c-FeOOH, Fe3O4, Fe2O3), NaCl and corrosion residual cementite (Fe3C). For more detailed examination, the cross-sectional line profiles of the alloy element on the corrosion scales of all samples were shown in Fig. 5. It can be seem that Cr, Mo and Al mainly concentrate in the interface between the corrosion scale and the steel substrate, whereas Ni is distributed all over this surface. From the EDS results, it can be shown that the amounts of dissolved oxides (solution/rust layer interface) are larger than insoluble oxides (rust layer/substrate interface). Because alloying elements such as Cr and Mo are less soluble than Fe, the amount of insoluble oxides in the HSLA steel is larger than the amount of dissolved oxides in the carbon steel. The dissolution process leads to the enrichment in Cr, Al and Mo in the rust layer. When the alloying element concentration is on the order of 0.1%, these protective oxides are generated equally [9]. Accordingly, the HSLA steels with higher alloy elements can form more protective rust layers than the carbon steel in seawater, which is proved by the SEM results. 3.2. Weight loss Fig. 6 shows the weight loss results in seawater. With the immersion time increasing, the corrosion rates of all samples show

some fluctuation at the beginning of the experiment, which are relating to the integrated effects of the Cl adsorption and the deposition of the corrosion products. On the one hand, the formation of corrosion scales can protect the steel from further corrosion in seawater; on the other hand, the sample can be accelerated an active dissolution by the absorption of Cl, which accelerates the dissolution of the steel matrix and destroys the protection ability of corrosion scales. After 1440 h immersion in seawater, the corrosion rates show obviously reduce, which is due to the formation of a stable corrosion scale. During corrosion process, it can be seen that each HSLA steel exhibits smaller corrosion rate than the carbon steel and the Steel A with the higher alloy content shows the smallest corrosion rate, indicating the effectiveness of alloy elements contained in the HCOST steel in seawater. 3.3. Potentiodynamic polarization test Fig. 7 shows the results of potentiodynamic (PD) polarization curves in seawater after 2 h immersion. It can be seen that the anodic current density continuously increases with an increase in the corrosion potential, which indicate that all samples show active dissolution behaviors in seawater.

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3- -FeOOH 4- NaCl 5 Fe2O 3

1

Intensity / CPS

0.08

1- -FeOOH 2- Fe3O 4

1

400

1,3

300

6-Fe3C

200

1 2,5

100

1

6

4 2 3,5

2,6 4 5,6 5 3,6

1

0 10

20

30

40

50

1 2 3 2 1 11 5 4 4

60

Steel A Steel B Carbon steel

0.07

Weight loss / g.m-2h-1)

(a)

0.06 0.05 0.04 0.03 0.02 0.01

70

2 Theta / degrees

0.00 480h

(b) 350 300

1,2

200

4320h

Fig. 6. Weight loss results of all samples at different times in seawater.

6-Fe3C

1 3,5

Steel A Steel B Carbon steel

-0.3

150

-0.4

100 2

2

3 2,4 2,5

2

1,6

1

1

1,3,6

66 2,4

1

1

1,5

1

55 4

0 -50 10

20

30

40

50

60

70

-0.5 -0.6 -0.7 -0.8

2 Theta / degrees

-0.9

(c) 350

1- -FeOOH 2- -FeOOH 3-Fe3O4

1 1

300

1,2

4-NaCl 5-Fe2O3

250

intensity / CPS

2880h

4-NaCl 5-Fe2O3

250

50

1440h

Time

E / V vsSCE

intensity / CPS

1- -FeOOH 2- -FeOOH 3-Fe3O4

1 1

960h

200

-5

-4

-3

-2

-1

0

1

2

log|I| / mA Fig. 7. Polarization curve results of all samples in seawater.

6-Fe3C

1

-1.0

3,5

150 100 50

2

2

2

3 2,4 2,5

1,6

1,3,6

66 2,4

1

1 1,5

1

1

1

55 4

0 -50 10

20

30

40

50

60

70

by Image-Pro plus software was shown in Table 2, which exhibits that the carbon steel exhibits the least defect ratio of all samples. As a result, the HSLA steel contains more activated sites and accelerates corrosion by forming much more micro-electrochemical cells between the grain boundary and the matrix by increasing the electrochemical reactivity. Therefore, the carbon steel exhibits lower corrosion rate in seawater.

2 Theta / degrees Fig. 5. X-ray diffraction results of corrosion scales for all samples in seawater.

It can be shown that each HSLA steel shows higher corrosion currents and more negative shift in corrosion potentials when compared with the mild steel and the corrosion potential was shifted in the negative direction and the anodic current density was increased as a result of adding Cr, Mo, Al and Ni. This shows that the mild steel show improved corrosion resistance than the HSLA steel, which is relate to the effect of the alloy element and the surface activate. The HSLA steel contains more Cr, Mo, Ni and Al, which can form more serious galvanic corrosion on its surface and accelerate the Fe matrix to erode during PD experiment. In addition, the HSLA steel is bainite and the carbon steel is pearlite, therefore the HSLA steel take on more defects of the steel matrix on its surface (such as FeC, dislocation, inclusion, cavity and lattice distortion) [10]. The defect ration (the percentage of the area of metal defects in the area of total reaction area) on steel surface

3.4. Electrochemical impedance spectroscopy For a better understanding of the effect of Cr, Mo, Ni and Al on corrosion behavior of the HCOST steel in seawater, EIS tests were performed in seawater after different immersion time in Fig. 8. It can be shown that EIS of all samples show the one capacitive arc at the first 144 h. After 144 h, the EIS of the HSLA steel show two capacitive loop and the carbon steel always show one capacitive loop, which is due to that the HSLA steel can form a compact corrosion scales in seawater (Figs. 2 and 3) as to introduce anther corrosion interface on its surface. According to the EIS in Fig. 8, the equivalent circuits were illustrated in Fig. 9. Fig. 9a is fitted to the impedance spectra with one-time constant and Fig. 9b is simulated to the impedance spectra with two time constants. In these models, Rs is the solution resistance, CPEf and Rf correspond to the corrosion scales capacitance and resistance, respectively. Rct is the reaction resistance. CPEdl is the double-layer capacitance. A constant phase element

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Steel A Steel B carbon steel

600

6hours

500

-lm (Z ) / ohm cm2

500

-lm (Z ) / ohm cm2

Steel A Steel B carbon steel

600

400 300 200

400 300 200

100

100

0

0 0

200

400

600

800

0

1000 1200 1400

300

Steel A Steel B carbon steel

Steel A Steel B Carbon steel

600

144hours

900

1200

1500

1800

720hours

500

-lm (Z)/ ohm.cm2

-lm (Z ) / ohm cm2

600

600

Re (Z ) / ohm cm2

Re (Z ) / ohm.cm2

700

336hours

500 400 300 200

400 300 200 100

100 0 0

-200

0

200 400 600 800 1000 1200 1400 1600

Re (Z ) / ohm cm2

0

300

600

900

1200

1500

1800

Re (Z) / ohm.cm2

Fig. 8. Electrochemical impedance spectra of all samples at different times in seawater.

(a)

CPEdl Rs R ct

(b)

CPEdl RS

CPEf R ct Rf

Fig. 9. Equivalent circuits used to represent the impedance results.

representing a shift from an ideal capacitor was used instead of the capacitance itself, for simplicity. The impedance of a constant n 1 phase element is defined as Z CPE ¼ ½Q ðjxÞ  [11–13], where Q is a proportional factor, x is the frequency and 1 6 n 6 1. The value of n seems to be associated with the non-uniform distribution of current as a result of the roughness and surface defect. It is well recognized that the polarization resistance (Rp) is inversely proportional to the corrosion rate. The Rp value is equal to Rct value during the first 144 h and sum of Rct and Rf values after 144 h of immersion. The Rp values obtained by analyzing

impedance spectra were presented in Table 3. The RP values increase with increasing in the corrosion time within 144 h of immersion, which is relates to the deposition of the corrosion scale. Each HSLA steel presents lower Rp values compared with the mild steel at the first 144 h, which is fit to the polarization curve results. The reason for this was related to the alloy element and the microstructure, as discussed in the part of the polarization curve research. After 144 h of immersion, each HSLA steel exhibits higher magnitudes of Rp compared with the mild steel and the calculated RP values increased with adding alloying elements, which is associated with the protective ability of corrosion scale. After 144 h of immersion, the stable corrosion scales had been formed on sample surface and the reaction ratio can be expressed as fel0 low:icorr ¼ iL ¼ nFD Cd : icorr is the diffusion current density, F is Faraday constant, D is diffusion coefficient of oxygen from the outer corrosion scales to the reaction interface of samples, d is the thickness of corrosion scales. It can show the corrosion rate is independent in the thickness of corrosion scale and the diffusion coefficient of the corrosion ion through the corrosion scale, which are relate to the protective ability of the corrosion scales. Cr, Mo, Ni and Al are help to form compact corrosion scales. Because alloying elements such as Cr, Mo, Ni and Al can help the HSLA steel to form a compact oxide or to enrich in the inner rust layer [14,15], which are all improve the protective property of corrosion scales. Accordingly, Cr, Mo, Ni and Al containing steels had more protective rust layer than carbon steel, as showed in Fig. 2. In addition, the research found that the fine structure materials can increase adhesion strength between the passive film and substrate because of the enhancement in the electron activity at grain boundary and possible pegging of the passive layer into grain boundary [16,17], therefore the HSLA steel with a fine grain size can be closely combine with the steel matrix. Consequently, the HSLA steel presents lower corrosion

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Table 3 Parameters of equivalent circuits for EIS of all samples at different immersion times in seawater. Time (h)

RS (X cm2)

Qdl (X1 cm2 sn)

ndl

Rct (X cm2)

4

Qf (X1 cm2 sn)

nf

Rf (X cm2)

Rp (X cm2)

6

Steel A Steel B Carbon steel Steel A

17.48 28.17 19.5 27.06

2.09  10 1.92  104 2.2  104 3.92  104

0.82 0.83 0.79 0.81

1088 1170 1299 1444

1088 1170 1299 1444

144

Steel B Carbon steel Steel A

24.87 21.36 52.45

4.34  104 2.80  104 3.40  104

78 0.83 0.61

1391 1599 100.6

3.37  104

0.64

1970

1391 1599 2070.6

336

Steel B Carbon steel

33.58 34.48

5.55  104 4.97  104

0.70 0.84

52.42 855.4

3.37  104

0.79

1289

1341.42 855

720

Steel A Steel B Carbon steel

109.7 88.64 33.8

2.06  104 3.86  104 1.12  103

0.66 0.66 0.77

267.6 484.8 908.6

3.87  104 3.14  104

0.80 0.74

1569 1094

1836.6 1578.8 908.6

rates and the Steel A shows the least corrosion rate after 144 h of immersion, which is in agreement with the loss weight result. 4. Conclusions The results obtained from a study of the effects of Cr, Mo, Ni and Al on the corrosion behaviors of the low alloy steel and the carbon steel in seawater had been presented. The following conclusions can be drawn from this investigation: (1) It can be shown that the Cr, Mo, Ni and Al containing steel decrease the corrosion rate for the HUST steel in seawater and increased corrosion rate at the beginning of experiments (2) Potentiodynamic polarization tests showed that all steels show active corrosion behaviors in seawater. (3) Impedance plots of all steels as a function of times exhibited similar corrosion process and revealed two typical corrosion behaviors. The Nyqusit plots showed one increased capacitance at the first 144 h, which was related to the formation of the outer rust layer; after 144 h of immersion. The HSLA steel exhibited two capacitance arcs and the carbon steel still showed one capacitance arc, which is ascribed to the formation of the inner rust layer. (4) During the first 144 h, the carbon steel shows the lowest corrosion rate of all sample and the corrosion rate depends on the crystal defect. After the 144 h, each HSLA steel show lower corrosion rate than the carbon steel and the Steel A shows the last corrosion rate, which is due to that the corrosion rate is dependent in the protection ability of corrosion scales. (5) The XRD result show that all steels produce the same compositions of corrosion scales such as a-FeOOH, c-FeOOH, Fe3O4, Fe2O3, NaCl and Fe3C.

(6) SEM and EDS results show the corrosion scales of the HSLA steel can form more compact corrosion scales in seawater. Cr, Mo, Ni and Al compounds promoted the formation of more protective corrosion scales in seawater. Cr, Al and Mo mainly concentrated in the interface between the rust layer and the matrix metal.

Acknowledgement The authors wish to thank the school financial support by Anhui University and Technology. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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