Enhancement of the corrosion resistance of zinc-aluminum-chromium coating with cerium nitrate

Accepted Manuscript Enhancement of the corrosion resistance of zinc-aluminum-chromium coating with cerium nitrate Jing-fei Qiao, Ming-ming Zhang, Ke-bing Zhang, Sheng-lin Zhang PII:

S0925-8388(16)31515-8

DOI:

10.1016/j.jallcom.2016.05.182

Reference:

JALCOM 37702

To appear in:

Journal of Alloys and Compounds

Received Date: 5 November 2015 Revised Date:

4 May 2016

Accepted Date: 17 May 2016

Please cite this article as: J.-f. Qiao, M.-m. Zhang, K.-b. Zhang, S.-l. Zhang, Enhancement of the corrosion resistance of zinc-aluminum-chromium coating with cerium nitrate, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.182. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Enhancement of the Corrosion Resistance of Zinc-aluminum-chromium Coating with Cerium Nitrate Jing-fei Qiao , Ming-ming Zhang ,Ke-bing Zhang, Sheng-lin Zhang *

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School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan,453007, China

Abstract

The present work aims at evaluating the effect of cerium nitrate on corrosion resistance of mild carbon steel coated by Dacromet in 3.5% NaCl solution. The scanning electron microscope (SEM) and energy dispersive X ray

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spectroscope (EDX) analysis were carried out to compare the surface structure and the composition of Dacromet coating and Dacromet +Re coating. The results of the electrochemical measurement showed that, compared with

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Dacromet coating, Dacromet+Re coating had a longer controlled sacrificial anodic protection function to steel substrate, a lower corrosion current density (icorr) and a higher corrosion potential (Ecorr). The modulus values of |Z| of Dacromet+Re coating is about 1 order of magnitude higher than that of Dacromet coating at low-frequency region.

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1. Introduction

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Keywords: Dacromet; Cerium nitrate; Corrosion resistance; Electrochemical impedance spectroscopy

Zinc-aluminum-chromium coating, also named as Dacromet, is a water-based and VOC [1-5]

. The coating was obtained through dipping

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(volatile organic compound) compliant coating

specimens in (or brushing specimens with) the slurry used for Dacromet, which consists principally of ultra-fine zinc and aluminum flakes together with chromate. The zinc and aluminum flakes align ini multiple layers forming a metallic silver gray coating. Applied as a liquid material, the coating becomes totally inorganic after baking and sintering at a certain temperature. This technique is a new kind of metal surface treating technique, which differs from either electroplating or hot dip of zinc or zinc-rich coating

* Corresponding author. Tel.: +86 373 3326335. E-mail address: [email protected] (Sheng-lin Zhang)

1

[6]

. Dacromet has excellent corrosion

ACCEPTED MANUSCRIPT protectiveness, strong covering capacity and freedom from hydrogen brittleness for steel or other metals. Hereby it was widely applied in automobile, ship craft, metal pipes and equipment, containers, and etc [7]. In recent years there are some studies of corrosion behavior of Dacromet systems. Hili Hu et

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al. [5] investigated the effect of chromate on the electrochemical behavior of Dacromet in seawater.

The self-healing of Cr(VI) may be confirmed by the electrochemical data obtained from their research. J.G. Liu et al [3, 8] studied the effect of hybrid SiO2 sol-gel and γ-APTS on the corrosion behavior of Dacromet. The nano-particles in SiO2 sols did enter into the micro-defects of

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Dacromet. The hybrid SiO2 gel/Dacromet composite system enhanced the erosion-corrosion resistance of Dacromet markedly. The γ-APTS treatment influenced the corrosion behavior of the

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organic/Dacromet composite systems mainly by restraining the corrosion of Dacromet through the reduction of the active surface by barrier effect. The pretreatment of Dacromet with γ-APTS led to an increased polarization resistance but a decreased total impedance |Z| relating to the untreated one after immersion for a period.

Regarding the application of rare earth elements on the surface treatment of metals, Aramaki , Ferreira et al.[10] and Abdel et al.[11] observed that the rare earth ions can form insoluble

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[9]

hydroxides (or oxides) in NaCl solution, which allows them to be used as a cathodic corrosion inhibitor. M. Tran et al.[12] studied corrosion behavior of steel in the presence of Y(III) salts in 3% NaCl solution. The influence of Y (III) concentration, the immersion time and the presence of

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oxygen were discussed [13].

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The present work investigates the effect of cerium nitrate (Ce(NO3)3), as a corrosion inhibitor, on the anti-corrosion behavior of mild carbon steel coated by Dacromet in 3.5% NaCl solution. The corrosion mechanism and corrosion resistance of the Dacromet coating were investigated by means of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and open circuit potentials (OCP). The morphologies and the chemical compositions of the Dacromet coatings were characterized by SEM and EDX.

2. Experimental

The mild carbon steel Q235 specimens (50 mm×25 mm×1 mm) were used as the substrates 2

ACCEPTED MANUSCRIPT materials. The pretreatment of the steel substrate surface is essential to making high quality coatings, which strongly depends on the contamination of the surface. Rectangle specimens were mechanically polished with Al2O3 sandpaper (up to 1200 grit). After polished, all specimens were degreased in acetone and methanol, then cleaned in distilled water, and finally dried in a warm air

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stream and used as coating substrates. The cleaned steel specimens were coated in water-based Dacromet slurries by dip-spin method. The ingredients of the water-based Dacromet slurry were shown in Table 1. The water-based Dacromet slurry with adding Ce(NO3)3 was labeled Dacromet+Re. After dip-spin Dacromet or Dacromet+Re slurries, the specimens were baked at for 10 min, and sintered at 300

for 30 min in an oven. Undergoing two cycles of the

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80

dip-spin and solidification process, the silvery specimens can be well prepared. The thickness of

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the coatings indicated by the TT300 thickness tester (Equipment Co., Ltd., Zhengzhou Times) is 12 µm around.

Table 1

Chemical compositions of Dacromet and Dacromet+Re slurries

Concentration in Dacromet (g.L−1)

Compositions

Flake aluminum powder Chromic acid anhydride Boracic acid Ethylene glycol Lauryl alcohol ethoxylates

ZnO

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Polyethylene glycol

270

Polyether-modified silicone defoamer

60

60 55

13

13

160

160

12

12

6

6

3

3

0.5

0.5 10

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Cerium nitrate

270

55

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Flake zinc powder

Concentration in Dacromet+Re (g.L−1)

The morphology and compositions of the coating formed were studied by SEM (AMRAY

1000B) with a field-emission gun operated at 20 kV and EDX (TN-5402). Electrochemical measurements were conducted in 3.5% NaCl solution in equilibrium with air

at 25℃ using CHI700B electrochemical workstation (CHENHUA Instrument Co., Ltd. Shanghai) with corresponding software for analysis. The electrolyte was prepared from reagent-grade chemicals and distilled water. A conventional three-electrode cell was composed of an Ag/AgCl electrode (SCE) used for the reference electrode, a platinum electrode used for the counter electrode, and the coated specimen (1cm×1cm of exposed surface area) used for the working 3

ACCEPTED MANUSCRIPT electrode. In potentiodynamic polarization experiment, the potential range was from -1.6 to -0.6 V versus the open circuit potential with a scanning rate of 0.5mV s-1. The electrochemical impedance measurements were carried out over the frequency range 105 Hz to 10-2 Hz with a 10mV

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peak-to-peak sinusoidal voltage. The open circuit potentials were measured with 4 Hz sampling frequency and take the arithmetic average within 2 min as the final value.

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Results and discussion

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3.1 Morphology and composition of the coating

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On average, three replicate specimens were tested for all tests in the studied condition.

SEM micrographs were used to compare the surface structure of Dacromet coating and

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Dacromet +Re coating, which is illustrated in Fig.1.

Fig. 1. SEM micrographs of (a) Dacromet coating and (b) Dacromet+Re coating

Fig.1 shows that, compared with that of Dacromet+Re coating (Fig.1b), the surface of

Dacromet coating is relatively coarse and have more micro-cracks and pinholes obviously (Fig.1a). The micro-cracks and pinholes, through which corrosive media can reach the metal substrate, will inevitably lead to decrease the corrosion resistance of Dacrome coating. It is evident that the incorporation of rare earth elements in Dacromet coating can decrease the micro-cracks and pinholes noticeably. Fig.2 is EDX analysis of Dacromet coating and Dacromet+Re coating in order to 4

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demonstrate the effect of cerium nitrate on the compositions of the coatings.

(a)

(b)

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Fig.2. EDX analysis of (a) Dacromet coating and (b) Dacromet+Re coating

The element Ce was easily observed in Dacromet+Re coating (Fig.2b). This indicated that

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there are a certain number of compounds of cerium, which deposited in the micro-cracks and pinholes, in Dacromet+Re coating after solidification process. It is anticipated that these compounds of cerium can effectively prevent the penetration of corrosive media and improve the corrosion resistance of Dacromet+Re coating.

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3.2 Open circuit potential

Fig. 3 shows the evolution of open circuit potentials of Dacromet coating and Dacromet+Re

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coating in 3.5% NaCl solution.

E O C Pⅰ(ⅰV ⅰvsⅰSC E ⅰ)

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-0.6

ⅰD acrom et ⅰD acrom et+R e

-0.7

-0.8

(ⅰ-0.86V ⅰ) -0.9

-1.0

-1.1 0

20

40

60

80

100

120

140

160

180

Im m ersionⅰTim eⅰ(day)

Fig.3. The open circuit potential curves of Dacromet coating and Dacromet+Re coating in 3.5% NaCl solution.

It can be seen from Fig.3, the evolution of the open circuit potential can be divided into four 5

ACCEPTED MANUSCRIPT periods. Period 1: The open circuit potential of Dacromet coating ( → ) and Dacromet+Re coating ( →

) shifts negatively at the early stage of immersion. Obviously, it is the penetration of the

electrolyte that caused the corrosion of the metallic flakes (zinc or aluminum) and resulted in the

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metal flakes to become active. The corrosion potentials (Ecorr) shifted continuously to more negative values with the increment of the exposed active area of zinc or aluminum during this period.

Period 2: The corrosion products generated by metallic zinc or aluminum and hydroxides or

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oxides of cerium precipitated in the micro-cracks and pinholes of the coating, and blocked the penetration of the electrolyte. Therefore, the active area of metal flakes was reduced and the →

for Dacromet coating,

positively shifted. According to the literature

[14]

→

for Dacromet+Re coating)

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corrosion potentials (

, the anodic protection effect of zinc or aluminum to iron acts

effectively as sacrificial anodes only when their potentials are more negative than −0.86V (vs SCE). Taking a view of the open circuit potential evolution, the sacrificial anodic protection of

during this period.

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metallic zinc or aluminum holds a more dominant position on protection of coating for metal

Period 3: The corrosion potentials (Ecorr) were more positive than −0.86V (vs SCE) and

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continued to shift to more noble at a much slower rate. It is certain that the major metallic Zn or Al was covered by the corrosion products and the physical shielding function holds a more dominant for Dacromet coating,

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position than the sacrificial anodic protection during this period ( → → ⅰfor Dacromet+Re coating).

During Period 4: Because aggregation of corrosion products causes local stress and

micro-cracks in the coatings, the corrosive medium, which may pass through the micro-cracks of the coating filled with the corrosion products, came into contact with the metal substrates with the extension of immersion time, the physical shielding function was broken ( → coating,

→

for Dacromet

for Dacromet+Re coating).

It is noteworthy that there is a demarcation line between the sacrificial anodic protection and the physical shielding function of the coatings for the steel substrates. After 40 days of immersion, the corrosion potential (Ecorr) of Dacromet exceeded the protective potential of the carbon steel 6

ACCEPTED MANUSCRIPT (0.86 V). This time for Dacromet+Re coating was 116 days. So, Dacromet+Re coating showed a long controlled the sacrificial anodic protection to substrate. This indicated that adding cerium nitrate in Dacromet can enhance corrosion resistance of Dacromet and prolong its lifetime. It is well known that zinc and aluminum are the active metals, especially in the form of

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powder, the corrosion process on zinc and aluminum in the electrolyte solution containing C1- is that following reactions occur [15-18]:

3e− → Al3+

(anodic)

(1)

Zn

2e− → Zn2+

(anodic)

(2)

2H+ + 2e− →H2↑

(cathodic)

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or

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Al

O2 + 2H2O + 4e− → 4OH−

(cathodic)

(3)

(4)

It can be deduced from the above electrochemical reactions that the cathodic reduction reactions will lead to the increment of OH- concentration and the elevation of pH value in the cathodic sites. When pH value reaches a certain value, the following reaction equilibriums will

Ce3+ + 3OH− → Ce(OH)3↓

(5)

2Ce3+ + 6OH− → Ce2O3 H2O↓

(6)

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or

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occur on the cathodic sites [19,20]:

these hydroxides or oxides of cerium, which are less conductive and have very stable [21]

,

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chemical properties because of the lanthanide series characteristics of rare earth elements

deposited in the micro-cracks and pinholes of the coating surfaces and can prevent corrosion medium, such as Cl-, from penetrating into the interior of the coating, acting as cathodic inhibitors that restrain the entire corrosion process by suppressing the cathodic reduction reaction[22-24], thereby the corrosion resistance of Dacromet+Re coating were enhanced.

3.3 Potentiodynamic polarization curves

The potentiodynamic polarization curves of Dacromet coating and Dacromet+Re coating are presented in Fig.4. The corrosion current density, icorr (which increases with corrosion rate), was 7

ACCEPTED MANUSCRIPT estimated by extrapolating the cathodic branch according to a Tafel analysis.

-0.6

Dacromet+Re Dacromet -0.8

-1.2

-1.4

-1.6 -7

-6

-5

-4

LogⅰIⅰ(A/cm 2 )

-3

-2

-1

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E corr(V SC E )

-1.0

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Fig.4. Potentiodynamic polarization curves for Dacromet coating and Dacromet+Re coatingin 3.5% NaCl solution.

Fig.4 shows that, compared with that of Dacromet coating, the polarization curve of

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Dacromet+Re coating has shifted left, that is, transferred to low corrosion current density. The -2

corrosion current density (icorr) of Dacromet+Re coating decreased from about 39.8µ A cm of

Dacromet coating to about 7.94µA cm2. Meanwhile, Fig.4 also shows that adding small amounts of cerium nitrate in Dacromet coating shifted the corrosion potentials (Ecorr) to more noble. Compared with that of Dacromet coating, the Ecorr of Dacromet+Re coating increased by about 77 mV.

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It should be noticed that, regardless of whether there is cerium nitrate in Dacromet coating, the shapes of the potentiodynamic polarization curves do not show a substantial variation. This indicates that cerium nitrate added in Dacromet coating does not change the dynamic behaviors of the electrode

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reaction and behave as a physical barrier.

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3.3 EIS measurements

Fig.5 showed impedance spectra of Dacromet coating and Dacromet+Re coating in 3.5%

NaCl solution with immersion time up to 150 days.

8

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(b1)

-120

-120

1d 7d 21d 60d 150d

-80

1d 7d 21d 60d 150d

-100

Z im age(ⅰK Ω.cm 2ⅰ)

-60

-40

-20

-80

-60

-40

-20

0

0

0

20

40

60

80

100

120

0

20

40

2

80

100

120

Z reat(K Ω.cm )

(a2)

(b2)

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100

100

|Z|(ⅰK Ω.cm 2 ⅰ)

10

|Z|ⅰ(ⅰK Ω.cm ⅰ2ⅰ)

60

2

Z reat(K Ω.cm )

1d 7d 21d 60d 150d

10

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1

0.1

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Z im age(ⅰK Ω.cm 2ⅰ)

-100

1

1d 7d 21d 60d 150d

0.1

0.01

0.01

10

-2

10

-1

10

0

10

1

10

2

3

10

10

4

Freqⅰ(H z)

(a3)

5

10

10

-2

10

-1

0

10

1

10

2

10

3

10

4

10

5

10

Freqⅰ(H z)

(b3)

-40

-30

-10

0 -2

10

10

-1

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-20

10

0

10

1

10

2

10

3

-60

1d 7d 21d 60d 150d

-50

Phase (degree)

1d 7d 21d 60d 150d

-50

Phaseⅰ(degree)

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-60

-40

-30

-20

-10

0 10

4

10

-2

5

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

Freqⅰ(H z)

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Freqⅰ(H z)

Fig.5. EIS spectra of Dacromet coating (a) and Dacromet+Re coating (b) after immersion in 3.5% NaCl solution for different times, (a1) and (b1) Nyquist plots, (a2) and (b2) Bode plots of |Z|mod, (a3) and (b3) Bode plots of phase

The results showed that the capacitive loops of high-frequency region in Nyquist plots of Dacromet coating (Fig.5(a1)) disappear after 1 day of immersion in 3.5% NaCl solution and turn to the straight line gradually. The slope of the straight line is not 1, which is the slope of Warburg tail in Nyquist plots [25]. Compared with Dacromet coating, the capacitive loops don't disappear in Nyquist plots of Dacromet+Re coating (Fig.5(b1)) throughout the total immersion. These 9

ACCEPTED MANUSCRIPT capacitive loops are all incomplete and their radiuses become larger and larger. In general, the capacitive loops of high-frequency region correspond to electrochemical process and reflect the information of the coating. Meanwhile, the capacitive loops of low-frequency region correlate with mass transport processes and reveal the information of the coating/substrate interface. From

capacitive loop is, the better the corrosion resistance of the coating.

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the electrochemical impedance in principle [26,27], at the same frequency, the larger the radius of the

Fig.5(a2)and(b2) show that, compared with Dacromet coating, Dacromet+Re coating has greater the impedance values at high-frequency region and low-frequency region. The modulus

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values of |Z| of Dacromet+Re coating is about 1 order of magnitude higher than that of Dacromet coating at low-frequency region. It was proposed that the impedance at low-frequency region

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could serve as an estimation of the barrier effect of a painted metal [28,29]. Generally, the larger this impedance value is, the better corrosion resistance of the coating. In addition, the modulus values of |Z| of Dacromet+Re coating almost remain constant when the frequency exceeds 40Hz, it means that Dacromet+Re coating has less capacitive components, such as micro-cracks and pinholes. This is consistent with the result of the SEM diagram (Fig. 1(b)).

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The main difference of Bode plots of phase between Dacromet coating and Dacromet+Re coating (Fig.5(a3) and (b3)) is that, during the immersion, the high-frequency process of Dacromet+Re coating shifts to higher frequencies. It may be related with the reduction of double [30]

, which should be

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layer capacitance due to the decreasing electrochemical active areas

attributed to the precipitation of hydroxides or oxides of cerium in the micro-cracks and pinholes

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of Dacromet+Re coating. So it can be expected that the seal and barrier effect induced by the deposition of hydroxides or oxides of cerium can improve the corrosion resistance Dacromet+Re

coating.

4. Conclusions

The addition of cerium nitrate to Dacromet coating not only decreases more micro-cracks and pinholes obviously and allows the formation of a more compact coating, but also enhance the corrosion resistance of Dacromet coating. Cerium nitrate can form insoluble hydroxides or oxides in NaCl solution. These hydroxides or oxides of cerium act as cathodic corrosion inhibitors that restrain the entire corrosion process 10

ACCEPTED MANUSCRIPT by suppressing the cathodic reduction reaction. But they can not change the dynamic behaviors of the electrode reaction, and then behave as a physical barrier.

Compared with Dacromet coating, Dacromet+Re coating had a longer controlled sacrificial anodic protection function to steel substrate and greater the impedance values at high-frequency

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ACCEPTED MANUSCRIPT Highlights: 1.Effect of cerium nitrate on corrosion behavior of Dacromet was studied. 2.Cerium nitrate reduced the surface defects of Dacromet. 3.Cerium nitrate enhanced the sacrificial anodic protection function of Dacromet.

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4.Cerium nitrate increased the impedance value of |Z| of Dacromet at low-frequency region.

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Prime Novelty Statement The present work aims at evaluating the effect of cerium nitrate on corrosion resistance of mild carbon steel coated by Dacromet in 3.5% NaCl solution. The electrochemical measurement showed that, compared with Dacromet coating, Dacromet+Re coating had a longer controlled sacrificial anodic

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protection function to steel substrate. The impedance value of |Z| of Dacromet+Re coating was about 1

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order of magnitude higher than that of Dacromet coating at low-frequency region.