Materials and Design 32 (2011) 750–755

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Materials and Design

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Characterization and corrosion behavior of electroless Ni–P/nano-SiC coatinginside the CO2 containing media in the presence of acetic acid

S.R. Allahkaram *, M. Honarvar Nazari, S. Mamaghani, A. ZarebidakiCenter of Excellence in High Performance Ultra Fine Materials, School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran,North Kargar, Tehran 11155-4563, Iran

a r t i c l e i n f o

Article history:Received 15 May 2010Accepted 23 July 2010Available online 5 August 2010

Keywords:E. CorrosionC. CoatingsA. Nanomaterials

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.07.030

* Corresponding author. Tel./fax: +98 2161114108.E-mail address: akaram@ut.ac.ir (S.R. Allahkaram)

a b s t r a c t

In this research, Ni–P and Ni–P/nano-SiC coatings were applied on the X70 steel substrate successfullywithout any surfactant. Then, CO2 corrosion in the presence of acetic acid (HAc) was investigated usingelectrochemical techniques. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS)and X-ray diffraction (XRD) techniques were used for surface analyses of the coatings. The electrochem-ical behavior of corrosion was investigated using polarization test and electrochemical impedance spec-troscopy (EIS). XRD pattern of Ni–P/nano-SiC coating was very similar to that of Ni–P coating. EDS resultsdemonstrated the presence of SiC particles in the coating. SEM images confirmed the presence of SiCnano-particles with almost uniform distribution in the coating. The corrosion current density was lessin the Ni–P and Ni–P/nano-SiC coated samples than uncoated X70 steel. Ni–P/nano-SiC coated samplehad the most corrosion resistance because of less effective metallic area available for corrosive media.The overall protection mechanism of Ni–P and Ni–P/nano-SiC coatings was achieved by formation of alayer of adsorbed hypophosphite anions (H2PO2

�).� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

One of the major problems in oil and gas industry is the CO2 cor-rosion of carbon steels [1–4]. In this type of corrosion, CO2 dis-solves in brine and as a result, carbonic acid produces whichmakes the media corrosive [1,3]. In addition, in most of the oiland gas fields the effect of organic acids especially acetic acid(HAc) as a corrosion promoter has been noted [5–7].

Electroless nickel–phosphorous (Ni–P) coating is widely used indifferent industries because of its high hardness, excellent corro-sion and wear resistances and its uniform thickness [8]. It can berationally anticipated that the inclusion of nano-sized particles inelectroless Ni–P alloy coating would be significant for broadeningthe scopes of these coatings in industry. Nano-sized particles couldendow the coating with special functionality. This has been pri-marily verified by the studies on electroless Ni–P alloy coatingco-deposited with hard particles such as SiC, B4C, Al2O3, etc. [9–11]. Among these coatings, the combination of Ni–P/SiC has provedto be one of the best-performing combinations [10–12].

Recently some attempts have been made to prevent the internalcorrosion of pipelines and tubes using Ni–P coating [13,14]; how-ever, it seems that there is a gap in this topic for the Ni–P/nano-SiCcoating and also the environments that are found in oil and gas

ll rights reserved.

.

industry like CO2 containing media that is accompanied with dis-solved acetic acid.

In this research, first the incorporation of SiC nano-particles onthe structure of electroless Ni–P matrix was studied. Then the ef-fects of Ni–P and Ni–P/nano-SiC coatings on the corrosion behaviorof X70 steel, in CO2 containing media in the presence of acetic acid,were investigated.

2. Experimental procedures

X70 steel samples, with nominal composition shown in Table 1,were coated by Ni–P and Ni–P/nano-SiC coatings. The shapes ofsteel samples were cylindrical with the exposed surface area ofabout 5.3 cm2. They were polished using 60–2000 grit silicon car-bide papers then degreased in acetone using an ultrasonic cleaningdevice and washed with ethanol.

For coating of X70 steel samples, in addition to the above men-tioned process, all the specimens were subjected to the followingpre-treatment and plating procedure:

1. Alkaline cleaning in a solution containing 20 g/l sodiumhydroxide (NaOH), 26 g/l sodium phosphate (Na3PO4), 26 g/lsodium carbonate (Na2CO3) and 5 g/l sodium silicate (Na2SiO3)at 75 ± 2 �C for 7 min.

2. Cleaning in 20 vol.% H2SO4 at room temperature (RT) for 30 s.3. Cleaning in 5 vol.% H2SO4 at RT for 30 s.

Table 1Chemical composition of the steel (wt.%).

C Si Mn P S Cr Ni Cu Mo Nb Co Al B Ti V Fe

0.037 0.141 1.291 0.006 0.013 0.011 0.156 0.024 0.207 0.018 0.015 0.055 0.006 0.017 0.039 97.964

(a.

u.)

Ni-P/nano-SiC

S.R. Allahkaram et al. / Materials and Design 32 (2011) 750–755 751

4. Electrocleaning in a solution containing 75 g/l NaOH, 25 g/lsodium sulfate (Na2SO4) and 75 g/l Na2CO3 at RT for 20 min. Thecurrent density applied was 10 mA/cm2 according to ASTM G1.

Between each step the specimen was rinsed with distilled waterat RT for 2 min. A commercial electroless nickel bath (SLOTONIP 70A from Schlotter) with sodium hypophosphite as reducing agentwas used to obtain the coatings. This bath provided Ni–P depositswith a high phosphorous content, 9–10% P. Temperature was set at90 ± 2 �C and pH was fixed at 4.6 ± 0.1.

For achieving pure Ni–P coating, deposition process took 2 h un-der magnetic agitation condition at a speed of 300 rpm. In the caseof Ni–P/nano-SiC coating, first a Ni–P film was deposited for 0.5 hunder the same condition as mentioned above. Then 4 g/l ofnano-sized SiC particles (average size 40 nm) were added to theplating bath by ultrasonic dispersion for 15 min, followed by depo-sition process for 2 h as stated above.

The corrosion tests were conducted in the formation water ob-tained from the oilfield with the composition shown in Table 2 atatmospheric pressure and the temperature of 22 �C. Prior to thecommencement of the tests, the solution was deoxygenated bypurging CO2 for at least 1 h and the purging was continuedthroughout the tests. At the commencement, 500 ppm acetic acidwas injected to the cell.

The morphology of the coatings was studied by scanning elec-tron microscopy (SEM) with a CamScan MV 2300 OXFORD instru-ment. Elemental analysis of the coating was determined by energydispersive spectroscopy (EDS) method. In addition, phase analysisof the coating was obtained using X-ray diffraction (XRD) tech-nique by a Philips X’Pert pro-instrument using Co Ka radiation.

Quench test was used to determine the adhesion of the coat-ings. In this test, specimens were heated at 300 ± 10 �C for 1 h,and then quenched in water at RT. The appearance of blisters orpeeling was taken as the evidence of inadequate adhesion.

Polarization tests were carried out in a three-electrode cellusing an EG&G potentiostat/galvanostat, model 273A. Platinumplate and Ag/AgCl electrode were used as counter and referenceelectrodes, respectively. The electrochemical impedance spectros-copy (EIS) tests were carried out using instrumentation coupledto a Solartron model SI 1255 HF frequency response analyzer(FRA). The EIS measurements were obtained in a frequency rangeof 100 kHz–0.01 Hz with an applied AC signal of 5 mV (rms) usingEIS software model 398.

20 30 40 50 60 70 80 90 1002 Theta (degree)

Inte

nsit

y

Ni-P

Fig. 1. XRD patterns of electroless Ni–P and Ni–P/nano-SiC coatings.

3. Results and discussion

3.1. Studies on as-deposited coatings

3.1.1. Surface analysesFig. 1 shows the XRD patterns of Ni–P coating and Ni–P/nano-

SiC composite coating. As it can be seen, there is only a single broadpeak around 52� which is associated with Ni for both of the coat-ings, which also shows their amorphous nature. In the Ni–P/

Table 2Composition of formation water.

Ions Na+ K+ Ca2+ Mg2+ Sr2+ Ba2+

Concentration (mg/L) 60,009 1010 3910 367 36 0.5

nano-SiC composite coating, SiC particles could not be detected.This could be because of the fine size of SiC particles and theirlow volume fraction as compared to Ni–P matrix [15]. Similarobservations have been made by other researchers [16].

To confirm the presence of SiC nano-particles in the coating,quantitative analysis of the surface was carried out using EDS tech-nique and the corresponding data are presented in Table 3. Siliconwas observed other than nickel and phosphorous. This confirmedthe presence of second phase silicon carbide particles in the Ni–Pmatrix.

The SEM images of the surface morphologies of as-depositedelectroless Ni–P and Ni–P/nano-SiC coatings are shown in Fig. 2.It can be seen that SiC nano-particles have been embedded in theNi–P matrix by co-deposition. Some of the SiC nano-particlesagglomerated to a certain degree although ultrasonic and magneticagitation had been used.

The distribution of SiC nano-particles through the thickness ofdeposit can be seen in the SEM micrograph of the cross-sectionas shown in Fig. 3. It is seen that SiC particles distributed almostuniformly throughout the thickness of the coating.

3.1.2. Mechanical properties measurementsMicro-hardness of electroless Ni–P and Ni–P/nano-SiC coatings

are shown in Table 4. Ni–P/nano-SiC composite coating has highermicro-hardness than Ni–P coating. It can be supposed that SiCnano-particles co-deposited in the composite coating could act asbarriers to retard the plastic deformation of Ni–P matrix and henceincrease the micro-hardness. Namely, SiC nano-particles as a sec-ond phase are capable of reinforcing the Ni–P matrix and impedefast propagation of dislocations in ductile Ni–P matrix.

3.1.3. Adhesion of coating to substrateAs mentioned in experimental procedures, in the case of Ni–P/

nano-SiC coating a layer of Ni–P deposited on the surface of thesubstrate prior to depositing the Ni–P/nano-SiC coating. This is

Fe2+ Cl� HCO3� CO3

2� SO42� NO3

� Total

95 100,350 569 1 1431 2 167,780.5

Table 3Compositions of the coatings characterized by EDS.

Type of deposited coating Composition (wt.%)

Ni P Si SiC (calculated)

Ni–P 91.79 8.21 – –Ni–P/nano-SiC 91.35 8.65 3.50 2.45

Table 4Micro-hardness of electroless Ni–P and Ni–P/nano-SiC coatings.

Type of deposited coating Micro-hardness (HV150)

Ni–P 605Ni–P/nano-SiC 695

752 S.R. Allahkaram et al. / Materials and Design 32 (2011) 750–755

an approved method for increasing the adhesion of Ni–P compositecoatings to the substrate [16]. The quench test observations re-vealed that there were not any blisters or peeling off betweenthe coating and substrate in both electroless Ni–P and Ni–P/nano-SiC coatings, which shows the good adhesion of these coat-ings to the substrate.

3.2. Corrosion studies

3.2.1. Surface morphologyFig. 4 shows the surface morphologies of electroless (a) Ni–P

and (b) Ni–P/nano-SiC coatings after polarization from �0.3 to+0.2 V vs. open circuit potential (OCP). The surface of Ni–P coatingwas consisted of both smooth and cracked sites. Similarobservations have been reported by Rahimi et al. for as-depositedelectroless Ni–P coating exposed to 3.5% NaCl solution at RT innon-deaerated condition [17].

The existence of both smooth and cracked sites on Ni–P coatingshows that localized corrosion has happened on this coating. Onthe other hand, a relatively uniform corrosion has occurred on

Fig. 2. SEM images of electroless (a) Ni

Ni-

P/n

ano-

SiC

Ni-

P

a

Fig. 3. SEM micrographs of the cross-section of electroless Ni–P/nan

the Ni–P/nano-SiC coating and no cracks were observed on that.Therefore, it can be deduced that Ni–P/nano-SiC coating has a bet-ter corrosion performance than that of Ni–P coating.

3.2.2. Electrochemical analysesThe polarization curves obtained for uncoated X70 steel, Ni–P

and Ni–P/nano-SiC coated samples are shown in Fig. 5. The electro-chemical parameters fitted from polarization curves are presentedin Table 5. It is seen that the corrosion potential (Ecorr) for the sam-ple coated with Ni–P/nano-SiC is �392.8 mV, which is more posi-tive than the uncoated and Ni–P coated specimens. Therefore, itcan be deduced that the Ni–P/nano-SiC composite coated sampleis a more thermodynamically steady system than the others [18].

The predominant cathodic reaction in the CO2 containing mediaaccompanied with acetic acid is the direct reduction of the aceticacid [19–21]:

HAc þ e� ! 1=2H2 þ Ac� ð1Þ

It can be seen that X70 steel had the highest cathodic currentdensity and the steel was in active dissolution state. Coating the

–P and (b) Ni–P/nano-SiC coatings.

b

o-SiC coating (a) low magnification and (b) high magnification.

Fig. 4. SEM images of electroless (a) Ni–P and (b) Ni–P/nano-SiC coatings after polarization from �0.3 to +0.2 V vs. open circuit potential in the oilfield formation watersaturated with CO2 in the presence of 500 ppm acetic acid.

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

-8 -7 -6 -5 -4 -3 -2

Log i (A/sqcm)

Pot

enti

al (

V,A

g/A

gCl)

Ni-P/nano-SiC

X70 Steel

Ni-P

Fig. 5. Polarization curves of X70 steel, electroless Ni–P and Ni–P/nano-SiC coatingsin oilfield formation water saturated with CO2 in the presence of 500 ppm aceticacid.

Table 5Electrochemical parameters fitted from polarization sweeps.

Type of sample Ecorr

(mV)ba

(mV/dec)bc

(mV/dec)icorr

(lA/cm2)

Uncoated X70 steel �584.2 25.4 29.6 4.5X70 steel with Ni–P coating �511.3 33.5 60.6 2.2X70 steel with Ni–P/nano-SiC

coating�392.8 29.9 43.1 1.1

S.R. Allahkaram et al. / Materials and Design 32 (2011) 750–755 753

sample with Ni–P decreased the anodic and cathodic current densi-ties significantly. This shows that there is a problem for the migra-tion processes of the electro-active species to the surface of steel.Therefore, it can be suggested that the Ni–P coating has blockedthe surface of steel and has acted as a barrier for active species.

The same results have been reported by other investigators[22–25]. It has been reported that electroless Ni–P is a barrier coat-ing, protecting the substrate by sealing it off from the corrosiveenvironments, rather than by sacrificial action. In addition, it hasbeen argued that the corrosion resistance of this layer is due tothe amorphous nature and passivity of Ni–P deposits. Amorphousalloys offer better resistance to corrosion attack than polycrystal-line materials because they have no grain or grain boundariesand also a glassy passive film can form on their surfaces [23–26].

The decrease in anodic/cathodic current densities was morepronounced in Ni–P/nano-SiC coated sample than that of Ni–P. Thiscan be attributed to the less effective metallic area available for thecorrosive medium in Ni–P/nano-SiC coated sample. The metallicsites are the suitable places for the reduction of active species,especially acetic acid. The decrease in metallic sites results inreduction of cathodic reactions. On the other hand, any decreasein the fraction of metallic sites causes a decrease in the numberof sites ready to be dissolved (i.e. reduction of anodic reactions).

It can therefore be deduced that SiC particles decrease the rate ofboth anodic and cathodic reactions which leads to a decrease incorrosion current density.

The evaluation based on electrochemical parameters also con-firmed this finding. The corrosion current density (icorr) of the spec-imen with Ni–P/nano-SiC composite coating was about 50% and24% of those of Ni–P coating and X70 steel, respectively, whichshowed that the X70 steel substrate coated with Ni–P/nano-SiChas a better corrosion performance than both uncoated X70 steeland the Ni–P coated specimen.

Fig. 6 shows the Nyquist plots obtained for uncoated X70 steel,Ni–P and Ni–P/nano-SiC coated samples at their respective OCP.The main difference between the EIS plots of the coated and un-coated X70 steel is the existence of an inductive loop for the un-coated sample, which indicates the occurrence of the formation/adsorption of corrosion products (FeCO3) on the surface of steel[19].

The Nyquist plots for the Ni–P and Ni–P/nano-SiC coatings ap-pear to be similar. They consist of a single semicircle in the highfrequency region signifying the charge controlled reactions. Itshould be noted that although these curves appear to be similarin shape, they differ in size. This indicates that the same funda-mental phenomena could have occurred in all these coatings butover a different effective area in each case.

The occurrence of a single semicircle in the Nyquist plots of thecoated samples shows that the corrosion process of these coatingsinvolves a single time constant indicating the existence of a doublelayer at coating/electrolyte interface. As it is known, each physicalphenomenon or reaction has a single time constant [27].

A similar conclusion of the existence of a single time constanthas been reported by Balaraju et al. [26] for the corrosion of elec-troless Ni–P coatings in 3.5% sodium chloride solution at theirrespective OCP in the deaerated condition. Also, it is known thatin CO2 corrosion the EIS measured at the corrosion potential re-flects mainly the characteristics of the anodic reactions [20].

-5

0

5

10

15

20

25

30

500 550 600 650 700 750 800 850 900 950

Real (Ohm.sqcm)

-Im

ag (

Ohm

.sqc

m)

X70 Steela

-50

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Real (Ohm.sqcm)

-Im

ag (

Ohm

.sqc

m)

Ni-P/nano-SiCNi-P

b

Fig. 6. Nyquist plots of (a) uncoated X70 steel and (b) Ni–P and Ni–P/nano-SiCcoatings in oilfield formation water saturated with CO2 in the presence of 500 ppmacetic acid.

754 S.R. Allahkaram et al. / Materials and Design 32 (2011) 750–755

Therefore, accordingly, it can be deduced that the fundamentalanodic reactions which have occurred at the coating/electrolyteinterface in the present study have also been confirmed by Balarajuet al. [26] experimental data. Therefore, the overall mechanism ofthe protection by electroless Ni–P coatings in the oilfield formationwater saturated with CO2 in the presence of acetic acid is the sameas dearated 3.5% NaCl solution.

The proposed mechanism is the preferential dissolution of nick-el which led to enrichment of phosphorus on the surface layer. Thisenriched phosphorous reacted with water to form a layer of ad-sorbed hypophosphite anions (H2PO2

�). This layer in turn blockedthe supply of water to the electrode surface, thereby preventingthe hydration of nickel, which is considered to be the first step toform either soluble Ni2+ species or a passive nickel film [23–26].

Since the purpose of this paper is investigating the anti-corro-sion performance of Ni–P and Ni–P/nano-SiC coatings, the preli-minary analysis was limited to the high frequency capacitiveloop of taken spectra. This loop is attributed to the charge transferresistance parallel to the double layer capacitance, both of whichare in series with the solution resistance between the surface ofspecimen and the reference electrode. This is the commonly pro-posed equivalent circuit model for the simple corrosion system,which is charge transfer control. Fig. 7 presents the equivalent cir-cuit used to analyze the impedance data.

Fig. 7. Equivalent circuit model of EIS test used for analysis of the corrosionbehavior.

The values of the charge transfer resistance (Rct) observed are366.2, 3417.3 and 5760.1 X cm2 for X70 steel specimen, Ni–Pand Ni–P/nano-SiC coatings, respectively. These data imply a bettercorrosion protective ability of NiP/nano-SiC composite coatingcompared to Ni–P coating and uncoated X70 steel. This seems tobe logical, because as mentioned before the effective metallic areaprone to corrosion is decreased in the case of electroless Ni–P/nano-SiC composite coating. These results are in good agreementwith the polarization data and confirmed that reinforcing themetallic Ni–P coating with SiC nano-particles can improve the cor-rosion resistance even in the oilfield corrosive media. Similarobservations have been reported by Lin et al. for electroless Ni–P/nano-SiC coating exposed to 3.5% NaCl solution at RT in non-deaerated condition [28].

4. Conclusions

1. SiC nano-particles have been successfully co-deposited withNi–P matrix on X70 steel to obtain Ni–P/nano-SiC compositecoating in the absence of surfactants.

2. According to surface analyses, both Ni–P and Ni–P/nano-SiCcoatings had amorphous structure. The nano-sized SiC particlescould not change the structure of the Ni–P coating.

3. The micro-hardness of electroless Ni–P/nano-SiC compositecoating was higher than that of Ni–P coating, due to the pres-ence and uniform distribution of nano-sized SiC particles.

4. In the CO2 saturated media containing acetic acid, Ni–P/nano-SiC and Ni–P coated X70 steel specimens showed less corrosioncurrent densities than uncoated X70 steel.

5. The Ni–P/nano-SiC coated specimen showed higher corrosionresistance than that of Ni–P coated specimen. This is attributedto the reduction of effective metallic area available for corrosivemedia.

6. The EIS data showed that the overall mechanism of the protec-tion by Ni–P and Ni–P/nano-SiC coatings in the formation watersaturated with CO2 in the presence of acetic acid is the protec-tion obtained by a layer of adsorbed hypophosphite anions(H2PO2

�).

Acknowledgments

Financial support from the Iranian Offshore Oil Company(agreement no. 2-87-4016-F-Gh) for this project is gratefullyacknowledged.

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