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NOTICE: this is the author’s version of a work that was accepted for publication in International Journal of
Greenhouse Gas Control. Changes resulting from the publishing process, such as peer review, editing, corrections,
structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may
have been made to this work since it was submitted for publication. A definitive version was subsequently published
in International Journal of Greenhouse Gas Control. [Vol. 7, (2011)218-224] doi:10.1016/j.ijggc.2011.10.005
Zinc phosphate and vanadate for corrosion inhibition of steel pipelines transport
of CO2 rich fluids
M. F. Morks, Ivan S Cole, Penny Corrigan
CSIRO Division of Materials Science and Engineering, Private Bag 33,
Clayton South, Victoria 3169, Australia
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15
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Abstract
17
The economic transport of CO2 rich fluids, both in the case of carbon capture and sequestration and
18
in transport of natural gas from CO2 rich fields requires the development of economical coatings for
19
pipeline steel. A possible system is sodium orthovanadate (Na3VO4) embedded in a zinc phosphate
20
coating. The effect of that coating on the inhibition of the oxygen reduction reaction on a mild steel
21
surface was investigated at different pH and vanadate concentration. Weight loss and electrochemical
22
polarization methods were applied to evaluate the corrosion rate and inhibition efficiency (η). Mild steel
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and zinc phosphate coated steel were immersed in slightly acidic de-ionized water (HCl/H2O) (pH 4)
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containing different sodium vanadate concentrations (0.0005, 0.001, 0.005 M). Vanadate-plus-zinc
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phosphate coating has high inhibition efficiency (η) of 99% by immersion in 0.001 M sodium vanadate
26
at pH 4. As the vanadate concentration increases (> 0.001 M VO43-) the inhibition efficiency decreases
27
to 25% at 0.005M sodium vanadate. The polarization of steel was performed at pH range of 1-9. The
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effect of adding sodium orthovanadate on the polarization behavior of steel at high pH was investigated.
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At high pH (>7) the inhibition efficiency of vanadate ions increases as the VO4-3 concentration dropped
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to 10 mgl-1. The effect of zinc phosphate aging time in the vanadate inhibitor on the corrosion rate was
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also investigated.
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Keywords: Zinc phosphate, CO2 corrosion, pipelines, Vanadate, Inhibitors, corrosion protection
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1. Introduction
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Both in carbon sequestration and capture (CCS) and in transport of natural gas from CO2 rich fields
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economical coatings are required to protect steel pipelines. The fundamental chemistry and
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electrochemistry is similar in both cases (given that a separate aqueous phase exists in the fluid being
38
transported in CCS).
39
40
CO2 impurities such as sulfide compounds are chemically dissolved in the phase and react carbonic
acids ( and sulfuric ) according to the following reaction:
(1)
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42
These corrosive species attack the steel surfaces and create cathodic and anodic galvanic cells.
43
At pH 6 the main corrosion process can be summarized by three cathodic (Eqs. (2a), (2b) and (2c)) and
44
one anodic (Eq. (3)) reactions [Ogundele et al., 1986; Damia´n et al., 2003; Nesic et al., 2001].
45
(2a)
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(2b)
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(2c)
48
(3)
49
50
CO2 corrosion, usually referred to as sweet corrosion, is one of the most severe forms of attack in
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the oil and gas production and transportation [Kermani et al., 2003; Lopez et al., 2003]. Carbonic acid
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(H2CO3) is a weak acid which has a strong base that oxidize ferrous cations (Fe2+) to form iron
53
carbonate (FeCO3) or bicarbonate (Eqs. 4-6). Iron carbonate (FeCO3) is commonly related with the
54
formation of protective layers on the steel surface [Videm et al., 1989; Videm et al., 1989; Heuer et al.,
55
1999].
56
Fe2+ + CO32- → FeCO3
(4)
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Fe2+ + 2HCO3- → Fe (HCO3)2
(5)
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Fe (HCO3)2 → FeCO3 +CO2 + H2O
(6)
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The iron activity may be attributed to the instability of oxides and carbonate formation, and to the
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support of cathodic reaction (Glezakou et al., 2009) induced by the high hydrogen ion concentration.
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In CO2 capture and storage (CCS), in order to avoid two-phase flow, CO2 need to be transported
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either in the supercritical or the liquid state-at pressures ranging from >5 to >10MPa. At these pressures,
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the solubility of water is limited (0.3-0.4 x 10-2 mole fraction) (Spycher et al., 2003). The pH value is
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changed depending on the water mole fraction and other additives (H2S, SO2, NO3-) in CO2 transport.
65
Free water condition is the ideal in CO2 transport where the pH is >4. As the water contaminant mole
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fraction increase, the tendency of carbonic acid formation increase that drop the pH value to 3.3. In
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presence of other contaminants (H2S, SO2, NO3-) pH value is dropped to<3 with a recent review
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indicating that high corrosion levels occur (Cole et al. 2010). Appropriate gas cleaning prior to transport
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may be able to limit the level of additional contaminates (Cole et al. 2010), however as indicated above
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if am aqueous phase exists that aqueous phase will have a pH between 3 and 4. Thus for both CCS and
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fot transport of oil or gas a coating that can protect steel at acid pH values below 4 is required. .
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Corrosion inhibition of steel pipelines in oil and fuel transport is usually performed by
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incorporating inhibitors in petroleum based oils during transports. The inhibitors, organic materials with
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amine groups, are often fed with the oils or gases under transport in the pipelines. The inhibitors are
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usually adsorbed on the steel surface through their active functional group and form a protective film
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that inhibits the oxygen-reduction reaction. Severe corrosion may occur when film forming inhibitors
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are removed from the steel by high liquid shear forces [Gerretsen et al., 1993]. Organic inhibitors such
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as benzimidazole and its derivatives have been employed to protect steel surfaces against corrosion and
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their inhibition mechanisms were investigated [Kuznetsov et al., 1970;; Makovei et al., 1983; Makovei
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et al., 1985; Voloshin et al., 1988; Popova et al., 1996].
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Zinc phosphate conversion coatings are an economical and environmentally friendly process to
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protect steel, Al and Mg alloys surfaces against corrosion in many industries such as automotive, steel
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forming, and as a primer for steel and aluminium paint for many years because they enhance the
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adhesion of the paint to the substrates [Sun et al., 2002; Zimmermann et al., 2003; Li et al., 2004; Akhtar
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et al., 2006]. Zinc phosphates have a unique crystal structure that is able to carry and store organic and
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inorganic inhibitors due to their porous structure. A lubricant such as soap (pH > 7) or mineral oil
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increases the corrosion resistance efficiency of zinc phosphate [Farias et al., 2009]. With the economy of
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production, environmentally friendly coating, speed of operation and ability to afford excellent corrosion
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resistance, wear resistance, adhesion and lubricative properties, zinc phosphate plays a significant role in
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the automobile, process and appliance industries [Lorin et al., 1974; Freeman et al., 1986; Rausch et al.,
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1990; Rajagopal et al., 2000]. The advantages of phosphate coatings make them suitable to protect steel
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pipelines at moderate pH level of >3.3 in oil transport.
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Vanadium compounds were reported as effective corrosion inhibitors in a number of anti-
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corrosion applications of aluminium [Buchheit, 1995; Guan et al., 2004; Seth et al., 2004; Schem et al.,
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2009; Wang et al., 2010]. Vanadium compounds inhibition is due to the formation of an insoluble oxide
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thin film (500 nm) over a wide range of pH and environmental conditions [Livage, 1996; Iannuzzi et al.,
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2007; Cuevas-Arteaga, 2008; Pulvirenti et al., 2009]. Monovanadate (V1), divanadate (V2),
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tetravanadate (V4) and pentavanadate (V5) are different vanadates species that change their electronic
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state in different pH. Polymerization of these oligomers to form orange decavanadates (V10) occurs
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during acidification [Heath et al., 1981; Larson, 1995; Tracey et al., 1995; Gorzsas et al., 2004;
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Aureliano et al., 2005]. At pH 4, only a signal from V10 is detected by 51V NMR, indicating complete
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polymerization of V1–V5. At pH 8.71 V10 partially de-polymerizes to give V2, V4, and V5 species.
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For carbon steel pipelines corrosion protection, zinc phosphate coating is an economic,
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environmentally friendly and easy to apply. In addition, the phosphate-plus-inhibitor system can
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enhance the corrosion protection even more through the unique microstructure of the phosphate crystals
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that can absorb and store the different kinds of inhibitors. This article focuses on the surface protection
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of steel pipelines at moderate condition (pH 4) using zinc phosphate coating and vanadates inhibitor.
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The preparation and testing of the corrosion performance of zinc phosphate layer-plus-vanadate inhibitor
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on mild steel by means of chemical and electro-chemical methods were investigated. The effect of
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vanadate concentration, aging time and pH on inhibition efficiency was assessed.
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2. Experimental procedure
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2.1. Materials and Chemicals
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Mild steel specimens of chemical composition C-0.13, Si-0.03, Mn-0.31, S-0.05, P-0.2, Ni- 0.01,
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Cu-0.01, Cr-0.01 and Fe-Balance (Wt. %) were used. Weight loss method was carried out to evaluate
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the inhibition performance of sodium vanadate on mild steel and zinc phosphate coated steel corrosion.
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Steel specimens of dimension 50x50x2 mm3 were ground to a 600 grit finish, degreased with ethanol,
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washed in double distilled water and finally dried with compressed air. The cleaned steel specimens and
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zinc phosphate coated steel were weighed before and after immersion in chlorinated de-ionized water
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(H2O + HCl) at pH4 for 2h in the absence and presence of various concentrations of sodium ortho-
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vanadate (0.0005, 0.001, 0.005 M) at room temperature. After immersion the samples were cleaned de-
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ionized water and ethanol and dried with compressed air. The weight loss measurements were
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performed using 4 digit electric balance and the inhibitor efficiency was calculated by applying the
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following formula:
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126
where, η is the inhibitor efficiency, Wu and Wi are the average weight-losses of test sample after
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immersion in acidic solution without and with inhibitor respectively.
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2.2. Zinc Phosphate Treatment
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Zinc phosphate bath was prepared by dissolving approximately 3.1 g zinc oxide in 2.7ml nitric
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acid (C: 61%) and 3ml phosphoric acid (C: 85%) in 100 ml de-ionized water. 0.01 g MnCO3 and 0.05
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MgCO3 were dissolved in the concentrated bath to prepare Zn-Mn-Mg phosphate bath. Mn phosphate
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bath was prepared by dissolving 3.7 g MnCO3 in 12 ml phosphoric acid in 100 ml de-ionized water. The
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phosphate bath was diluted to 1 litre with de-ionized water. The bath acidity was adjusted at pH at 2.5
135
using sodium hydroxide. Upon immersion in the phosphate bath, steel panels chemically react with
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primary zinc phosphate (Zn(H2PO4)2) solution and insoluble zinc phosphate crystals (hopite-
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Zn3(PO4)2.4H2O) of grey color are grown and adhere to the surface. In Mn phosphate bath the steel
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reacts with primary manganese phosphate and a dark grey layer of manganese tertiary phosphate is
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grown.
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The surface of steel specimens was polished (emery grade: 180 and 400), degreased with ethanol
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and rinsed with deionized water before immersion in diluted zinc phosphate bath at 60 oC. The
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immersion time is 5 min followed by rinsing with deionized water and drying with compressed air.
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The surface morphology of zinc phosphate crystals was observed using a Philips XL30 FE-SEM
with a LinkISIS X-ray analysis system (Oxford Instruments).
A JEOL JDX-3530M X-ray diffractometer system was employed to analyse the phase structure
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of the zinc phosphate coatings.
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2.3. Electrochemical Polarization Measurements
150
For electrochemical polarization investigations, both the zinc phosphate coated steels and mild
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steel strips of the same composition were cleaned as mentioned in the previous section and fixed in a
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perspex electrochemical cell with an exposed area of 3.14 cm2. The electrochemical measurements were
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carried out in a conventional three-electrode cell. The working electrode was the mild steel specimen
154
(coated or uncoated). A saturated calomel electrode (SCE) and a titanium mesh were used as the
155
reference and counter electrodes respectively. All polarization measurements were carried out at room
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temperature. The specimens were immersed in the electrolyte for a period of 30 min to allow them reach
157
the equilibrium state before running the polarization measurements. Electro-chemical polarization
158
studies were carried out using a Solartron potentiostat / galvanostat- model 1280B unit. The polarization
159
measurements were carried out at different pH (1-9) by changing the pH of double ionized water (>10
160
MΩ) using hydrochloric acid and sodium hydroxide solutions. All the measurements were made in a
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200 ml working solution.
162
The potentiodynamic polarization measurements were carried at a potential range of -0.8 to -
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0.2V vs. reference electrode. The sweep rate was 0.2mVs-1. The Tafel method was adopted to acquire
164
the values of the corrosion current (Icorr). The inhibition efficiency (η) was calculated from the corrosion
165
current by the following formula:
166
167
where
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respectively.
and
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2.4. ICP-AES Analysis
are the corrosion current densities in the absence and presence of inhibitor
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An Agilent 730 Simultaneous Axial atomic emission spectrophotometric ICP-AES system was
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applied to detect and evaluate the elemental metal ions concentration (Fe, V, Zn) in the solutions after
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immersion of the steel and zinc phosphate coated steel in the mild acidic de-ionized water at pH4 for 2
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hours. The solutions were diluted to 1:50 by deionised water.
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Results and Discussions
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3.1.
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Figure 1 shows the scanning electron microscope images of Zn, Zn-Mn-Mg and Mn-phosphate
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coatings. The different phosphate coatings have different crystal morphologies such as needles, flowers,
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and scale shapes. The crystals morphology is related to the nucleation and growth mechanism of
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phosphate crystals on the steel surface. Bath acidity, type of metal-ion (Mn, Mg, Ni,…), and metal
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surface pretreatment affect the crystal morphology. Mn and Mg metal ions in the phosphate bath have a
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positive effect on refining and improvement of the phosphate crystal structure as shown in Fig. 1. In zinc
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phosphate free Mn-Mg bath the crystal size is bigger (Fig. 1a) compared to zinc phosphate bath with
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Mn-Mg and Mn-phosphate bath (Fig. 1b and1c). This is attributed to the activity of metal ions (Mn and
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Mg) that creates huge numbers of active sites on the steel surface to initiate the phosphate crystal
187
growth.
Phosphate Microstructure
188
The cross-section observed by SEM (Fig. 2) provides information about the phosphate layer
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thickness (500- 2000 nm) as well as the growth mechanism of the phosphate layer. It was concluded that
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the phosphate layer is composed of huge numbers of phosphate crystals which grew on the active sites
191
of the steel surface. The growth mechanism of phosphate crystals is affected by many factors including:
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steel surface roughness, surface chemical activation, type of phosphate bath, and steel microstructure.
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However, the crystals grow from nano-size to micron size during growth and as a result, overlapping of
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crystals may occur during growth. The crystals overlapping can be observed in the cross-section SEM
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micrograph. Overlapping creates channels among the phosphate crystals that host inhibitors, organic
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paint, minerals oils that inhibit the corrosion by preventing the oxidation reaction of Fe from occurring.
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Hopeite (Zn3(PO4)2·4H2O) is the phase structure of the zinc phosphate coating deposited on the steel
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surface as analysed by X-ray diffraction (Fig. 3). Hopeite is a hydrated zinc phosphate with crystals in
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the orthorhombic system. The chemical reaction involved in phosphate deposition is the transfer of
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primary phosphate salt into tertiary phosphate salt according to the following formula:
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3Zn(H2PO4)
202
(Primary phosphate)
→
Zn3(PO4)2
(Tertiary phosphate)
+
H3PO4
+ H 2↑
(Free acid)
203
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3.2.
Polarization of Mild Steel and Zinc Phosphate at different pH
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Figure 4 shows the polarization behaviour of mild steel and zinc phosphate coated steel at different
206
pH. The corrosion current (icorr), and corrosion potential (Ecorr) of mild steel and zinc phosphate on mild
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steel are listed in Table 1, along with the calculated inhibition efficiency of the zinc phosphate coating.
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Zinc phosphate and mild steel have similar polarization behaviour at different pH. At low pH (1-3) there
209
is a negative shift of polarization potential from -500 to -650 mV as the pH increases from 1 to 3. A
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typical feature of these polarization curves was that the hydrogen evolution reaction (cathodic curve)
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was remarkably accelerated by lowering the pH (increasing HCl concentration). However, the anodic
212
curves obtained showed very similar current densities, regardless of the HCl concentration. At pH 4, the
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polarization potential of mild steel is significantly shifted towards positive direction that recorded the
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highest potential value of -159 mV. On the other hand, zinc phosphate coated steel recorded a potential
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range of -320 to -430 mV at pH range of 4-9. The matching of corrosion potential values of the mild
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steel and zinc phosphate coated steel at pH 1-3 indicate an erosion of zinc phosphate coating by the
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acidic electrolyte during the relaxation period (30 min). For steel and zinc phosphate, the corrosion
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current decreases obviously as the pH increases, indicating a decrease in corrosion rate (Table 1).
219
Erosion of zinc phosphate coating is significantly higher at low pH as the inhibition efficiency of zinc
220
phosphate is nil at pH range of 1-3. The inhibition efficiency of zinc phosphate increased to nearly 70%
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at pH>3. The results revealed an enhancement in corrosion resistance by applying zinc phosphate
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coating on mild steel.
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3.2. Vanadate inhibition and weight loss test
225
The effect of vanadate concentration on the inhibition of mild steel and zinc phosphate coated
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steel studied by immersion in mild acidic de-ionized water at pH 4 containing different sodium vanadate
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concentrations: 0.0005, 0.001, and 0.005 M. The inhibition efficiency and change in weight loss of steel
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strips and zinc phosphate coated steel at different vanadate concentration are shown in Fig. 5. Based on
229
the weight loss data, it is observed that zinc phosphate coatings exhibit high inhibition performance (η)
230
of 86% and 99% in mild acidic solution containing 0.0005 and 0.001 M vanadate, respectively. On the
231
other hand, the vanadate has a lower inhibition performance of 25 and 55% on the corrosion of steel at
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vanadate concentration of 0.0005 and 0.001 M respectively. Vanadate with molar concentration of 0.001
233
M has effective inhibition efficiency for both zinc phosphate coated steel and steel corrosion. As the
234
vanadate concentration increases to >0.001M, the inhibition performance decreases. At vanadate
235
concentration of 0.005M, the steel weight increased after immersion, indicating a growth of a film on
236
the surface. Vanadate ions (VO43-) probably react with the steel surface by chemical reaction to form an
237
iron vanadate layer on the steel surface that reduces the rate of dissolution of iron in HCl.
238
ICP-AES Atomic emission spectrophotometric analysis of the acidic solutions containing
239
0.005M vanadate after immersion of steel and zinc phosphate coated steel samples are listed in Table 2.
240
Fe metal ions were found in the solution used for steel immersion, indicating the reaction of steel surface
241
with the vanadates acidic solution. The zinc phosphate layer prevented the dissolution of iron because
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there was no iron detected in the solution used for immersion of zinc phosphate coated steel. Also, zinc
243
has not been detected which confirms the stability of the zinc phosphate layer in the acidic solution.
244
Figure 6 shows SEM and EDS elemental analysis for a layer deposited on the steel surface after
245
immersion in the acidic solution with 0.005M sodium vanadate for 2 hours. An increase in the weight
246
after immersion of the mild steel sample in the acidic solution contains 0.005M sodium vanadates for 2
247
hours was measured, indicating the development of a layer on the surface. EDS analysis of this layer
248
showed the presence of V, Fe, and oxygen. From the results of atomic analysis and SEM, it was
249
concluded that a vanadate conversion reaction occurred on the steel surface during immersion. The
250
vanadate ions are converted to vanadium pentoxide by consumed the electron released by anodic
251
desolving of iron. A thin layer of vanadium oxide was detected by XRD, EDS and atomic analysis. The
252
reactions involved in vanadate conversion coating are iron dissolution (anodic reaction) and cathodic
253
conversion of vanadate anions (cathodic reaction) into vanadium pentaoxide. The possible reactions are:
254
255
Fe → Fe2+ + 2e-
(iron dissolution)
VO43 + 2e- → V2O5 (Vanadium pentaoxide)
256
257
3.3 Aging time and Vanadate Concentration
258
The potentiodynamic polarization curves for zinc phosphate in alkaline-ionized water containing
259
5, 10 and 20 mg vanadate at pH 9 are shown in Fig. 7. The potential shifted to positive value and the
260
corrosion current density decreased as the vanadate concentration reduced from 20 to 5 mgl-1. The
261
inhibition performance of zinc phosphate coatings increased as the aging time of the zinc phosphate
262
samples in vanadate solution was increased from 1 to 2 hours. The results indicated a higher corrosion
263
protection at low concentration of vanadate inhibitor. The zinc phosphate coating has two advantages in
264
steel protection: as a solid anti-corrosion inhibitor and as a carrier for vanadate inhibitor or any other
265
effective inhibitors. Phosphate crystals adhere well with the steel surface and that prevents the loss of
266
inhibitor film from the surface by gas or liquid shear forces during CO2 transport. Although there is no
267
available evidence of the presence of vanadate ions attached to zinc phosphate crystals yet, it is likely
268
that vanadate ions are adsorbed and adhered onto the phosphate crystals and steel surface through the
269
channels of the phosphate structure. For high inhibition performance, the optimum concentration for
270
vanadate is 5 mgl-1 in alkaline medium and 15-20 mgl-1 in acidic medium. At present, it is not known
271
why the inhibitor is working effectively at very low concentration.
272
3.4 Applicability to CCS or oil and gas applications
273
A requirement of a coating for both the CCS and the oil and gas transport application was good
274
corrosion protection at moderately acidic pH values. The results in this paper have shown that the
275
combination a zinc phosphate coating and a 0.001 M vanadate solution can lead to 99% efficiency of
276
corrosion inhibition in a saline solution at pH of 4. Thus it is suggested is a good system for future
277
exploration. Such future exploration would need to consider
278
1. How to embed the vanadate in the zinc phosphate coating
279
2. How the system performed under a wider range of acid pH’s
280
3. How the system performed under supercritical CO2 at the pressures likely in CCS.
281
4. Conclusions
282
Zinc phosphate, as solid inhibitor layer for carbon steel surface, has the advantage of the ability
283
to store inhibitors due to the unique structure of the phosphate crystals. The key observations are:
284
(1) SEM images of different phosphates showed a decrease in phosphate crystal size by adding Mn
285
and Mg metal ions in Zn phosphate bath.
286
(2) Zinc phosphate layer has a total thickness of 0.5-2 µm under the experimental conditions.
287
(3) Phosphate crystals are grown from nano-size to micron size and overlap during growth to create
288
channels of unique structure for hosting inhibitors.
289
(4) Hopeite is the main phase structure of phosphate crystals
290
(5) Polarization curves revealed the erosion of zinc phosphate is significantly higher at low pH and
291
292
293
294
295
the inhibition efficiency of zinc phosphate is nil at pH range of 1-3.
(6) Phosphate crystals are adhered well with the steel surface that may reduce the loss of inhibitor
film from the surface by gas or liquid shear forces during oil transport.
(7) Zinc phosphate coatings exhibit high inhibition performance (η) of 86% and 99% in mild acidic
solution containing 0.0005 and 0.001 M vanadate, respectively.
296
(8) The inhibition efficiency of zinc phosphate coatings increased to nearly 70% at pH>3.
297
(9) Vanadium pentaoxide film is grown on mild steel by immersion in acidic solution containing
298
0.005M sodium vanadate.
299
300
301
Acknowledgment
The authors acknowledge the financial support from CSIRO, CMSE-Clayton.
302
303
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Figure Captions
387
Figure 1 SEM micrographs of different phosphate crystals: a) Zn, b) Zn-Mn-Mg and c) Mn phosphate
388
Figure 2: SEM micrographs of Zn-phosphate cross-section
389
Figure 3: X-ray diffraction pattern of zinc phosphate coating on mild steel surface
390
Figure 4: Potentiodynamic polarization curves of zinc phosphate and mild steel at different pH
391
392
Figure 5: Effect of vanadate concentration on the corrosion (weight loss) of mild steel and zinc
393
phosphate
394
395
Figure 6: SEM and EDS analysis of deposited layer on mild steel surface after immersion for 2 hours in
acidic de-ionized water contains 0.005M vanadate
396
397
Figure 7: Effect of immersion time and vanadate concentration on the polarization behaviour of Zn
398
phosphate coatings.
399
400
401
402
403
404
405
406
407
408
409
Table 1: corrosion current and inhibition efficiency of Zn phosphate on mild steel at different pH
410
pH
1
2
3
4
5
6
7
8
9
Zinc Phosphate
Ecorr, V
icorr, A
-0.51
6.60 e-4
-0.58
9.60 e-5
-0.67
1.50 e-5
-0.43
6.70 e-7
-0.394
3.80 e-7
-0.438
3.5 e-7
-0.50
2.6 e-7
-0.39
2.27 e-7
-0.326
3.2 e-7
Mild steel
Ecorr, V
icorr, A
-0.50
2.5 e-4
-0.58
9.6 e-5
-0.63
5.6e-6
-0.159
1.7 e-6
-0.157
7.5 e-7
-0.28
3.5 e-7
-0.23
8.2 e-7
-0.39
6.9 e-7
-0.263
5.2 e-7
η, %
0
0
0
66
47
68
67
38
411
412
413
414
Table 2: Atomic analysis data for mild acidic de-ionized water solutions contain 0.005M sodium
vanadate at pH 4 (water/HCl) after immersion zinc phosphate coated steel and steel samples for 2hours.
415
Sample Reference
Cu
Fe
Na
V
Zn
Zinc coated steel, ppm
<0.05
<0.01
300
215
<0.2
Steel, ppm
<0.05
4. 3
310
200
1.6
416
417
418
419
420
421
422
423
424
a)
425
c)
b
)
426
427
428
429
430
Figure 1 SEM micrographs of different phosphate crystals: a) Zn, b) Zn-Mn-Mg and c) Mn
phosphate
431
432
433
434
435
436
437
Zn phosphate
438
439
440
441
442
443
444
445
446
451
452
453
20k
Fe (310)
Fe (220)
450
30k
Fe (200)
449
hopite
hopite
hopite
hopite
Relative intensity
448
Fe (211)
Fe (110)
40k
447
10k
20
40
60
80
100
Diffraction angle, 2θ (degree)
454
120
455
Figure 3: X-ray diffraction pattern of zinc phosphate coating on mild steel surface
456
457
458
pH 6
pH 7
pH 8
pH 9
pH 1
pH 2
pH 3
pH 4
Zinc Phosphate
462
E, V
461
pH 1
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
-0.4
-0.6
-0.8
-1.0
-9
10
10
-7
1x10
-5
log i, A
10
-3
-1
10
-0.2
-0.4
E, V
460
mild steel
0.0
-0.2
459
-0.6
-0.8
-1.0
10
-8
10
-6
-4
1x10
log i, A
10
-2
463
464
465
466
467
468
469
470
471
472
473
476
477
478
479
480
80
60
40
Steel
Zn Phosphate
pH 4
2.0
steel
Zn Phosphate
Wt. loss, mg
475
100
Inh. Effeciency, (η) %
474
1.5
1.0
0.5
0.0
20
0
1
2
3-
3
4
5
VO4 conc., mM
0
1
2
3
4
5
3-
VO4 conc., mM
481 Figure 5: Effect of vanadate concentration on the corrosion (weight loss) and inhibition
482 efficiency of mild steel and zinc phosphate
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
o
Fe
EDS
506
V
507
V
508
509
510
511
512
Figure 6: SEM and EDS analysis of deposited layer on mild steel surface after immersion
for 2 hours in acidic de-ionized water contains 0.005M vanadate
513
514
515
516
517
518
519
520
521
522
0.0
0.0
523
-0.1
524
ZnPhos in 5mg vanadate-1h
ZnPhos in 5mg vanadate-2h
pH 9
-0.1
-0.2
527
E, V
526
-0.3
-0.3
Ecorr = -0.340 V
icorr = 1.17 e-7
Ecorr = -0.363
icorr = 3.2 e-7
-0.4
E, V
-0.2
525
Zn Phos in 5mg vanadate_pH 9
Zn Phos in 10mg vanadate_pH 9
Zn Phos in 20mg vanadate_pH 9
Zn phosphate_pH 9
-0.4
-0.5
-0.6
528
529
-0.5
1E-9
-0.7
1E-8
1E-7
1E-6
logi, A
1E-5
1E-10 1E-9
1E-8
1E-7
1E-6
1E-5
log i, A
530
Figure 7: Effect of immersion time and vanadate concentration on the polarization
531
532
533
534
535
536
537
behaviour of Zn phosphate coatings.