International Journal of Greenhouse Gas Control 7 (2012) 218–224
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International Journal of Greenhouse Gas Control
journal homepage: www.elsevier.com/locate/ijggc
Mn–Mg based zinc phosphate and vanadate for corrosion inhibition of steel
pipelines transport of CO
2
rich fluids
Magdi F. Morks, Penny A. Corrigan, Ivan S. Cole
CSIRO Division of Materials Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia
article info
Article history:
Received 13 July 2011
Received in revised form
14 September 2011
Accepted 26 October 2011
Available online 30 November 2011
Keywords:
Mn–Mg–zinc phosphate
CO2 corrosion
Pipelines
Vanadate
Inhibitors
Corrosion protection
abstract
The economic transport of CO
2
rich fluids for carbon capture and sequestration requires the development
of economical coatings for pipeline steel. A possible system is sodium orthovanadate (Na
3
VO
4
) embedded
in a modified zinc phosphate coating with Mn–Mg additives. The effect of this coating on the inhibition
corrosion on a mild steel surface was investigated at different pH and vanadate concentration. Weight
loss and electrochemical polarization methods were applied to evaluate the corrosion rate and inhibition
efficiency (). Mild steel and modified zinc phosphate coated steel were immersed in slightly acidic
de-ionized water (HCl/H
2
O) (pH 4) containing different sodium vanadate concentrations (0.0005, 0.001,
0.005 M). The Mn–Mg–zinc phosphate-plus-vanadate coating has a high inhibition efficiency () of 99%
when formed by immersion in 0.001 M sodium vanadate at pH 4. As the vanadate concentration increased
to 0.005 M the inhibition efficiency decreased to 25%. The polarization of steel was performed at pH
range of 1–9. The effect of adding sodium orthovanadate on the polarization behaviour of steel at high
pH was investigated. At high pH (>7) the inhibition efficiency of vanadate ions increases as the VO
4
-3
concentration dropped to 0.05 mM. The effect of Mn–Mg–zinc phosphate aging time in the vanadate
inhibitor on the corrosion rate was also investigated.
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
In carbon sequestration and capture (CCS), the transport of CO
2
rich fluids from the point of capture to the point of storage requires
economical coatings to protect steel pipelines. In CCS, in order
to avoid two-phase flow, CO
2
needs to be transported either in
the supercritical or the liquid state – at pressures ranging from
>5 to >10 MPa. At these pressures, the solubility of water is lim-
ited (0.3–0.4 × 10
-2
mol fraction) (Spycher et al., 2003). If water is
present above the solubility limit then the pH value of the separate
water phase changes depending on the water mole fraction and
other contaminants (H
2
S, SO
2
, NO
3
-
) that may be present in CO
2
transport. Cole et al. (2011) mapped out the likely conditions that
may prevail in a pipeline into four different regimes
A — Very low contaminant levels and extremely low water content.
B — Low contaminant levels and water content below the solubility
content.
C — Low contaminant levels and water content above the solubility
content.
D — Moderate contaminant levels and water content above the
solubility limit
The purity of the gas stream is controlled by pollutant con-
trol measures at the source of CO
2
(e.g., power plant) and by gas
conditioning prior to piping the gas (Lee et al., 2009). The first
regime is typical of CO
2
transport in enhanced oil recovery (EOR)
in the USA (under Kinder Morgan guidelines (Kinder Morgan CO
2
Company, 2001)) and would prevail if CO
2
were extracted using
monoethanolamine (MEA) in a plant with strong pollution control
measures and gas conditioning to lower the water content below
the pressure solubility limit (500 ppm in CO
2
). The second regime
would occur if gas conditioning was limited or there was a limited
source of H
2
O into the pipe. The third could occur in the absence
of gas conditioning, or with additional sources of H
2
O while the
fourth would occur without gas conditioning and limited cleaning
of the gas at source.
In regimes C and D where an aqueous phase will exist within
the bulk CO
2
fluid this aqueous phase will absorb CO
2
via:
CO
2
(g) + H
2
O(l) ↔ H
2
CO
3
(aq) (1)
H
2
CO
3
↔ H
+
+ HCO
3
-
(2)
HCO
3
-
↔ H
+
+ CO
3
2-
(3)
Such absorption would render the aqueous phase acidic (pH
approximately 3.1 from carbonic acid alone) and under these
conditions the cathodic reactions may occur either by the direct
reduction of hydrogen ions, or via carbonates:
2H
+
+ 2e
-
→ H
2
(4)
1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijggc.2011.10.005