5
Parer No.
128
The NACE !nternatlonal AnnuaK Conference and Conos;on Show
INFLUENCE OF LIQUID FLOW VELOCITY ON C0 2 CORROSION:
A SEMI-EMPIRICAL MODEL
C. de Waard
Koninklijke/Shell Laboratory
P.0.Box 38000, 1030 BN
Amsterdam, The Netherlands
U. Lotz
Shell Internationale Petroleum l\.1ij. B. V.
P.O.Box 62, 2501 AN
The Hague, The Netherlands
A. Dugstad
Institutt for Energiteknikk
P.O.Box40
N-2007 Kjeller. Norway
ABSTRACT
C0 2 corrosion rates of csrbon steel obtained from a large number of experiments in a high pressure test
loop with various strictly controlled environments and fl.ow conditions were fitted to a semi-empirical
model equation. Tills model combines a contribution of the flow-independent kinetics of the c.orrosion
reaction with one from flow dependent mass transfer of dissolved C0 2 by means of a resistance model.
The model can be adjusted to reflect the influence of microstructure and composition of the steel.
Keywords: carbon dioxide, corrosion, kinetics, test loop, liquid flow velocity, mass transport, steel
composition, microstructure, modei
INTRODUCTION
The Kjeller Sweet Corrosion-II project was carried out in 1988 a...'ld 1989 at the Institutt For
Energiteknikk (IFE) in Kjeller. Norway. Details oft.his project have been published recently1.
The test loop was made of 80 mm ID duolex stainless steel, 'Mth a volume of about 110 I.
Publication Right
Copyright by NACE International. NACE International has 00€n given first rights of publication of this manuscript. Requests for permission to
publish thi; manuscript in any form, in part or in whole must be made in writing to NACE International, Publica1ions Division, P.O. Box 218340,
Houston. Texas 77218-8340. The material presented and the views expressed in this papBr are solely those of the author(s) and are no!
necessarily endorsed· by the Association. Printed In the U.S.A.
,The composition of the te~:: '.'.-::;uid was kept essentially constant at a predetermined pH iind Fe++
concentration by means of a filter and ion exchangers.
The test specimen consisted of neutron activated steel coupons, made from a selection of carbon steels, or
low alloy pipe or tubing materials. Most of the experiments were performed with steel St-52
(DIN 17100), which is similar to steel ASTM A537 Gr 1. The coupons were mounted in a row in the
middle of the pipe, dividing the latter in two halves. Corrosion rates were determined from the outside of
the pipe by monitoring the decrease in radioactivity. The test Juratiou of mo.st tests were 2-3 days. Most
of the test series comprised measurements at 3.1, 8.5 and 13 mh with one of the following temperatures:
20, 40, 60 or 90 °C. C0 2 pressures ranged from 0.3 to 20 bar.
It should be noted that this was the first time that substantial data became available on the effect of
veiocity on C02 corrosion under well defined turbulent flow conditions, i.e. in a pipe, while the
composition of the environment was constant and not changed by accumulation of corrosion products.
Furthermore, corrosion rates were often higher than those predicted by means of earlier predictive
equation.s2. This prompted further attempts to find a model which can describe the corrosion rate as a
function of flow velocity. Preliminary results of such modelling has been reported earlier. 3
THEORETlCAL BACKGROUND
When mass transfer rates of corrosive species cannot keep pace with the reaction kinetics of the corrosion
reaction, an equilibrium reastion (corrosion) rate vcor will be established which can be approximated as
follows 3 4 :
(I)
where kr and km i:ile ihe:: rate const<1nts associated >vith the reaction kinetics of the corrosion reaction (e.g.
the charge transfer reaction) and with the mass transfer of the dissolved C0 2 from the bulk of the solution
to the surface of the steel, respectively.
Eq.1 can be written as:
1
1
Vcor
Vr
1
Vm
--=-+--
(2)
where vr is the highest possible reaction rate i.e. when mass transfer is infinitely fast.
V m is highest possible mass transfer rate of the corrosive species:
(3)
The mass transfer coefficient km depends on the thickness of the concentration bow1cary layer, which in
tum depends on the flow geometry. For turbulent flow through a pipe 3 :
0.7
km
=
Cm
Dca2
0.5
v
u
0.8
(4)
d0.2
i28/2
where Cm is a constant, Dc0;z is the diffosion coefficient of C02 in water, v is the kinematic viscosity of
water, U is the liquid velocity and dis the characteristic length of the mass transport geometry, which
equals the hydraulic diameter of the pipe in this case. Eq. 3 can be written as:
_
Vm -cm
D~
0.7
o.5
v
0 8
u·
(5)
Q:2HpC02
C1
where H is Henry's constant for the solubility of C02 . V m is almost temperature independent for
temperatures between 20 and 80 °C, because the temperature influences in Dc0:2• v and H cancel out. Tbs
is demonstrated in Figure 1. The units used here are: Hin mole/m3/bar, and Dcai and v in m 2/s. For this
reason the formula used for V m was written as:
Uo.s
(6)
V m = c,,,' ()2 pC02
cl"
where c,,,' is a temperature independent constant to be fitted to the data.
For the charge transfer controlled reaction rate, the following equation was used:
(7)
where T is the absolute temperature and pHco2 is the pH of pure water and C02 .
This equation is very similar to the model e:iuat1on used before2, with the exception ofpHc02 being used
instead ofFeC03 saturation pH, plfsat, which simplifies the calculation and which appeared to be equally
guud or beLt~r for- the fit of th~ v~"i.01.?I:! models.
For the temperature range of 10-80 °C, analysis of the temperature dependence ofC02 solubility and
H 2 C03 dissociation constants showed that the temperature dependence (tin °C) of the pH can be
approximated by
pHc02 = 3.82 +
0.00~84 t
-0.5 log(pCOJ
(8)
This equation was used instead of the empirical one used previously2:
pH{:02 = 3.71+0.00417 t -0.5 log(pC02 )
(9)
because the slightly lower wuues corresponded better with the actual ones found experimentally in the
IFE testloop.
Initially, a term c5 log(Fe) was added, but this was later abandoned as its influence appeared to be weak
and uncertain.
12813
··;
... :·
DATA FITTll\lG APPROACH
In order to arrive at a model equation wbch describes the various trends as clearly as possible, iilltial
modelling WM restricted to data which were likely to be free of complications caused by conditions where
the build-up of protective scale could be expected. The measurements at temperatures of 90 °C can be
expected to be influenced by such scales, and were excluded for this reason.
With cll models tried, disturbine d~viations from predicted corrosion rates were observed with a number
of data points ai high C0 2 pressures, possibly also resulting from scale formation. In order to restrict the
influence of these data, the C0 2 pressure was restricted to <6.5 bar.
For 42 out of 103 test series made by JFE for steel St-52, the corrosion rate at 13 mis was lower than at
8. 5 mis. The reason for this effect is not completely clarified; it has been proposed that at high flowrates
the carbide layer which is formed does not survive the shear stresses 1.
In order to avoid the complications occurring at high flowraies, corrosion rates at 13 mis were excluded
when they were lower than :those at 8.5 mis in the same test.
In a spreadsheet, this could be done by using an appropriate "computed criterion" for selection from the
main database.
For the data fitting various non-linear regression techniques were used. In Excel computer spreadsheets
the "Solver" add-in was used, while to obtain more statistical detail a shareware program "Nonlin" was
-<
used-.
RESULTS
The iiJghest correlation coefficient was obtained from the fit of Equations 6 and 7 to 221 data points
obtained by excluding C02 pressures :?:6.5 bar and temperatures of90 °C, and also corrosion rates at 13
mis when these were lower than at 8.5 mis in the same test series. This fit was arbitrarily chosen for
forther reference, also because it is more conservative than the fit where all points at l3 mis were
;uclu<leu. Fur i:lill> case, Equations 6 and i become ( Vr and Vmin mmly, T in K, pC0 2 in bar, U in mis,
din m):
log(Vr) = 4.93 -
1119
-:r+ 0.58
log(pC02) -0.34 (pHactual - pHco2
)
(lOa)
and
u
0.8
(!Ob)
V m = 2-45 ().2 pC0 2
d
which have to be combined in the resistance model, Eq_2_ Figure 2 demonstrates the quality of the
correlation obtained, with a coefficient of determination, R 2, of0.91. The percentage difference between
observed corrosion rates and those predicted with the model roughly follows a normal distribution, as
shown fr1 Figure 3. The standard deviation for this distribution is about 25%.
Effect of Liquid Flow Velocity
Without the mass trnnsfertenn, a best fit of Yr only to the same data yields a R 2 of only 0.71,
demonstrating the necessity ofinduding mass transfer in the interpretation. In Figure 4 the correlation is
exa.rnined as a function offlowrate . The correlation at 0.1 mis appears to be bad. It should be noted tli.at
12814
the flow is uniikely to be turbulent in this case.
The dependence of the corrosion rate on liquid flow velocity decreases with increasing pH, as
Jemonstrated in Figure 5. This is important for practical situations, where dissolved FeC03 can increase
the pH sinificantly.
Figure 6 gives an impression of the quality of the fit to individual measurements as a function of flow
velocity.
Effect of pH
It appeared that t..lie effect of pH is best modelled by ir1clusion of a term in V r> rather than by taking
proton reduction into account as a second cathodic reaction. In Eq. lOa the effect of pH is expressed in
terms of a pH change w.r.t. pure C0 2 saturated water. It may be noted that this can be written in terms of
rHact (the actual pH) by combining Eq.8 and lOa (tin "C):
log(Vr) = 6.23 -
1119
t+zn + 0.0013 t + 0.41 log(pC0 2)
-0.34 pHact
(11)
suggesting a simple linear relation between corrosion rate and H+ concentration at high mass transport
rates, when Yr is controlling the corrosion rate. At lower flow rates, however, the influence ofVmis such
that the overall dependence on pH will be less, as can be seen in Figure 5.
Ir: practical situations the pH will often be controlled by the presence of dissolved FeC03 • Considerable
supersaturation ofFeC03 can occur6 because of its slow precipitation kinetics, and this causes high pH
values to b~ possible_ The precip!tation rate can be estimated with an equation proposed by Johnson and
Tomson7 :
FeC0 3 prec.rate(mole/s) = area x Kpr= {~[Fe++][ CO 3
]-JK"'-}
(12)
where Ksl' is the solubility product ofFeC03 , 'Kp= is a temperature dependent rate constant, and "area"
is the exposed surface area of steel per unit volume of water. The corrosion rate is then found from Eq. l 0
by inserting the pH where the Fe++ concentration is such that the precipitation rate equals the corrosion
rate. This makes direct comparison with earlier test data difficult when the degree of supersaturation was
not weU. defined.
Effect of Temperature
'With all model equations for the kinetically controlled reaction rate V r it was observed that the
coefficients c 1 and c 2 were highly interdependent. Cross correlation coefficients found were often about
-0.9. Small changes in the database used for the best fit often resulted in a large change in the individual
values of c 1 and c 2, the latter controlling the effect of temperature on corrosion rate. Figure 7 shows the
fit obtained for different temperatures.
·
It should be appreciated that the temperature dependence of the corrosion rate expressed by Eq. lOa is
reduced at lower flow rates, because then the contribution ofEq. l Ob, which is almost temperature
independent, becomes more important_
At high temperatures and C0 2 pressures, iron carbonates or -oxides can be formed, which can lower
corrosion rates appreciably_ Figure 7 demonstrates the overprediction of the corrosion rates at 90 °C.
128/5
In previous work2 , the effect of ::.-::aling was accounted for by a multiplication factor FscaJe, which may be
seen as expressing the degree of coverage by protective scale. For the data shown here, a conservative
factor was found by trial and error:
log
2400
CFscale)= -T- 0.44 log(pC02) --6.7
(13)
with Fscale :S l
which is very similar to the factor proposed before, only the coefficient -0.44 was changed from -0.6. The
effect of application of this factor is shown in Figure 8, and appears to give a significant improvement.
However, the improvement for data v:ith pC0 2 > 6.5 but at other temperatures was still marginal, and
still resulted in overprediction.
Effect of C02 Partial Pressure
The relationship between C0 2 partial pressure and corrosion rate is commonly formulated as:
log (V0Jr) = n log (pC02 ) +constant
(14)
=
where n 0. 7. Although Vr, (Eq. (7), conforms to such a relationship, it is not a priori dear that this is
also the case for V cor' which contains also a contribution from V m in the resistance model. However,
;alculatioru using Eq. I 0 showf'Ai the log-log relation to be well obeyed at constant flew rate, and the
values for n obtained in this way are shown in Figure 9. It appears that n is not really a constant, but
depends on flow rate and temperature. At low flow rates n increases, and can even approach the value 1,
which has also been found experimentally by some Japanese researchers 8 .
STEEL COMPOSITION AND MICROSTRUCTURE
-;-he iniriai i.i1tetpretation of the data was confined to a pipeline steel DIN St-52, containing 0.08 % Cr and
0.18 % C, and having a ferritic/pearlitic structure as expected for a normalised steel. Although the
majority of the measurements were made with this steel, corrosion rate measurements were also carried
out for 15 alternate low alloy steels. They each were exposed at 60 °C, at pH 4, 5 and 6, C02 pressures
slightly above 2 bar, and flow velocities of 3 .1, 8.5 and 13 mis. Data for 0. I mis were not inciuded
because the mode! equations appear to be Jess applicable for these flow rates, as discussed above. A
Cor-Ten A steel and a 3.5 % Ni steel were excluded from the evaluation in an attempt to make the
influence of C and Cr as clear as possible. The remaining 13 steels were tentatively separated in 8
nonnalised steels, and 5 quenched and tempered (Q&T) steels, based on reported metallographic
structure.
The equations obtained above for St-52 gave a complete Jack of correlation for the other steels, and it
came quickly obvious that they cannot be used for other steels without taking the difference in
composition into account. Numerous attempts were made to include this in the model equations Eq. 1-3.
The best results were obtained by separating the steels' into two groups, corresponding to normalised and
Q&T steels.
Normalised steels
The preferred model is described by applying correction factors Fer and Fe to Vcor and Yr, resp. Also, a
baseline formula for Cr%=0 and C% =O is needed for Vr and V m' since Eq.'s 6 and 7 already contain a
12816
.. : .. :.::
:.:;;
.. ··
contribution for the presence of Cr and C in St 52. The dependency on pC0 21 temperature and pH were
maintained, however:
1119
-y+ 0.58
log(Vr) = 4.84
log(pC02 )
-0.3 4 (pHactual - PHc0 2 )
u
(15)
0.8
(16)
Vm=2.8 Q2PC02
d"
Vcor ' = V cor Fer
Fer= 1+(2.3±().4).Cr"/o
(17)
Fe= I+ (4.5±1.9).C%
(18)
Here the influence of Cr acts on both parts ofEq. l, while the effect of the carbon is only on the kinetic
(flow-independC::nt, e.g. charge transfer) part ofEq.1, Vr.
The error levels indicated in Eq.'s 17 and 18 arc Ix standard error. The effect on the correlation observedpredicted corrosion rates is shown in Figure I 0, demonstrating a reasonable correlation, with R2=0.83.
Q&T steels
When it was attempted to find a Fer and F c for this case, they appeared to differ significantly from Eq.'s 17
and 18 above. The value obtained for Fe was Yery near to 1, and statistically there was a 44% chance that
it is not different from I . For this reason it was decided to only use Fer in this case:
1
Veor' = Vcor Fer
Fer= 1+(1.4±0.3).Cr%
(19)
(20)
Fe= 1
with the following baseline formula for C% = 0 and Cr% = 0:
log(Vr)
1119
= 5.07 -y- + 0 58
log(pC02 )
(21)
-0.34 (pHactual - PHc02 )
U0.8
Vm=2.7 ~pC02
d
(22)
The correlation obtained after appiication of this correction is shown in Figure 11. Although the resulting
correlation is not as good as with the normalised steels, this correction also results in a significant
improvement, with R2 = 0.80.
12817
.... ..
.. ·.
DlSCUSSION
Preliminary calculations indicate that the magnitude of the mass transfer rate obtained with V m in Eq. I 0
cannot be explained on the basis of transport ofC02 (Vm too small) or H 2 C03 (Vm too large) only.
However, not all H 2 C03 needs to be transported to the steel from the bulk of the solution~ a major part
can be formed directly from C0 2 present near the surface. This would imply that the corrosion rate is for
the main part controlled by the hydration rate of C02 to H 2 C03 at the steel's surface, and that V m is
increased to reflect this w.r.t. the diffusion ofH2 C03 mol~cules. Another possibility is that V m also
contains a contribution from the mass transfor ofH... ions to the steel's surface. Attempts to fit this
concept into the model resulted in a considerable loss of correlation, however. For the above reasons,
Eq. I 0 should be regarded .l!S a semi-empirical e.quation to describe the effect of flow rate. Comparison of
the initial modelling results"', which are quite similar to the present results for St-52 steels, showed good
correspondan~e with theoretical predi~ons by Nesic and Post1ethwaite9 .
The effegt of carbides on the C02 corrosion rate suggests that lamellar cementite can act as a cathodic
depolariser after initi~ corrosion has increased the area of the carbides by dissolving interspersed ferrite.
This can lead to the formation of a coherent network of carbides on the surface. These carbides have a
different (lower) overvoltage for the reduction ofH+ ions JO, and can be expected to interfere mainly with
the flow-independent part of the corrosion reaction, described by Yr· Another possibility for the increase
in corrosion rate associated with the presence of a cementite layer has been proposed by Crolet c.s. 11 ,
who showed that acidffication of the liquid imprisoned inside this porous layer may explain the obsenied
increase.
Steels with more homogeneously distributed carbides like in tempered martensite and in bainitic structures
are not expected to form such a network because here the carbides show !ess coherence, and the
corrosion rate should be less affected by the carbon content of the steel in this case. The results suggest
that the effect of C in normalised steels only shows up when the flowrate is sufficiently high.
The effect of Cr appears to be compatible •.vith a partial blockage of rhe surfa<..:1::, probabiy by Cr-oxidt:s,
which interferes with the corrosion reaction as a whole. The different effect of Cr observed for normalised
and for Q&T steels may be caused by the formation of more Cr-carbides in the latter, leaving less Cr to
form protective oxides. Since this is a strong function of the tempering treatment applied, the correlation
obtained is in fact surprisingly good, suggesting that these tempering treatments are similar in practice.
Still, correlation with actually measui:-ed dissolved Cr concentrations should even be better, but is not
available at present.
The difference in the baseline formulas, wiLlJ Cr"h=O and C%=0, for both types of steel indicates that
martensitic steels could corrode somewhat faster than nonnalised ones at low C levels (but enough to get
martensite), but the effect is rather uncertain and possibly net statistically significant. Figure 12 gives an
example to demonstrate the differences obtained for the two groups of steels.
It should be appreciated that the model proposed here to account for differences between various low
alloy steels should be regarded as tentative, and only applies to conditions where protective films do not
form. Other model equations may describe the observed eftects equally good. Furthermore, the
development of a carbide network on the surface of a normalised steel is a time-dependent process, and
the proposed model may lose validity for long exposure times. Still, the predicted effects may be useful as
a first estimate for the order of magnitude of the influence to be expected of microstructure and
composition.
12818
.- ·=
;
..
CONCLUSIONS
For conditions where substantial protective films are not formed, the proposed model gives a good
description of experimental C02 corrosion rates and the effect ofliquid velocity. The model can also
serve to understand many of the observations and features of older models.
..
In order to predict corrosion rates in practical situz.tions e.g. multiphase pipelines, the model should
only be used while taki.'lg the effect of dissolved iron and FeC0 3 precipitation kinetics into account. .
"
Carbide films left behind as a result of corrosion can have a significant effect on C02 corrosion rate,
of which the extent can be estimated from steel rnicrostructure and composition.
UST OF SYMBOLS
v
H
d
T
t
~
~
pC0 2
fC0 2
u
Fscale
Fer
Fe
Pl-Ic02
P~ct
corrosion rate, rnm/y
reaction rate, mm/y
mass transfer rate, mm/y
reaction rate constant, s-I
mass transfer coefficient, s·l
diffusion constant ofC02 , rn 2/s
kinematic viscosity of water, m 1 /s
Hen.--y's constant for C0 2 , mole/m31bar
hydraulic diameter, rn
absolute temperature, K
temperature, °C
solubility product ofFeC03 , moI2fkg2
FeC03 precipitation rate, kg 2/m2/mol/s
partial C0 2 pressure, bar
C0 2 fugacity, b!!!"
liquid velocir;, mis
correction factor for presence of protective FcC03 scale
correction factor for presence of chromium in steel
correction factor for carbon content of steel
pH of dissolved C02 in pure water
actual pH in presence of dissolved salts
REFERENCES
l
?.
3
4
5
6
A. Dugstad, L Lunde and K Videm, "Parametric Study of C0 2 corrosion of carbon steel",
NACE CORROSION/94, paper 14.
C. de Waard, U. Lotz and D.E. Milliams, "Predictive Model for C02 Corrosion Engineering in
Wet Natural Gas Pipe.lines", Con-osion 47, 12 (1991) 976.
C. de Waard, U. Lotz, "Prediction ofC02 corrosion of carbon steel", NACE CORROSION/93,
papcr 69.
U Lotz, "Velocity Effects in Flow Induced Corrosion", NACE CORROSION/90, paper 27.
P.H Sherrod, Nonlinear Regression Analysis Program, version 3, 1994.
A Dugstad, "The importance ofFeC03 supersaturation on the C0 2 corrosion rate of carbon
steels", NACE CORROSION/92, paper 14.
121319
7
8
9
I0
lI
M. L. Johnson., M.B. Tomson, "Ferrous carbonate precipitatio11 kineics and its impact on C02
corrosion", NACE CORROSION/91, paper 268.
T. Murnta, E. Sato, R. Matsuhashl, "Factors controlling corrosion of steels in COrsaturated
environments", Advances in C02 corrosion, Vol. l, NACE 1984.
S. Nesic, J. Postlethwaite, "A Predictive Model for C02 corrosion", NACE Canadian Region
West~m Conference, Februari 1994, Calgary.
G. Wranglen, An Introduction to Corrosion and Protection of Metals, Chapman ·and Hall, London
(1973).
I. Crolet, S. Olsen and W Wilhelmsen., "Influence of a layer of undissolved cemetite on the rate of
C0 2 corrosion of carbon steel", NACE CORROSION/94, paper 4.
3.5E-02 - , . - - - - - - - - - - ' - - - - - - - - - - - - - - - - ,
D
0.7
H---
vo.s
3.0E-02
2.SE-02
2. 0 E-02
-+---+--+--_,__-+--+---+---+-__,--<-1----+--+---+--+--+--+---+--+--+---+
0
10
20
30
40
50
60
70
80
90
100
temperature, °C
Figure I.Temperature dependence ofpa..-t ofEq. 5 for mass tr~nsfer r~te of'C0:.:
70
D
60
50
E
E 40
-0
~
2
0
30
'O
Q)
a.
20
10
0
0
10
20
30
40
50
60
70
observed, mmly
Figure 2. Correlation for 221 observations of St-52 corrosion rates with model equation.
128110
VJ
_CJ
0
0
'-
c
([)
~
C1l
::i
E
i3
-100 -80
-60 -40 -20
0
20
40
60
80
100
predicted-observed, %
Figure 3. Distribution of errors in predicted corrosion rates from previous figure. The dotted line
represents a calculated normal cumulative distribution with a standard error of25% and a mean of-4%.
50.0
;
3.1 mis
40.0
0
e: 30.0
"O
Q)
1:5 20.0
:0
([)
Q_ 10.0
0.0
2..0
0. 0 ~-'--=--+---+----+----!
0.0
10.0
20.0
30.0
40.0
6.0
4.0
obsen.ed, mm/y
obsen.ed, mm/y
f
"C'l}
u
'O
<!)
Ci.
700
60.0
~
70.0
8.5 mis
0
50.0
40.0
~
40.0
30.0
-gE
0
30.0
20.0
10.0
0.0
0.0
E
60.0
50.0
>.
0
13 mis
/
/
¥ 20.0
D.
20.0
40.0
60.0
10.0
0.0
0.0
20.0
40.0
60.0
80.0
obsaf\ed, mm/y
obsen.ed, mrrJy
Figure 4. Correlation diagrams for different flowrates for best fit with Eq.10.
128111
---
14.0
1 bar C02, 40 'C
12.0
10.0
8.0
<E
E
6.0
4.0
2.0
Figure 5. Effect offlowrate and pH at 40 °C, 1 bar C02
35
30
25
0
~
E 20
2
~
;::; 15
0
0
i
J
0
.
/
D
pl 1-5.65
1
D
I
I
I
I
2
4
6
8
I
I
10
12
14
16
18
20
velocity, mis
Figure 6. Example of measured corrosion rates (squares) compared to predicted behaviour (lines); 40 °C,
2-5 bar C02, steel St-52.
128/12
::>-.
~
25.0
20
0
-u
~
CL
0
D
40 °C
~ 40.0
,,- 30.0
~ 20.0
0
D
0
50.0
>.
20.0
-r::T 15.0
10.0
'5
·c
0
D
'5
5.0
0.0 ~::._---+--------;
0.0
10.0
20.D
~
0
10.0 D
0 .0 - i = = - - - - - - - - - - j
0.0
20.D
40.0
0.
observed, mmfy
<' ao.o
observed, mm!y
I so ·c
~
!60.Q~DD
<!)
0
:g
0.
90
100.0
80.0
60.0
E
E
40.0
·c
0
40.0
20.0
0.0 + = - - - - + - - - - - 1 - - - - i
0.0
20.0
40.0
60.D
20.0
0.0 ~----+------<
0.0
50.0
100.0
observed, mmly
observed, mm/y
Figure 7. Correlations obtained. with Eq.10 for various temperatures.
~::~1
E 60.0.
!
~-~
~
D
D
~
_/
/
D [
o
__
O
i~~~~u
0.0
~
0.0
20.0
40.0
I
60.0
80.0
observed, mmly
Figure 8. Correlation for 90 °C data, after multiplying with scale factor
128113
0.85
\
\
\
----- -
...
\,\
Q)
0
Q.75
'._"
a_
><
Q)
N
0
------· 20 °C
....
\
c
i:::
-
\.
\
0.8
60 °C
.
. --..:::.
..., ·.
..........:
...........__
0.7
-~.....~
(.)
0.
0.65
06
40 °C
~~~~....,.~~
I
I
·-·················-·········-··
1
·9
7
flowrate, mis
3
11
13
15
Figure 9. Dependence ofpC02 exponent non flowrate and temperature. pH is that ofFeC03 saturated
water, d=0.05 m.
50.0..,.
45.0
..
40.0
z:
.
35.0
E 30.0
E
·a
25.0
"'
~
"t::
., 20.0
'" ...
E. 15.0
Ill
:-
5.0
·iyir
0
II
""'II
~~
"'
"' . .
10.0
0.0
"'
.. . ....
... 11
.a ....
"'"' ...
5
10
I<
.... .
&II
"' "
Iii
Ill
.
"'
"'
15
20
25
30
35
40
45
50
observed m m/y
~igure I 0. Correlation between observed and predicted corrosion rates for normalised steels after
... pplying correction factors for composition.
i28/14
40.0
lii
35.0
~
E 25.0
E
ho!
c:;
~
~ 15.0
"
~,::t~
0
5
10
.
~
..
.
.
.
30.0
/
"
..
30
35
..,""
.. ..
"'ra
25
20
15
40
observed mmJy
Figure 1 I. Correlation between observed and predicted corrosion rates for Q&T steels a.."l:er applying
correction factors for composition.
50°C 1 bar C02 pH6 Gm/s
6.0
8.0
7.0
5.0
4.0
.;::.
~
E
E
E 3.0
E
3.0
2.0
1.0
0.0
'V
co ci
C'! ci
Q&T
Nonnalised
Figure I 2. Exampie of calculated effect of composition oflow aHoy steels on C02 corrosion rates.
128115
·-:·.:
%Cr