Parer No. 5 128 The NACE !nternatlonal AnnuaK Conference and Conos;on Show

INFLUENCE OF LIQUID FLOW VELOCITY ON C02 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 C02 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 C02 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 00n 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. C02 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 constvith the reaction kinetics of the corrosion reaction (e.g. the charge transfer reaction) and with the mass transfer of the dissolved C02 from the bulk of the solution to the surface of the steel, respectively. Eq.1 can be written as:

1 1 1 --=-+--Vcor Vr 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 :

k = m

0.7 Dca2

Cm 0.5 v

0.8 u d0.2 (4)

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:

0.7 0 8 _ D~ u

Vm -cm o.5 Q:2HpC02 (5) v 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 m2/s. For this reason the formula used for V m was written as:

Uo.s V m = c,,,' ()2 pC02

cl" where c,,,' is a temperature independent constant to be fitted to the data.

(6)

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 2C03 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.

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; ... :

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 C02 pressures, possibly also resulting from scale formation. In order to restrict the influence of these data, the C02 pressure was restricted to

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 C02 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):

1119 log(Vr) = 6.23 - t+zn + 0.0013 t + 0.41 log(pC02) -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 ofV mis 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 :

FeC03 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 c1 and c2 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 c1 and c2, 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 C02 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:

2400 log CF scale)= -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 pC02 > 6.5 but at other temperatures was still marginal, and still resulted in overprediction.

Effect of C02 Partial Pressure The relationship between C02 partial pressure and corrosion rate is commonly formulated as:

log (V 0Jr) = 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 researchers8 .

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 pC021 temperature and pH were maintained, however:

1119 log(Vr) = 4.84 -y- + 0.58 log(pC02 )

-0.34 (pHactual - PHc02 ) (15)

0.8 u

Vm=2.8 Q2PC02 d"

(16)

V cor ' = V cor Fer Fer= 1+(2.3().4).Cr"/o (17)

Fe= I+ (4.51.9).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 observed-predicted 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= (19) 1 +(1.40.3).Cr%

Fe= 1 (20)

with the following baseline formula for C% = 0 and Cr% = 0:

1119 log(Vr) = 5.07 -y- + 0 58 log(pC02 )

-0.34 (pHactual - PHc02 ) (21)

U0.8 Vm=2.7 ~pC02 (22)

d

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 2C03 (Vm too large) only. However, not all H2C03 needs to be transported to the steel from the bulk of the solution~ a major part can be formed directly from C02 present near the surface. This would imply that the corrosion rate is for the main part controlled by the hydration rate of C02 to H2C03 at the steel's surface, and that V m is increased to reflect this w.r.t. the diffusion ofH2C03 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

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 FeC03 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.

v

H d T t

~ ~ pC02 fC02 u Fscale Fer Fe Pl-Ic02

P~ct

UST OF SYMBOLS corrosion rate, rnm/y reaction rate, mm/y mass transfer rate, mm/y reaction rate constant, s-I mass transfer coefficient, sl diffusion constant ofC02 , rn2/s kinematic viscosity of water, m1/s Hen.--y's constant for C02 , mole/m31bar hydraulic diameter, rn absolute temperature, K temperature, C solubility product ofFeC03 , moI2fkg2 FeC03 precipitation rate, kg2/m2/mol/s partial C02 pressure, bar C02 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 A. Dugstad, L Lunde and K Videm, "Parametric Study of C02 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. 3 C. de Waard, U. Lotz, "Prediction ofC02 corrosion of carbon steel", NACE CORROSION/93,

papcr 69. 4 U Lotz, "Velocity Effects in Flow Induced Corrosion", NACE CORROSION/90, paper 27. 5 P.H Sherrod, Nonlinear Regression Analysis Program, version 3, 1994. 6 A Dugstad, "The importance ofFeC03 supersaturation on the C02 corrosion rate of carbon

steels", NACE CORROSION/92, paper 14.

121319

7 M. L. Johnson., M.B. Tomson, "Ferrous carbonate precipitatio11 kineics and its impact on C02 corrosion", NACE CORROSION/91, paper 268.

8 T. Murnta, E. Sato, R. Matsuhashl, "Factors controlling corrosion of steels in COrsaturated environments", Advances in C02 corrosion, Vol. l, NACE 1984.

9 S. Nesic, J. Postlethwaite, "A Predictive Model for C02 corrosion", NACE Canadian Region West~m Conference, Februari 1994, Calgary.

I 0 G. Wranglen, An Introduction to Corrosion and Protection of Metals, Chapman and Hall, London (1973).

l I I. Crolet, S. Olsen and W Wilhelmsen., "Influence of a layer of undissolved cemetite on the rate of C02 corrosion of carbon steel", NACE CORROSION/94, paper 4.

3.5E-02 -,.----------'----------------,

3.0E-02

2.SE-02

D 0.7 H---

vo.s

2. 0 E-02 -+---+--+--_,__-+--+---+---+-__,--

-100 -80 -60 -40 -20 0 20 40 60 80 100 predicted-observed, %

VJ _CJ 0 0 '-

c ([)

~ C1l ::i E i3

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%.

0.0 2..0 4.0 6.0 obsen.ed, mm/y

700 ~ 8.5 mis 60.0 0 f 50.0 0

"C- 40.0 'l}

30.0 u 'O 20.0 10.0 0.0

0.0 20.0 40.0 60.0 obsen.ed, mrrJy

50.0

; 40.0

e: 30.0 "O Q)

1:5 20.0 :0 ([) Q_ 10.0

3.1 mis 0

0. 0 ~-'--=--+---+----+----! 0.0

70.0 >. 60.0 E 50.0 E -g 40.0

~ 30.0 20.0 D.

10.0 0.0

0.0

10.0 20.0 30.0

obsen.ed, mm/y

13 mis 0 /

/

20.0 40.0 60.0 obsaf\ed, mm/y

40.0

80.0

Figure 4. Correlation diagrams for different flowrates for best fit with Eq.10.

128111

35

30

25

~ E 20 2

~ 15 ;::; i 0

J 0

::>-. 25.0 ~ 20.0 -r::T 15.0 -u 10.0 '5 ~ 5.0

20 c

0 D 0

0 D

D 0

0 >. 50.0 40 C ~ 40.0 ,,- 30.0

~ 20.0 '5 ~ 10.0 D

0

CL 0.0 ~::._---+--------; 0. 0 .0 -i==----------j 0.0 10.0

observed, mmfy

0.85

0.8

c Q) i::: Q.75 0 a_

>< Q) N 0.7 0 (.)

I 0. 0.65 I 06

1

\ \. \ ....

\ \ \ ...

\,\ '._" .

... , .. .

--..:::.

- ------ 20 C

----- - 40 C

60 C

..........: . ..........__

-~ ..... ~

~~~~....,.~~ ---

3 7 9 11 13 15 flowrate, mis

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 .. 35.0

z: .. E 30.0

'"

Iii E ...

Ill

a 25.0 &II "'

.... ~ .... .. .. "t:: 20.0 .. ..

"' .,

... 11 I< " E. "'

II 15.0

Ill ""'II .a ....

:- ~~ "' .. 10.0 "' "' "' .. 5.0 "' .. ..

iyir 0.0

"'

0 5 10 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 ..

30.0 ..

~ 25.0 " E E .. ho! "" c:; .., ~ " .. ~ 15.0 ~ .. ~,::t~ . .. "'ra

0 5 10 15 20 25 30 35 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.

50C 1 bar C02 pH6 Gm/s

8.0 7.0

~ E E

3.0 2.0 1.0 0.0

Nonnalised

6.0

5.0

4.0 .;::. E 3.0 E

Q&T

'V co ci

C'! ci

Figure I 2. Exam pie of calculated effect of composition oflow aHoy steels on C02 corrosion rates.

128115

-:.:

%Cr