mechanistic prediction models

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Mechanistic prediction models

Mechanistic Prediction Models Of CO 2 Corrosion

Introduction

In order to assess and predict corrosion in oil and gas production, it is necessary to

define the chemistry contained in the oil and gas environment in line with mechanism of

the corrosion reactions. Sometime, the corrosion reactions on the steel surface attack at

the different ways which may lead to misinterpretation. Many variations of damages

due to corrosion indicate how corrosion mechanism happens in multi chemical

reactions. Corrosion attacking pipeline material forms various model damages such as:

pitting, cracking and scale detachment (spalling). Thus, prediction

corrosion mechanism is a significant importance tool to enhance the understanding of

corrosion behavior. The main concepts of mechanistic models are the interrelation of

chemical reactions or physical changes of state. The corrosion model is developed by

using information about standard state properties of all species of interest with a

formulation for the Gibbs energy. The thermodynamics model is used to predict the

concentration and activities of both ionic and molecular species in the systems.

Factors Influencing CO2 Corrosion

a. pH

Normally, the higher pH, the lower is the corrosion rate. The pH is influenced bychanging the H+ ions concentration, temperature, pressure, and ionic strength. The

effects of the dissolved iron bicarbonate will also increase pH . The increase of pH

causes film thickening and film becoming more dense and protective. Dependency of

pH to the temperature and pressure is expressed in the following expression:

b. Temperature

The temperature influences the conditions of protective iron carbonate layers. At

temperature below 60oC, Hydrogen evolution acts a rate determinating step and

carbonate scale does not form well. Carbonate scale governs corrosion rate at the range

temperature of 60 - 100oC when protective films is formed. Such temperature is called

as scaling temperature which is formulated as:

Tscale= 2400/(6.7+0.6Log(fCO2)) ..(2)

The effect of temperature of the diffusion coefficient is given by:

c. The effect Of CO2 Partial Pressure

In the case of no protective film, an increase in CO2 partial pressure, corrosion rate will

increase. The higher the partial pressure of CO2, the higher is CO32- ions concentrationwill have higher supersaturation (at the high pH) results in increasing of corrosion rate.


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In the case of non-ideal gas, the total pressure must be corrected with CO2 fugacity

which is formulated as:

CO2=a.pCO2. (4)

Where a is the fugacity coefficient

d Effects Of Flow

Flow induced corrosion is due to combination effects of mechanical and

electrochemical forces. The flow results in thinner boundary layer which allows

dissolved oxygen in water to corrode the surface more quickly. Higher flow will also

increase the wall stress that can cause localized corrosion and surface damage.

e Effects Of Acetic Acid (HAc)

The presence of HAc can change the mechanism of the anodic dissolution of iron

through competitive adsorption of acetate ions, CH3COO- (or Ac-) and HCO-3. The

fact that carbonic acid (H2CO3) is not fully dissociated in the solution, it provides a

reservoir of H+ ions which contribute the additional cathodic reactions. The increase of

corrosion rates is also attributed to the forming of thinner iron carbonate films since the

formation of iron acetate.

d. Effects Of Fe2+ Concentration

The effects of Fe2+ ions on corrosion rate are influenced by their ability to form iron

carbonate. It has been commonly accepted that solid iron carbonate scale precipitates onthe steel surface when the concentrations of Fe2+ and CO3 2- ions in the CO2 water

solution exceed the solubility limit. Protective scale will not form when scaling

tendency very low although Fe2+ have achieved a saturation value. In this condition,

the iron carbonate film is porous and non protective which does not have effects in

reducing corrosion rate.

Corrosion Rate Calculation

The mathematical model of corrosion occurring at the metal surface can be expressed in

the several equations from Equation 5 to Equation 8. These corrosion mechanisms are

based on several assumptions which can be described as follows: convective diffusion(Eq. 5), molecular diffusion (Eq. 6) and diffusion via solid film (Eq. 7). Corrosion

mechanism which happens in solutions as a combination of mix gases can be expressed

as:

Convective diffusion reactions through the mass transfer boundary layer.

Molecular diffusion through the liquid in the porous outer scale:

There is always a very this film and dense film at the metal surface acting as a surface

barrier. Using an assumption that outward diffusion of Fe2+ is neglected and the filmcontinuously grows through a cyclic process, the outer scale of film is controlled by


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mass transfer reaction. Then, solid state diffusion through the inner film can be

formulated as:

By eliminating the unknown interfacial concentration co and ci, from equations, the

following equation is obtained for the corrosion rate of steel due to mixed species will

become:

In case when inner solid film does not form, Equation 7 can be neglected. Thus,

Equation 8 will become:

Where:

Km is mass transfer coefficient of species I (m/s)

is bulk concentration of species i (mol/m3)

is the interfacial concentration of species i at outer scale/solution interface (mol/m3)

is diffusion coefficient for dissolved species i (m2/s)

is outer scale porosity

is tortuosity factor

is interfacial concentration of species i

is the thickness of outer film sacle

A is Arhrhenius constants

Tk is temperature (Kelvin)

cs is surface temperature

The total corrosion rate equals to the sum of the corrosion caused by each species.

Based on the description of the CO2 corrosion process, a schematic of the CO2

corrosion process is shown in Figure 1 below.

Scale Properties

The corrosion process happens via a direct heterogeneous solid state reaction at the steel

surface which acts as a solid state diffusion barrier. The amount of scale retained on the

metal surface depend on time, hydrodynamic stresses, chemical reaction, precipitation

rate, change of mass scale removal of the outer scale. Empirically, competition growth

between scale formation and scale damage is formulated:

This equation is used to calculate the changes of the scale, thus the change in mass of

the outer scale can be calculated.

Experiment Setup


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Electrodes

A three-electrode set-up was used in all electrochemical experiments. A rotating

cylinder electrode with a speed control unit was used as the working electrode. A

platinum or graphite was used as a counter electrode. Glass cell was fitted with graphite

electrodes as auxiliaries electrode and a Ag/AgCl as a reference electrode.

Specimen preparation

The working electrodes were carbon steels which have chemical compositions as can be

seen in Table 1. It was used a cylinder rod of 1.2 cm2 in diameter and 1 cm thickness.

Before immersion, the specimen surfaces were polished successively with 240, 400 and

600 grit SiC paper, rinsed with alcohol, and degreased using acetone. Some of the

experiments were repeated in order to obtain the reproducible results.

Cell Solutions

The schematic of the experimental setup are shown in Figure 2. The experiments were

performed both in flow and stagnant solutions condition. The total pressure was 1 bar,

the glass cell was filled with 1 liter of distilled water and 3% wt NaCl which was stirred

with magnetic stirrer. Then, CO2 gas was bubbled through the cell (at least one hour

prior to experiments) in order to saturate and de-aerate the solution. Temperature was

set using a hot plate. After the solution was prepared, the pH was measured to reach the

pH set by using NaHCO3 as buffer solutions.

Simulation of flow condition test was conducted using rotating cylinder electrode

(RCE). A cylindrical working electrode was screwed onto an electrode holder at thecenter of the cell for rotating in the RCE. The Linear Polarization Resistance (LPR)

technique was used to measure the polarization resistance, Rp, and the corrosion rate.

The procedure is similar to that described in ASTM G 5-94 - Standard Reference Test

Method for Making Potentiostatic and Potentiodynamic Polarization Measurements.

Results And Discussions

Effects Of Temperature And pH On Corrosion Rate

The effects of increasing temperature on the corrosion rate in solutions containing 3%

sodium chloride and different pH is shown in Figure 3. The model prediction was

assumed that there was no inner solid film formation. The barrier film was due to only

outer porous film. The results were shown that both of the experimental and predictions

results presented an increased of the corrosion rate. Comparison of those two results,

there were clearly a good agreement. The effect of temperature increased corrosion rate

fast at the pH 3.8. While, at the pH 6, the corrosion rate increased slowly. The corrosion

rate was not reduced significantly, at pH 6, which reflects a relatively porous, detached

and un-protective of scale. It may relate to the fast formation of the scale. All the results

were also revealed that at the range temperature of 50oC to 60oC, corrosion rate started

to decrease that associated to the scaling temperature. This showed that electrochemical

reactions and mass transfer also increased with temperature.


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Effects Of Acetic Acid And Rotation Rate On Corrosion Rate

The mechanistic calculation of effect of acetic acid concentration on the corrosion rate

in solutions containing 3% sodium chloride and CO2 gas is shown in Figure 4.a. It was

an evident that the corrosion rate calculated using mechanistic corrosion formula was

not in good agreement with the experiments. The data from the calculation showed thatwhen HAc was present, corrosion rate was consistently increased as HAc was increased.

But, from the experiments results, effects of HAc in increasing corrosion rate happen

only until concentration less than 200 ppm. At the HAc concentration more than 200

ppm, the corrosion rate was decreased.

The effect of rotating rate in 3% sodium chloride solutions at pH of 3.8 and at 22C is

shown in Figure 4.b. An increase in rotating rate up to 6000 rpm results an increase of

corrosion rate. These data were obtained both from experiments and calculation results.

Effects of flow in changing corrosion rate are caused by the changes of mass transfer

boundary layer that have a prime role in corrosion behavior.

Conclusion

The mechanistic prediction models of CO2 corrosion have shown a satisfied model in

calculating corrosion rate, predicting corrosion mechanism and simulating the growth of

corrosion products. Using this prediction, the parameters influencing corrosion such as:

rate of controlling step corrosion reaction, rate of ions dissolution in the solution can

easily be observed and analyzing. Through studying CO2 corrosion mechanism, it can

be concluded that CO2 corrosion produce a very thin films (1-5 m). The film

properties such as: porosity, contour surface appearance and quality of the film change

with time and influenced by solution compositions. The decrease of corrosion rate in thehigher of pH is controlled by the film formation. The other factors affecting corrosion

rate are the scale formation rate and the scale damage rate. The scale formation rate

includes both the corrosion rate and precipitation rate have also role in corrosion

mechanisms.

Acknowledgments

The authors are thankful to Universiti Teknologi PETRONAS for providing grant and

facilities for the research.

References

[1] L. G. S. Gray, B. G. Anderson, M. J. Danysh and P. R. Tremaine, Effect of pH and

Temperature on the Mechanism of Carbon Steel Corrosion by Aqueous Carbon

Dioxide, Nace Corrosion Annual Conference, paper 1999, paper no. 40,

[2] Yuhua. Sun, Tao Hong, CO2,Corrosion in Wet Gas Pipelines at Elevated

Temperature, Corrosion, 2002, paper 2281

[3] Fritz.J., Dolores, Film Formation and CO2 corrosion in the presence of Acetic Acid,

Master Theses, Ohio University, 2004


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[4] Keith George And Srdjan Nesic Electrochemical Investigation And Modeling of

Carbon Dioxide Corrosion of Carbon Steel In The Presence of Acetic Acid, Nace

Corrosion Annual Conference, 2004, paper 4379

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[8] Wei Sun, Kinetics of Iron Carbonate and Iron Sulfide Scale Formation in C

CO2/H2S Corrosion, PhD Dissertation, Ohio University, November, 2006

[9] J.E.Oddo, M.B. Tomsom, Simplified Calculation of CaCO3 Saturation at High

Temperature and Pressures in Brine Solutions, SPE of AIME, 1982, paper 1583.

[10] Ismail. C. Mokhtar., Predictiong CO2 Corrosion With The Presence of Acetic

Acid, PhD theses, UMIST, 2005

[11] Hedges, Bill and Lorraine McVeigh, The Role of Acetate in CO2 Corrosion: The

Double Whammy, Proceedings of the NACE Annual Corrosion Conference, 1999.

paper no. 12.

[12] S. Nesic, J. Postlethwaite, S. Olsen, An Electrochemical Model for Prediction of

Corrosion of Mild Steel in Aqueous Carbon Dioxide Solutions, Corrosion, Vol 52, No

4, April 1996, p 280.

[13] ASTM G 5-94 - Standard Reference Test Method for Making Potentiostatic and

Potentiodynamic Polarization Measurements

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