Modeling and Reducing of Corrosion Rate in the Oil Sectors

1 Assist. Prof. Dr. Ala'a Abdulrazaq Jassim, Dr. Abdulzahra Khdier, Eng.Mohammed Nabeel

Basrah University, Engineering College, Basrah, Iraq1E-mail: alaahade88@yahoo.com, P.O.Box:1458 Al-Ashaar Mail

Key wards: corrosion rate, weight loss, operation conditionsAbstract

The petroleum industry contains a wide variety of corrosive environments. Most of the corrosion difficulties in refineries are due to inorganic compounds such as water, H2S, CO2, sulfuric acid, and sodium chloride.

Buzurkan site is one of the oil fields lies in south east of Missan governorate (one of the industrial cities in the southern of Iraq). The crude oil is extract from the neighbor oil fields. The crude oil of these fields is characterized with low temperature, high viscosity and high content of brines as compared with the other oil fields, so it is important to remove the brines and gases from the crude oil through the wash tanks.

Corrosion occurrences in the cover of tank due to the liberate of sour gases especially Carbon Dioxide, Hydrogen Sulfide which presence in the crude oil, and due to presence of water vapor, this lead to produce of carbonic acid and sulfur acid, which cause this problem in the tank cover and the transporting gas pipelines.

The aim of the present work is to develop a mathematical model for studying and investigating the influence of operation conditions such as pH, and crude oil temperature, on the corrosion rate. A comparative study between theoretical and experimental results were evaluated and discussed under different operation conditions.

Also, the methods of washing tanks metal protection were examined by using various types of coatings. The experimental results predicted from weight loss method by immerse the specimens in corrosive mediums Crude Oil as fully and partially with and without epoxy coating were analyzed. The theoretical and experimental results for both types of immersing and partial immersing samples approved the proportional relation between the temperature and the corrosion rates.

IntroductionThe word Corrosion stands for material or metal deterioration or surface

damage in an aggressive environment. The overall chemical reaction of reagents with a metal is divided into two largely independent electrochemical reactions:1-Anodic reaction2-Cathodic reaction

Corrosion Rate Measurement MethodsNumerous methods have been evolved for the measurement of different types

of corrosion such as:1-Weight Loss Determination Method2- Electrochemical Methods Corrosion in the Petroleum Industry The petroleum industry contains a wide variety of corrosive environments. Corrosion problems occur in the petroleum industry in at least three general areas: 1) Oil production and gas fields,

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2) Transportation and storage, 3) Refinery operations.

Oil production and gas fields: Oil production and gas fields consume a tremendous amounts of iron and steel pipe, tubing, casings, pumps, valves, and sucker rods. Leaks cause a loss of oil and gas and also permit infiltration of water and silt, thus increasing corrosion damage. Saline water and sulfides are often present in oil and gas wells. Corrosion in wells occurs inside and outside the casing. Surface equipment is subject to atmospheric corrosion. In secondary recovery operations, water is pumped into the well to force up the oil.

Refinery operation Most of the corrosion difficulties in refineries are due to inorganic compounds such as water, H2S, CO2, sulfuric acid, and sodium chloride, and not to the organics themselves. For this reason, the petroleum industry has much in common with the chemical industry.

Description of Buzurkan Site: Buzurkan site is one of the oil fields which lie in south east Missan governorate (Missan governorate is one of the industrial cities in the south of Iraq) about 75 km from the city centre. The crude oil is extracted from the neighboring oil fields. The crude of these fields is characterized with low temperature, high viscosity and high content of brines as compared with the other oil fields such as Basrah governorate oil fields. Therefore, it is important to remove the brines and gases from the crude oil through the washing tanks. The crude is passed through a series of different processes as shown in figure 1. The washing tanks of this field and gas pipes which leave from the upper part of these tanks suffer from corrosion problems, which blight the chock abutments so crackup the tank cover adding to the occurring of corrosion phenomena in tank base so there is a requirement to do. Some of the protection processes must be done for these parts to stop this negative phenomenon to continue so that the system is suffering from precipitation of unknown materials which lead to close the stream partially in the pipes which transport the gases. After extraction of crude oil from the neighboring reservoirs, it flows in pipes to Degassing Stations to free it from gases then the oil trundle line which comes from the degassing station with a pressure of about (1.5 kG/cm2) and a temperature of about (40oC) in summer or (28-30oC) in winter. Then, it is injected with fresh water (3-5Vol.%) and after adding the demulsified chemical, the oil enters the crude oil heater in order to raise the oil temperature due to its high viscosity. The outlet temperature of oil after heating is about (65-70oC) then it flows to enter the Washing Tanks with pressure about (1kG/cm2).The capacity of a washing tank is about 10,000 m3 and it is made from carbon steel. The oil enter the tank through a vertical pipe, the gases leave washing tank to an upper stream through it. This pipe is holding on circular pipe involving holes, the circular pipe level is under the water level to attain the principle of sedimentation. The gas leaves the wash tank with a high percent of CO2/H2S and other gases due to heating through the crude heaters. When the oil reaches the desirable level, it goes out through the pipe, the crude oil leaves the washing tank with (500-1000 mg/l) of salts and B.S.W more than (1Vol.%) at a temperature of (55oC) and a pressure of

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(0.9 kG/cm2). After this stage, the Crude Charge Pumps raises the flow pressure of crude oil to (5KG/cm2) and the temperature is about (53oC) then the oil enters the Electrical Desalts to free it from any other brines at a temperature of (51oC) and a pressure of (4.5 kG/cm2), treated crude should not be more than (30 mg/l) salts content and (0.1Vol. %) water content, then it is sent to the Storage Tanks (28000 m3) in capacity and under (2KG/cm2,45oC).

Figure 1 Shows a Schematic Diagram for Oil Buzurkan FieldHowever, the main corrosion problems are divided into three parts:

1- Corrosion occurrences in the base of tank due to the high concentration of brines and chloride of water which lead to pitting corrosion.2-Corrosion occurrences in the tank cover due to the liberation of sour gases especially Carbon Dioxide, Hydrogen Sulfide which is present in the crude oil, and due to the presence of water vapor; this leads to produce carbonic acid and sulfuric acid, which cause this problem in the tank cover and the transporting gas pipelines.3- Corrosion occurrences in the middle section of the washing tank.

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Theoretical aspectA comprehensive model has been developed for the computation of corrosion rates of carbon steels in the presence of carbon dioxide, hydrogen sulfide, and aqueous brines. The model combines a thermodynamic model that provides realistic speciation of aqueous systems with an electrochemical model for partial cathodic and anodic processes on the metal surfaces. The partial processes take into account the oxidation of iron and reduction of hydrogen ions, water, carbonic acid and hydrogen sulfide. Thus, the objective of this section is to develop a mathematical model that covers the

following items-:1-Utilize a comprehensive thermodynamic model to compute the activities of species that participate in corrosion processes;2-Include the partial cathodic and anodic processes that are responsible for CO2 corrosion according to the mechanisms which are determined in the literature;

Anodic Reaction Description The mechanism of anodic dissolution of iron has been extensively investigated in acidic solutions [1] while several variations of the mechanism have been proposed; the dependence of the dissolution rate on the activity of hydroxyl ions is generally accepted. The mechanism proposed by Bockris et al. [2], is as follows:Fe + OH- FeOH + e- ... (1)

FeOH FeOH+ + e- ... (2)

FeOH+ Fe2+ + OH- ... (3)

The current density for (Fe) dissolution is given by:

... (4)

Where: = 1.5

The exchange current density can be expressed as follows [3]:

...

(5)

According to Smart and Bockris [4], c = 1.6.

The activity of hydroxyl ions is calculated from the following equations as shown below [5]: ... (6)

And:

... (7)

The reversible potential is calculated from the Nernst equation [6] and depends on the concentration of (Fe2+) ions as follows: ... (8)

Cathodic Reactions for CO2 Corrosion

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In CO2 corrosion, cathodic processes may be due to the reduction of four separate species, i.e., H+, H2O, H2CO3 and HCO3

- in acidic solutions, the reduction of H+ is the dominant cathodic reaction:H+ + e- 0.5 H2

... (9)

It is generally accepted that the H+ reduction reaction may proceed under activation or mass transfer control [6]. According to the basic electrochemical kinetics [6], the current density for H+ reduction can be written as follows [7]:

... (10)

The activation current density for proton reduction can be written as follows:

... (11)

Where: αH = 0.5 [2].The potential (E) is calculated from Nernst equation as shown below:

... (12)

So, the exchange current density is given by [7]: ... (13)

In equation (13), the reaction orders with respect to the activities of H+ and H2O have been obtained from the studies of Bockris et al. [2] and Smart and Bockris [4]. The limiting current densities in equation (10) results from diffusion limited transport of protons to the metal surface and can be calculated as [8]:

... (14)

the current density can be expressed as [7]: ... (15)

As for proton reduction, α H = 0.5.

Also, the potential (E) is calculated from Nernst equation as follows:

... (16)

Thus, the exchange current density is given by the following equation [7]: ... (17)

The H2CO3 reduction is under activation or chemical reaction control so,

... (18)

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Where the activation current is given by [7]:

...

(19)

The value of transfer coefficient can be assumed equal to that for H2O reduction. Also, the reversible potential is equal to that for H2O reduction. The exchange current density for H2CO3 reduction can be expressed as [9]:

... (20)

The value of can be obtained if the activity coefficient is known. Following Nesic et al. [9], the limiting current density can be expressed as

follows:

... (21)

The value of DH2CO3 is equal 10-5 cm2/s, KH2CO3 is equal 1.70*10-3 and H2CO3 is

equal 0.039 s-1 respectively [10]. For all partial processes, the generalized Arrhenius

equation can be expressed as follows [11],

... (22)

According to mixed potential theory, the value of corrosion current is equal to the sum of cathodic current densities which is equal the anodic current density. In other meaning: ... (23)

Results and discussion:In this section, the experimental data obtained from the weight loss method are

examined. They are analyzed and discussed for evaluating the corrosion rate under different operating conditions. Also, the theoretical results from the mathematical model are compared with the experimental data. The experimental results predicted from the weight loss method by immersing the specimens in corrosive mediums Crude Oil as fully (case 1) and partially (case 2) with and without epoxy coating. This is to show the effect of changing the operation conditions such as (Crude oil temperature, Water Salinity and Crude oil acidity) on the weight of specimens, which are related with the corrosion rate. The coatings used to reduce the corrosion phenomena are: Red Oxide as Prime Coat and Penguard Top Coat, Hard Top AS (Epoxy Coating No.1), D-99 (Epoxy Coating No.2) and Coal tar Epoxy Mastic 35670 (Epoxy Coating No.3) as Topcoat. However, the symbols A, B, C and D refer to without coating, with coating No.1, 2 and 3 respectively.

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A Comparative Study for the Corrosion Rates Resulting from Theoretical and Experimental Tests for Crude Oil The theoretical results which were predicated from the developed mathematical models have been compared with the experimental data for both cases (1 and 2) type (A) as shown in figures (2) and (3). The results depict the proportional relation between corrosion rate versus crude oil temperature.

Figure (2) The corrosion rates at various temperatures pH=6.5

Figure (3) the corrosion rates at various temperatures , pH=6

Effect of Temperature:- According to equation (22), increases in temperature led to increasing the applied current density for all partial reactions. It must be known that the applied current density has proportional relationship with exchange current density as shown in equations (5), (13), (18), and (20). Tables (1) and (2) can explain these statuses.

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0

0.5

1

1.5

2

2.5

3

3.5

0 20 40 60 80 100

Temperature (C)

Corr

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te (m

m/y

r)

TheoreticalExp. Case 1

Exp. Case 2

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Temperature (C)

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m/y

r)

Theoretical

Exp. Case 1

Exp. Case 2

Table (1) Values of applied current density at different temperatures.

Table (2) Values of exchange current density (ioH) at different

temperatures and different (pH) values.

T(oC)

pH 20 40 60 80

6.5 11.94404*10-9 12.03075*10-9 12.09787*10-9 12.15006*10-9

6.0 21.23985*10-9 21.39405*10-9 21.51339*10-9 21.60620*10-9

5.5 37.77039*10-9 38.04460*10-9 38.2568*10-9 38.42187*10-9

5.0 67.166322*10-9 67.65392*10-9 68.03133*10-9 68.32483*10-9

Also, the effect of temperature can be shown in equations (8), (12) and (16) which shows that the increase in temperature will decreases the reversible potential. Consequently, increasing the corrosion current densities for anodic and cathodic reactions according to the equations (4), (11), (16) and (19) are due to reducing the value of the reversible potential. Tables (3) and (4) illustrate the relation between temperature with the reversible potential and cathodic current density (iH).

Table (3) Values of potential from Nernst equation at different temperature

T(oC)

20 40 60 80

3.162277*10-7 0.061991425 0.036201995 0.010412565 0.01037686410-6 0.091069008 0.06726338 0.043457752 0.019652124

3.162277*10-6 0.120146591 0.098324765 0.076502939 0.05468111410-5 0.149224173 0.12938615 0.109548127 0.089710103

T(oC) i*H * 10-5

20 2.1333611440 2.14884868160 2.16083600380 2.170158142

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[H+]

Table (4) values of the cathodic current density at different temperature

T(oC)

pH 20 40 60 80

6.5 2.01684781*10-9 3.2393276*10-9 4.5493465*10-9 4.9867235*10-9

6.0 2.84001077*10-9 4.7894368*10-9 7.2848463*10-9 10.381723*10-9

5.5 3.26088036*10-9 5.6434212*10-9 8.9952610*10-9 13.482381*10-9

5.0 3.42120753*10-9 5.9806411*10-9 9.7167003*10-9 14.888547*10-9

Case (1), refers to the fully immersed specimens in the crude oil, figures (4),

(5), (6) and (7) show the effect of temperature, within a range between (20-80)oC on the corrosion rates as a function of time for different types A, B, C and D. Figure (4), illustrates that the corrosion rate for type (A) increases up to (0.19511) mm/yr for (300) days, while it increases up to (0.30635), (0.565657) and (0.647545) mm/yr for the same period of time as shown in figures (5), (6) and (7) respectively. For type (B), the experimental results show that the corrosion rates have varied as follows (0.02655), (0.041538), (0.041236) and (0.077329) mm/yr as shown in the previous figures. Also, For type (C), the results obtained show a good resistance for corrosion but when compared with type (B), the results show that type (B) has a better resistance for corrosion than type (C), the corrosion rates for this case have varied as follows (0.050735), (0.060427), (0.066681) and (0.07296) mm/yr for a period of time reaching to (180) days. Also, for the same period of time as for type (C), the corrosion rates for case (D) have varied as follows (0.05412), (0.063691), (0.070032) and (0.076374) mm/yr. For case (2), which refers to the partially immersed specimens in the crude oil, figures (8), (9), (10) and (11) show the effect of temperature, within a range between (20-80)oC on the corrosion rate. However, these figures revealed that when the temperature increases then the corrosion rates have increased. Figure (8), shows that the corrosion rate for type (A) increases up to (0.49767) mm/yr for (300) days, while it increases to (0.589957), (0.80723) and (1.56737) mm/yr for the same period of time as shown in figures (9), (10) and (11) respectively. For type (B), the experimental results show that the corrosion rates varied as follows (0.03358), (0.0388), (0.043877) and (0.103669) mm/yr as shown in the previous figures. For type (C), the results obtained show good resistance for corrosion but when compared with type (B), the results show that type (B) has a good resistant for corrosion than type (C), the corrosion rates for this case have varied as follows (0.041474), (0.071777), (0.080633) and (0.089566) mm/yr for a period of time reaching to (180) days. Also, for the same period of time as for type (C), the corrosion rates for type (D) varied as follows (0.062767), (0.077245), (0.112986) and (0.148772) mm/yr. However, these figures revealed that when the temperature increases, then the corrosion rate has increased.

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The sharp decline after nearly (50) days of testing has occurred due to forming precipitation (scale formation) on the metal surface. After this period, polishing process for this metal surface is carried out to show corrosion continuously. However, the theoretical and experimental results for both types of immersing justified the proportional relation between the temperature and the corrosion rates. Therefore, temperature is considered one of the design variables that show a large influence on the corrosion phenomena. The rate of corrosion of iron and steel in aqueous acid solutions increases, with increasing temperature. Also, the rate constant for the corrosion reactions in neutral solutions increases with increasing temperature range. In general an increasing in temperature range of a corroding system has four main effects:1- The rate of chemical reaction is increased.2- The solubility of gases in solution decreased.3- The solubility of some of the reaction products may change, resulting in different corrosion reaction products.4- Viscosity is decreased, and any thermal differences will results in increased circulation.

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0 50 100 150 200 250 300 350

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A

B

C

D

Figure (4) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 1, T=20oC, pH=6.5]

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Figure (5) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 1, T=40oC, pH=6.5]

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0 50 100 150 200 250 300 350

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r)A

B

C

D

Figure (6) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 1, T=60oC, pH=6.5]

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B

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Figure (7) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 1, T=80oC, pH=6.5]

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B

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Figure (8) The corrosion rates as a function of time for types A, B, C and D (Crude oil) [Case 2, T=20oC, pH=6.5]

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B

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Figure (9) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 2, T=40oC, pH=6.5]

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0 50 100 150 200 250 300 350

Time (day)

Corr

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n Ra

te (m

m/y

r)A

B

C

D

Figure (10) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 2, T=60oC, pH=6.5]

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0 50 100 150 200 250 300 350

Time (day)

Corr

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n Ra

te (m

m/y

r)

A

BC

D

Figure (11) The corrosion rates as a function of time for types A, B, C and D (crude oil) [Case 2, T=80oC, pH=6.5]

Conclusions1- A mathematical model have been used for calculating the corrosion rates. The results show a good agreement between theoretical and experimental results. The experimental results have obtained from weight loss method. The accuracy was more than 75%.2- The effect of operation temperature on the corrosion rate was analyzed. The values of corrosion rates varied as follows (0.49767), (0.589957), (0.815324) and (1.56737) when the temperature varied between (20-80)oC for partial immersing and without coating.3- In all above cases, the experimental results show that when the specimens immersed as partially have corroded more than specimens which were immersed fully.4- Coating No.(2) shows a good resistant against corrosion phenomena all for Buzurkan site operation conditions. Nomenclatures

Symbol Description UnitAs Area of specimens in2

Ad Debye-Huckel Constant. -A Area m2

Awj Atomic weight of elements g/molActivity of Hydroxyl ion, Hydronium ion, Water and

Carbonic acid respectively. -

D Diffusion coefficient of carbonic acid cm2/sE Cell potential V

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Eo Standard cell potential VF Faraday constant (96500) C/equivalentGo Free energy change J/mol

Corrosion current density A/cm2

Exchange current density for Fe dissolution A/cm2

Anodic current density A/cm2

Factor related with exchange current density A/cm2

The activation and limiting current densities for hydrogen evolution

A/cm2

I Ionic strength. -R Gas constant (8.314). J/mol.Ko

T Temperature Ko

TO Reference temperature Ko

Symmetry coefficient. -Chemical reaction constant. s-1

SubscriptsH Hydrogen ion. -

H2O Water molecule. -H2CO3 Carbonic acid. -

References [1] Drazic D.M., Iron and its Electrochemistry in an active state, in B.E. Conway, J. O’M. Bockris, R.E. White, eds., Modern Aspects of Electrochemistry, No.19: Plenum Press, NewYork, NY, p.69-192.[2] Bockris J. O’M., Drazic D., Despic A.R., Electrochem. Acta, v.4, p.325-361 (1961).[3] Shreir L.L., Corrosion (Metal/ Environment Reactions), (1978).[4] N.G. Smart and J. O’M. Bockris, Corrosion, v.48, p.277-280 (1992).[5] An Overview of Data Analysis and Data Entry for OLI Program, OLI Systems Inc., 2006.[6] Vetter K.J., Electrochemical kinetics: Academic Press, New York, NY (1967).[7] Anderko A. and Young R.D., “Simulation of CO2/H2S Corrosion Using Thermodynamic and Electrochemical Models”, OLI Systems Inc., 2004.[8] Nesic S., Key issue related to modeling of internal corrosion of oil and gas pipeline-A review, Corrosion Science, v.49, p.4308-4338 (2007).[9] Nesic, S., J. Postlethwaite, S., Olsen, Corrosion, Vol.52, p.280-294, (1995).[10] www.wikipedia.com[11] Perez N., Electrochemistry and Corrosion Science, Kluwer Academic Publishers, p.75 (2004).

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