Corrosion Prevention of Crude Oil Steel Pipelines In Swampy Soil Using Cathodic Protection Method

Transcription

1 Corrosion Prevention of Crude Oil Steel Pipelines In Swampy Soil Using Cathodic Protection Method 1 Emmanuel J. Ekott, 2 Emmanuel J. Akpabio and 1 Ubong I.Etukudo 1 Department of Chemical Sciences, Heritage Polytechnic, Eket, Nigeria. 2 Department of Chemical & Petroleum Engineering, University of Uyo, Uyo, Nigeria ABSTRACT: A long time exposure of steel pipes underground causes their surface to corrode. The increasing cost of tubular goods failures due to corrosion in the oil and gas industry has increased interest in and importance of corrosion problems and their solutions. Metal dissolution and sudden cracking of tubing and casings are mainly caused by the presence of carbon dioxide, hydrogen sulfide, oxygen, and water. The presence of larger volumes of water in aggressive environments intensifies the severity of corrosion, while oil tends to form a protective oily film on the metal surfaces. Corrosion of buried steel pipeline can adequately be controlled by catholic protection method. In this study steel materials were buried in a swampy soil and their corrosion rates compared after forty-nine days. Cathodic protection impressed current was applied to one of the steel materials. The circuit was connected with a zinc anode, a 12 volt DC power source with the aid of copper wires, and the soil acted as electrolyte. Corrosion rate of the metals were determined using weight loss method, prior to washing and oven drying. After twenty eight days, the weight loss for protected steel was small compared to weight loss for unprotected steel and zinc anode. The protected steel lost a total of 0.0lg after forty nine days. However, the unprotected steel and zinc anode lost a total of 0.17g and 11.04g respectively. This result suggests that the catholic protection was effective. Hence, further finding on the actual voltage range required for effective cathodic protection should be studied. Key Words: Corrosion, cathodic protection, swampy soil, steel pipe, anode, electrolyte. INTRODUCTION Corrosion is possibly the most important and costly cause of problems encountered in oil production systems. Corrosion requires special consideration during the design and fabrication of production equipment and the operation of the process. Corrosion detection, monitoring, and control are paramount considerations when seeking maximum equipment life, minimum cost, and maximum safety. Corrosion can occur anywhere in the production system from well bottom to final transfer of produced gas or oil to the refinery. Cathodic protection involves the application of a direct current from an external source to a metal surface immersed in an electrolyte to oppose the discharge of corrosion current from anodic areas. When such a protection system is installed, all exposed portions of the protected metal surface become a single cathodic area. Two methods are used: sacrifical anodes and impressed current: Sacrificial Anodes: The choice of material used as sacrificial anodes is limited to those that are less noble in the galvanic series than those to be protected. For example, for the protection of steel the materials used as sacrificial anodes are usually aluminum, magnesium and zinc because of the great potential difference between them and steel. Zinc is used in low resistivity soils and water. Aluminium is excellent in saline water and also has a high energy capacity per anode weight. This relates to the rate at which the anode is consumed in use. For example, typically magnesium is consumed at an approximate rate of 17 pounds per ampere per year, zinc at a rate of approximately 26 pounds per ampere per year and aluminium alloy at approximately 7 pounds per ampere per year, for a similar system. Impressed Current: For many systems, the amount of protective current required is too large for a practical size of sacrificial anode. In these situations it is more practical to use a silicon/iron alloy as an anode by connecting it to the positive side of a DC generator, at the same time connecting the negative side to the metal to be protected. In this way, generated currents can be used to make the protected metal cathodic. It is always important to ensure that anodes are properly installed so that minimum electrical resistance exists between anode and the surrounding electrolyte. For example, anodes used to protect structures should be placed in areas of low soil resistance with low resistance material packed around the anode to serve as a backfill. It is also important to minimize stray currents. In general, sacrificial anodes are used where the required amounts of protective current are small and well distributed, such as along a pipeline. They are also limited to soils and waters of low resistivity. On the other hand impressed currents are used to generate much larger currents and require an external power source. Impressed currents are most often used to protect storage tanks. Cathodic ISSN : X Vol. 3 No. 1 Feb-Apr

2 protection is used effectively to provide external protection to oil and gas lines and vessels, but is not effective in the protection of inner surfaces. The Electrical Circuit In addition to a voltage source, there also needs to be a completed electrical circuit consisting of an anode, a cathode, and an electrolyte. The Anode: The anode is the part of the metal surface that corrodes that is, the metal dissolves in the electrolyte. This loss of electrons is called oxidation. The iron ion goes into solution, and the two electrons are left behind in the metal. The Cathode The cathode is that portion of the metal surface that does not dissolve. It is the site where chemical reactions that absorb the electrons generated at the anode. The electrons generated as the iron dissolves at the anode and travel through the metal to the cathodic surface area. There are two primary reactions possible at the cathode, the hydrogen evolution reaction and the oxygen absorption reaction. Most metals are thermodynamically unstable. Their degradation is a universal reaction caused by the fact that the oxide of a metal has lower energy than the metal itself. This property makes refined metals to corrode. Corrosion of metal has been described by Douglas (2000) as the physical and chemical wearing away of a metal by a chemical on electrochemical agent. Corrosion is essentially a reaction between a metal with its environment especially with oxygen, carbonic acid, sulphuric acid or by electrochemical metal ion transport hence, it is usually controlled by inhibitions, resistant alloys, and nonmetallicmaterials on protective coatings (Allen and Raymond, 1990). When severely damaged by corrosion steel pipes lose their strength, ductility and other mechanical properties, hence, they cannot serve the purpose for which they were designed for. Corrosion is a major problem faced by process industries and the society at large. In the process industries, transportation of some commodities takes place in underground steel pipes. The corrosion of such pipelines and other equipment generates hazardous system malfunctioning as well as costly damages and repair costs (Akpabio et al, 2011). Most of the commodities pollute the environments, farmland, aquatic lives and drinking water sources. This also behooves on the process industries to clean up and prevent future occurrence. (Okoroafor, 2004) several countries have accounted loses due to corrosion. In the United States (U.S), the total direct cost of corrosion losses is estimated at 276 billion dollars per year. Similarly, the United Kingdom in2006 estimated that corrosion expenditure amounts to about 4.5% of the turnover of the U.K chemical and petro-chemical industry. Much of this total expenditure represents investment in materials and protection practices to manage corrosion in new equipment. However, a significant proportion arises from failure to identity and mitigates well known corrosion risk at the ISSN : X Vol. 3 No. 1 Feb-Apr

3 design stage, and the U.K study estimated that 15% of the total estimated cost may be saved by the application of existing carrion prevention technology (Richardson and Cottis, 2006). Corrosive attack on a metal starts on the metal surface, hence any modification on the surface of the metal or its environment changes the rate of corrosion reaction. Based on this knowledge, scientists have designed methods such as painting, use of proper alloys, electroplating, and catholic protection (Douglas, 2000). Okah Avae (1996) described catholic protection as the most popular method of protecting underground metallic structures. Ekott, Akpabio and Etukudo (2012) recorded 96% protection in their experiment on cathodic protection of steel buried in soil. Cathodic protection involves using impressed current on the metal to be saved (cathode) so as to suppress the electrochemical reaction on its surface. When properly applied, this can inhibit corrosion. In its simplest form, it is achieved by attaching a sacrificial anode (such as zinc, magnesium or aluminum) to the metal to be protected thereby making the iron or steel the cathode in the cell formed. The sacrificial anode having a more negative electrode potential than the protected cathode, eventually corrodes away, ceasing its protective action unless it is promptly replaced. Figure 1 Schematic of Impressed Current and Sacrificial Anode Systems MATERIALS AND METHOD Swampy soil samples were collected from a swamp and filled in two plastic vessels measuring 15,000cm 3 each. Three metal samples two steel rods measuring 50cm in length and 2.75mm in diameter, and a zinc sheet measuring 5cm by 15cm (75cm 3 ) were collected from a commercial shop in Eket, Nigeria. The impressed current was gotten from a 12 volt DC rectifier and a 12 voltage accumulator while an electronic weighing balance (model: D-46397) was used for all weight measurements. With the aid of a copper wire, one of the steel rods was connected to the negative terminal of the DC source, while the zinc anode was connectedto the positive terminal of the power source. The copper (cathode) and zinc (anode) were buried in the soil and the rectifier connected to the A.C. mains and switched on. See the arrangement in figure 2. After seven days the metals were unearthed and detached from the circuit. They were rinsed with water, over dried for one hour at 100 o C, weighed and the circuit re-installed before buried in the soil then powered. Water was added to the soil sample daily to maintain moist and swampy characteristics; voltage supply from the DC source was monitored to avoid deviation. The Weight losses of the metal samples were noted over a period of forty nine days. ISSN : X Vol. 3 No. 1 Feb-Apr

4 Figure 2 Principle of Cathodic Protection RESULTS AND DISCUSSION Table 1 shows the result for the experiment at the end of 21 days, no weight loss was recorded for the steel with Catholic protection while weight loss was recorded for the un-protected steel. Furthermore, after four weeks (28 days) it was observed that the protected steel began to show very little weight loss. For the duration of the experiment a total weight loss of 0.01g was recorded for the protected steel while the unprotected steel weight lost was 0.17g. Table 1: Weight loss of materials in grams Number of Weight loss for Weight loss for Weight loss for days un-protected protected steel zinc anode g 0.00g 2.16g g 0.00g 3.34g g 0.00g 4.75g g 0.01g 6.15g g 0.01g 7.31g g 0.01g 8.46g g 0.01g 11.04g ISSN : X Vol. 3 No. 1 Feb-Apr

5 Figure 3: A graph of weight loss against number of days Comparing the result of this study with previous study conducted on loamy soil as reported by Ekott, Akpabio, and Etukudo (2012), the weight loss recorded for the zinc anode is higher in the latter than the former. This difference may be as a result of elemental composition or water content in the different soil samples hence, soil analysis is recommended to decipher such marked difference in corrosion rate. Table 2. Standard EMF Table (Adapted from Total Manual) ISSN : X Vol. 3 No. 1 Feb-Apr

6 All of the corrosion problems that occur in oil and gas production systems are due to the presence of water, in either large amounts or just traces. This corrosion process is known as the wet corrosion process and is electrochemical in nature. As stated above, wet corrosion is an electrochemical process. As corrosion occurs, an electrical current passes through the corroding metal. For current to flow, there has to be a voltage source and a completed electrical circuit. Different metals require different amounts of energy when being refined. This in turn gives them differing tendencies to corrode. This energy can be measured and is shown in the Galvanic or Electrochemical series, which is a progressive comparison of the electromotive force (EMF) of each metal when immersed in water. The electromotive force is the voltage required to lose or gain electrons (or to be oxidized/reduced). Potential values of EMF are a function of both the metal and the chemical and physical characteristics of the water. Absolute values also depend upon temperature, velocity, and other factors, but for most purposes, it is sufficient to compare voltages in water under similar conditions. As shown in Table 2, just as metals high up in the table require the most energy to refine, they also are most eager to corrode, whereas the metals at the bottom of the table exhibit least energy to refine and are the least eager to corrode. CONCLUSION By comparing the tabulated results obtained from the experiment, all the metals lost weight but at different rates. The protected steel lost only a small amount of weight compared to the unprotected steel. The zinc anode lost far more amount of weight than the unprotected steel rod. The protected steel was sufficiently protected by the zinc anode and the impressed current voltage applied, hence the effectiveness of cathodic protection for steel (cathode) and zinc (anode) practically, for controlling corrosion of buried steel pipes. Fube done on soil of saline type and the actual value of direct voltage to be applied for cathodic protection should be researched on. This will enhance the life span of buried steel pipes so as to reduce the economic losses. RECOMMENDATION Corrosion is generally combated by a complex system of monitoring, preventative repairs and careful use of materials. Monitoring methods include both off-line checks taken during maintenance and on-line monitoring. Off-line checks measure corrosion after it has occurred, telling the engineer when equipment must be replaced based on the historical information he has collected. This is referred to as preventative management. On-line systems are a more modern development, and are revolutionizing the way corrosion is approached. There are several types of on-line corrosion monitoring technologies such as linear polarization resistance, electrochemical noise and electrical resistance. On-Line monitoring has generally had slow reporting rates in the past (minutes or hours) and been limited by process conditions and sources of error but newer technologies can report rates up to twice per minute with much higher accuracy (referred to as real-time monitoring). This allows process engineers to treat corrosion as another process variable that can be optimized in the system. Immediate responses to process changes allow the control of corrosion mechanisms, so they can be minimized while also maximizing production output. In an ideal situation having on-line corrosion information that is accurate and real-time will allow conditions that cause high corrosion rates to be identified and reduced. This is known as predictive management. REFERENCES [1] Akpabio, E.J., E. J. Ekott and M.E. Akpan (2011) Effective oilfield equipment maintenance via inhibition of microbiologically influenced corrosion. Environmental Research Journal 5(2): [2] Allen R.D. and Raymond J.R. (1990) Relationship of Regional Water Quality to Aquifer Thermal Energy Storage.Environment Problems and Solutions Edited by T. NeyatVeziroglu, Hemisphere Publishing Corporation, New York, USA, pp [3] Douglas, V. (2000): Encyclopedia American United State of America: Grolier Incorporated. Pp [4] Ekott, E. J., E. J. Akpabio and U.I Etukudo (2012) Cathodic Protection of Buried Steel Oil Pipelines in Niger Delta. International Journal of Advances in Engineering, Science and Technology 2 (#3): [5] Okah Avae, B.E. (1996). The Science of Industrial Machinery and Systems Maintenance (2 nd ) Ibadan: Spectrum books limited. [6] Okoroafor, C, (2004) Cathodic Protection as means of saving national asset.j. Corr. Sci. Tech., 1.1 (Special Edition), 1-6. [7] Richardson J.A. and R.A. Cottis (2006).Corrosion in the process Industries, Encyclopedia of Chemical Processing, Edited by Sungyuu Lee, pp [8] Nalco Energy Services (2004) CAPEX College, Oil Field Chemicals Training Manual pp ISSN : X Vol. 3 No. 1 Feb-Apr