Abstract: The purpose of this study, was to investigate the application of sacrificial anode and impressed current cathodic protection techniques as a tool for determining the optimum zinc consumption, protection potential and protection current for steel in saline water at a given environment of temperature, time, pH and solution velocity. Weight loss method was used to determine the amount of zinc consumption, while the electrochemical polarization methods were used to determine the protection current and potential. Box-Wilson experimental design was used to design the set of impressed current experiments. Zinc consumption increase with temperature, time and solution velocity, while it decreases with pH of solution. Protection potential and current were analyzed using mathematical models. These models were estimated by regression analysis methods.
INTRODUCTION
Among the various corrosion control methods available, cathodic protection is a major technique adopted to control the corrosion of steel. Cathodic protection system is aimed to shift the potential of the steel to the least probable range for corrosion (Parthiban et al., 2008). In practice, it is most commonly used to protect ferrous materials and predominantly carbon steel (Lindley and Rudd, 2001). One of the most corrosive environments is the saline water (Zahran and Sedahmed, 1997). Equipment used in these environments, such as desalination industry and cooling systems using seawater as a coolant suffer severely from corrosion owing to the presence of the highly aggressive chloride ions in high concentration. Chloride ions have the ability of destroying oxide films which may protect metals such as steel, stainless steel and copper and its alloys. To overcome this problem two approaches are possible, namely: (1) either use expensive alloys to build desalination equipments; or (2) to use cheaper alloys along with cathodic protection. The use of expensive alloys in building equipment increases the capital costs of the plant. In most of the cases it was found that using low cost alloys along with cathodic protection is more cost effective than using expensive alloys. There most important types of cathodic protection are the Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP), also known as galvanic cathodic protection. In ICCP, direct current (dc) source is connected with its positive terminal to the auxiliary electrode (anode) and its negative terminal to the structure to be protected; in this way, current flows from the electrode through the electrolyte to the structure. While in SACP, the auxiliary anode is composed of a metal more active in the Galvanic Series than the metal to be protected and the impressed source of current can then be omitted. Sacrificial metals used for cathodic protection consist of magnesium-base and aluminum-base alloys and, zinc. Sacrificial anodes serve essentially as sources of portable electrical energy. They are useful particularly when electric power is not readily available, or in situations where it is not convenient or economical to install power lines for the purpose (Shrier, 2000). The saline and acidic are very corrosive environment and the study of corrosion inhibition process is very important (Omotosho et al., 2012; Ekuma et al., 2007, 2008; Alagbe et al., 2006; Bazargan-Lari and Bazargan-Lari, 2009). The present study, was an attempt to apply the above two cathodic protection methods for the corrosion prevention of steel pipe carrying saline water. The effect of temperature, flow rate, pH and time were studied in present work.
MATERIALS AND METHODS
Sacrificial anode system: Experimental work of sacrificial anode system was carried out to determine the consumption rate of zinc in artificial sea water (4% w/v NaCl/distilled water) using weight loss for various conditions of temperature (0-45°C), flow rate (5-900 L h-1), pH (2-12) and time (1-4 h). Working electrode was tube specimen of low carbon steel with dimensions of 13.50 cm length, 2.68 cm inside diameter and 0.31 cm thick. The composition of steel specimen was as follows: wt.%, C, 0.1648; Si, 0.2540; Mn, 0.5101; S, 0.0062; Cr, 0.0253; Ni, 0.0090; Cu, 0.1511, V, 0.0034 and the remainder is Fe. Anode electrode was zinc strip with dimensions of 12.50 cm length, 1.00 cm width and 0.60 cm thick. The composition of zinc specimen was as follows: wt.%, Al, 0.12; Pb, 0.0034; Cu, 0.0017; Cd, 0.0033; Fe, 0.0032; Sn, 0.0023 and the remainder is Zn. The cleaning procedure of low carbon steel tubes (cathode) and zinc strips (anode) before and after each experimental test was done as mentioned everywhere (Yaro et al., 2011a; Khadom et al., 2010). The apparatus shown in Fig. 1 was used to obtain the experimental data. After adjusting the operating conditions, the zinc strip was weighted and fixed at the inlet of the steel tube by rubber stopper and was electrically connected by an insulated copper wire to the steel tube outlet. The zinc strip is extending along the steel tube to ensure uniform current and potential distribution along the tube wall.
| Fig. 1: | Schematic diagram of apparatus used in sacrificial anode test system |
The seawater was pumped from the vessel through the flow meter to measure the desired flow rate, then it flow from the bottom to the top of steel tube to return to the vessel again. After each run the zinc strip was rinsed in distilled water and brush to remove the corrosion products, dried with clean tissue then immersed in the benzene and acetone, dried again and then re-weighted to determine the weight loss. The steel tube is cleaned in similar way.
Impressed current system: For impressed current system, the experimental work was carried out to determine the potential and current density required in cathodic protection using weight loss and polarization techniques for various conditions of temperature (0-45°C), rotating velocity (0-400 rpm) and pH (2-12). Working electrode was cylinder specimen of low carbon steel with dimensions of 1.58 cm length, 2.33 cm outside diameter and 0.18 cm thick having the same above chemical composition. Auxiliary electrode was a rod of high conductivity graphite, 4.50 cm length and 0.8 cm outside diameter. The cathodic potential is determined with respect to Saturated Calomel Electrode (SCE). A lugging capillary bridge leading to the reference electrode is mounted near the center of cathode to within 1 mm from the side of the cathode. The opening of the capillary tube near the cathode is equal to 1 mm in diameter. The rotating shaft was made of brass and Teflon. The steel specimen was mounted between two Teflon spaces fitted with O-ring seals to prevent electrolyte contacting the brass rod. The electrical contact to the cylindrical specimen was made through the rotor and a carbon brush contact. Electrochemical cell: Consists of a spherical flask with six necks, one with large opening located in the middle used to input the shaft which combined with steel specimen and the others with smaller openings located around the middle large opening. One of them was used to input the reference electrode, one was used for thermometer, two were used for auxiliary electrodes and the latter was used for aerated. The capacity of electrochemical cell is 1 L. Apparatus shown in Fig. 2 was used to find polarization curves, protection potential and protection current.
RESULTS AND DISCUSSION
Sacrificial anode system: To investigate the rate of zinc consumption during the cathodic protection of carbon steel pipe carrying 4% NaCl solution, 256 experiments were conducted using the factorial experimental design, each variable was discrete into four levels, such that for temperature (0, 15, 30, 45 °C), flow rate (5, 300, 600, 900 L h-1), pH (2, 5, 8, 12) and time (1, 2, 3, 4 h). For the present system the electrochemical cell responsible for cathodic protection is Zn/NaCl/Fe. The anodic reaction is:
| (1) |
and the cathodic reaction is one of the following reactions:
| (2) |
| (3) |
| (4) |
The cathodic reaction is depending on the nature of seawater but reaction of O2 reduction towards the wall of the carbon steel pipe is assumed predominate.
| Fig. 2: | Schematic diagram of apparatus used in impressed current test system |
| Fig. 3: | Zinc consumption with time for different temperatures at flow rate = 600 L h-1 and pH = 8 |
Time effect: Figure 3 shows the rate of zinc consumption (dissolution) which is instead of corrosion rate of steel, with time at different temperatures, different flow rates and different pH, respectively. The rate of zinc dissolution increases with increasing time and this is a normal case. But this increasing is not equally with time where the dissolution rate in the first hour is more than second hour and so on. The reasons of that are attributed to continuous growth of the corrosion products layer with time which affects the transport of oxygen to the metal surface and the activity of the surface and hence the corrosion rate. Also, the cathodic reactions will result an increase in pH with time either by the removal of hydrogen ions Eq. 2 or by the generation of hydroxyl ions Eq. 2 and 4, both reasons are reduced the corrosion rate of steel and hence the dissolution rate of zinc.
Temperature effect: Figure 4 shows the effect of temperature on the rate of zinc dissolution with time with different flow rates and with different pH’s, respectively. The increase in the rate of zinc dissolution with increasing seawater temperature (particularly from 15 to 30°C) may be explained in terms of the following effects:
| • | A temperature increase usually increases the reaction rate which is the corrosion rate and according to the Freundlich equation (Shrier, 2000): |
| (5) |
The rate constant (k) varying with temperature according to Arrhenius equation:
| (6) |
Equation 5 and 6 indicates that the k is increased with increasing temperature and then the corrosion rate which leads to increasing the rate of zinc dissolutions.
| • | Increasing seawater temperature leads to decreasing seawater viscosity with a consequent increase in oxygen diffusivity according to stokes-Einstein equation (Konsowa and El-Shazly, 2002): |
| (7) |
| Fig. 4: | Zinc consumption with flow rate for different temperatures at time = 4 h and pH = 5 |
As a result of increasing the diffusivity of dissolved oxygen, the rate of mass transfer of dissolved oxygen to the cathode surface increases according to the following equation:
| (8) |
With a consequence increase in the rate of zinc dissolution.
| • | The decreases in seawater viscosity with increasing temperature improve the seawater conductivity with a consequent increase in corrosion current and the rate of corrosion |
| • | On the other hand, increase of temperature reduces the solubility of dissolved oxygen with a subsequent decrease in the rate of oxygen diffusion to the cathode surface and the rate of corrosion |
It seems that within the present range of temperature effects 1, 2 and 3 are predominating.
Flow rate effect: Figure 5 shows the effect of solution flow rate on the zinc dissolution with time, with different temperatures and with different pH’s, respectively. It can be seen that the dissolution rate of zinc increases with increasing the flow rate. This may be attributed to the decrease in the thickness of hydrodynamic boundary layer and diffusion layer across which dissolved oxygen diffuses to the tube wall of steel with consequent increase in the rate of oxygen diffusion which is given by Eq. 8. Then the surface film resistance almost vanishes, oxygen depolarization, the products of corrosion and protective film are continuously swept away and continuous corrosion occurs. The flow rate of seawater may also caused erosion which combined with electrochemical attack (Khadom, 2010).
| Fig. 5: | Zinc consumption with flow rate for different pH’s at time = 4 h and temperature = 45°C |
| Fig. 6: | Zinc consumption with time for different pH’s at temperature 30°C and flow rate = 600 L h-1 |
pH effect: Figure 6 shows the effect of pH on dissolution of zinc with time, with different temperatures and with different flow rates, respectively. It can be seen from this figure that the rate of zinc dissolution increases with decreasing of pH (particularly at range of pH 5 to 2). Within the range of about 5 to 12 the corrosion rate of steel and hence dissolution rate of zinc is slightly dependent of the pH, where it depends almost on how oxygen rapidly reaches to the metal surface. Although it was expected that at very high of pH value (12), the dissolution rate of zinc is much reducing because the steel becomes increasingly passive in present of alkalis and dissolved oxygen, but the nature of electrolyte (sea water) prevents that where chloride ions depassivate iron even at high pH. Within the acidic region (pH<5) the ferrous oxide film (resulting from corrosion) is dissolved, the surface pH falls and steel is more direct contact with environment. The increased rate of reaction (corrosion) is then the sum of both an appreciable rate of hydrogen evolution and oxygen depolarization. These results agree with our previous work (Yaro et al., 2011a).
Impressed current system: Mathematical modeling was used every where to develop a relation among variables (Khadom et al., 2009; Okeniyi et al., 2012). To investigate the protection potential and protection current required for cathodic protection. Statistical and Central composite rotatable design (Box-Wilson) (Ahmed et al., 2009; Ekuma and Idenyi, 2007; Ekuma, 2008) were adopted to design the set of experiments. For the purpose of design the operating range of variables are first specified according to the following:
| • | X1: | Temperature range between 0 to 45°C |
| • | X2: | Rotating velocity range between 0 to 400 rpm |
| • | X3: | pH range between 2 to 12 |
Number of experiments (N) is calculated according to the following equation depending on the number of variables, p, (X1, X2 and X3):
| (9) |
The relationship between the coded variables (Xj, where j = 1, 2, 3) and the corresponding real variables were:
| (10) |
| (11) |
| (12) |
| (13) |
Table 1 shows the coded and real values of the experiments to be conducted.
Protection potential Ep: To find the potential required in cathodic protection, the weight loss technique is adopted. The experimental results of the Corrosion Rate (CR) of steel cylinder specimen at different conditions without impressed current were shown in Table 2. The results show increase in corrosion rate of steel with increasing temperature and rotating velocity while there is a pronounced decrease with pH increase. The reasons of that are similar as mentioned above. Mathematical and statistical analysis are powerful way for representing the dependent and independents variables (Yaro et al., 2011b).
| Table 1: | Coded and real values of experiments according to central composite rotatable design (3-variables) |
| *Center point is repeated 3 times to asses experimental reproducibility | |
| Table 2: | Corrosion rate (CR) experimental results for designed variables expressed as corrosion rates (by weight loss) in absence of applying impressed current |
| *Milligrams per square decimeter per day | |
A regression analysis of the objective function (corrosion rate) as function of temperature, rpm and pH leads to the following equation with 0.956 correlation coefficient:
| (14) |
Equation 14 shows that the Corrosion Rate (CR) increases with increasing temperature (X1), rotating velocity (X2) and with decreasing of pH (X3). X1 has effect about four times of X2, but X3 is very effective on corrosion rate especially in linear term. There is no interaction between any variable with other. The variation in coefficients is due to varying the range of each variable. Fig. 7 shows the observed values versus predicted corrosion rate values. Figure 8 and Table 3 show the relation between the potential and corrosion rate. It can be seen that the Ep is shifted to more negative direction with increasing temperature, velocity and with decreasing of pH. That means the Ep is more negative with increasing of corrosion rate. It can be seen from Table 3 that the Ep is slightly varying with variables except at very low pH region, where Ep is relatively high (in active direction) compared with other variables. Equation 15, with 0.98 correlation coefficient, shows the predicted value of Ep.
| (15) |
Equation 15 shows that the protection potential (Ep) is slightly more negative with increasing temperature (X1) and rotation velocity (X2). X2 has very low effect on Ep. pH (X3) decreases leads to more negative of Ep. There is an interaction only between X1 and X3 with low effect.
| Fig. 7: | Observed versus predicted values of corrosion rate (CR) of cylindrical steel specimen with different conditions |
| Table 3: | Results of protection potentials with different conditions |
Polarization: The characteristic of cathodic polarization curves with variation of temperature, velocity and pH can be illustrated. Figure 8 shows polarization curves provide information about effects of changes in potential as a function of current density. Since the electrolyte is seawater (saltwater), the concentration polarization type is predominant (Jezmar, 2002). From polarization curves, it can be determined the free corrosion potential, Ecorr, limiting current density, iL and initial protection current density, ip1 (Peabody, 2001). Where Ecorr is determined when the potential becomes approximately constant with decreasing current. The limiting current plateau is not well defined, thus the following method will be adopted to find iL values:
| (16) |
where, i1 and i2 are the current associated with E1 and E2. ip1 can be determined by intersect of Ep (which is determined previously) with cathodic polarization curve (Trethewey and Chamberlain, 1996).
| Fig. 8: | Variation of protection potential (Ep) of steel in seawater with corrosion rate at X1 = 9.5°C, X2 = 85 rpm and X3 = 4.11 as pH |
| Fig. 9: | Cathodic polarization curve of carbon steel in seawater at X1 = 9.5°C, X2 = 85 rpm and X3 = 4.11 |
From Fig. 9 and other polarization curves one can see that the Ecorr is more negative with increasing temperature and with decreasing velocity and pH. While iL increases with increasing temperature and velocity and with decreasing pH. The high value of iL means the high corrosion and vice versa. ip1 is also proportional to the corrosion rate where in high corrosive media (high temperature and velocity and low pH), one can see that the ip1 is largely raised in contrast to the Ep which is slightly varying with conditions as mentioned previously. The results of Ecorr, iL and ip1 are summarized in Table 4 for various conditions.
| Table 4: | Results of Ecorr, iL and ip1 which obtained from polarization curves. |
| Table 5: | Initial and steady values of protection current |
| * Run 19 is added to check effect of velocity on stability time | |
Protection current iP: The results obtained from polarization curves for current required for cathodic protection were listed in Table 4. These values were almost unstable with time due to scale formation on the surface of steel that reduces the current consumption. The data shows decrease the cathodic protection current density from an initial value (iP1) to a fairly steady values iP. For high temperature regions, iP is more stable than for low temperature regions. This is because the high temperatures enable to form the scales on the surface greater than the low temperatures. With increasing velocity relatively (0-150 rpm), iP is more stable due to increase the corrosion products with increasing velocity. Further increasing in velocity leads to remove the scales and delay in stability of iP. iP is more stable with reducing the pH from 7 to 4.11 due to increasing the corrosion product with lowering of pH. But with very low value of pH iP becomes less stable due to the dissolution of scale. The results obtained from figures, such as Fig. 10; of protection currents for various conditions are summarized in Table 5.
| Fig. 10: | Variation of protection current density versus time of carbon steel in seawater at X1 = 9.5°C, X2 = 85 rpm and X3 = 4.11 |
CONCLUSION
The study of sacrificial anode cathodic protection of short steel tube using zinc strip extended axially in the pipe revealed that under the present range of conditions, the rate of zinc consumption increases with increasing time, temperature and flow rate and with decreasing of pH. The zinc consumption with very low pH is very high and the cathodic protection becomes unreliable. The study of impressed current cathodic protection of rotating vertical steel cylinder in sea water showed that the protection potential and protection current are highly depend on variable of research.
ACKNOWLEDGMENT
This study, was supported by Baghdad University, Chemical Engineering Department which is gratefully acknowledged.