The corrosion of underground structures has greater impact on our lives than many of us realize since it touches practically every aspect of our society. The buildings that we work in, the bridges and overpasses that we cross, the power that comes into our homes, the water that we drink are designed and built within the constrains imposed by this form of corrosion . The fundamental cause of deterioration of underground structures is soil corrosion. Topical examples of soil corrosion are related to oil, gas, and water pipeline; buried storage tanks (a vast number are used by gas stations); electrical communications cables and conduits; anchoring systems; and well and shaft casings. Such systems are expected to function reliably and continuously over several decades . When a gas or crude oil pipelines failed, there is a high degree of environmental, human and economic consequence .
Corrosion in soil is a complex phenomenon. Several important variables have been identified that have an influence on corrosion rates in soil; these includes particle size, moisture content, degree of aeration, pH, redox potential, resistivity, soluble ionic species (salts) and microbiological activity. The relative important of variables changes for different materials, making a universal guide to corrosion impossible .
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Over the years there have been many reports on the corrosion behaviour of metals in soil. For most part, however, these studies have been concerned with underground structures made from ferrous alloys [3, 4-13]. Much less is known about the corrosion behaviour of nonferrous metals in soils [14, 15]. The current research work is an attempt to give some light on the corrosion behavniour of Cu, Al, Al-Cu and C-Steel in soil solution of jubail industrial city in Saudi Arabia by weight loss and electrochemical measurements. Jubail is the most important industrial city in Saudi Arabia (Fig. 1). It is a complex of petrochemical plants, an iron works and a number of smaller companies, plus a Royal Saudi Naval Base. It held the Middle East's largest and the world's 11th largest petrochemical company, SABIC. Moreover, It is home to the world's largest seawater desalination plant. It provides 50% of the country's drinking water through desalination of the water from the Arabian Gulf .
Cu (99.99%), Al, Al-Cu and C-Steel were used as metallic materials. The Chemical composition of Al, Al-Cu and C-steel are given in Table 1. Specimens of size 10-13 mm in diameter and 40-50 mm in length were cut from the respective metal rods.
Soil was taken from ~1.5m underground in the middle of Jubail industrial city. The extracted soil solution was prepared by mixing soil with de-ionized water until saturation. The resultant soil solution has a chemical composition of 28.5 Ca2+, 2.8 Mg2+, 4.7 Na2+, 4.8 K+, 7.0 Cl-, 118.3 HCO3-, 34.0 NO3-, 105.0 SO42-+ and 0.9 PO43- in mg L-1. The values of pH and resistivity of the above solution are 7.12, 4854.37 ohm cm, respectively.
Sulfate reducing bacteria (SRB) were considered as the major bacterial group involved in microbiologically influenced corrosion (MIC)  and mostly abundant in various types of soil environments. So, the tested soil solution was examined to see if it contain sulfate reducing bacteria (SRB) or not and the result was positive.
Weight loss measurements
The specimens were polished successively with metallographic emery paper of increasing fineness up to 1000 grit, washed with de-ionized water, degreased with acetone, dried with stream of air; and weight prior to the immersion in soil solution. In order to identify the immersed specimens, stamped code numbers were used. Four specimens (Cu, Al, Al-Cu and C-Steel) were removed from each solution after specific immersion periods of 1, 2, 4, 10 and 15 weeks. The corrosion products were removed using the respective pickling solutions . The specimens were then dried, reweighed and the corrosion rate (CR) was calculated in mmy-1. To predict the mathematical model, the data for 15 weeks of immersion were used. The maximum pit depth (P, mm) varied exponentially with the time of exposure (t, year) in accordance with the power-low equation :
which can be written in as:
Where K is the first year pit depth (mm) during time of exposure t (year) and n' is a constant. The first year corrosion rate is an important parameter not only for soil corrosivity determination but also for long-term corrosion forecasting.
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Electrochemical measurements, including open circuit potential (OPC), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), were performed in a three-electrode cell, each metal specimen (Cu, Al, Al-Cu or C-steel) was used as the working electrode, a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.
Prior to the electrochemical measurements, the working electrodes were treated as described in the weight loss measurements; and then immersed in the tested soil solution. The open circuit potential of the electrodes was followed with time over 1.5h interval. After reaching steady state potential the electrode impedance was recorded as Nyquist plots. During EIS measurements, an AC disturbance signal of 10 mV was applied on the electrode at the open circuit potential. The measuring frequency ranged from 0.1 to 30000Hz. The potentiodynamic current-potential curves were carried out by changing linearly the electrode potential from the starting (-1500 mV) potential with respect to SCE towards more positive direction with the required scan rate (1 mV s-1) till the end of the experiment (+1500 mV). A Potentiostat/Galvanostate (ACM Gill AC instrument model 655) was used for the electrochemical measurements. All impedance data were fit to appropriate equivalent circuits using computer program ZSimDemo 3.20.
2.5. Scanning electron microscopy (SEM)
Surface film morphology of the studied materials after prolong immersion (15 weak) in the tested soil solution was studied using SEM technique.
Finally, it must be noted that all measurements were conducted in stagnant soil solutions at ambient temperature in well closed systems.
Experimental Results and Discussion
3.1. WL measurements
3.1.1. Time dependence of corrosion rates
The corrosion rates for Cu, Al, Al-Cu and C-Steel for 15 weeks period are given in Table 2. It can be seen that the corrosion rates depend on the nature of the studied materials. Higher corrosion rates were detected for C-steel and lower ones for Cu. However, in all cases the corrosion rate was tend to decrease with the immersion time except that for Al; as the corrosion rate in the latter case showed limited decrease with time until the fourth weak and then tends to increase with time. Actually, corrosion can be monitored by visual inspection as was shown in Fig. 2. The corrosion was dramatically observed in Al-Cu and C-steel after 1 week of immersion (Fig. 2a), while in Al, after 4 weeks (Fig. 2b). After 15 weeks of immersion all tested materials were attacked appreciably except that of Cu (Fig. 2c).
Practically, all corrosion in soil environments is electrochemical in nature . At the anode, some metal, M, goes into solution as an ion, leaving behind its negatively charged electrons:
In aerated alkaline and neutral soil environments, the cathode reaction can be given as:
The hydroxide ions () will migrate through the soil environment toward the positive metal ions that have left the anode. When positive and negative ions meet they form solid corrosion products that tend to fasten to the metal surface and give some protection by slowing down the anode and cathode reactions (i.e. corrosion rate) . This corrosion layer is not truly passivating and corrosion will continue, although at a reduced rate.
3.1.2. Kinetic study
When modeling the data relative to the corrosion rate, Eq. (2) was used for all the studied materials. Figure 3 shows the obtained log-log plots while Table (3) shows the values for , K and correlation coefficients r2 for each line. The results in Table 3 can be interpreted as follows:
Regarding the exponent n' for the studied materials, it was in the range of 0.52-0.99. The value of n' was close to 0.5 for Cu and Al-Cu indicating a diffusion control mechanism and n' = 0.99 and 0.76 for Al and C-Steel, respectively, indicating a gradual change from diffusion control mechanism to charge transfer control mechanism .
According to K values the increasing order of Jubail soil solution corrosivity on the studied materials can be given as follows:
Cu < Al < Al-Cu < C-Steel
By using K values one can estimate the durability factor which is defined as the ratio between the corrosion rate of C-Steel and corrosion rate of non-ferrous metal. It is an important parameter, which will be of immense help to designers in the selection of durable engineering materials for a particular area . The determined durability values for Cu, Al, Al-Cu and C-steel are 63.04, 19.97, 2.44 and 1, respectively. Obviously the durability data indicate that non-ferrous metals viz. Cu, Al and Al-Cu have better durability factors than that of C-steel. Hence the corrosion susceptibility of the studied materials can be given in the following order:
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Cu < Al < Al-Cu < C-steel
The correlation coefficient r2 for the analysis was 0.90 or greater, indicating an excellent correlation of the data. Additional evidence of the quality of fit is presented in Fig. 4 in which predicted values of pit depth in terms of mm are plotted against the corresponding experimental values for Cu, Al, Al-Cu and C-steel at different immersion periods. Reasonable agreements between experimental and predicted results are observed.
3.2. Electrochemical measurements
3.2.1. OCP measurements
Figure 5 represents the potential-time curves of the studied metals and alloys in the tested soil solution. As observed, the steady state potential depends on the nature of the studied materials. Cu being the most noble while Al being the most active of theses materials. For Cu, C-steel and Al, the obtained open circuit potentials were in the same order as that for standard electrode potentials of the following reactions :
However, Al-Cu alloy shows more noble OCP as compared with Al. This behaviour may be attributed to the intermetallic particles (i.e. Al2Cu). Many electrochemical studies have reported that the Al2Cu phase is noble with respect to the Al matrix [23-25]. For example, Scully et al.  measured the OCP for the bulk Al2Cu phase and pure Al in deaerated containing solutions of pH ranging from 2 to 10. The OCP results showed that the Al2Cu phase has amore positive potential than that of pure Al by as much as 750 mV depending on solution pH.
3.2.2 EIS measurements
Typical Nyquist plots for the studied materials in the tested soil solution at ambient temperature are displayed in Fig. 6. The EIS spectra of all materials exhibited a single capacitive semicircle, However another capacitive semicircle may be made out in the EIS spectra of all materials when the data in the high frequency regions was magnified. One such case has been illustrated in Fig. 7, which is the Nyquist plot for Cu. The presence of two capacitive semicircles indicated two different types of reactions occurring on the metal surface. This has been explained in detailed elsewhere . However, Fig. 6 showed that the corrosion resistance in the tested soil solution depends mainly on the material nature; where larger (uncompleted) capacitive loop was obtained for Cu and the smaller one for C-steel.
In order to supply quantitative support for discussion of the previous experimental EIS results, an appropriate model (ZSimDemo 3.2) for equivalent circuit quantification has been used. Figure 8a represents the best simulation between the measured and the calculated EIS data for Cu corrosion in the tested soil solution in accordance with the circuit Rs(Q1(R1(Q2Rct))), which is shown in Fig. 8b. Similar simulations were obtained for the other materials but are not shown. In this equivalent circuit, Rs represents the soil solution resistance. R1 and Q1 are the resistance and capacitance of the passive layer, respectively. Rct and Q2 are the charge transfer resistance of the corrosion process and the double layer capacitance at the metal surface, respectively. As shown, the classical capacitance was substituted by the constant phase element (Q) so that the phenomena related to heterogeneous surfaces may be taken into account. The constant phase element (Q) is defined as :
where the coefficient A is a combination of properties related to both the surface and the electroactive species. The exponent n has values between -1 and 1. A value of -1 is characteristic of an inductance, a value of 1 corresponds to a capacitor, a value of 0.5 can be assigned to diffusion phenomena.
The impedance parameters Rs, R1, Rct , Rp (Rct+R1) n1, n2, Q1 and Q2 were evaluated by using ZSimDemo 3.2 software and are listed in Table 4. It must be noted that the fitting quality was evaluated by chi-squared () values of 10-3, which are shown in Table 4. As observed there was no significant variation in Rs values except for C-Steel system, as it gives lesser value than that of the other materials. This decrease may be attributed to the slight increase in the ionic concentration of the solution caused by iron dissolution ; especially WL data revealed that C-steel has the highest susceptibility to corrode in the studied soil solution. n2 values show good agreement with n' values obtained kinetically from WL measurements. However, Rp values indicated that the corrosion susceptibility of the studied materials follows an order similar to that obtained previously:
Cu < Al < Al-Cu < C-Steel.
3.2.3. PDP measurements
Figure 9 shows the polarization curves of Cu, Al, Al-Cu and C-steel in the tested soil solution at ambient temperature. The polarization parameters such as corrosion potential (), anodic () and cathodic () Tafel slopes and corrosion current density () of all studied systems were estimated by using Tafel ruler and listed in Table 6. Moreover, the polarization resistance (Rp) can be determined by the following equation :
The calculated Rp values were also involved in Table 5. Analysis of polarization data indicates that:
the recorded values for different materials give similar order to that obtained from OCP measurements,
anodic and cathodic Tafel slopes were larger than 120 mV decade-1; the reason probably lies in the growth and dissolution of surface film on the electrode and the reaction of the subvalent metal ions ,
the variation of values depends mainly on the nature of the investigated materials revealing that Cu has the lowest corrosion rate while C-steel has the highest corrosion rate,
Rp values agree well with the analogous values obtained from EIS measurements.
3.3. SEM analysis
SEM micrographs for Cu, Al, Al-Cu and C-steel specimens after immersion in soil solution for 15 week were shown in Figs. 10-13. The different observations that obtained from these figures were attributed to the metal type and can be summarized as follows:
For Cu specimen (Fig. 10), the surface is nearly intact as the polishing stripes remained perfectly visible on it even after exposure. The most interesting feature is the appearance of few rod shaped bacteria species, perhaps SRB, attached randomly to the metal surface. As observed, there is no biofilm formation on Cu surface after this prolonged immersion period as a biofilm is a structured community of microorganisms encapsulated within a self-developed polymeric matrix and adherent to the surface. It was evidence that Cu has a reputation that no micro-organism colonise it, as copper is poisonous to living organism . But some natural environments may influence bacterial metabolic activities and resistance to toxic agents .
For Al and Al-Cu specimens (Figs. 11 and 12, respectively), the environment attack was very clear especially in the case of Al-Cu alloy (Fig. 12). The dark area represents the inorganic corrosion products while the light area represents the formation of biofilm. In the case of Al, the biofilm was thin with low surface coverage (Fig. 11A) and with high power zoom, a rod-shaped microbial species were detected (Fig. 11B, panel X) which hold together through an exopolymer substance (Fig. 11B, panel Y). While in the case of Al-Cu specimen, a compact biofilm with high surface coverage was obtained and when high power zoom was done, microbial colonies are clearly observed (Fig. 12B).
For C-steel specimen (Fig. 13), a wide spread biofilm surrounded with dark stone-like corrosion products (see Fig. 13A). Moreover, the biofilm was also impregnated by such particles but of smaller size (see Fig. 13B) and when high-power zoom was done, a rod and spherical-shaped microbes within the biofilm are obviously observed (Fig. 13C). Moreover, high power zoom for the inner layer showed hexagonal plates with attached rod-shaped bacteria (Fig. 13D). Recent study for C-steel corrosion in natural sea water over 1 and 2 years immersion period evaluated the appearance of hexagonal plates with attached rod-shaped bacteria which was identified as SRB . While Arzola et al  observed hexagonal plates of ferrous sulfide (FeS) at API X-70 steel surface after 24 h of immersion in a H2S-saturated solution at room temperature. So, the observed hexagonal plates in Fig. 13D can be identified as the biogenic product (FeS) of SRB species.
The previous observations obtained from SEM micrographs emphasized the existence of SRB species (rod-shaped bacteria) in the tested soil solution with activity varies with the metal type. Depending on the obtained surface morphology both corrosion rate (i.e. the extent of corrosion products) and microbial activity for the studied materials can be correlated in the same increasing order as follows:
Cu < Al < Al-Cu < C-Steel
3.4. Corrosion behaviour and soil corresivity
Corrosion in a particular soil environment is often attributed to several soil variables which interact to produce the soilâ€™s corrosivity aggressiveness. The American water works association (AWWA) has developed a complex numerical soil corrosivity scale that is applicable to cast iron alloys. A severity ranking is generated by assigning points for different variables presented in Table 6. So when the soil parameters are evaluated separately, the appropriately points is allocated to each result depending on the extent to which the factor contributes to the corrosivity of soil. The points are then summed, and a final corrosivity index is reported. 10 points or more indicates that the soil tested is corrosive, whereas below 10 suggested that the soil is not corrosive . In the present study the corrosivity of the tested soil solution can be evaluated depending on the following parameters:
Water content: Water constituents the essential electrolyte that supports electrochemical corrosion reactions in water saturated or unsaturated soils. The studied soil environment is classified as saturated soil solution. So, water plays an important role as electrolyte that supports electrochemical reaction and using AWWA C-105 standard, a corrosive index of 2 was obtained
pH: soil pH typically varies between 5 and 8. In this range, pH is generally not considered to be the dominant variable affecting corrosion rates. The pH of the tested soil solution fall in this range (pH=7.12) and using AWWA C-105 standard, a corrosive index of 0 was obtained.
Soil resistivity: Resistivity has historically been used as an indicator of soil corrosivity. Since ionic current flow is associated with soil corrosion reactions, high soil resistivity will usually slow down corrosion reactions. According to resistivity value (4854.37 ohm cm) for the tested soil solution, a corrosive index of 0 (zero) was obtained.
Redox potential (degree of aeration): The redox potential is essentially a measure of the degree of aeration in a soil environment. A high redox potential indicates a high oxygen level . In the present study, the tested soil solution was extracted in naturally aerated de-ionized water, so it was expected to gain a significant level of oxygen (high redox potential, <100) and using AWWA C-105 standard, a corrosive index of 0 (zero) was obtained. However, this oxygen content may decrease with immersion time due to the electrochemical reactions on the metal surface in closed system. Low oxygen content (low redox potential, >0) may provide an indication that conditions are conductive to anaerobic microbiological activity (Figs. 10-13) Using AWWA C-105 standard, a corrosive index of 5 (five) was obtained. With respect to this unstable parameter, the average corrosive index of 2.5 was finally given.
Chlorides: Chlorides ions generally participate in the dissolution reactions of many metals. Furthermore, their presence tends to decrease the soil resistivity. In the present study, low concentration of chloride ions (7.0 mg L-1) was detected for the tested soil solution. This may be attributed to remoteness of the area that the soil sample was collected from it.
Sulfates and/or sulfides: Sulfate ions are generally considered to be more benign in their direct corrosive action toward metallic materials than chlorides. The presence of sulfates also poses a major risk for metallic materials since these ions are nutrients to SRB species that convert these benign ions into highly corrosive sulfides. The tested soil solution contains both sulfate ions (105.0 mg L-1) and SRB species. So, by using AWWA C-105 standard, a corrosive index of 3.5 was obtained.
In view of the evaluated soil solution parameters, the final corrosivity index was summed to be 8. According to AWWA C-105 standard, an index below 10 suggests the soil solution is not corrosive.
3.5. Concluded remarks
Comparative analysis for the corrosion susceptibility of Cu, Al, Al-Cu and C-steel in soil solution of jubail industrial city was evaluated at ambient temperature using weight loss, EIS, PDP and SEM techniques. The following points give the most important remarks that obtained from the present study:
Weight loss measurements illustrated that the corrosion rate (mm y-1) of the studied materials is time dependence.
It was found that the weight loss of the studied materials varied with time according to the power-low equation.
Kinetic data revealed that the corrosion of Cu and Al-Cu are diffusion control while Al and C-steel show gradual change from diffusion control to charge transfer control.
By using K values for the studied materials, it was found that nonferrous materials (Cu, Al and Al-Cu) have better durability factors than C-steel and the following order can be given:
Cu < Al < Al-Cu < C-Steel
Good consistency between the data obtained from EIS and PDP measurements was obtained.
SEM analysis for the studied materials surface after prolonged immersion period revealed that a biogenic corrosion layer was formed on the metal surface with density depends on the metal type.
SEM analysis showed that the existence of SRB species (rod-shaped bacteria) in biofilms was clear cut especially in the case of C-steel as hexagonal plates of ferrous sulfide was obviously detected in the inner corrosion layer.
Soil solution corrosivity was estimated in accordance to AWWA C-105 standard which implies that it is not corrosive.
Good correlation between corrosion rates and the estimated soil solution corrosivity was obtained.