Petroleum and fuels are stored in tanks buried underground. Underground storage tanks are mainly made up of steel which is considered more economical. Corrosion is one of the main environmental concerns, and is continuously a critical issue for Petroleum and Chemical industries[1]. Underground storage tanks are susceptible to corrosion at its surface. Corrosion includes rusting, discolouration and tarnishing of the surface[2]. It can be reduced but cannot be stopped as it is a thermodynamically spontaneous reaction[3]. In rusting process, the metal is degraded to form ferric rust, a redbrown compound, which is a sure sign of electrochemical oxidation of the metal. Almost all metals, with the exception of gold and platinum, will corrode in an oxidising environment forming compounds such as oxides, hydrides, and sulphides [4].

The degradation of metals by corrosion is a universal reaction [4]. Metal loss from external corrosion reduces the life of a tank and often results in ruptures. Rupture of underground fuel storage tank due to corrosion can result in fuel leaks and seepage into the ground with devastating ecological consequences[1]. It is important to realise the corrosive attack on a metal and hence, any modification of the surface or its environment can change the rate of corrosion. Clearly, maintenance of underground storage tanks should be in such a manner that corrosion does not result in an unscheduled interruption in the process[5].

The corrosion in storage tanks depends on many factors such as the soil resistivity, pH of soil, the moisture content of soil, etc. Many methods to estimate corrosion rate are available. Weight-loss measurement is currently the most direct and reliable method, but time-consuming and information-limited. Electrochemical measurement, which tests faster with more information, can be used in the field detection combined with other methods, which is a non-destructive, quantitative technique and a powerful tool for researching the corrosion process [6].

In practical way, the corrosion rates are non-zero, hence, the threat of corrosion-induced failures is actually increasing with time. New technologies are developed which enabled to make improvements and repairs before failures actually occur to a possible extent [5].

Paints coatings and cathodic protection are the main means employed to protect steel tanks against corrosion. Provided maintenance is adequate, there should be little concern about corrosion. However, field observation reveals the maintenance procedures that are not always sufficient. Therefore, corrosion remains as one of the dominant factors which leads to storage tanks’ structural failures [7].

A vast majority of these underground fuel storage tanks are made of carbon steel and coated to prevent corrosion and contamination of stored product. But these steels have inadequate alloy additions to be considered as corrosion-resistant. All coatings contain some defects that expose the tank and thus undergo a variety of corrosion failure models in underground environments, including general corrosion, pitting corrosion and stress corrosion cracking [1].

Considering these failures, corrosion models are used. One of the corrosion models is MTCF (Mean Time to Corrosion Failure).For tanks installed for more than 10 years, a corrosion model test was conducted to examine the soil environment around the tank and its relationship with the metal underground storage tanks. A statistical model was used to evaluate the relationship between the aggressiveness of the environment and the rate of corrosion and to predict the remaining life of the underground storage tank prior to corrosion failure. An example of such method is Mean Time to Corrosion Failure (MTCF) [8].

Corrosion in underground storage tanks has become an important issue which needs to be figured out before any economical loss. Hence, the basic objective of the present work is to be set as the investigation of the average life of underground storage tanks using suitable numerical model. As Mean Time to Corrosion Failure (MTCF) model is the proven model for predicting the average life of the underground storage tanks based on soil and tank characteristics, it has been used to determine the average life of underground storage tanks based on corrosion rate, resistivity and other characteristics of tank and soil.

Numerous models are available to stimulate the rate of corrosion of underground storage tanks. In many cases, the actual underground storage tank is not available to measure the thickness of the tank wall directly, and/or it is not feasible to collect values for soil properties. In such cases, informed judgments are used, or a range of probable values is identified and used in corrosion model [9].

In 1979, a statistical method was developed for assessing and predicting the failure of unprotected steel Underground storage tanks are based on soil variables surrounding the tanks [10]. The MTCF (Mean Time to Corrosion Failure) model was originated from a survey of 2000 steel walled, non-cathodically protected underground storage tanks in US and Canada. By the late 1980s, the model had been used at over 22,000 sites in North America and subsequently refined based on observations following additional tank excavations [9].

Based on their study, the tank failure from external corrosion ranged from 5 to 35 years. The corrosion rate was not solely a function of tank age. The MTCF depends upon characteristics of the tank backfill material (e.g., soil moisture content, soil resistivity and sulfide content) along with the tank age. The model assumes that uniform and pitting corrosion are the primary corrosion mechanisms [11]. The MTCF Model is based on four model variables which affect the corrosion rate of underground storage tanks. Those variables are given by,

• chloride ions - causes the resistivity to be lower;
• soil moisture - primarily affects the activity of the chloride ions, if the moisture content is below 17.5%, the chloride ion concentration has a significant effect on the corrosion rate;
• pH - for lower pH (< 7.0) of the soil, the higher the corrosion rate, if the pH is above 7.0, the corrosion rate of the soil yields a longer service life;
• resistivity - follows the chloride ion concentration in that higher resistivity which results in a lower chloride ion content and a lower corrosion rate.

The MTCF for an unprotected carbon steel underground storage tank without cathodic protection is described by equation[1] as given by,

(1)

where,
T_f is the mean age to leak, in years,
R is the soil resistivity in ohm-centimeter,
S is the capacity in gallons of the UST, in Imperial gallons,
M is related to the soil moisture content, where

S_u is a factor related to the sulfide content of the soil,

To study the MTCF model, available experimental data were used[13]. The experimental corrosion data were taken for seven pilot storage tanks, but only six storage tanks are considered. The tanks were fabricated from mild steel. These tanks were coated with corrosion resistant pigment red lead (Pb₃O₄). The tanks were cylindrical in shape with dimensions of diameter: 16.5 cm, height: 42 cm. The environment used for experimental study by them are given in Table 1.

The tanks were removed, properly cleaned, washed, dried and re-weighed after six months. The weight-loss technique [14,15] was used to calculate the corrosion rate as given by,

(2)

The corrosion rate was estimated; where

W = Weight loss(mg)
D = Density of sample (g/cm3)
A = Area of specimen exposed to corrosion (cm2)
T = period of exposure (hours)

The corrosion rate, change in soil resistivity, density and pH for different periods is given in Table 2.

Table 1. Details of Underground storage tanks used for Corrosion study [13]

Table 2. Details of experimental results of underground storage tanks [13]

To determine the average life of underground storage tanks using MTCF model equation 1, the information of corrosion rate, resistivity and other characteristics of tank and soil are necessary as per equation 1. The available experimental data from literature for six different underground storage tanks have been considered as mentioned in Table 1. The average life of these six underground storage tanks has been determined by MTCF model using the experimental data of density, corrosion rate, soil resitivity, and pH after 92, 255, and 419 days.

The average life of the underground storage tanks for six tanks using MTCF model were calculated based on experimental data[13]. The MTCF were calculated by considering the corrosion rate and soil resistivity data for 92 days, 255 days and 419 days respectively. The estimated MTCF are shown in Table 3.

The comparative bar chart for MTCF is shown in Figure 1. It can be observed that there is no significant change in the value of MTCF for the data taken for different time periods. The estimated maximum MTCF is 27.3 years and minimum is 17.4 years. Hence, in worst condition, the life of considered tank would be around 17.4 years and in best condition it would be up to 27.3 years. The predicted results are in well agreement with literature data as the tank failure from external corrosion ranged from 5 to 35 years [11]. Hence the main finding of the present work is the applicability of MTCF model; it can be successfully used for the determination of the average life of the underground storage tanks.

Table 3. Details of estimated MTCF in years

Figure 1. The comparative bar chart for MTCF

Now-a-days, there are many new and advanced techniques for the protection of underground storage tanks. Cathodic protection and effective coating are used to prevent corrosion at the surface of underground storage tanks [16] and either of them is effective [17]. They are known to increase the age of tanks. The different corrosion protection techniques are given in Table 4.

Application of corrosion inhibitors to underground storage tanks acts as an another protective measure. It is known that surface reactions are strongly affected by the presence of foreign molecules according to surface chemistry. Corrosion being surface reaction, can be controlled using inhibitors which adsorb on the reacting metal surface. Adsorption is the attachment of molecules directly to the surface. The technique of adding inhibitors to the environment of a tank is a well known method of controlling corrosion in many branches of technology. A corrosion inhibitor can restrict the rate of the anodic process or the cathodic process by simply blocking active sites on the metal surface[4].

Table 4. The different corrosion protection techniques[4]

Cathodic protection is a very well known method of preventing corrosion on tanks situated in electrolytes. Practically, the tanks constructed of iron or steel (including stainless steel) are provided with cathodic protection where the electrolyte is soil. It has become a standard procedure for Underground storage tanks [18].

The long-term performance of a coating is affected significantly by its adherence ability to the material it is applied, as poor adhesion will allow corrosion products to undercut the coating film from areas of damaging the tank. Coating experts often say that surface preparation is the most important factor in determining the success of any protective coating system as stated by coating experts [19]. Many factors in surface preparation affect the coating which includes residues of oil grease, rust on the surface, etc which can decrease adhesion or mechanical bonding of coating to the surface of the tank [20]. For an organic coating, surface pre-treatment of the steel is most important. The pre-treatment consists of use of primers or coupling agents, solvent decreasing and acid pickling etc, which forms a chemical bridge between the steel tank surface and the organic coating [21].

Corrosion in underground storage tanks is an important issue which need to be known before any loss. In the present paper, an attempt has been made to investigate the average life of underground storage tanks using Mean Time to Corrosion Failure (MTCF) model based on experimental data of corrosion rate, resistivity and other characteristics of tank and soil. 17.4 and 27.3 years are the minimum and maximum value of the average life of underground storage tanks, respectively that have been estimated by MTCF model. In worst condition, the life of considered tank would be around 17.4 years and in best condition, it would be up to 27.3 years for six underground storage tanks considered. Considering the determined average life of the tanks, they can be replaced before any inconvenience due to corrosion failure. The model results are in well agreement with the available data, as the tank failure from external corrosion ranged from 5 to 35 years. Hence, MTCF model can be recommended for the determination of the average life of the underground storage tanks.