Corrosion of carbon steel in formic acid as an organic pollutant under the influence of concentration cell

The presence of contaminants in water even in small amounts can cause considerable corrosion damages of metals. This is due to free corrosion effect or the formation of concentration cell of pollutants resulting in a galvanic effect. The current work was devoted to study the effect of formic acid (CH2O2) as an organic pollutant on the corrosion rate of carbon steel under different operating conditions. It includes an investigation of galvanic corrosion caused by the establishment of concentration cell of formic acid under different operating conditions. The ranges of operating parameters were formic acid concentration of 10 10 M and temperature of 32 50 °C. The results showed that increasing formic acid concentration to 10 M leads to an increase in the corrosion rate by up to 7.6 times that in the water of 0.1N NaCl. In addition, the corrosion rate in each terminal in concentration cell also increased by up to 2.3 times. Pumping of air in formic acid solution led to a considerable increase in the corrosion rates and enhances the concentration cell effect which increases the galvanic currents. High increase of corrosion rate was noticed by pumping the air at high temperature reaching up to 4 times depending on temperature. In general, the galvanic currents were high initially and decreased with time due to the formation of corrosion product layer. The increase in temperature from 25 to 50 C caused an increase in the galvanic corrosion rate reached up to 2 times in formic acid solution. In addition, the galvanic currents were noticed to decrease with temperature while the corrosion rate of each terminal was increased.

amounts of acid rain becomes larger than buffer capacity [2]. The effect of corrosion on structures due to the presence of acid in most areas is mainly local nature. However, acidification of soil and water may cause to increase corrosion of buried structures, including water pipes. Besides, there is a common problem in the first place, where the long-range transport of existed pollutants in air have a considerable influence. The metal corrosion due to atmosphere resulting from acid deposition is often a local problem appears in areas close to the source of contamination. The main reason of this type of corrosion is the dry deposition of air pollutants. The elevated gases from industrial plants dissolve in the rain to form acid rain. The corrosion effect of acid rain is depending on the materials and on the amounts of contamination [2]. Galvanic corrosion is one of the most common types of corrosion, resulting from two metals connected by a conductor in a corrosive medium.
Galvanic corrosion occurs also when a certain metal is exposed to a concentration difference of some specific corrosive species what so called concentration cells [1]. The rates of galvanic corrosion and the potential difference over a galvanic couple terminals are dependent on the electrochemical properties of the metals, and environmental variables such as oxygen content, temperature, salts concentration, solution properties, and flow rate.
In addition to that the corroding system geometry, a larger flexibility in the material selection may be possible if dissimilar materials can be coupled without significant damage [3]. The presence of some kinds of pollutants in the water or liquids with which an industrial equipment deals can cause an appreciable corrosion rate which is influenced by process conditions such as temperature and flow rate. In this context, different pollutants No.27-(6) 2020 Journal of Petroleum Research & Studies (JPRS) E78 are possible to be present in water. Formic acid is a weak organic acid and slow to react causing a considerable corrosion damage for a long term of exposure. However, it is more difficult to control formic acid corrosion at the elevated temperatures [4]. Oxygen plays an important key in corrosion process but it is not usually presented in the dissolved liquids.
At the drilling stage , oxygenated liquids are injected for the first time. The drilling mud may cause corrosion of the casing of wells, drilling equipment, pipelines, and the equipment of mud handling. The aim of present work is to investigate the effect of formic acid as an organic pollutant on the corrosion rate of carbon steel especially when establishing the concentration cell of formic acid on carbon steel metal under different process conditions.

Experimental Work:
Corrosion of carbon steel was investigated in a formic acid containing solution. The chemical composition of the carbon steel under study as shown in Table 1. The experimental work presented here involved three parts. Firstly, the corrosion of carbon steel specimen was determined as a free corrosion test in formic acid of different concentrations that were: 10 -4 , 10 -5 , and 10 -7 M. Secondly, concentration cell experiments were carried out by connecting different concentrations of formic acid with 0.1N NaCl solution to determine the corrosion rates in this case. Therefore, in concentration cell experiments the electrolytes were two separated solutions on contains polluted solution with formic acid CH 2 O 2 concentration and the other was 0.1N NaCl solution. Thirdly the formic acid solution is aerated by pumping air at different temperatures of 32 o C and 50 °C. The salt used was pure NaCl. The formic acid used was purchased from the local market with a molecular weight of 46.03 g/g mole and density of 1.22 g/cm 3 . The distilled water used in experiments with a conductivity of 6.63 μS, pH of 6.86, oxygen solubility of 6.08 ppm at laboratory temperature 27 °C . Ethanol was used to clean the specimens. It was supplied by FLUKA with a assay of 99.9 %. The specimens were cut into coupons of dimensions 40 × 40 mm. One side of the specimen was exposed to the solution while other was completely insulated, thus the total surface area of 1600 mm 2 . The specimens were holed from the center by fine screw for the purpose of holding it in the solution. The area of hole was very small and negligible compared to the total exposed area of the specimen. The Then, they were washed with brushing using plastic brush in running tap water to remove part of the corrosion products. After that it is washed by distilled water, dried using clean tissue, followed by immersion in ethanol for 30 s. Then using electrical oven at about 80 ºC for 3 minute [5,6] The specimens were stored in a desiccator over high activity silica gel until use. The specimens were weighted by accurate balance to obtain their weights before corrosion test. After the solution reached the required temperature, the two specimens were electrically connected by a wire to measure galvanic currents variation with time by using Zero Resistance Ammeter (ZRA) where one specimen was connected to the (+ve) and the other to the (-ve) terminal. The coupon was mounted by connecting it on holding board using a fine screw. The effect of the screw was ignored. During each experimental run, galvanic potential variation with time was measurement by using Standard Calomel Electrode SCE bridged. The galvanic current and galvanic potential of the couple specimens were measured for 2 hours immersion time in the solution. After each test, the specimens were weighted by highly sensitive balance of accuracy 0.1 mg (SAUTER type).
The corrosion rate for two similar metals calculated in gmd by: Where ∆W the weight loss in gram, A is the area in m 2 , and t is the time in day.  Figure (2) that the beginning of the run a high corrosion potential is found decayed quickly with time. This potential decay is due to the oxide film formation on the metal surface and due to the reduction in surface activity. After the immersion this film undergoes reductive dissolution and the corrosion potential decreases with time reaching the asymptotic value [7]. It can also be seen that the corrosion potential shifts to more negative values with increasing temperature. This is due to the escape of O 2 from the solution as shown in Table (2). This is in agreement with [8]. In Figure (3) at 25 °C and the OCP exhibits a potential shift to more negative. The reduction in potential over time is due to continuous corrosion [9]. The initial reduction of OCP is because a partial dissolution of the formed film followed by the posterior passivation film growth [10,11]. The anodic iron dissolution in formic acid solution produces hydrated ferrous ions. The metallic iron dissolves in the form of hydrated ferric ions which means that the passive film is mainly ferric oxide, Fe 2 O 3 as in the following equations [12]:

No.27-(6) 2020 Journal of Petroleum Research & Studies (JPRS)
E81 These two reactions occur simultaneously causing the passive layer at a constant thickness, which increases with the increase in the anodic potential. Table 3      promote the reaction kinetic on hydrogen ion reduction on the metal surface the factor that enhances the corrosion [5,16]. In addition, Table 3 indicates the rate of corrosion increases with increasing in acid concentration. The study of Osarolube et al. [17] stated that the

Effect of Temperature
Figures (6 and 7) show the effect of temperature on potential of CS couple in 0.1N NaCl and 10 -5 M CH 2 O 2 respectively. The temperature also has an effect on the solubility of air in the water. Generally, one can see a trend that the high temperatures have a lower redox potential. This seems reasonable since the increased temperature increases the kinetic energy of the oxygen molecules which increases the possibility for them to leave the solution. This is in agreement with previous works [18,24]. From Figure 6, it is evident that the potential at T= 40 o C and 50 °C after 5 and 10 minutes respectively is shifting towards positive values and became higher than at T=32 °C. Then it becomes lower than T=32 °C after 90 and 45 minutes respectively. While, , it can be seen from Figure (7)   The maximum value is apparently dependent on the temperature. It can be seen that at highest temperature (50 °C ) the current is lowest. This is attributed to the low oxygen solubility [14]. At the beginning, the metal surface is active which causes higher corrosion rate. As the process time proceeds, a corrosion product layer starts to form on the surface which leads to a decrease in the activity of the metal surfaces and restrains the diffusion of oxygen to the surface and thus the galvanic corrosion decreases. The higher the temperature and the higher the galvanic current are needed, but this difference decreases with time.  Equations (6) to (10) are the anodic and cathodic reactions occurring on both specimens:

Effect of Pollutants Concentration
And in the acid side: 2H + + 2e − → H 2 (hydrogen evolution) 74.233 and it decreases to 37.1 in 10 -5 M CH 2 O 2 at same temperature) due to the increase in evolution of hydrogen reaction which causes a greater metal surface dissolution [19]. The temperature increase of acidic solutions influences the corrosion rate of materials in several ways: (i) it increases the rate of electrochemical reaction. As the system in current work is under kinetic (activation) control, the increase in temperature is very influential [20], (ii) The temperature increases causes an increase in the solubility of the reaction products which may results in different corrosion reactions (iii) viscosity decreases which causes an increase in the oxygen diffusivity [21,22].   Table 4 for concentration cell coupling with Table 3 for free corrosion indicates that: These trends of corrosion rate agree with the concept of mixed potential theory. In addition, Table 4 indicates that the corrosion rate of both terminals is increased with the temperature.

Conclusions:
The following conclusions are drawn for the investigated ranges of operating conditions in the current work: solution side. When a metal exposed to 0.1N NaCl solution becomes in concentration cell with formic acid, its corrosion rate increases considerably. The percent increase depends on coupled acid concentrations reaching up to 2.3 times when the concentration of formic acid is 10 4 M. In addition, high potential difference is established between the two terminals causing high galvanic current.
3. When air is bubbled in formic acid solution, the O 2 concentration increases causing an appreciable increase in the CR ranging from 2 to 4 times depending on temperature.
While in salt solution the CR increases by about 5 % at 32 °C and more than 2.5 times at 50 °C. In addition, the increase in the O 2 concentration shifts potential considerably to more positive direction.