Corrosion rate of carbon steel under synergistic effect of seawater parameters including pH, temperature, and salinity in turbulent condition

3.1 Simultaneous effect of temperature and velocity

Figure 3 illustrates simultaneous effect of temperature and velocity. As Figure 3 shows, at the stagnant condition, corrosion rate is increased by temperature increment up to 40°C.

This result is consistent with the previous achievements (Melchers, 2005; Melchers & Chernov, 2010). In this case, as there is no agitation in the solution, the mass transport of species to the corroding surface is rate controlling and the corrosion rate is mostly under the control of concentration polarization. In the case of concentration polarization controlled processes, the value of limiting current density (i_L) can determine the rate of corrosion. Equation (2) illustrates the parameters affecting the magnitude of limiting current density in a surface without diffusion limiting deposits (Bardal, 2004).

where i_L is limiting current density, is oxygen diffusion coefficient, z is solution viscosity, F is Faraday number, δ is thickness of diffusion boundary layer, and c_B is oxygen concentration in solution.

Temperature can cause an increase on i_L through affecting a couple of parameters and δ). The effect of temperature on follows the Arrhenius expression and is described according to Equation (3) (Bardal, 2004).

where A and Q are constant in water, R is universal gas constant, and T is temperature in Kelvin.

As Equation (3) shows, grows by temperature. On the contrary, as temperature is increased, a kind of molecular and ionic agitation will be created in the solution that reduces the thickness of diffusion layer. Both increasing and decreasing δ that happen through the temperature growth results in the increment of i_L based on Equation (2), which causes an increase in the rate of corrosion (Bardal, 2004).

This result is also in agreement with the achievements of two previous researches on the effect of temperature on the corrosion rate of carbon steels (Melchers, 2005; Melchers & Chernov, 2010). As reported previously, corrosion rate generally is increased by the increment of temperature. However, as the temperature increases more, a different effect of temperature on corrosion rate can be observed.

In stagnant condition, as the temperature is increased more than 40°C, corrosion rate is decreased, which seems to be different from previous research reports. The reason is because the increment of temperature up to the high level causes decrease in the amount of oxygen concentration in the solution and decrease in the rate of cathodic reaction and total corrosion rate consequently.

According to Figure 3, samples show a kind of similar trends in 200 and 300 rpm. In both conditions, corrosion rate decreases as the effect of temperature increases. In these high velocities, corrosion rate is not under the control of concentration polarization anymore because velocity facilitates the ion transport in solution and corrosion rate is not affected by limiting current density as was in stagnant condition. In the absence of concentration polarization issue, temperature decreases the rate of corrosion first by providing a suitable condition for the formation of surface deposit layer and second by causing the decrease in the rate of cathodic reaction through the limitation of dissolved oxygen.

At temperatures up to 40°C, corrosion rate increases as velocity grows, which is confirmed by the research of Melchers (2004). To explain the increasing effect of velocity on corrosion rate at these temperatures, the effect of velocity on i_L should be considered. As velocity increases, the thickness of diffusion boundary layer (δ) decreases, which results in an increase in i_L as Equation (2) and the rate of corrosion consequently. The relationship between corrosion rate and velocity in laminar and turbulent fluids with diffusion controlled corrosion on the surface of clean metal is as Equation (4) (Bardal, 2004).

This effect is in consistent with the left part of Figure 9 (Bardal, 2004). As Figure 9 shows, at high flow rates, erosion and removal of deposit layer take place and corrosion rate increases. Figure 10 also shows that the effect of fluid velocity on corrosion rate increases through increasing the value of limiting current density. As mentioned, the trend of corrosion rate variation under the effect of temperature is similar at 200 and 300 rpm and they are totally different from stagnant trend.

Figure 9

Corrosion rate as a function of (flow velocity)^1/2 (active metal) (Bardal, 2004).

Figure 10

Schematic over voltage curves showing the effect of flow velocity on the corrosion rate through changing i_L (active metal) (Bardal, 2004).

As Figure 3 shows, corrosion rate is generally increased by velocity, which is acceptably consistent with the approaches of former research (Melchers, 2004).

3.2 Simultaneous effect of salinity and velocity

The simultaneous effect of salinity and velocity is presented in Figure 4. According to Figure 4, generally, by increment of salinity, corrosion rate increases at all velocities, because high salinity results in a higher value of solution conductivity, which increases the rate of corrosion (Mofodi, 2004). This result is totally in agreement with the outcome of previous research at Persian Gulf, which shows an increasing effect of salinity on the rate of corrosion (Ray, Lee, Little, & Lemieux, 2006). The same result achieved by Syed (2006).

The graphs of corrosion rate variation as a function of salinity show similar trends at 200 and 300 rpm and the slope of these two curves is obviously sharper than stagnant one. This kind of similarity between the curves of 200 and 300 rpm differentiates the stagnant condition from turbulent one. According to this figure, at room temperature, corrosion rate is increased by the increase in salinity because the salinity is capable of increasing solution conductivity. In moderate and high velocities, the effect of salinity on corrosion rate is somehow negligible.

At higher salinities, corrosion rate is more susceptible to velocity change and more variation in corrosion rate occurs with the constant increase in velocity. The reason for this phenomenon is that, at higher salinities, the rate of oxygen distribution is limited, so the concentration polarization becomes controlling. In this situation, while the corrosion rate is under the control of concentration polarization, the influence of velocity gets very significant. Velocity assists the distribution of dissolved oxygen, which causes a decrease in the amount of concentration polarization and grows the rate of corrosion consequently. In all salinities, corrosion rate is increased by velocity increment through the effect of velocity on growing i_L, which causes an increase in the rate of corrosion. This relationship is presented in Equation (4) and Figure 10 (Bardal, 2004).

3.3 Simultaneous effect of pH and velocity

The simultaneous effect of pH and velocity is shown in Figure 5. Referring to Figure 5, in all pH values, corrosion rate is increased by velocity through the effect explained in Figures 9 and 10, which are on the basis of velocity effect on i_L.

In all velocities, corrosion rate is decreased by pH increment. Higher pH values provide more favorable media for the formation of calcareous deposits on the surface of carbon steel samples in seawater environment (Hack & Guanti, 1988; Hartt et al., 1984; Lin & Dexter, 1988). The precipitation of Ca(OH)₂ by the pH increase is more likely to occur by the direct combination between calcium and carbonate ion as illustrated in Equations (5) to (8) (Elbeik et al., 1986).

Equation (5) shows the production of carbonate ion through the reaction between the atmospheric carbon dioxide and water. At the pH increase, Equations (6) and (7) are displaced to right. Consequently, the increase in the carbonate ion concentration happens and Equation (8) displaces to right, leading the direct formation of calcium carbonate (Elbeik et al., 1986). Magnesium compounds, Mg(OH)₂ in particular, could also contribute to the protective character of calcareous deposits at the pH values above 9.5. However, calcium carbonate is thermodynamically more stable. The formation of surface deposits results in a decrease in the rate of corrosion because the deposit layers are not usually electron conducting; thus, the cathodic reaction has to take place at the metal surface underneath the deposits. This forms a diffusion resistor in series with the usual diffusion resistance in the boundary layer and Equation (2), which is valid for a clean surface, changes to Equation (9) (Elbeik et al., 1986).

where t_B is thickness of the deposit, P_B is oxygen permeability of the deposit, and t_B/P_B refers to the diffusion resistance of the layer of deposited substances (Elbeik et al., 1986). The formation of deposit layer at higher pH values results in a decrease in the amount of i_L [Equation (9)], which causes a reduction in the rate of corrosion.

At lower pH values, the corrosion rate of carbon steel samples is shown to be more susceptible to velocity increase according to Figure 5. This issue can be explained considering the increasing effect of velocity on the rate of corrosion as presented in Figures 9 and 10. As higher pH values are more favorable condition for the formation of calcareous deposits, by pH increment, the alloy gets more resistant to corrosion rate increase through the resistive effect of deposit layers. The trends of corrosion rate variation under the effect of pH are similar at 200 and 300 rpm and also different from that of stagnant condition, which presents the sharp difference between the effects of turbulent and stagnant media on the rate of corrosion.

3.4 Simultaneous effect of salinity and temperature

Figure 6 illustrates the simultaneous effect of salinity and temperature. At the salinity of 20 (Figure 6) by the increment of temperature, corrosion rate at first increases, which is expected according to the previous studies (Melchers, 2005; Melchers & Chernov, 2010), and then decreases. This variable effect of temperature on corrosion can be considered by Figure 11 (Bardal, 2004). As the left part of Figure 11 shows, corrosion rate is increased first by the temperature. The increasing effect of temperature on the rate of corrosion is explained through its influence on i_L increment as shown in Equation (2). However, at higher temperatures, the amount of oxygen concentration is reduced by the temperature increase, causing lower i_L, which results in the reduction in corrosion rate as described in Figure 12. This figure represents the effect of oxygen concentration reduction on i_L and corrosion rate consequently. As Figure 6 shows, the trend of corrosion rate variation by temperature effect is similar at salinities of 30 and 40, and in both cases, corrosion rate increases as temperature grows. At higher salinities, the rate of oxygen distribution is low and the concentration polarization becomes controlling. In these situations, temperature increase can cause a growth in the rate of corrosion.

Figure 11

Effect of temperature increase on the corrosion rate in an open system (Bardal, 2004).

Figure 12

Effect of oxygen concentration on limiting current density (i_L) and corrosion rate (Mofodi, 2004).

In stagnant condition and mid range of temperature, corrosion rate is decreased by salinity. The difference between this trend and the trend in Figure 4 is related to controversial effects of salinity on corrosion rate. Salinity is capable of decreasing the rate of corrosion as it decreases the amount of dissolved oxygen and applying reduction in the rate of cathodic reaction. On the contrary, salinity can increase the rate of corrosion by increasing the solution conductivity. In Figure 4, at mid and high ranges of velocity, velocity can cancel out the decreasing affect of salinity because, although salinity increase causes decrease in oxygen concentration, velocity enhances the oxygen arrival to the surface and the concentration polarization can be moderated. Thus, the increasing effect of salinity on corrosion rate becomes important and corrosion rate is increased as a result of increase in solution conductivity. However, in Figure 6, not only we do not have the moderating effect of velocity (as it is in stagnant condition) but also we are in the mid range of temperature that enhances the decreasing effect of salinity on the rate of corrosion. As both temperature and salinity are increased, the amount of soluble oxygen concentration is decreased and the rate of corrosion decreases accordingly.

3.5 Simultaneous effect of pH and temperature

Figure 7 represents the simultaneous effect of pH and temperature. As presented in Figure 7, by increasing pH, corrosion rate generally reduces in all temperatures. The reason is that, at higher pH values, there is more possibility of calcareous deposits formation as illustrated in Equations (5) to (8), which makes a reduction in the rate of corrosion by providing a kind of diffusion boundary layer on the surface of metal. In all pH values, as temperature grows, corrosion rate at first is increased and then gets down. As explained before, at first, temperature increment increases the rate of corrosion by the increase of i_L value as Equation (2). This effect is presented in the left part of Figure 11. However, when the temperature passes the peak of curve presented in Figure 11, the reduction in the amount of oxygen concentration results in a decrease in i_L value (Figure 12) and reduces the rate of corrosion consequently.

3.6 Simultaneous effect of pH and salinity

In Figure 8, the simultaneous effect of pH and salinity is presented. Considering the simultaneous effect of salinity and pH according to Figure 8, by increasing pH, corrosion rate generally decreases at all salinities because of the pH effect on the formation of deposit layer as described in Equations (5) to (8), and this result is in agreement with the achievements of former researchers (Hack & Guanti, 1988; Hartt et al., 1984; Lin & Dexter, 1988). At pH values up to 9, corrosion rate increases as the salinity grows, similar to the previous studies’ trends (Melchers, 2005; Melchers & Chernov, 2010). The reason for this phenomenon can be explained through the effect of salinity on increasing the value of solution conductivity, which results in an increase in the rate of corrosion. As it is obvious in Figure 8, the effect of salinity on corrosion rate variation is more significant at pH values lower than 9, but at pH 10.5, salinity does not apply a noticeable effect on the variation of corrosion rate because, when pH increases, the calcareous deposits will form on metal surface and make the metal less resistive to corrosion rate variation under the effect of salinity.