Question

a) Using appropriate data sources (to be cited correctly) make a reasonably accurate sketch of Evan’s diagram for iron immersed in vigorously agitated diluted hydrochloric solution at pH 5 at 30 °C. The surface to volume ratio of this iron piece is small. In other words, there is no change in solution pH.

The diagram should exhibit correct corrosion potential and corrosion current density.

(b) Describe what could happen if the solution become stagnant. Using appropriate data sources to modify Evan’s diagram accordingly.

Answer 1

(a) Evan’s diagram for iron immersed in vigorously agitated diluted hydrochloric solution at pH 5 at 30 °C.

-0.1 T -0.15 ;..----+ 1 Fe 025 0.5 -0.55 t -06 i i-- 2H. +20,-H2 -7-6.5-6-6.5-6 ㅢ.5-4-3.5-3 Log (M(A))

(b) Modified Evan’s diagram for stagnant solution.

When one of the reactions is limited by the rate of transport of the reactant to the metallic surface being corroded, the situation increases in complexity as illustrated in the polarization plot of the system above. The system represented here is similar to the previous one, that is, pH of five at 30°C, with the exception that the environment is stagnant. In this situation the reduction of oxygen now becomes a possible second cathodic reaction.

-0.3 -0.7 -7-65-6-5.5-6-4.5 -3.5-3-25 Log (/lI(A))

O₂ + 4H⁺ + 4e^- → 2H₂ O

In order to model the polarization plot, the total cathodic current corresponding to the sum of the currents of the hydrogen reaction and oxygen reduction has to be balanced by the single anodic current. The intercept where the opposing currents are balanced occurs at an E_corr of −0.33 V vs. SHE and, since the surface area is still 1 cm² , an i_corr of 8.2 µA cm⁻² or 0.1 mm y⁻¹ . It should be noted that the current for the reduction of oxygen is constant across the potential range shown in Fig. and has the value of 6.3 µA cm⁻² .