Mathematical Modeling of the Formation of Calcareous Deposits on Cathodically Protected Steel in Seawater

Abstract

A first principle mathematical model of the formation of calcareous deposits on a cathodically protected steel rotating disk electrode in seawater is presented. The model includes equations which transport phenomena, electrochemical reactions, precipitation reactions, and a homogeneous reaction involved in the formation of calcareous deposits on an electrode surface. Predicted concentration profiles show that a high concentration of OH-ions on the electrode surface leads to the formation of calcareous deposits. The calcareous deposits contain mostly CaCO3, but the initial deposits are predicted to contain more Mg(OH)2 than CaCO3. The predicted calcareous deposits on the electrode surface reduce the active surface area available for the electrochemical reactions, which results in a decrease in the cathodic current density. The predicted current density as a function of time during the formation of deposits agrees qualitatively with experimental data.

The Effect of Temperature and Local pH on Calcareous Deposit Formation in Damaged Thermal Spray Aluminum (TSA) Coatings and Its Implication on Corrosion Mitigation of Offshore Steel Structures

“…At first, the cathodic reaction increases the [OH − ] at the steel surface (Figure 2b) which results in a pH increase. Such an increase is hindered when the local pH is high as at 10.19 at 30 • C and 9.54 at 60 • C. Those high pHs are favorable for Mg(OH) 2 precipitation, which will be further proved in Section 4.3.3 [30]. This is a very thin base layer (measured as 6 µm) as observed in Figure 5b,e.…”

“…Thus, the CO 2 is also being consumed to form CaCO 3 , delaying the pH stabilization as the CaCO 3 is still being formed. Once both layers are fully formed, it acts as a natural barrier against further steel corrosion [6,9,27,30]. In summary, two events are taking place even after the maximum peak at 30 • C; (i) local formation of OH − and (ii) consumption of CO 2 for the formation of CaCO 3 .…”

“…Lee and Ambrose [30] mentioned that although the amount the dissolved oxygen content in water increases with a decrease in temperature, the diffusion coefficient of oxygen is diminished as well as the hydrogen evolution reaction, which can explain the lower pH necessary to precipitate Mg(OH) 2 One can wrongly assume that CaCO 3 will precipitate in acidic media at 60 • C, once its precipitation pH is found to be 6.6. However, it should be noted that the neutral point of water is defined as 7 at room temperature (25 • C), which is shifted to a different value at higher temperature.…”

Electrochemical Characteristics in Seawater for Cold Thermal Spray-Coated Al–Mg Alloy Layer

“…The reduction reaction of dissolved oxygen and water generates OH -ions near the coating surface resulting in the formation of calcareous deposits on the coating surface. It is known that the calcareous deposits are mixtures of CaCO 3 and Mg(OH) 2 [25,26]. The mechanism for the formation of calcareous deposits [26,27] is believed to be (1) Mg 2?…”

Numerical Simulation of Galvanic Corrosion between Carbon Steel and Low Alloy Steel in a Bolted Joint

“…The publications of Murer et al [10][11][12] and Shi and Kelly [13] in this context also gave an extended insight into the topic, especially in the very important choice of boundary conditions. New studies of Sun et al [14], who applied the mathematical approach of Yan et al [15] to the modeling of deposit formation under seawater conditions, clearly introduce a possible way of a useful model built up for the mentioned purpose. The following studies and results are based on the progress achieved by them.…”