CC513 - 2008 Assignment #5

Cathodic protection and calcareous deposits

This problem is a particular application of the blocking or passivation of an electrode surface by forcing the accumulation of some ions that can eventually precipitate and block the surface. For this problem, imagine a situation where you would have to design a sacrificial CP system for the protection of the four main pillars of an oil drilling platform. These pillars are basically sealed steel cylinders, one meter in diameter and 25 m long in their immerged section. The sacrificial anodes are zinc bars 100 cm long, 12 cm wide and 12 cm thick, with their back tightly screwed onto the steel surface. Answer the following questions knowing that the temperature of the seawater decreases exponentially as a function of depth, changing from 25^oC at the surface to 10^oC at a depth of 25 m and that the diffusion coefficient of CO₃^2- can be described by the following expression:

D_t = D₂₅o_C (1 - 0.043×(25 - t))

• where:

  • D_t is the diffusion coefficient of CO₃^2- at temperature t

  • t is the temperature in ^oC

  • D₂₅o_C is the diffusion coefficient of CO₃^2- at 25^oC (5 x 10^-5 cm² s^-1)

• Question 1

Given that the corrosion rate of unprotected steel is 1.4 mm y^-1, estimate the total corrosion current for one pillar.

• Question 2

Given that the zinc anodes can provide a sacrificial current corresponding to a corrosion rate of zinc of 8 mm y^-1, evaluate the number of anodes that would be required to reduce the corrosion of steel by a factor of ten. Assume that the total corrosion current calculated in Question 1 remains the same to balance a constant cathodic process, the reduction of oxygen.

• Question 3

In order to reduce the consumption of anodes over time you would like force the precipitation of calcareous deposits onto your steel surface because you know that, by doing so, you can cut down the corrosion of steel a hundred fold. Knowing that the level of Ca²⁺ in seawater is 0.015 m and that the diffuse layer is approximately 5 micrometer thick.

Question 3a: Calculate the current density that would be required to force the precipitation of insoluble aragonite onto the steel close to the water line;

Question 3b: How many sacrificial anodes per m² would be required to provide this initial protective current ?

Question 3c: What would the impact on the current density requirements of attempting to deposit aragonite in an agitated sea ? Please be specific in your answer.

• Question 4

Calculate the current density required to precipitate the aragonite at the bottom of the pillar. Assume that the K_sp of aragonite is the same.