Corrosion Protection of an Oil Platform Using Sacrificial Anodes

Steel structures immersed in seawater can be protected from corrosion through cathodic protection. This protection can be achieved by an impressed external current or by using sacrificial anodes. The use of sacrificial anodes is often preferred due to its simplicity.

The principle for cathodic protection using sacrificial anodes is quite simple: the steel structure is electronically connected to a less noble metal, for example aluminum, which causes the sacrificial anode to be anodically polarized and the steel structure to be cathodically polarized when the electrodes are immersed in seawater. The anodes are dissolved through anodic dissolution of the metal while oxygen reduction takes place at the surface of the steel structure. The supply of oxygen is what often limits the current density for oxygen reduction, which means that a limiting current of an almost constant value over a few hundreds of millivolts in potential is obtained at the surface of the steel structure.

Figure 1: Polarization behavior of the sacrificial anodes (blue) and steel surface (red).

Figure 1 above shows the schematic polarization of the sacrificial anodes and of the oxygen reduction reaction at the surface of the steel structure. The red curve represents the polarization of the steel surface while the blue curve is the polarization of the sacrificial anode. On the left, the currents at the steel structure (red) and at the sacrificial anodes (blue) are plotted as functions of the electric potential measured relative to a common reference. The plot to the right shows the electric potential as a function of the logarithm of the absolute value of the current. As shown in the left graphs, oxygen reduction is achieved at the steel surface along the range of the cathodic limiting current represented by the flat horizontal part of the red curve. In the right plot, the vertical part of the red curve represents oxygen reduction. The system operates at the point where the cathodic current (red) is equal in size (but opposite in sign) as the anodic current.

The shape of the blue curve changes depending on the number and design of the anodes in the system, and the designer of the system needs to ensure that the different parts of the steel structure are well within the corrosion protected range of potentials (the “flat” part of the red cathodic curve); otherwise the structure is not fully protected and may start to corrode. The width of the oxygen reduction part of the curve is a few hundred millivolts. In addition, the anodes have to be able to deliver the required potential to keep the given current.

The first step in the design of a cathodic protection system is therefore to investigate the potential of the steel structure assuming a constant cathodic current (oxygen reduction). The potential has to be well within the required range where oxygen reduction protects the structure and also avoiding hydrogen evolution, which may eventually cause hydrogen embrittlement.

This example is based on a Recommended Practice for Cathodic Protection Design by DNV (Ref. 1).

Model Definition

Figure 2 and Figure 3 show the model geometry. The sacrificial anodes are placed relatively close to the oil platform. The radius of the inner cylinder is chosen so that the main part of the charge transport occurs within this cylinder. The outer cylinder is modeled as an infinite Element Domain, which rescales the equations to represent an approximately thousand times larger cylinder.

Figure 2: Model geometry.

Figure 3: Close-up view of the cylindrical sacrificial anodes and the oil platform structure.

In seawater, the composition is assumed to vary to a very small extent and diffusion of the ions that carry the current is negligible compared to the contribution from migration of these ions in the electric field. This assumption, together with the boundary conditions, allows using a primary current density distribution analysis on the system where only the influence of ohmic effects in the given geometry are taken into account.

At the sacrificial anode surfaces, a constant potential is set assuming a relatively fast kinetics. This assumption implies that a very small change in surface overpotential leads to a very large change in current density and it is therefore reasonable to set a constant potential. Grounding the electronic phase potential (setting it to zero), the boundary condition for the electrolyte phase potential is:

(1)

The value for the equilibrium potential of the aluminum anodes is -1.05 V vs Ag/AgCl in this model.

At the steel surface, it is assumed that oxygen reduction takes place at a limiting current density, limited to the rate of transport of dissolved oxygen to the surface, and that the surface is sufficiently protected so that metal dissolution currents can be neglected. This yields a constant normal current density boundary condition at the steel surfaces of the structure. The boundary condition for the steel surface is:

(2)

The value for the limiting current is 0.1 A/m2 in this model.

All other boundaries are insulated.

The model can be easily extended to secondary current density distribution analysis in order to add the kinetics of the electrode reactions in a second stage, see for instance the Anode Film Resistance Effect on Cathodic Corrosion Protection example. A tertiary current density distribution analysis that also accounts for the transport of charged species is also possible in the Corrosion Module, although this would require the use of different physics interfaces.

Results and Discussion

Figure 4 shows a slice plot of the potential in the electrolyte,

. The potential close to the steel structure surface varies several hundreds of millivolts, depending on position. The further the distance from an anode – the lower the potential, an expected result since the current in the electrolyte flows from the anodes to the steel cathode.

Figure 4: Slice plot of the electrolyte potential.

One can relate the electrolyte potential,

, to the electrode potential, shown in Figure 1, by considering a reference electrode placed in the electrolyte in close vicinity to the steel surface. The electric potential of a reference electrode,

, is

(3)

The electric potential of the steel surface,

, is constant due to the high conductivity of the metal, and the potential of the steel surface versus the reference electrode becomes

(4)

In this model we chose the metal potential as ground, and assuming a Ag/AgCl reference we get:

(5)

Figure 5 shows the potential of the steel surface according to Equation 5. The parts of the steel surface with the highest (most anodic) values in this plot are the least protected.

Figure 5: Steel platform potential vs a Ag/AgCl reference.

Figure 6 shows a close-up of one of the structure legs. The inside bottom part of the leg is the part most susceptible to corrosion.

Figure 6: Potential on one of the legs of the platform structure.

Finally, Figure 7 shows the current densities on the anodes, which are of interest because their magnitudes are directly proportional to the consumption rate of the anode metal. The highest current density for the anodes is about four times the lowest current density.

Figure 7: Current densities on the anodes.

Reference

1. Det Norske Veritas, Recommended Practice Cathodic Protection Design, NDV-RP-B401, October 2010.

Corrosion_Module/Cathodic_Protection/oil_platform

Modeling Instructions

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Global Definitions

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Parameters 1

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Name	Expression	Value	Description
sigma_sea	5[S/m]	5 S/m	Seawater conductivity
i_oxygen	-0.1[A/m^2]	-0.1 A/m²	Limiting current for oxygen reduction at steel structure
Eeq_Al	-1.05[V]	-1.05 V	Anode equilibrium potential vs. Ag/AgCl