A cathodic protection system is a corrosion cell in which the structure to be protected is the cathode. Sacrificial or galvanic systems are corrosion cells of the differential metal type. The earliest experiments on cathodic protection were performed with zinc anodes that were electrically connected to copper plates immersed in seawater. From galvanic series, an arrangement would produce a cathode (copper) and an anode (zinc). In the large galvanic cell so formed, the zinc cylinder corroded away in a manner to protect the copper substrate. This method of cathodic protection can be used with other combination of metals providing the necessary current to the metal to be protected.

In a electrolyte (e.g. seawater) when two metals are electrically connected to each other, electrons will flow from the more active metal to the less active metal due to the difference in the electrical potential (driving force). When the most active metal (anode) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active (cathode) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals.

Sacrificial Anode Material

From the galvanic series, magnesium heads the list as the most anodic metal and is widely separated from iron. Magnesium coupled to iron provides sufficient galvanic potential to provide positive protection. The sacrificial anode system is inherently a safer system than impressed cathodic protection systems because the normal potentials generated are insufficient to damage coatings present on the surface to be protected. Because of the low potentials generated, sacrificial systems can be used only in low-resistance soils, i.e., with a resistivity less than 3000 W cm.

Anode Efficiency

A prospective sacrificial anode must possess a large number of electrons per unit mass and should deliver these electric charges efficiently. Thus the electrical output of an anode is given by current capacity which is expressed in Ah kg^-1 or kg A^-1 y^-1. The value of the current capacity is determined by the electrochemical equivalent, the density and the efficiency of the anodic material. The electrochemical equivalent, which is dependent on the atomic weight and valence, is a characteristic of the anode material. However efficiency is determined by a number of factors including nature of the environment, operating current density and metallurgical microstructure. It is apparent that if the cathode reaction rate on the anode is low then the efficiency will be high, so that there is minimum self corrosion. Similarly large operating currents will yield high anode efficiency. It should be added that the type of corrosion attack experienced by the anode also significantly affects the magnitude of the anode efficiency. For instance, severe pitting and intergranular attack may result in a chunk of the anode to become detached without complete consumption of the electric charge in that piece.

Protective Current Requirements

Assumptions of protective current requirements and bare metal areas.

• For bare metal in the ground, a current of 11 to 22 mA/m² of bare metal surface has been found adequate, except under extreme or unusual conditions. This value must then be modified to suit the particular conditions.
• For coated pipe, the current required is difficult to estimate without field tests. The primary reason is the unknown condition of the protective coat which can vary from nearly 0% to 98% coverage.

FPSO Sacrificial Anodes

FPSO (Floating Production Storage & Offloading)

Sacrificial Anodes on the Hull

If the vessel is a new build, a life cycle economic comparison will almost certainly favor the use of galvanic anodes over impressed current. Anodes selected are normally platform-sized anodes 160 - 230 kg modified for flush mounting. Aluminum alloys are preferred based on higher efficiency coupled with lighter weight.