Cathodic protection This is method of reducing or preventing corrosion of a metal by making it a cathode in the electrolytic cell. This can be achieved by means of an externally impressed current or sacrificial anode. An electrolyte is needed to ensure the passage of current through the part to be protected. This is effective only in soils or aqueous media where part to be protected is immersed. It is not effective in the atmosphere.
(1) Impressed –current method an external DC power supply is connected to the metal be protected. The negative terminal of power supply is connected to the part to be protected and the positive to an Auxiliary or inert anode eg: graphite. Steel scrap, Al, Si-Fe are also can be used. Si-Fe and graphite are suitable for ground-beds-buried or sea-bed for marine projects.
Applications: pipe-lines, underground cables of Al, Pb; storage tanks, heat-exchangers, steel-gates exposed to sea water, hulls of ships, highways and bridges.
(2) Sacrificial anode (or galvanic coupling) in this metal which has more negative electrode potential than the structure to be protected is connected electrically to the part or structure to be protected. The structure is protected at the sacrifice of another metal. Mg alloys, Zn, Al-5%Zn are widely used. These anodes are replaced as soon as consumed.
Applications: under-water parts of ships, ship hull, underground pipes, steel water tanks, water heaters, condenser tubes, oil-cargo-ballest tanks. Galvanized sheet is sacrificial protection of steel (Zn on steel).
Anodic protection This is based on the formation of a protective film on metals by externally applied anodic currents. An external current icrit is initially applied impressed on the metal so as to passivate it. Then the current density is reduced to ipassive and maintained at that value to ensure the passive film does not dissolve. Material must exhibit passivity in corrodent eg: Ni, Fe, Cr, Ti and their alloys. A potentiostat is used to maintain the metal at a constant potential w.r.t a reference electrode. If the control is lost temporarily and the potential strays into the anodic region, the corrosion can be disastrously high. The primary advantage is its applicability in extreme corrosive environments with low current requirements.
Comparison of Anodic and Cathodic protection
Anodic | Cathodic | |
Applicability | Active-passive matals/alloys | All metals/alloys |
Nature of corrosive medium | Weak to aggressive | Weak to medium |
Cost: Installation Maintenance | High Very low | Low Medium to high |
Operating conditions | Can be accurately determine | Determined by empirical testing |
Significance of applied current | Direct measure of protected corrosion rate | Complex to indicate corrosion rate |