Cathodic Protection of Tower Anchors Presented by Tom McKigney Brance-Krachy Co., Inc. Houston Texas to The Society of Broadcast Engineers September 10, 2002

THE CORROSION CELL When two dissimilar metals are connected in an electrolyte such as soil, a corrosion cell is formed. One metal becomes the anode and suffers corrosion while other metal becomes the cathode and corrosion is prevented. Anodic and cathodic areas exist on the surface of all steel structures due to slight variations in material. Composition, local stresses, differences in coating condition and availability of oxygen are all factors in creating this. In the absence of cathodic protection corrosion will eventually occur.

ELEMENTS OF A CORROSION CELL ANODE & CATHODE ELECTRICAL POTENTIAL ELECTROLYTE METALLIC PATH

THE GALVANIC SERIES

Anchor Corrosion Cell

Stray Current Corrosion

ANCHOR CORROSION MITIGATION 1. Galvanizing and Epoxy Coatings 2. Concrete Encasement. 3. Electrical Isolation. 4. Cathodic Protection. Galvanic Anodes Impressed Current When used in conjunction with Cathodic Protection the other methods can be beneficial in mitigating corrosion problems.

Galvanizing and Epoxy Coatings Not a solution Galvanizing depletes rapidly underground Only effective against atmospheric corrosion Epoxy coatings can accelerate failure if used alone Holidays give rise to the small anode large cathode effect

Concrete Encasement Used for structural strength May provide some corrosion protection Corrosion can cause concrete failure Cracks in concrete may set up corrosion cells

Electrical Isolation

CATHODIC PROTECTION SYSTEMS The cathodic protection of metals has been around since 1824. Sir Humphrey Davy attached zinc plates on the hulls of British naval vessels to retard corrosion of the copper sheathing. Cathodic Protection continues to be an effective method for mitigating the destructive action of corrosion. The principal of cathodic protection involves the introduction into the electrical circuit a metal that is more electro-negative than the existing anodic and cathodic areas. This additional metal becomes the anode and will corrode while providing current to the metal it is protecting, thereby overcoming the local anodic area and making them cathodic.

CATHODIC PROTECTION

GALVANIC SYSTEMS The classic Cathodic Protection solution to a galvanic corrosion problem is to introduce an anode of a suitable alloy of magnesium or zinc which will suffer corrosion and can be sacrificed in protecting the cathode. This approach is used in galvanic systems.

Anchor Cathodic Protection

IMPRESSED CURRENT SYSTEMS As an alternative solution, the protective current needed to make the structure cathodic may be obtained by applying low voltage Direct Current derived from normal ac mains supply. This is achieved by transformer /rectifiers which supply DC to specially designed anodes which will dissipate large currents without themselves suffering significant wastage.

IMPRESSED CURRENT SYSTEM

Cathodic Protection System Design Requirements Anode Selection depends on: Soil Resistivity Current Requirement Design Life

Maintenance and Record Keeping Check system and record Potential Readings on a regular schedule. This should be done at least once a year.