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galvanic protection

Posted by tomh on April 18th, 2003 11:17 AM
In reply to Rotting Lolley colums by Tom on April 17th, 2003 12:18 PM [Go to top of thread]

Your question can serve as valuable lesson to others that may have steel columns, fasteners, tanks or pipes on or in the ground. Basically, steel posts act as a cathode in ground contact. Iron will give up electrons to the ground causing steel to become weakened and suseptible to oxidation and failure. An anode must be used as a sacraficial metal to prevent this failure. Zinc is most often used for electrolytic protection.

Any construction involving steel in grond contact must provide sacraficial metals (anode) so that the iron or steel (cathode) is not oxidized by this natural electrolytic oxidation process. Ideally, any column in contact with the ground would be galvenized. The zinc in the galvanized metal would be sarificial, but the steel would remain intact for years. Zinc-containing paint can also serve this purpose. Greater protection is possible using a zinc rod sunk within a few inches of the steel column (imbedded through the footer) to provide galvanic protection. Consider incorporating a cathodic protection in your replacement system.

The principle is no different than the galvanic protection in your hot water heater, and is critical to the long-term structural integrity of steel components in ground contact. So, I disagree that your rotting columns are due to a failure to shed water. Steel corrodes in the ground because of an electrical differential, and that differential is increased when moisture is present and may be aggrevated by alkalinity in the concrete.

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