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PLATTLINE Zinc Ribbon Anodes

Technical ArticlesTechnical Article #5 - Cathodic Protection of Above Ground Storage Tank BottomsGovernment regulations, environmental issues and economics have made cathodic protection of aboveground storage tank bottoms increasingly important. Internal corrosion occurs, generally, from crudeand other stored products carrying water into the tank. The extraction process of the product itself will

leave a water with an extremely high salt content, which settles to the bottom of the tank corrodingthe bottom from within. This circumstance can eventually cause product leakage - in fact, there are anestimated 20,000 leaking tanks each year in the United States alone. Repair costs, production down-time, product loss, clean-up costs and regulation penalty costs are just a few of the reasons that tankowners have turned to adequate corrosion prevention of AST Bottoms.Plattline Zinc Ribbon Anodesare used over othergalvanic anode or impressed current systemsbecause thesand between the two bottoms will hold any moisture,thereby creating an electrolyte (aconductor) and corrosioncan begin. The moisture is introduced by leaks in the tank ortankcondensation. Independent laboratory testing has shownthat the pitting rate in highly conductive sandcould be greatenough to perforate a tank bottom in as short as three (3)years. This electrolyte willactivate Plattline Zinc RibbonAnodes and galvanically protect the steel bottom. When thesandbecomes dry, Plattline will regulate itself to onlybecome active when the environment is corrosive.Magnesiumhas a much stronger driving potential, is not self regulatingand will consume itself at amuch greater rate, thus shorteningthe life of the application. Also, magnesium is not as flexiblea

material and does not easily lend itself tovarious design configurations.

Application to Install PlattlineA liner is usually installed as secondary containment whenretrofitting tanks with a new tank bottom.

The anode is surroundedby approximately 6" to 10" of clean, dry sand andplaced on top of theliner.There are several design configurations used for Plattlineinstallations SeeDiagram 1. Thespecific design is determined by several factors - ease of installation, tank diameter andotherengineering criteria. Usual spacing between the anode ribbonis 4 to 6 feet.

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Once the zinc ribbon is installed, several zincreference cellsare placed in the sand electrolyte. Lead wires to the outsideof the tank enable the tankowner to monitor the cathodicprotection after installation and the tank is returned to

service.Multimeters, or voltmeters are used to take these electricalpotential measurement readings.See Diagram 2. After installation of Plattline, the new bottom is thenwelded in place, checked forleakage and repaired and thetank is returned to service.

Conclusions

In the actual monitoring of a 140 ft diameter tank with a double bottom, one major oil companyconcluded that:

Zinc Ribbon Anodes used in a cathodic protection system only becomes effective when the sandcushion between the bottoms conductive or contaminated with moisture.

Generally, the closer the anode spacing, the better the cathodic polarization. Under the rightconditions, 8ft spacing appears to work well.

Zinc Ribbon Anodes can effectively achieve cathodic protection of a double bottom when theelectrolyte resistivity conditions are low enough to allow electrical current flow.