Cathodic Protection of Steel Tank Bottoms by the Use of Magnesium Anodes
- J.R. James (The Dow Chemical Company) | R.L. Featherly (The Dow Chemical Company)
- Document ID
- Society of Petroleum Engineers
- Transactions of the AIME
- Publication Date
- December 1948
- Document Type
- Journal Paper
- 238 - 244
- 1948. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 4.3.4 Scale, 4.2.3 Materials and Corrosion, 4.2 Pipelines, Flowlines and Risers, 4.1.9 Tanks and storage systems
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The replacement or reconditioning of oil storage tank bottoms due to externalcorrosion is an expensive maintenance item to the oil industry.
Cathodic protection as a means of mitigating this problem has proved verysuccessful in the past few years.
One pipe-line company recently used magnesium anodes to cathodically protecttwo 55,000 bbl and two 20,000 bbl tanks. The tank bottoms were protected asindividual units with considerable factual data being taken on one of the20,000 bbl tanks. Twelve 60-lb anodes were installed about 2~ ft from the tankon approximately 17-~ft centers around the tank.
Previous installations and tests have shown that sufficient protection isprovided when the tank-to-soil potential using a copper-copper sulphatehalf-cell as a reference electrode is -0.90 volts at the edge of tank.
Potential measurements taken a few months after completing the installationindicated that the potentials were higher than were needed for protection.Resistors were then inserted in the individual anode circuits which increasedthe life of the anode installation and still provided adequate protection forthe structure.
All connecting pipe lines were then insulated from the tank. This limited theanode current strictly to the tank resulting in a greater degree of protectionwith a still longer anticipated anode installation life.
The corrosion of metallic structures in contact with soil or water has been avery serious and costly problem for centuries. The annual loss to our nation'spipelines alone is estimated at $200,000,000. Mitigation of this problem byvarious methods has been carried on quite successfully for a number of years.An electrical method called cathodic protection has been used with considerablesuccess to control corrosive influences on pipelines, tank bottoms, cablesheathing and other buried metal structures.
Corrosion of oil storage tank bottoms has been a serious problem among the oilindustries for years. In addition to damage costs and loss of oil, thereplacement of one tank alone represents several thousand dollars. Areconditioning program at periodic intervals was instituted by severalorganizations and found to be a very expensive process.
A few years ago two of the pipeline companies installed cathodic protectionunits at their tank farms the average current required for protection of thetank bottoms varied from 0.7 to 1.0 milliampere per square foot. The resultsindicated that adequate protection was obtained at a cost of approximately 10pct of the previous reconditioning program.
The total installation cost for this tank was less than $300 which amounts to$20 per year based on an expected installation life of 15 years. This figurerepresents an annual cost of one per cent compared to the cost of replacing thetank bottom.
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