API – Cathodic Protection of Aboveground Petroleum. Storage Tanks: “ Galvanic anodes method is not practical for protection of large bare structures.”. API Cathodic Protection of Aboveground Petroleum Storage Tanks Scope : This covers the procedures. Practices and guidance for effective cor. Cathodic Protection of Tanks API – Download as Powerpoint Presentation . ppt), PDF File .pdf), Text File .txt) or view presentation slides online. Cathodic.

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The electrode of an electrochemical cell at which oxidation corrosion occurs. The electrode of an electrochemical cell at which a reduction reaction occurs.

API — RP defines corrosion as the deterioration of a ali resulting from a reaction with its environment. Corrosion of dathodic structures is an electrochemical process.

For the corrosion process to occur, areas with different electrical potentials must exist on the metal surface. These areas must be electrically connected and in contact with an electrolyte.

There are four components required for a corrosion cell: The role of each component in the corrosion cell is as follows:. At the anode, the metal corrodes by releasing electrons and forming positive metal ions.

For steel, the anodic reaction is:. At the cathode, chemical reactions take place using electrons released at the anode. No corrosion takes place at the cathode. One common cathodic reaction is:.

API – Cathodic protection for tank bottoms – Apiexam

The metallic path serves as a path for electrons released at the anode to flow to the cathode. The electrolyte contains ions and conducts current from the anode to the cathode by ionic movement. The electrolyte contains both negatively charged ions called anions and positively charged ions called cations that are attracted to the anode and cathode, respectively.


Moist soil is the most common electrolyte for external surfaces of the tank czthodic, while water and sludge generally are the electrolytes for the internal surfaces. If you have difficulty remembering the 4 elements of a electrochemical cell, use the acronym ACME. Besides, that a look at this video I prepared. If you cannot see the video above, go to youtube here: Electrochemical corrosion in a tank bottom. There are many forms of corrosion. The two most common types relative to tank bottoms are general and localized pitting corrosion.

API – 651 Cathodic protection for tank bottoms

In general corrosion, thousands of microscopic corrosion cells occur on an area of the metal surface resulting in relatively uniform metal loss. In localized acthodic corrosion, the individual corrosion cells are larger and distinct anodic and cathodic areas can be identified. Metal loss in this case may be concentrated within relatively small areas with substantial areas of the surface unaffected by corrosion. Differences in electrochemical potential between adjacent areas can result from uneven distribution of alloying elements or contaminants within the metal structure.

Potential and physical differences between the weld metal, the heat affected zone and the base metal are the aapi fore behind preferential weld corrosion, with mechanisms such as galvanic corrosion, stress corrosion, etc. Just like potential differences in a metal can generate corrosion, also Ion concentration gradients in the electrolyte can provide a potential.

Differential aeriation can also generate corrosion. The part of the metal exposed to higher oxygen concentration acts as cathodic region and part of the metal exposed carhodic oxygen concentration acts as anodic region.

Consequently, poorly oxygenated region undergoes corrosion. Differential aeriation under a tank bottom can happen if the soil has clay, debris, or other type of contamination. On the other side, if you can change the composition of an electrolyte adding a corrosion inhibitor, that would reduce the corosion rate, but this is not used much in tank bottoms and not the subject of API — Soil corrosion is a damage mechanism affected by a lot of parameters. Soil resistivity is the most common used parameter to determine corrosivity.


Salts present in the soil electrolyte affects the current carrying capacity of the soil and therefore corrosion rates. Moisture content, pH, oxygen concentration, and other factors interact in a complex fashion to influence corrosion. For 2 years now Apiexam. No one else is doing it.

No one 561 is giving away the information. Several of my readers have reported how they have used the free material in Apiexam. Skip to content Skip to primary sidebar By Eng. To protect metal from corrosion, there are several things that can be made.

A tank bottom can make good use of cathodic protection and coatings. The provision of an insulating coating to the structure will greatly reduce the current demand for cathodic protection. It is currently in the fourth edition. The following is what the Body of knowledge for the exam says. The inspector should have a practical understanding and be familiar with the information contained in RPrelated to:.

Operation and Maintenance of Cathodic Protection Systems. In this article, we are going to cover the first point, specifically the basic mechanism of a corrosion cell. The following definitions of section 1 are neccesary anode: A chemical substance containing ions that migrate in an electric field. For the purposes of this recommended practice, electrolyte refers to the soil or water adjacent to and in contact with the bottom of an aboveground petroleum storage tank, including the contaminants and chemicals contained therein.

The loss of electrons by a constituent of a chemical reaction.