Determination of Need for Cathodic Protection Almost any tank of certain size that is in contact with soil will need a cathodic protection in the bottom, because coatings are not perfect NACE SP , so it will be a moment when corrosion starts to act. With more reason if the surface is not coated. If analysis of these data dictate that the rate of corrosion is too high or expected to be too high, a cathodic protection system should be considered. Before a tank is built, there needs to be some kind of study to determine the feasibility of cathodic protection systems.
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Carlos F Molina To protect metal from corrosion, there are several things that can be made Coat the surface with paint so there is no contact between the metal and the electrolyte. Coat the surface with a another metal that is sacrificial metal that protects the steel. Cathodic protection from sacrificial anodes — no power supplies. Cathodic protection by the use of an impressed current from an electrical source.
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: 1.
Corrosion of Aboveground Steel Storage Tanks 2. Determination of Need for Cathodic Protection 3. Methods of Cathodic Protection for Corrosion Control 4. 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. Corrosion of steel 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: an anode, a cathode, a metallic path connecting the anode and cathode, and an electrolyte.
The role of each component in the corrosion cell is as follows: a. At the anode, the metal corrodes by releasing electrons and forming positive metal ions. For steel, the anodic reaction is: b. 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: c. 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 bottom, 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.
In general corrosion, thousands of microscopic corrosion cells occur on an area of the metal surface resulting in relatively uniform metal loss. In localized pitting 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 riving 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 lower 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. No one else is doing it. No one else is giving away the information.
Several of my readers have reported how they have used the free material in Apiexam. In the following post, I will be sharing with you an important information. Keep tuned.
API RP 651
API – 651 Cathodic protection for tank bottoms