- 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.
API 651 – CATHODIC PROTECTION OF ABOVEGROUND STORAGE TANK BOTTOMS
The inspector should have a practical understanding and be familiar with the information contained in RP-651related to:1. Corrosion of Aboveground Steel Storage Tanks2. Determination of Need for Cathodic Protection3. Methods of Cathodic Protection for Corrosion Control4. Operation and Maintenance of Cathodic Protection Systems
The following definitions of section 1 are neccesary
anode: The electrode of an electrochemical cell at which oxidation (corrosion) occurs.
cathode: The electrode of an electrochemical cell at which a reduction reaction occurs
CORROSION OF TANKS
API – 651 RP defines corrosion as the deterioration of a metal resulting from a reaction with its environment. 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.
d. 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. A – Anode, C – Cathode, M – Metallic Path, E – Electrolyte. 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.
FACTORS AFFECTING THE RATE OF CORROSION
-The composition of the metal is a factor in determining which areas on a metal surface become anodes or cathodes. 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.
-Physical and chemical properties of the electrolyte influence the location of cathodic and anodic areas on the metal surface. 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 – 651.
-Soil characteristics. 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.
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