API – 651 Cathodic protection for tank bottoms

By Eng. 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.
  • Pasivation.
  • Anodization.
  • 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 most effective protection for a tank bottom is a combination of a corrosion resistant coating and a cathodic protection system (says API 571 and NACE RP0193). The provision of an insulating coating to the structure will greatly reduce the current demand for cathodic protection.


API 651 – CATHODIC PROTECTION OF ABOVEGROUND STORAGE TANK BOTTOMS

Let´s concentrate in what you should study for the API 653 exam (due March 17, 2017). API 651 RP was created in 1997 to present procedures and practices for achieving effective corrosion control on aboveground storage tank bottoms through the use of cathodic  protection. It is currently in the fourth edition.
The following is what the Body of knowledge for the 2017 exam says
The inspector should have a practical understanding and be familiar with the information contained in RP-651related 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. We´ll cover the rest of the first point in the next article and we´ll continue from there.

DEFINITIONS

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

electrolyte: 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
oxidation: The loss of electrons by a constituent of a chemical reaction

CORROSION OF TANKS

ELECTROCHEMICAL CELL

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:

corrosion-equation-1
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:

Reaction at the cathode

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.

WE ARE ALMOST THERE

For 2 years now Apiexam.com has been delivering information about what is required to pass the API 653 exam. So far we have covered some 90% of all the material needed to pass the exam at the first try. 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.com to get certified. Apiexam will continue to deliver information until 100% has been treated.

In the following post, I will be sharing with you an important information. Keep tuned.

 

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