API 651: Cathodic Protection of Aboveground Petroleum Storage Tanks Flashcards
A concrete tank pad, constructed on a stable, properly prepared subsoil can eliminate what three things? (API 651-14, Section 5.3.3.1)
1) eliminating intrusion of groundwater
2) soil-side corrosion, and
3) the need for cathodic protection
What is the most common material used as a pad beneath storage tank bottoms? (API 651-14, Section 5.3.2)
Clean sand is the most common material used as a pad beneath aboveground storage tank bottoms.
Why are thick-film, laminated, and chemical- and corrosion-resistant linings primarily installed in tanks? (API 651-14, Section 5.4.4)
The purpose of various types of internal tank linings is to mitigate internal corrosion threats and should not be considered as sufficient justification to eliminate the need for external cathodic protection of aboveground storage tank bottoms.
What are the advantages of a sacrificial anode (galvanic) cathodic protection system? (API 651-14, Section 6.2.2)
a) no external power supply is required;
b) installation is relatively easy;
c) capital investment is low for small diameter tanks;
d) maintenance costs are minimal;
e) interference problems (stray currents) are rare;
f) less frequent monitoring is required.
Why can secondary containment liners in a dike render a cathodic protection system ineffective? (API 651-14, Section 5.4.3.3)
Cathodic protection systems are rendered ineffective when an external liner is installed between the anodes and the tank bottom to be protected, because the external liner acts as a shield to the flow of cathodic protection current necessary for protection.
What are the disadvantages of a sacrificial anode (galvanic) cathodic protection system? (API 651-14, Section 6.2.3)
a) driving potential is limited;
b) current output is low;
c) use is limited to low-resistivity soils;
d) not practical for protection of large bare structures;
e) very short life expectancy in low-resistivity soils.
How can impressed current anodes be installed when bedrock is encountered at a shallow depth? (API 651-14, Section 9.3.2.2)
Sometimes it is necessary to install an anode in a location where rock is encountered at a shallow depth, or where soil resistivity increases markedly with depth. Such sites can be coped with by a horizontal installation of anodes. A ditch is excavated to whatever depth is practical, and a horizontal column of coke breeze is laid therein, usually square in cross section. The anode is laid horizontally in the center of this column.
What can be done to improve the current distribution to the center of tank bottoms? (API 651-14, Section 9.3.2.3)
In some instances, to improve current distribution to the center of the tank, it may be desirable to install anodes in holes which are bored at an angle under the perimeter of the tank bottom or bored horizontally underneath the tank bottom by directional drilling.
When cathodic protection is successfully applied to a tank bottom, the metal surface being protected acts as the __________ of an electrochemical cell.
Cathode
How do galvanic cathodic protection systems supply the current required to stop corrosion? (API 651-14, Section 6.2.1)
Galvanic systems use a metal more active than the structure to be protected to supply the current required to mitigate corrosion
A sand cushion is placed on top of an old steel tank bottom and a new steel bottom is installed without providing any corrosion prevention measures for the new steel bottom. What will be the likely fate of the new steel bottom? (API 651-14, Section 7.2.4.3)
The old bottom forms a shield that collects the cathodic current flowing through the ground and prevents cathodic protection of the new bottom. Unless the anodes are installed between the two steel bottoms (see Figure 8), or the old bottom is removed, electrically isolated, or coated with a nonconductive material, a galvanic cell can develop between the old and new bottom. Industry experience has shown that if a conductive electrolyte exists between bottoms, the current flow and metal loss will be from the new bottom, resulting in premature failure of the new bottom.
What are some advantages and disadvantages of impermeable membrane secondary containment systems? (API 651-14, Section 7.2.5)
a) There are several advantages to the installation of an external liner:
b) An external liner provides a means of detecting and containing leaks and preventing ground contamination if leaks occur.
c) An external liner eliminates the natural current flow between the old bottom and the new bottom, thus, reducing the accelerated failure of the new bottom due to galvanic corrosion.
d) An external liner may reduce entry of groundwater into the space between the bottoms
e) Installing an external liner could be disadvantageous for the following reasons.
f) An external liner makes the future addition of cathodic protection difficult.
g) An external liner acts as a basin to contain water or any other electrolyte that might wet the sand between the old and new bottoms, thus, increasing corrosion rates.
Sacrificial anodes intended for use in soil environments are packaged in what kind of backfill? What is the purpose of the backfill? (API 651-14, Section 7.3.5.1.3)
Special backfill, consisting of a proper mixture of gypsum, bentonite, and sodium sulfate, promotes anode efficiency, lengthens anode life, and keeps the anode environment moist.
At which electrode of an electrochemical cell does oxidation (corrosion) occur? (API 651-14, Section 3.2)
Anode
What is “cathodic protection?” (API 651-14, Section 3.7)
A technique to reduce corrosion of a metal surface by making the entire surface the cathode of an electrochemical cell.
