API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks Flashcards
- 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)
protection: A technique to reduce corrosion of a metal surface by making the entire surface the cathode of an electrochemical cell.
- What is a “continuity bond?” (API 651-14, Section 3.11)
A metallic connection that provides
electrical continuity.
- What is meant by “electrical isolation?” (API 651-14, Section 3.17)
An electrical circuit
where electrical current flows from certain areas of a metal to other areas through a solution capable of conducting electricity (electrolyte).
- What is a “galvanic anode?” (API 651-14, Section 3.25)
- What is meant by “structure-to-electrolyte voltage?” (API 651-14, Section 3.47)
(also structure-to-soil potential or pipe-to-soil potential): The voltage difference between a metallic structure and the electrolyte which is measured with a reference electrode in contact with the electrolyte.
- What are four major factors that influence the severity of internal corrosion? (API 651-14, Section 4.3)
a)
b)
c)
d)
a. Conductivity (a function of dissolved solids).
b. Suspended solids.
c. pH level.
d. Dissolved gases such as CO2, H2S, or O2,
- What are four factors that can limit the effectiveness or even preclude the use of cathodic protection on aboveground tank bottoms? (API 651-14, Section 5.1.4)
a)
b)
c)
d)
a. Foundations such as concrete, asphalt, or oiled sand.
b. An impervious lining between the tank bottom and anodes
such as in secondary containment systems.
c. High resistance soil or rock foundations.
d. Old storage tank bottoms left in place when bottom is
installed.
- When a native soil pad is to be used beneath an aboveground tank, what characteristics of the soil can be measured to conduct a corrosion analysis of the site? (API 651-14,
Section 5.3.7.1)
Determination of aggressive ions such as
chlorides and sulfates along with measurement of pH and resistivity are helpful for further corrosion analysis.
- When cathodic protection is successfully applied to a tank bottom, the metal surface being protected acts as the of an electrochemical cell. (API 651-14, Section 6.1)
Cathodic protection is a technique for
preventing corrosion by making the entire surface of the
metal to be protected act as the cathode of an electrochemical cell.
There are two systems of cathodic protection:
a. Galvanic.
b. Impressed current.
- 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 struc-
ture to be protected to supply the current required to stop corrosion (see Table 3 for a partial galvanic series). The more active metal is called an anode, commonly referred to as a galvanic anode or a sacrificial anode. The anode is electrically connected to the structure to be protected and buried in the soil.
- What are the advantages of an impressed current cathodic protection system? (API 651-14, Section 6.3.2)
a)
b)
c)
d)
a. Availability of large driving potential.
b. High current output capable of protecting large structures.
c. Capability of variable current output.
d. Applicability to almost any soil resistivity.
- What are the disadvantages of an impressed current cathodic protection system? (API 651-14, Section 6.3.3)
a)
b)
c)
d)
e)
f)
The disadvantages of impressed current systems include:
a. Possible interference problems (stray currents) on foreign structures.
b. Loss of AC power causes loss of protection.
c. Higher maintenance and operating costs.
d. Higher capital cost for small installations.
e. Safety aspects of rectifier location.
f. Safety aspects of negative lead connection.
g. More frequent monitoring.
- What are some advantages and disadvantages of impermeable membrane secondary containment systems? (API 651-14, Section 7.2.5)
There are several advantages to the installation of a sec-
ondary containment system:
a. Provides a means of detecting and containing leaks and preventing ground contamination if leaks occur.
b. Eliminates the natural current flow between the old bottomand the new bottom, thus reducing the accelerated failure of the new bottom due to galvanic corrosion.
c. May reduce entry of groundwater into the space between the bottoms. The integrity of tank-to-membrane seals is critical in this case.
Installing a secondary containment system could be disadvantageous for the following reasons:
a. Makes the future addition of cathodic protection virtually impossible.
b. Membrane 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.
c. Could entrap hydrocarbon products, thereby requiring
extra precautions for hot work.
- 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)
Magnesium and zinc anodes prepackaged in spe-
cial backfill are readily available in a number of size and
weight configurations to meet various current output and
anode life design requirements. The use of a special backfill with anodes is desirable for installation in soil environments.
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.
What two factors determine the number of sacrificial anodes required to provide cathodic protection for aboveground storage tanks? (API 651-14, Section 7.3.5.1.4)
a)
b)
total current requirements and the expected individual
anode discharge rate in the soil.
In impressed current systems, what types of anodes are generally preferred for soil installations? (API 651-14, Section 7.3.5.2.1.2)
Graphite, high silicon cast iron, or mixed metal
oxide anodes are generally preferred for soil installations.
Platinized niobium, tantalum, and titanium are best suited for water rather than soil installations.
How are current requirement tests for impressed current cathodic protection systems conducted? (API 651-14, Section 7.3.5.2.2.2)
Current requirement tests are conducted by
forcing a known amount of current from the temporary anode bed through the soil and onto the tank to be protected. The degree of protection at various locations around the tank, and under the tank center if possible, is evaluated using potential measurements. This testing allows approximation of the current required to protect the tank. Current requirement tests should be conducted with an adequate liquid level in the tank to maximize contact of the tank bottom with the cushion
material.
What are three generally accepted criteria for determining if adequate cathodic protection has been achieved? (API 651-14, Section 8.2.2)
A negative (cathodic) potential of at least 850 m V
with the cathodic protection current applied. This potential shall be measured with respect to a saturated copper/copper sulfate reference electrode (CSE) contacting the electrolyte. Voltage drops other than those across the tank bottom-to-electrolyte boundary must be considered for valid interpretationof this voltage measurement.
A negative polarized potential of at least 850 m V relative to a CSE. (One common method of measuring polar-
ized potential is by using the “instant off” technique.)
A minimum of 100 m V of cathodic polarization
measured between the tank bottom metallic surface and a stable reference electrode contacting the electrolyte. The formation or decay of this polarization can be measured to satisfy this criterion.
What is the standard method of determining the effectiveness of cathodic protection? (API 651-14, Section 8.3.1)
The standard method of determining the effectiveness
of cathodic protection on a tank bottom is the tank-to-soil potential measurement.
How should packaged galvanic anodes be installed? (API 651-14, Section 9.2.1)
Packaged anodes should be inspected to ensure integ-
rity of the container and should be kept dry during storage. If
individually packaged anodes are supplied in waterproof containers, that container must be removed before installation. Electrical continuity between the anode and lead wire should be tested without compromising the integrity of the package.
Packaged galvanic anodes should be back-filled with compacted native soil. Figure 11 shows a typical galvanic anode installation.
- Why must the coke breeze backfill around impressed current anodes be correctly installed? (API 651-14, Section 9.3.1.3)
Impressed current anodes can be buried vertically,
horizontally, angled, or in deep holes. Impressed current
anodes are typically installed in carbonaceous backfill such as coke breeze. If the backfill is installed properly so that there are no voids around the anode, much of the current reaching the anode is conducted to the backfill by electrical contact. This promotes consumption of the backfill instead of the anode and substantially lengthens the effective anode life.
Carbonaceous backfill also tends to reduce total circuit resistance by lowering anode-to-soil resistance.
- What is the weak point in all anodes? (API 651-14, Section 9.3.1.3)
cable connection to the
anode; this is the weak point in all anodes,
- To what should the negative lead of a cathodic protection rectifier be connected? (API 651-14, Section 9.3.4.3)
To the structure to be protected
- What will happen if the positive and negative output leads from an impressed current cathodic protection rectifier are reversed? (API 651-14, Section 9.3.4.3)
If the leads are reversed, with the positive lead
mistakenly attached to the tank, the tank bottom will serve as an anode and rapid corrosion failure can result.
- What can happen if underground wire attached to the positive rectifier terminal is not completely insulated? (API 651-14, Section 9.3.5.1)
If not completely insulated, the wire may discharge current (act as an anode), which will result in corrosion of the wire and rapid failure of the cathodic protection installation.
- What is the recommended frequency of cathodic protection surveys? (API 651-14, Section 11.3.1.2)
Prior to energizing a new cathodic protection sys-
tem, measurements of the native structure-to-soil potential should be made. Immediately after any cathodic protection system is energized or repaired, a survey should be conducted to determine that it operates properly. An initial survey to verify that it satisfies applicable criteria should be conducted after adequate polarization has occurred.
- What is the recommended frequency for checking sources of impressed current? (API 651-14, Section 11.3.2.2)
All sources of impressed current should be
checked at intervals not exceeding two months.
How long should records related to the effectiveness of cathodic protection be retained? (API 651-14, Section 11.4.7
Records related to the effectiveness of cathodic protection should be retained for a period of 5 years unless a shorter period is specifically allowed by regulation.
