Nace CP 1, Course Manual

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Basic Chemistry and Basic Corrosion Theory 2:20
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
For example, if a steel bulkhead were discharging 875 mA over a period of
four years, how much metal would be lost?
Wt = KIT = kg
K = From Table 2.1, the loss rate is 9.1 kg/A-yr
I = 875 mA = 0.875A
T = The 4 year loss would therefore be:
9.1kg/A-yr x 0.875A x 4 years=31.9kg=70.3 lb
If the loss is occurring over the entire bulkhead, it will probably be
insignificant. If, on the other hand, the bulkhead is coated and loss is taking
place only at holidays, then several penetrations might occur in the short
time of four years.
Faraday’s Law is also very useful for determining the expected life of
cathodic protection anodes. Knowing the anode material and expected
output of an anode, you can calculate the life expectancy. These
calculations are beyond the scope of this course but are covered in
advanced cathodic protection courses.
Polarization
Polarization
is the deviation from the open circuit potential of an electrode
resulting from the passage of current. There are two major types of
polarization.
The fundamentals of electricity, chemistry, and electrochemistry form the
basis for an understanding of corrosion and cathodic protection.
As current flow continues over time, polarization occurs at both the anode
and cathode. Polarization lowers the potential difference between the anode
and cathode areas, causing a reduction in the corrosion current and the
corrosion rate.
Depolarization is a condition that counters the effects of polarization.
Depolarizers include:
•
Dissolved oxygen
•
Microbiological activity
•
Water flow