1. METL 1313 Introduction to Corrosion Lecture2
Basic Electricity

2. Basic Electricity
Corrosion and cathodic protection are electrochemical phenomena.
Electrical instruments are often used in corrosion testing.
The need to understand various electrical terms, laws, circuits and equipment is paramount when working with corrosion and cathodic protection.

3. Basic Electricity
Knowledge of the above is essential for anyone entering the field of corrosion/cathodic protection technology.

4. Electrochemistry
The branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity by chemical changes.

5. Electrons
Subatomic particles that revolve around a nucleus and carry a negative charge.
They also help hold matter together, similar to mortar in a brick wall.

6. Voltage
Voltage or potential, is an electromotive force or a difference in potential expressed in volts.
Voltage is the energy that puts charges in motion.
Can also be defined as electrical pressure.

7. Voltage Voltage is measured in volts, millivolts, and microvolts.
In corrosion work all three units are used.
The following shows their relationship:
1.000 volt millivolts
0.100 volt millivolts
0.010 volt millivolts
1 millivolt volt
volt microvolt

8. Voltage Common symbols for voltage are:
Emf electromotive force - any voltage unit
E or e voltage across a source of electrical energy (e.g. battery, pipe-to-soil potential)
V or v voltage across a sink of electrical energy (e.g. resistor)

9. Voltage
You will be concerned with voltage when making various measurements in corrosion and cathodic protection work.
Among those voltages measured are pipe-to-soil potentials, voltage drops across resistances or along pipelines.

10. Current
Current is the flow of charges along a conducting path and is measured in amperes.
Current is frequently abbreviated as amps, milliamps, or microamps.
In corrosion work we use all three units.
The following shows their relationship:

11. Current Relationships
1.000 ampere milliamperes
0.100 ampere milliamperes
0.010 ampere milliamperes
0.001 ampere milliampere
ampere microampere

12. Ampere
The ampere is the common unit of current, and is equal to a flow rate of charge of 1 coulomb per second.
One coulomb is the unit of charge carried by 6.24 x 1018 electron charges.

13. Current Common symbols for current flow are: I any amperage unit
mA milliamperes or milliamps
μA microamperes or microamps
Direct current flows constantly in one direction in a circuit.
Alternating current regularly reverses direction of flow, commonly 100 or 120 times per second.

14. Resistance
Resistance is the opposition or measure of difficulty that electric charges encounter when moving through a material.
The resistance of a conductor is defined as the ratio of the voltage applied to the electric current which flows through it.
The ohm is the common unit of resistance measurement.

15. Resistance
Resistance may also be measured in milliohms (0.001 ohm) or in megohms (1,000,000 ohms).

16. Resistance Common symbols for resistance are: R, r
Ω (Greek letter omega)
Resistance is very important in corrosion and cathodic protection work.

17. Resistivity
The resistance of a conductor of unit length and unit cross-sectional area.
The symbol used for resistivity is the Greek letter (ρ) pronounced rho).

18. Resistivity
An intrinsic property that quantifies how strongly a given material opposes the flow of electric current.
If the resistivity of a material is known, the resistance of a conductor such as a cable or pipeline of known length and cross-sectional area can be calculated from the formula: R = ρ x L / A

19. Resistivity
Resistivity is constant for a given material.

20. Cross-sectional Area Calculations
Square or rectangular shape: A (cm2) = h x w where: h = height (cm) w = width (cm)
Circular shape: A (cm2) = πr where: r = radius (cm)

21. Resistance to Current Flow
Resistance to current flow is lowest for:
Low-resistivity (high-conductivity) media
Short length for current flow
Large cross-sectional area for current flow

22. Resistance to Current Flow
Resistance will be greatest for:
High-resistivity (low-conductivity) media
Long path length for current flow
Small cross-sectional area for current flow

23. Resistivity of Common Materials
Material Resistivity (ohm-cm)
Aluminum x 10-6
Carbon x 10-3
Copper x 10-6
Iron x 10-6
Steel x 10-6
Lead x 10-5
Magnesium x 10-6
Zinc x 10-6
Ice x 108
Rubber x 1016
Water (tap) x 103
Water (sea) x 101
Soil (varies) x 102 to 5 x 105

24. Scientific Notation
Scientific notation uses exponents where the multiplier 10 is raised to a power.
For example: 1 x 102 = 1 x 10 x 10 = x 10-2 = 1 x .1 x .1 = .01

25. Resistivity
The common unit of resistivity measurement for an electrolyte is ohm-centimeter.
Electrolytes prevalent in corrosion and cathodic protection work includes soils and liquids (water).

26. Resistivity
Since electrolytes do not usually have fixed dimensions (the earth or a body of water, for example), resistivity is usually defined as the resistance between two parallel faces of a cube 1 cm on each side or 1 cm square.

27. Resistivity
Electrolyte resistivities vary greatly. Some electrolytes have resistivities as low as 30 ohm-cm (seawater) and as high as 500,000 ohm-cm (dry sand).
The resistivity of an electrolyte is an important factor when evaluating the corrosivity of an environment and designing cathodic protection systems.

28. Electric Circuit
An electric circuit is the path followed by an electric current.
Electrical laws govern the relationships in electric circuits.

29. Ohm’s Law Relationships
Where E or V = Voltage (electromotive force) I = Current (amperes) R = Resistance (ohms) E or V = IR I = E/R R = E/I

30. Ohm’s Law
If two of the three electrical variables are known, you can compute the third.
If you have measured the voltage and current in a cathodic protection circuit, for example, you can easily calculate the circuit resistance.

31. Ohm’s Law Triangle
An easy way to use the Ohm’s Law triangle is to place your thumb over the quantity you are seeking.
To solve for resistance, as in the above example, place your thumb over the R and you can see that R = E/I.
Solve for voltage by placing the thumb over the E and see that E = I x R.