1. Basic Corrosion Theory
METL 1313
Introduction to Corrosion
Lecture3
Basic Corrosion Theory

2. What Is Corrosion?
One general definition of corrosion is the degradation of a material through environ- mental interaction.
This definition encompasses all materials, both naturally occurring and man-made and includes plastics, ceramics, and metals.

3. Corrosion
In this course our focus will be on the corrosion of metals, with emphasis on corrosion of carbon and low-alloy steels used in many structures both above and below ground.

4. Corrosion
A significant amount of energy is put into a metal when it is extracted from its ores, placing it in a high-energy state.
Those ores are typically oxides of the metal such as hematite (Fe2O3) for steel or bauxite (Al(OH3) for aluminum.

5. Corrosion
One principle of thermodynamics is that a material always seeks the lowest energy state.
Most metals are thermodynamically unstable and will tend to seek a lower energy state, which is an oxide or some other compound.

6. Corrosion
The process by which metals convert to the lower-energy oxides is called corrosion.
Corrosion of most common engineering materials at near-ambient temperatures occurs in aqueous (water-containing) environments and is electrochemical in nature.

7. Corrosion
An aqueous environment is also referred to as an electrolyte.
In the case of under-ground corrosion, moist soil is the electrolyte.
The corrosion process involves the removal of electrons (oxidation) from the metal, and the consumption of those electrons by some other reduction reaction, such as oxygen or water reduction.

8. Corrosion
The oxidation reaction is commonly called the anodic reaction.
The reduction reaction is called the cathodic reaction.
Both electrochemical reactions are necessary in order for corrosion to occur.
The oxidation reaction causes the actual metal loss but the reduction reaction must be present to consume the electrons liberated by the oxidation reaction.

9. Corrosion
Oxidation and reduction reactions are sometimes referred to as half-cell reactions and can occur locally (at the same site on the metal) or can be physically separated.
When electrochemical reactions are physically separated, the process is referred to as a differential corrosion cell.

10. Corrosion
A site where metal is being oxidized is referred to as the anode or anodic site.
Electric current in the form of positively charged ions (defined as a positive flow of charge) flows from the metal surface into the electrolyte.

11. Corrosion
Positive current flows in the electrolyte to the site where oxygen, water, or some other species is being reduced.
Those sites are referred to as the cathode or cathodic sites.

12. Corrosion
There are four necessary components of a differential corrosion cell.
An anode must be present
There must be a cathode
There has to be a metallic path electrically connecting the anode and cathode. (normally, that is the metal itself.)
The anode and cathode must be immersed in the same electrically conductive electrolyte (normally, moist soil or water).

13. Corrosion
Underground corrosion of pipelines and other structures is often the result of differential corrosion cells of which a variety of different types exist.
Differential corrosion cells include (differential aeration cells) which can develop where different areas on a metal surface is exposed to different oxygen concentrations in the electrolyte.

14. Corrosion
Differential corrosion cells can be created by differences in the nature of the pipe surface or the soil chemistry.
Galvanic corrosion is a form of differential cell corrosion in which two different metals are electrically coupled and exposed in an electrolyte.

15. How Do We Detect Corrosion?
The electrochemical nature of the corrosion process provides opportunities to detect and mitigate corrosion of underground structures.
We can monitor the voltages and the currents associated with the corrosion process.
When a piece of metal is placed in an electrolyte, such as soil, or water, a voltage or corrosion potential will develop across the metal/electrolyte interface because of the electrochemical nature of the corrosion process.

16. Corrosion
We cannot directly measure metal/electrolyte interface voltage or corrosion potential, but by using a voltmeter and a reference cell, we can measure the voltage or corrosion potential of metals in an electrolyte.
Reference cells or electrodes are commonly referred to as half-cell electrodes.

17. Corrosion
The measured voltage is referred to as, a corrosion potential, solution potential, open circuit potential, or native potential of a metal in the particular environment in which the measurement is being made.
For soil environments, the most common reference cell/electrode used is the copper/copper sulfate reference electrode (CSE or Cu/CuSO4).

18. Corrosion
Potential measurements can be used to estimate the tendency of a metal to corrode, or relative resistance of different metals to corrosion in a given environment.
Noble metals, such as gold and platinum, have more positive potentials and are more resistant to corrode than are the more common engineering metals such as steel and aluminum.

19. Corrosion
A galvanic series is a list of metals and alloys arranged according to their relative corrosion potentials in a given environment.
Table 1.1 shows a galvanic series for metals and other materials in neutral soils and water.
Table 1.1 shows that carbon has the most positive potential of the materials listed and magnesium has the most negative potential.

20. Practical Galvanic Series

21. Corrosion
Potentials measured for different metals in a galvanic series vary somewhat, depending on the nature of the environment.
The relative position of the metals is similar for natural environments such as soil and seawater.

22. Corrosion
Another use for corrosion potential measurements is to establish whether galvanic corrosion is likely to occur.
When two metals are electrically coupled in an environment, the more negative (active) member of the couple will become the anode in the differential corrosion cell.

23. Corrosion
The more positive (noble) member of the couple will become the cathode in the cell.
In general, the severity of corrosion due to galvanic couples tend to increase as the difference in potential between the two members of the galvanic couple increases.

24. Corrosion
The galvanic series in table 1.1 indicates that, when copper is electrically coupled to commercially pure aluminum in neutral soil or water, the copper will become the cathode, accelerating corrosion of the commercially pure aluminum.

25. Practical Galvanic Series

26. Corrosion
Table 1.1 also shows that the potential of mild steel can differ depending on whether the surface is clean or covered with mill scale.

27. Practical Galvanic Series

28. Corrosion
The potential of metals is a function of electrolyte properties, including pH, ion concentration, oxygen, and moisture content.
The potential differences that develop on underground pipelines and other structures as a result of electrolyte factors can result in severe corrosion.

29. Corrosion
Potential measurements are commonly used on underground pipelines to detect the presence of different types of differential corrosion cells.
An electrical connection is made to the pipe, and the potential of the pipe is measured with respect to a reference electrode placed over a pipeline.
The afore mentioned process is shown schematically in Figure 1.2.

30. Pipe/Soil Measurement

31. Corrosion
Normally, the reference electrode is connected to the negative lead of a digital voltmeter to obtain a negative reading.

32. Pipe/Soil Measurement

33. Corrosion
With this type of measurement, the most negative regions of the structure are the anodes and are undergoing accelerated corrosion due to the differential corrosion cells.

34. Practical Galvanic Series

35. Corrosion Measurements
Current measurements can be used to detect differential corrosion cells if the anodes and cathodes are large.
Large corrosion cells create long-line currents that can be detected by measurements made over pipelines or other underground structures.
Through Ohm’s law (V = IR), where V is voltage, I is current, and R is resistance) we know that current flow through an electrolyte will create a voltage gradient.

36. How Do We Mitigate Corrosion?
The principal methods for mitigating corrosion on underground or submerged structures are coatings and cathodic protection (CP).
Coatings are normally intended to form a continuous film of electrically insulating material over a metallic surface to be protected.

37. Coatings
The function of a coating is to isolate the metal from contact with the surrounding environment (preventing direct contact the metal).
Coatings interpose such a high electrical resistance that electrochemical reactions cannot readily occur.

38. Coatings
In reality, all coatings, regardless of overall quality, contain voids or holes.
Holes in coatings are referred to as holidays.
Holidays are formed during application, transportation, or installation of mill-coated pipe.
Holidays in coatings also develop in service as a result of degradation of the coating, soil stresses, or movement of the pipe in the ground.

39. Coatings
Degradation of coatings in service can also lead to coating disbondment from the pipe surface, further exposing metal to the underground environment.
A high corrosion rate at a holiday or within a disbonded region can result in a leak or rupture, even where the coating effectively protects a high percentage of the pipe surface.

40. End