1. Scope and Language of Corrosion
Chapter 1:
Scope and Language of Corrosion

2. Purpose
When a study of corrosion is first undertaken, it is often assumed that corrosion is a single simple reaction.
If cost and availability were not factors in material selection, only the very best materials would ever be chosen.
Consideration of materials such as gold or platinum must be dismissed, one must think in terms of affordable substances for practical use in homes, industries, automobiles, etc.

3. Purpose
Any number of alloys or classes of alloy may serve as the best choice for a particular application.
Learning when to choose which material to use, comes with experience and knowledge.
All materials have certain advantages as well as disadvantages.
Materials degrade by various mechanisms of attack in different types of environments.

4. Corrosion
Most people are familiar with corrosion in some form or another; in particular the rusting of an iron fence, a tin can, steel pilings, boats, or the rusting of an iron nail.

5. Corrosion
You may have noticed that the steel frames at new construction sites may have a green, orange, or reddish color.
Coatings are applied to protect the frames against rusting.
After installation, and particularly on bridge construction, steel surfaces are recoated with other materials that may lend a better appearance or added corrosion protection.

6. Corrosion
Developments in low-alloy steel technology permit the erection of iron structures without using paints or coatings under certain circumstances.
Some special alloys known as corroding steels, tend to rust initially, but then appear to stop rusting almost completely.
weathering steel bridge

7. Corrosion
All underground water, gas, and oil pipelines that crisscross the nation is subject to corrosion.
Corrosion can attack piping both internally, and externally.

8. Corrosion
It is probably safe to say that almost everyone, is somewhat familiar with corrosion, which is defined as follows:
Corrosion is the deterioration of a substance (usually a metal) or its properties due to reactions with its environment.
Note that the words chemical or electrochemical are not used in this definition.
The deterioration of wood, ceramics, plastics, etc., must also be studied by the corrosion engineer and is included in the term corrosion.
Attention must also be directed to the idea that properties, as well as the material itself, can and do deteriorate.

9. Corrosion
In discussing corrosion, it is necessary to use technical terms which are rarely used outside the field of corrosion.
Terms and definitions should not be regarded as the only correct ones; other definitions may serve as well or even better.
The modern corrosion engineer or technician must find suitable materials for a great variety of corrosive environments and temperature ranges.

10. Economic Significance
It is unfortunate, that many people, outside of industry as well as many within industry will often observe corrosion and simply accept it as an inevitable problem.
Something can and should be done to keep the environment safe, and prolong the life of many metallic materials exposed to corrosive environments.

11. Economic Significance
Corrosion control becomes a major factor in the very important area of dollars and cents.
The greatest interest and concern for corrosion, stems from its practical effects and how those effects may be avoided.
There is no general agreement as to what should be included in calculating the monetary loss to corrosion.
There is ample evidence that the annual cost of corrosion in the United States amounts to at least a trillion dollars.

12. Economic Significance
As further indication of the importance of corrosion control, studies on corrosion are reported in many journals, books magazines, and receives increasing attention from industry, government, and technical societies.
NACE International, formerly known as The National Association of Corrosion Engineers, was formed in 1943 to devote exclusive attention to the particular subject of corrosion.

13. Corrosion Around Us
All metals and alloys can and generally do corrode.
In looking about, it is easy to find metallic corrosion occurring all around us, as well as finding some metals which do not appear to be corroding at all.

14. Corrosion Around Us
The following are some examples of common systems, and areas susceptible to corrosion.
The automobile is an absolute corrosion testing device.
Materials must be selected for the following uses: Atmospheric, High- temperature, Closed cycle water, Wear, Fatigue, and Crevice exposures.
Many different metals, plastics, paints, plating, and inhibitors are used to prolong the life of the auto parts.
The auto body, and certainly the muffler, are generally seen in various stages of corrosion.
Looking at the car’s body may probably reveal corrosion taking place in numerous locations.
Even the once-bright bumper may show either corrosion pin holes or more serious attack.

15. Corrosion Around Us

16. Corrosion Around Us
The water pipes of an older home, particularly if they are made out of steel or galvanized steel, will usually show some evidence of corrosion.
Particularly at joints where the galvanized coating might have holidays, or at dissimilar metal junctions such as a brass valve.
Barnyard roofs made of steel or galvanized iron rust through in time.
Newer aluminum roofs (of the proper aluminum alloy) look bright and shiny and appear to be free of corrosion.

17. Corrosion Around Us

18. Corrosion Around Us
The green coatings on copper or brass roofs of churches and municipal buildings develops as a result of corrosion.
Copper-bearing metals that have reacted with certain components of the atmosphere, develop the green coating frequently called a patina or aerugo.

19. Corrosion Around Us
The patina coating is actually considered a protective corrosion layer that retards continued reaction and is also pleasing to the eye

20. Corrosion Around Us
In the case of an aluminum canoe, corrosion seems almost negligible, but a closer look will often reveals tiny pits of corrosion attack.
A tiny pit may not appear serious since it’s size is limited to such a small area, but even a small leak might be considered very serious.

21. Corrosion Around Us
Metals exposed to seawater will generally corrode more readily.
Ship hulls (inside and out), pilings, drilling rigs, oil platforms, and other equipment exposed to highly conductive marine environments must be protected in some manner.

22. Corrosion Around Us
Concrete degradation of bridges, highways, buildings, and drainage ducts is often caused by environmental interaction rather than by mechanical forces.
When corrosion control is not exercised during initial construction, tax payer’s money is often required for the restorations of failed structures and facilities.

23. Various Metals
Of the approximately 117 elements known to man, about 91 are metals.
Each element has different mechanical, chemical, and physical properties.
Although all metals can corrode, they each may corrode in a given situation to a different degree, and probably in a different manner.
About half of 91 metals have been alloyed to make more than 40,000 different alloys, with many more to come.

24. A Natural Process
Almost all material should be expected to deteriorate with time when exposed to ‘the elements.”
Corrosion is a perfectly natural process, as natural as water flowing downhill.
If iron were exposed to air and water, rust would be expected to develop within a matter of hours.
If copper, brass, aluminum, or stainless steel were substituted for iron, a given degree of corrosion might take longer, but some corrosion would still be anticipated.

25. A Natural Process
Instead of forming rust (a form of iron oxide), some oxide of copper, aluminum, or chromium may form very slowly and coat the bare metal.
An oxide coating, even if extremely thin, could form a partial barrier to continued attack, and slow down the rate of corrosion almost to a standstill.

26. Material Selection
For a specific application, a certain alloy may be superior to another.
Many materials compete for the same application because of factors other than corrosion.
Cost is a major factor; other factors may include one or a combination of the following:
Safety, cost.
corrosion behavior.
weldability.
Suitable mechanical properties (tensile strength, impact resistance, fatigue, etc.)
High or low temperature strength (ductility).

27. Measuring Corrosion
When corrosion occurs, a weight gain or loss is often experienced.
This weight change is most commonly used as the measure of the extent of corrosion.
The units usually used are as follows:
Weight Change: Weight loss or gain per unit area per unit time.
Dimension Change: Loss of metal thickness per unit time.
Mechanical Property Change: Percent loss in tensile strength, yield strength, ductility, or other mechanical property.

28. What Causes Corrosion?
There is now little or no controversy about what factors cause most forms of corrosion on metals.
Current thinking in the case of usual corrosion is firmly grounded in electrochemical theory.
Various formulas and equations have been devised that describe the chemical reactions which make up most corrosion processes.
There are three basic kinds of corrosion:
Chemical
Electrochemical
Physical