CORROSION BEGINS WHEN THE CONDITIONS ARE RIGHT

When exposed to the Earth's atmosphere under the typical conditions, most metals have a tendency to slowly return to their natural state......as a metallic ore.

(NOTE: The only metals which do not commonly corrode under the average conditions present at the Earth's surface are gold, platinum and palladium.)

Example:  Iron in auto body steel exposed to the atmosphere "oxidizes" and changes back to Iron Ore (Ferric Oxide Fe₂O₃).......better known as rust. Corrosion causes nearly 400 billion dollars  in damage in the U.S. alone each year and it is estimated that most nations spend about 5% of their total income in combating or repairing corrosion damage.

"A RECIPE FOR RUST "

Take 2 parts iron (FE).

Combine with 3 parts Oxygen (O).

Add water and salt to taste.

Increase temperature to speed up process.

TEMPERATURE: Generally corrosion rates increase with increases in temperature. This is due to to several interrelated factors:

1.Higher temperatures tend to promote the corrosion reaction kinetics. Therefore except in cases where oxygen is free to escape, higher temperatures boost the corrosion rate. (Essentially makes the corrosion reaction go to completion more quickly)

2.Corrosive by products will have a higher diffusion rate at higher temperatures and thus will be delivered to the corroding surface more efficiently.

(Note: In open systems certain corrosive gases actually have lower solubility at higher temperatures. Thus an increase in temperature may result in a decrease in the corrosive gas solubility and accordingly a decrease in the corrosion rate as well.)

pH: Almost without exception the rate of corrosion increases with decreasing pH (increased acidity). This is due to corresponding increase in the H+ (hydrogen ion) concentration as well as the associated increase in the solubility of most other potentially corrosion enhancing substances.

OXYGEN AND OXIDIZER LEVELS: As one would suspect oxygen acts as an oxidizing agent in the corrosion process as do other common oxidizers such as Cl2 (chlorine gas)  and Br2 (bromine gas). As the concentration of these agents rises, corrosion rates increase.

See Also;

Types of Corrosion

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