HOW DOES CORROSION FORM ON A METAL BODY?
Requirements for Corrosion
Corrosion is nature's method whereby metals and alloys return to their un-refined naturally occurring forms as ores and minerals. Metals such as iron, nickel, zinc, aluminum and copper occur naturally as oxides as well as sulfides and carbonates (meaning they are combined with the elements oxygen, sulfur or carbon and oxygen). When ore is refined this natural and most stable state (ore) is reversed to produce the actual metal which is less stable under natural conditions than the ore. A large amount of energy is applied to the ore (blast furnaces etc.) as the metal is converted from its oxide, carbonate, sulfide etc. Given the right conditions and following the
law of entropy (DEFINITION: ENTROPY IS THE NATURAL TENDENCY OF ALL SYSTEMS TO PROCEED FROM AN ORDERED AND LESS STABLE STATE TO A MORE DISORDERED AND MORE STABLE STATE) these high energy and more ordered metals tend to revert to their low energy state and less ordered ore condition.
Thus with most metals, including iron and steel alloys, this means that under most natural conditions unless special precautions are followed, metals will combine with oxygen to form rust and corrosion.
FOR CORROSION TO OCCUR THREE REQUIREMENTS MUST BE MET:
1.The metal needs to be in contact with an electrolyte. (DEFINITION: ELECTROLYTES ARE SOLUTIONS WHICH CONDUCT ELECTRICAL CURRENT AND SUPPORT IONIZED PARTICLES.) Usually the electrolyte is water but other liquids as well as gases in some instances may serve as electrolytes as well.
2.The presence of dissolved substances in the electrolyte is necessary. These substances can be dissolved gases such as oxygen, chlorine or bromine gases or dissolved ions such as H+ (hydrogen ion) all of which serve as strong corrosive agents. Pure distilled water with no dissolved substances will not function as an electrolyte and metals subjected to it will not corrode.
3.The presence of a corrosion cell whereby two portions of the metal surfaces become electrically connected via an electrolyte, salt bridge etc. A portion of the metal surface then becomes anodic (corrodes) giving up metal ions and another portion becomes cathodic, gaining metal ions.
(SIMPLIFIED CORROSION CELL NEUTRAL PH.)
Oxidization takes place at the anode (+) while reduction takes place at the cathode (- )... Ie.
Anode dissolves while cathode remains intact.
This microscopic corrosion cell represents only one of trillions of such cells that can occur on every metal surface. A corrosion cell can occur on the molecular level.
These cells usually are produced by three factors:
1. Irregularities in the metal's surface produced by the original metalworking, forming or extruding.
2. Differences in the composition of the metal's surface. In alloyed materials such as most steels this may be due to different microstructures or to micro crystals of different metals. Additionally, inclusions at or beneath the metal surface may create anode/cathode couplings. These impurities may be introduced when the metal was still molten or due to impurities actually being pressed into the surface by shaping, rolling or finishing operations.
3. Stresses induced from forming , welding etc.
An automobile body or other painted or coated steel structures serve as havens for much larger corrosion cells that can form and grow in the pores or breaks in its painted surfaces!
Virtually all forms of corrosion occur by the same basic chemical reaction as fire, "oxidation", and as the name of the reaction implies the presence of oxygen or oxidizers is not only required for the reaction to begin but to continue. Essentially the metal combines with oxygen to form a new substance rust or corrosion.
Note: Less common processes such as high temperature sulfidation and certain other reactions may also considered to be corrosion).
The corrosion process is a natural and normal occurring electrochemical reaction, that takes place when metal objects (as on an automobile body) are exposed to an oxidizing environment usually under wet or moist conditions.
Please see also:
Types of Corrosion A Recipe for Rust
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