When exposed to the Earth's atmosphere under
typical conditions, most metals have a tendency to slowly return to their natural
state......as a metallic ore.
are only three metals which do not commonly corrode under the average conditions
present at the Earth's surface and the gold is the cheapest of these!( Platinum
and palladium being the other two.)
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
RECIPE FOR RUST"
2 parts iron (FE).
with 3 parts Oxygen (O).
water and salt to taste.
up the heat to speed up process.
Generally corrosion rates increase with increases in
temperature. This is due to to several interrelated factors:
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.
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.)
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.
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.
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.
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.)
takes place at the anode (+) while reduction takes place at the cathode (- )...
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
usually are produced by three factors:
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.
Less common processes such as high temperature sulfidation and certain
other reactions may also considered to be 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.