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Corrosion - a simple explanation

What is corrosion?
corr3

Steel gets rusty, copper gets covered with verdigris and other non precious metals are broken down in similar ways. We call it corrosion. The material corrodes as it reacts chemically with its environment. Almost all common corrosion is electro-chemical. It takes place in galvanic ”cells” and works in about the same way as a flashlight battery.
The flashlight battery has a central carbon staff (cathode or negative terminal) and a casing of zinc (anode or positive terminal). The space between them is filled with a
paste or sludge (electrolyte). When the circuit is closed (the light turned on) electrones flow
from the anode (the casing) to the cathode (the carbon staff) through the electrolyte (the sludge). The electrones return to the zinc casing via the glowing light-bulb. The casing is broken
down by the galvanic process (corrosion) and may eventually start to leak.


The cathode is precious. The anode is non precious

Galvanic cells are created when, for example, two different metals get in touch with an electrolyte (such as plain water). This kind of corrosion is called galvanic.
Galvanic cells can be utterly small and arise on a single metal surface - partly due to the different mixtures of utility metals, partly on account of contaminations like oxide scale and remains
of slag. When one microscopic particle is not as precious as the other corrosion may occur.
In a galvanic cell, the most precious metal (or other conductive material, like graphite) is always the cathode, the least precious always the anode.

Moisture and oxygen
Moisture and oxygen are needed to make steel rust. Both are present in the air.
When the relative humidity of the air exceeds about 60% the metal surface is covered by a very thin moist film that acts as an electrolyte.
If the steel surface is soiled by salt, dust, etc, rust may form at less humidity, since the dirt binds the moisture. Outdoors in Sweden, the air is mostly humid enough for the formation of rust.
Indoors, the air is heated and generally too dry for corrosion. When the temperature drops like in a factory that is closed for vacation, the risk of corrosion is immediately at hand

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The rusting of steel
When the film of moisture forms on the steel, lots of galvanic cells, or ”flashlight batteries” are formed.
The current of electrons flows from the more precious particles of the steel (cathodes) to the less precious (anodes), through the moist film (the electrolyte) back to the more precious particles. Rust is formed where the electrons leave the anode.
Eventually the entire surface will be covered by rust, that also spreads inwards, into the steel. The rust will continue to form as long as there is oxygen and moisture.

A screw can become a cathode or an anode
When a steel screw is used on a piece of sheet-copper, the screw becomes the anode, since copper is more precious.


TEEL SCREW
CURRENT
MOISTURE
COPPER

It will rust away quickly, due to the considerable difference in potential.
When the same screw is used on a piece of less precious sheet-metal, like aluminum, the screw becomes the cathode and does not rust. But the sheet-aluminum becomes the anode and will corrode.

 



STEEL SCREW
CURRENT
MOISTURE
ALUMINUM

Factors that determine corrosion speed
In order to make it easier to evaluate the relationship between metals, the electrochemical potential of the metal has been determined by a method of electric measuring. The concept of potential will provide an idea about how precious or non precious a metal really is.
The speed of corrosion depends on the difference in potential between the metals or the metal grains, the conductivity of the electrolyte, the access of oxygen as well as the size of the anode and cathode surfaces.

Difference in potential
The greater the difference in potential between the metals, the faster the corrosion of the anode. Without special precautions, only metals of the same, or nearly the same, potential should be combined for use in a moist environment.

The conductivity of the electrolyte

MOISTURE FILM OF POOR CONDUCTIVITY
BLUE = ANODE
ORANGE = CATHODE

If the conductivity of the electrolyte is poor (fresh water) the corrosion will concentrate to the area adjacent to the contact point between the anode and the cathode.
If the conductivity is good (salt water), the extension of the


MOISTURE FILM OF GOOD CONDUCTIVITY
BLUE = ANODE
ORANGE = CATHODE

corrosion will be considerably larger.
Outdoors, the speed of corrosion is affected by air pollution. Highly polluted industrial and city air is most aggressive. Second is salty sea air. In pure countryside environment metals hold up a lot better.

The effect of oxygen
If the oxygen is stopped from reaching the metal surface there won´t be any rust. The more oxygen, the faster rust will form (up to a limit). Ina beam that is hermetically sealed there will not form any rust since no oxygen will enter. In closed heating systems the oxygen will eventually be consumed and the formation of rust will stop.

The size of the anode and cathode surfaces

A larger cathode surface will be in contact with more oxygen. And the smaller the anode surface is in relation to the cathode surface, the more concentrated the corrosion current from the anode will be. The combination of small anode surface/ large cathode surface means an increased risk of corrosion

GREAT RISK OF CORROSION
BLUE = ANODE
ORANGE = CATHODE

 

 



SMALL RISK OF CORROSION RISK OF CORROSION
BLUE = ANODE
ORANGE = CATHODE

 
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