Corrosion

The corrosion process involves the deterioration of a substance, usually a metal, or its properties because of the reaction with its environment.

Electrochemical

The formation of the surface layer, while relatively thin can form an effective barrier against further attack, and thus slow the rate of the corrosion process.

Iron oxide

The anode is the part of the metal that corrodes (i.e dissolves in the electrolyte).

Anode

Cathode

Metal pathway

Electrolyte

The greater the metal loss

• E=IR
• E is the driving voltage of the circuit.
• I is the current magnitude
• R is the resistance of the circuit.

The Anode

The It is the more noble region on the electrode (metal surface) where the electrons are consumed.

Reduction of material

The potential differences

The return path connects the anode and the cathode and allows passage of electrons, generated the anode to the, when corrosion takes place on a metal surface there is always a metal pathway joining the anode to the cathode

And electrolyte is a medium that conducts Ionic (rather than electronic) current.

Anode, cathode, return Path, and electrolyte must be present for corrosion to occur. The effort of removing one or more of them is Corrosion Control

• Anode
• Cathode
• Return path
• Electrolyte

It increases the efficiency rate of the corrosion reaction

If the anodes and cathodes to remain in the same place for a period of time, but corrosion is localized, and referred to is pitting corrosion.

Mill scale is found on new iron and steel in the form of blue-black layers, of iron oxide, some of which are harder than the parent metal. The nanoscale is electrically positive reactive to the iron or steel, so it is Product to the parent metal.

A blue-black layer of iron/iron oxide 

Cathodic relative to substrate

Generally removed prior to painting

Galvanic Series is a list of metals in order of their corrosion potentials, with the most easily corroded, or most active, at the top, and the least easily corroded, or least active, at the bottom

• Magnesium
• Zinc
• Aluminum
• Carbon steel
• Cast-iron
• Copper
• Stainless Steel
• Silver
• Gold
• Platinum

When dissimilar metals are connected, the most active (or anodic) metal corrodes more rapidly, while the more noble is better protected corrodes less rapidly.

As the potential difference between these two dissimilar metals increases, the rate of Galvanic corrosion increases.

Corrosion rates increase as potential differences between metal increases.

Oxygen

Temperature

Chemical salts

Humidity

Corrosion reactions are electrochemical in nature and usually accelerated with increasing temperature, therefore corrosion proceeds faster in warmer environments than colder ones.

Chemical salts increased the rate of corrosion by increasing the efficiency (connectivity) of the electrolyte.

It also acts as a hygroscopic material extracting moisture from the air which increases the corrosion and non-immersed areas.

Time of wetness refers to the length of time atmospherically exposed substrate has sufficient moisture to support the corrosion process.

• 1 Chemical/Marine
• 2 Chemical with high humidity
• 3 Marine with high humidity
• 4 Chemical with low humidity
• 5 Rural with low humidity
• 6 Rural

A very severe environment causes rapid rusting. Airborne salts and chemical pollutants may serve to stimulate corrosion. Humidity and seawater provide electrolytes, which also hasten the process.

This environment is highly corrosive, because of its gases, chemicals, and high humidity.

General corrosion results in a relatively uniform loss of material over the entire surface.

Thinning over the affected surface.

Localized corrosion occurs at discrete sites on the metal surface. The areas immediately adjacent to the localized corrosion normally corrode too much lesser extent.