Corrosion can be generally defined as;” Degradation of a metal (material) by chemical or Electro-chemical means. It is obvious that two mechanisms are involved, Firstly an Electrical Circuit and secondly a Chemical Reaction.
Electrical Circuit
In corrosion circuit the current is always D.C. (Direct Current). For corrosion circuit to exist three things are needed: Anode, Cathode and Electrolyte.
1. An Anode
Is a positively charged area? (It becomes positively charged because the atoms release two electrons), the iron atom has 26 of each, 26 protons and 26 electrons, in its passive state
When the two electrons are released the atom still has its 26 protons, but now only 24 electrons. (In this state the atom is now an ion, positively charged by two units and written as Fe++) (An ion is a charged particle, and can be positive or negative, a single atom or a group of atoms, known as a molecule.)
This losing of electrons can be shown as: - Fe→Fe+++2e (The Fe++ is called a positive iron ion).
2. A Cathode
Is a negatively charged area (where there are more electrons than needed in its passive state). At the cathode the electrons enter into the electrolyte to pass back to the anode.
3. An Electrolyte
Is a substance, which will conduct a current and be broken down by it, (dissociate into ions). Water, Acids, alkalis and salts in solution are very efficient electrolytes.
As the electrons pass into the electrolyte it is dissociated or (separated) into positive and negative ions, as shown by the formula: -2eH2O→2H++2OĦ.
The couple electrons back with the Hydrogen ions to form two full Hydrogen atoms, which join together to form Hydrogen gas. The hydroxyl ions return to the anode through the electrolyte carrying the electrons.
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| Corrosion Triangle |
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| Osmotic or Hygroscopic Blisters |
MATERIAL | KNOWN POTENTIAL AV. VALUES |
Graphite | + 0.25 v |
Silver | - 0.1 v |
Nickel 200 | - 0.15 v |
Copper | - 0.35 v |
Mill Scale | - 0.4 v |
Mild Steel | - 0.7 v |
Aluminium Alloys | - 0.9 v |
Zinc | - 1.0 v |
Magnesium | - 1.6 v |
The Chemical Reaction
Only the chemical reaction, (the formation of corrosion products), occurs at the Anode.The positive iron ions, Fe++, receive the returning hydroxyl ions and ironically bond together to form iron hydroxide, which is hydrous iron oxide, rust, and is shown by the formula: Fe++ + 2OĦ →Fe (OH)2
Corrosion only occurs at the Anode, never at the Cathode.
The corrosion triangle shows the three elements needed for corrosion to occur, Anode, Cathode and Electrolyte. If any one of these three is removed from the triangle, corrosion cannot occur. The one most commonly eliminated is the electrolyte. Placing a barrier between the electrolyte and the anodic and cathodic areas, in the form of a coating or paint system does this. If electrolyte is not in direct contact with anode and cathode, there can be no circuit, and so no corrosion.
Certain factors can increase the reaction rate, listed below are some of these.
1. Temperature.
Steel, is thermodynamically unstable metal.
The hotter steel is faster in corrosion than the other cooler one.
2. Hygroscopic Salts
(Hygroscopic-tending to observe)
A hygroscopic salt is one, which will attract water and dissolve in it.
When salts are present on a substrate (top of the surface) and a coating is applied over them, water will be drawn through the film and the resulting solution builds up a pressure under the film.
Eventually the film is forced up to form blisters.
These blisters are called osmotic or hygroscopic blisters, and are defined as ‘pinhead sized water filled blisters’.
Sulphates and Chlorides are the two most common salts, chlorides predominant in marine environments, and sulphates in industrial areas and sometimes agricultural.
3. Aerobic conditions
(Presence of oxygen) By introducing oxygen into the cathodic reaction the number of Hydroxyl ions doubles.
This means that double the number of iron ions will be passivated and therefore double the corrosion rate. Shown by: 2H2O + O2 + 4e → 4OH-
4. Presence of some types of bacteria
On the metal surface, for example Sulphur Reducing Bacteria, better known as (SRB), or MEMs, Metal Eating Microbes.
5. Acids and alkalis
6. Bi-metallic contact.(corrosion) Otherwise known as Bi-Metallic Corrosion.
Metals can be listed in order of nobility. A noble metal is one, which will not corrode. In descending order, the further down the list the metal is, the more reactive it is, and so, the more anodic it is, the metal loses its electrons to become reactive ions. The degree of activity can be expressed as potential, in volts. The list can be called Galvanic List, Electro Motive forces series or the Electro-Chemical series.
Mill scale
Is immediately above steel on the galvanic list.
This means that mill scale is Cathodic to steel, and if left on the surface of steel will accelerate the corrosion of the steel substrate.
Mill scale is formed during the rolling operation of steel sections e.g. RSC, RSA, RSJ.
The oxides of iron form very quickly at temperatures in excess of 580°C
The first oxide formed is FeO, iron oxide, the next is Fe3O4 and last of all Fe2O3. Common names in order are Wustite, Magnetite and Haematite.
These oxides are compressed during the rolling operation to produce blue mill scale.
The thickness of mill scale varies from 25 to 100 um(Microns)
When it has been removed by any surface preparation method, it can never re-cur.
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