BGAS Grade 1 Painting Inspector Course
BGAS Grade 1 Painting Inspector Course Fees 450 $ | BGAS Grade 1 Painting Inspector Course Exam Fees To be informed | BGAS Grade 1 Painting Inspector Course Duration 25 Days | BGAS Grade 1 Painting Inspector Course Location Muscat
Corrosion
Corrosion can be generally defined as degradation of a metal by chemical or electro-chemical means.
BGAS Grade 1 Painting Inspector Course covers From this definition it is obvious that two mechanisms are involved, firstly an electrical circuit and secondly a chemical reaction.
Electrical circuit
In a corrosion circuit the current is always direct current (DC). It is conventionally thought that a current passes from positive + to negative -, ie from anode to cathode. In fact electrons are flowing in exactly the opposite direction, from cathode to anode. For corrosion circuit to exist three things are needed:
Anode
An anode is a positively charged area. It becomes positively charged because the atoms release two electrons each, thus causing an imbalance between protons and electrons, positive and negatively charged units.
In its passive state, the iron atom has 26 of each, protons and electrons, when the two electrons are released the atom still has 26 protons, but now only 24 electrons.
In this state the atom is now an ion, overall 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 à Fe++ +2e. The Fe++ is called a positive iron ion. An ion can be positive or negative and is a charged particle, an atom or a group of atoms.
Cathode
A cathode is a negatively charged area where there are more electrons than needed in its passive state. These are electrons released from the anode. At the cathode the electrons enter into the electrolyte to pass back to the anode.
Electrolyte
An electrolyte is a substance, which will conduct a current and be broken down by it, (dissociate into ions). Water is the most abundant electrolyte and also very efficient. Acids, alkalis and salts in solution are also very efficient electrolytes. As the electrons pass into the electrolyte it is dissociated into positive and negative ions, as shown by the formula: -2H2Oà2H+ + 2OH-.
Simultaneously the electrons couple back with the hydrogen ions to form two full Hydrogen atoms, which join together diatomically to form hydrogen gas. This is termed as being evolved, or given off from the cathode. The hydroxyl ions return to the anode through the electrolyte carrying the electrons.
The corrosion triangle, as shown below, can illustrate the electrical circuit. The electron circuit can be seen to be from anode A, to cathode C, through the electrolyte E, back to A.
The chemical reaction
From the above we can see that no chemical reaction, (combination of elements) has occurred at the cathode, or in the electrolyte. The chemical reaction, the formation of corrosion products, only occurs at the anode. BGAS Grade 1 Painting Inspector Course covers The positive iron ions, Fe++, receive the returning hydroxyl ions and ionically bond together to form iron hydroxide, which is hydrous iron oxide, rust and is shown by the formula: Fe++ + 2OH-à Fe (OH)2 It is now apparent that corrosion only occurs at the anode, never at the cathode, hence the term cathodic protection.
If a structure can be made to be the cathode in a circuit, it will not corrode. 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. The basic corrosion reaction, as explained above, occurs fairly slowly at ambient temperatures.
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