Corrosion - 1 Flashcards
Corrosion can be generally defined as degradation of a metal by ,,,,,,,,, or ,,,,,,,,, means
chemical or electromechanical
Two mechanisms are involved in corrosion, firstly ,,,,, and secondly ,,,,,,,,.
An electrical circuit & Chemical reaction.
In a corrosion circuit the current is always ,,,,,
direct current (DC).
It is conventionally thought that a current passes from ,,,,,, to ,,,,,,, but electrons are flowing the opposite direction, ,,,,,,
- positive + to negative
(anode to cathode)
-from cathode to anode
For a corrosion circuit to exist three things are needed
- Anode
- Cathode
- Electrolyte
An anode is a ,,,,,, charged area which becomes positively charged because the atoms ,,,,,, two electrons each
positively
release
Anode:
In its ,,,,,, state, the iron atom has 26 of each, protons and electrons
passive
Anode:
when the two ,,,,,, are released, the atom still has its 26 ,,,,,,, but only ,,,,,,, . In this state the atom is now ,,,,,
electrons
protons
24 electrons
an ion
Anode:
overall positively charged iron by two units and written as ,,,
Fe++
An ,,,,,, is a charged particle and can be
positive or negative, a single or a group of atoms, known as a molecule
ion
Anode:
This losing of electrons can be shown as: ,,,,,,,.
The Fe++ is called ,,,,,,
-Fe → Fe++ + 2e
a positive iron ion
Anode:
An ,,,,, can be positive or negative and is a
,,,,,,,, an atom or a group of atoms
ion
charged particle
Anode:
A passive iron atom Fe has ,,,,proton & ,,,,,electrons
26
26
Anode:
An iron ion Fe++, has ,,,,,, protons and ,,,,,, electrons
26
24
Cathode:
A cathode is ,,,,,, charged area
negatively
Cathode:
A cathode is a negatively charged area where there are ,,,,, electrons than needed in its ,,,,,,,.
more
passive state
Cathode:
At the cathode the electrons enter into the ,,,,,, to pass back to the ,,,,,
electrolyte
anode.
Electrolyte:
An electrolyte is a substance, which will ,,,,,,,and ,,,,,,,
conduct a current and be broken down by it, (dissociate into ions).
,,,,,, is the most abundant electrolyte and
also very efficient
Water
,,,,,,,, and ,,,,,, in solution are also very efficient
electrolytes.
Acids, alkalis and salts
As the electrons pass into the electrolyte it is ,,,,,,, into positive and negative ions, as shown by the formula:,,,,,,,,,,, Simultaneously the electrons couple back with the ,,,,,,, ions to form ,,,,,,,, atoms, which join together ,,,,,, to form hydrogen
gas. This is called being ,,,,,, or ,,,,,,,
dissociated
-2H2O→2H+ + 2OĦ
hydrogen
two full hydrogen
diatomically
evolved
given off from the cathode
Electrolyte:
The hydroxyl ions return to the ,,,,, through the electrolyte carrying the ,,,,,,,
anode
electrons.
The chemical reaction:
In corrosion, chemical reaction, the formation of corrosion products, only occurs at the ,,,,,,
anode
If a structure can be made to be the ,,,,, in a circuit, it will not corrode
cathode
The chemical reaction:
The corrosion triangle shows the three elements needed for corrosion to occur are ,,,,,,
anode, cathode and electrolyte.
The chemical reaction:
The one most commonly eliminated element is
the ,,,,
electrolyte
The chemical reaction:
If ,,,,,, is not in direct contact with anode and cathode, there can be no circuit and so no corrosion.
electrolyte
The chemical reaction:
factors can increase the reaction rate, listed below are some of these are; ,,,,,
Temperature
Hygroscopic salts
Aerobic conditions
SRBs or MEM
Bi-metallic contact
The chemical reaction:
,,,,,,,and ,,,,,, are the two most common salts,
Sulphates
Chlorides
The chemical reaction:
chlorides predominant in ,,,,, environments
marine
The chemical reaction:
sulphates predominant in ,,,, and ,,,,.area
industrial
sometimes agricultural
The chemical reaction:
By introducing oxygen into the cathodic reaction the number of ,,,,, doubles
hydroxyl ions
The chemical reaction:
A ,,,,, metal is one, which will not corrode
noble
The chemical reaction:
The degree of activity can be expressed as ,,,,,
potential, in volts
The chemical reaction:
The list can be called a ,,,,, list
galvanic
The chemical reaction:
when the free potentials of the metals are ,,,,, it can also be called the ,,,,, or ,,,,,,,
known
electromotive forces series or
electrochemical series
The chemical reaction:
Mill scale is formed during the ,,,, operation of steel sections eg RSC, RSA, RSJ
rolling
The chemical reaction:
The oxides of iron form very quickly at temperatures in ,,,,
excess of 580oC
The chemical reaction:
During rolling operation, The first oxide formed is ,,,, the next is ,,,,,, and last of all ,,,,
iron oxide (FeO), wustite,
Fe3O4, magnetite
Fe2O3, haematite
The chemical reaction:
These oxides are compressed during the rolling operation to produce ,,,,,
blue mill scale
The chemical reaction:
The thickness of mill scale varies from ,,,,
25-100um.
The chemical reaction:
When mill scale has been removed by any surface preparation method, it can ,,,,,,
never recur
