Corrosion Flashcards
What is corrosion
Chemical reaction or electrochemical reaction between a metal and its environment
This leads to either
- removal of the metal
- formation of oxide
- formation of another chemical compound
The laws in thermodynamics
Two laws
Energy can neither be created nor destroyed.
When corrosion occurs naturally it releases free energy, as it is a spontaneous process.
Whenever a spontaneous change occurs it must release free energy from the system to the surrounding at constant temperature and pressure.
Corrosion process
An atom is a positive nucleus surrounded by negatively charged electrons
The overall charge of the atom is zero. As the negative charge of the electrons is equal to the positive nucleus.
They normally have 6 electrons.
Electrons may be added or taken away from the atom. When this happens the overall charge of the atom is no longer zero. This conduction is called
Ionic.
If the atom looses a electron it because positive.
If it gains an electron it becomes negative.
First step of corrosion process
Metal atoms change their state from being metallic ( no charge on atom) to being ionic (charge) this is by losing at lease one electron.
This process of corrosion goes on at atomic level. Each atom loses one or more electrons. ( normally no more then three) to becom ionic
The reaction in which metal is changed from its metallic state to ionic state is known as -
A Anodic reaction.
This is part of an overall reaction involving the metal and other elements present in the environment.
This is called oxidation.
This is what happens to the anode.
This is the anodic reaction. Lose of electrons
Cathodic reaction
The free electrons formed in the anode reaction must be used up if the reaction is to proceed.
This takes place at the site where the free electron is used. ( cathode) this is known as the cathodic reaction.
Definition of the corrosion process
Degradation of a metal by an electrochemical reaction with its environment.
For corrosion to take place need four main factors must apply
An anode
A cathode
An electrolyte
An electrical connection between the cathode and anode.
Electricity is the passage for electrons between the two defined points.
This occurs through a metal connecting wire this is called the current.
It can also pass through suitable aqueous solutions
The amount of charge carried by the electron when given a electron flow at a constant rate is measured in amperes.
The driving force causing the current to flow is measured in volts.
Confusion between conventional flow and electron flow.
Conventional flow flows from the cathode (-) to the anode (+) like a battery circuit.
Electron flow is the other way around. The current goes from the anode (+) to the cathode (-). From the anode to the structure protecting the structure.
Types of corrosion
Dissimilar metal corrosion Disslimilar phases in the metal grains Dissimilar energy levels “ Ion concentrations “ oxygen concentrations
Dissimilar metal corrosion
( galvanic corrosion)
When dissimilar metals are placed in the same electrolyte a potential difference exists between them. In battery terms ( flow) the anode is the negative and the cathode is the positive electrons flow from the neg to the pos.
To determine which of the two metals will be the cathode it depends on where on the galvanic series the metals sit.
Galvanic series
Lower the series they more likely be the anode.
Series
More noble
-gold
-silver
-copper
-steel
-aluminium
-zinc
-magnesium
Less nobleCrevice corrosion
This corrosion is associated with cervices 20-100 micrometers. And commonly invoked chloride ions in the electrolyte.
The oxygen in the bulk of the electrolyte will be replaced more easily then that trapped in the crevice.
This is very aggressive corrosion due to the lack of oxygen.
This creates a low oxygen situation within the crevice impedes the cathodic process.
The increase of hydrogen ion concentration accelerated the metal dissolution process.
The electrolyte becomes very acidic and the metal within the crevice will corrode while outside will be catholically protected.
Pitting corrosion
This type of corrosion initiates uniformly one the surface of the steel under water. This consumes oxygen by the normal cathode reaction.
In a water drop there will be more oxygen around the edges of the drop from the air outside the droplet. Then in the centre.
Another way of looking at it is that the oxygen has a long distance to travel through solution to get the the cathode in the centre of the drop then at the edges it has less distance to travel.
This will make the centre cathode become anodic and start to Corode. Pushing ions to the out er edges of the rain drop.
Inter granular corrosion
Intermetallic.
Compounds that are formed from the metal atoms and having indentifiable chemical formulae be either be anodic or cathodic to the patent metal.
Compounds
These are between metals and non metallic elements. Hydrogen carbon silicon nitrogen.
Plain carbon steel is a two phase metal some grains are cathodic while others are anodic.
Stress corrosion cracking
These conditions necessary for SCC to occur are
Tensile stress
Suitable environment
Sensitive metal
Appropriate temperature and ph range.
Corrosion fatigue.
There are many similarities between SCC AND CORROSION fatigue. But the biggest difference is
Fatigue cracking is under. DYNAMIC stress
SCC is under STATIC STRESS.
Stages of fatigue cracks
Slip bands
Very small cracks
Extension of the crack along favourable paths
Small crack propagation at right angles to the stress.
Erosion corrosion
Two main types
LAMINAR- fluid flow in pipelines
IMPINGMENT. turbulent conditions.
Fretting corrosion
Requirements.
Interface must be under load
Virbation or repeated movements between the interfaces
This appears on risers or clamps.
Other factors affecting corrosion rates
Temperature
Water flow
Ph of the water.
PH SCALE
0———-7———-14
Acid. Neutral. Alkali
