Corrosion Flashcards
What is corrosion?
The natural phenomenon whereby a metal converts to oxides, hydroxides and sulphates
What two actions cause corrosion?
Chemical
Electrochemical
How does corrosion occur?
The tendency for metals to return to their natural state
What sort of metals do not corrode and why?
Noble metals as they are already in their stable state
What are the two chemical changes that happen during corrosion?
The metal that is attacked/oxidised undergoes an anodic change, whilst the corrosive agent is being reduced and undergoes a cathodic change
What is electrochemical action?
Otherwise known as galvanic action
When two dissimilar metals are in the presence of an electrolyte, they corrode
What is microbiological corrosion?
Corrosion due to bacterial/ fungus/mould attacks
What is steel corrosion?
An intergranular cracking of the metal which is caused by a combination of stress and corrosion
What is filiform corrosion?
Special form of oxygen crevice corrosion which occurs on metal surfaces having an organic coating system
What happens if filiform is not treated asap?
Can cause intergranular corrosion
How can filiform corrosion be removed?
Using glass bead-blasting material with portable abrasive equipment and/or sanding
How can ALclad be susceptible to corrosion?
If the surface is scratched , the underlying material is attacked by corrosion
Too many solution heat treatments can affect the thickness of protection
How can passive film be made to be resistant to corrosion?
By adding a tight non-porous layer of oxide to the surface
How can corrosion take place in alloy grain structures?
The boundaries may be prone to corrosion which can lead to pitting and intergranular corrosion
What are the effects of corrosion on steel?
Red rust will form on its surface
What are the effects of corrosion on aluminium?
Experience crevice, stress and fretting corrosion. It appears mainly as a white/grey powder
What rapidly corrodes with aluminium?
Mercury
What effects does corrosion have on magnesium ?
Magnesium is already very easy to corrode. It creates grey powders when it is corroded and severe pitting is seen on the surface
What effect does corrosion have on copper?
Green powder is seen
What are the influencing factors that make a material more susceptible to corrosion?
Metal type Dissimilar metals Exposed surface areas Temperatures Electrolytes are present Oxygen and organisms are present Mechanical stresses Exposure time
What is filiform usually found on?
acidic surfaces
How do you identify stress corrosion?
Dark rings around the rivet heads or fasteners
Wearing away of parts/components
Smoking rivets - black round the rivet and give off black burnt looking streaks
Intergranular cracking caused by corrosion
Failure beyond yield point
What does cold expansion and shot peening do?
Increase cracking
What is the list of called to show how much certain metals will react and corrode?
Galvanic scale
What does the rate of galvanic corrosion depend on?
The size of the surface of metals
What is pitting corrosion most common in?
Magnesium and aluminium alloys
How is pitting corrosion identified?
White powder
How do you describe pitting corrosion?
It starts on the surface and shows up as a white powdery substance and it extends down vertically
What can pitting corrosion do to the strength of the structure?
Decreases its strength
What is intergranular corrosion?
An attack along the grain boundaries of the metal with little or no indication on the surface
What is exfoliation corrosion?
An advanced form of intergranular corrosion where surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries below the surface
What is fretting corrosion?
When vibration causes the surface to rub together resulting in abrasive wear.
It can cause severe pitting
Where does fretting corrosion form?
Forms between 2 highly loaded surfaces which are not designed to move against each other
How can you reduce fretting corrosion?
Use of lubrication or dampening methods
What is microbial corrosion?
The attack of a surface sure to bacteria, fungi or mould
They produce corrosive chemicals such as hydrogen, sulphide, ammonia or inorganic acids
What can bacteria be?
Aerobic or anerobic
What do aerobic bacteria require to live and how do they accelerate corrosion?
They need oxygen to live
Accelerate corrosion by oxidising sulphur to produce sulphuric acid
How do anaerobic bacteria create corrosion?
They oxidise inorganic compounds which result into chemical reactions that cause corrosion
How can you treat microbial corrosion?
Use of biocide
What is crevice corrosion?
A form of concentration cell corrosion
Corrosion of metal in a metal to metal joint and corrosion at the edge of a joint
How can crevice corrosion be reduced?
Sealing of joints and use of protective coatings in the fraying surface areas
