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