6.4 Flashcards
What does corrosion convert metal into?
A metallic compound such as an oxide, hydroxide or sulphate
What 2 actions causes corrosion to occur?
Chemical or electrochemical
Why does corrosion occur?
Because of the tendency for metals to return to their natural state
Which kinds of metals don’t corrode?
Noble metals such as gold or platinum
Which metals corrode the easiest?
The most active ones (those which tend to lose electrons easily, such as Al and Mg)
Why is titanium and aluminium said to be corrosion resistant?
They form a passive oxide film on their surface which is impermeable and prevents further oxidisation
How is corrosion prevented from aluminium alloys?
Anodised, primed and top coated
How is steel protected from corrosion?
Cadmium or zinc plating
How is magnesium protected from corrosion?
Special chemical and electro-chemical treatments and paint finishes
How can you tell is steel has corroded?
Red rust (ferric oxide)
How can you tell if aluminium has corroded?
Crevice stress and fretting corrosion
White/grey powdery material
How to tell if magnesium has corroded?
A grey powderthat has severe pitting of the surface
How to telll if copper has corroded?
Green powdery product
How can filiform corrosion be identified?
By its characteristic work-like trace of corrosion products beneath the paint film
When does filiform occur?
When the humidity of the air is 78-90% and the surface is slightly acid
What happens if filiform is not removed?
It can become intergranular corrosion
What is stress corrosion?
Intergranular cracking of the metal caused by a combo of stress and corrosion
How does galvanic corrosion occur?
When 2 dissimilar metals make contact through an electrolyte
Which metals would you find pitting corrosion on?
Aluminium and Magnesium
How would you identify pitting?
A white powdery deposit appears on the surface of the metal
What is intergranular corrosion?
An attack along the grain boundaries of the metal with little to no indication on the surface
What is exfoliation corrosion an advanced form of?
Intergranular corrosion
Which type of products is exfoliation corrosion likely to occur?
Wrought products such as extrusions, thick sheets and thin plates
Hat does microbial corrosion include the actions of?
Bacteria, fungi, moulds
What does aerobic bacteria need?
Oxygen
What do fungi feed on?
Organic materials
What is crevice corrosion a form of?
Concentration cell corrosion
