6.4 Flashcards
What is corrosion
Corrosion is the natural phenomenon whereby a material reverts back to its natural form
Why Is aluinium vulnerable when left on concrete?
When the concrete gets wet the lime in it makes the water slightly alkali which corrodes the concrete
What are noble metals
Gold, platinum, silver
Un-reactive metals, cathode.
What is a passive film?
An invisible layer of oxidised metal on the metals surface which prevents it from further erosion
What are the 3 requirements for corrosion to form
- an electrical potential difference within the metals
- a conductive path between the two areas
- some form of electrolyte covering the two areas
What is galvanise if corrosion
Two dissimilar metals … etc
Anode is corroded
What is electrolytic corrosion
Caused by an external source of EMF
What is microbiological corrosion
Bacteria fungi mould can produce corrosive chemicals. Commonly found in fuel tanks.
What is stress corrosion
Intergranular cracking
Corrosive environment raises the stresses causing materials to fail below their yield point
What is filiform corrosion
Oxygen cell concentration
Worm like appearance.
Under paint surface.
Can be a start to intergranular corrosion.
78-90% humidity, surface slightly acidic
Lead to intergranular corrosion if not removed
Can be removed using glass bead blasting, sanding etc in early stages
What is pitting corrosion
Common in magnesium and aluminium alloy
Starts as white powdery deposit
Decreases strength and can lead to stress corrosion
Can be a start to intergranular
What is intergranular corrosion
Along grain boundaries
Little/no indication
Paint, primer cladded aluminium can help protect the base material
What is exfoliation corrosion
An advanced form of intergranular
Surface grains lifted by the expanding corrosion products
Wrought products are more prone due to their elongated structure.
Fretting corrosion
Forms between 2 highly loaded areas
Rub together resulting in abrasive wear
Protective oxide film is constantly removed
Lubrication helps prevent it
Black or gray deposits found around the surfaces
Crevice corrosion
Stagnant solution in crevices
In metal-metal joints
Corrodes edge of joint
Prevented by using sealant
