6.4 Corrsion Flashcards
Corrosion occurs due to what 2 actions
Chemical and electrochemical
What is erosion
Destruction due to mechanical action
Why do gold and platinum not corrode
Because they are naturally/chemically pure
What type of change does the metal being corroded go through and what type of change does the corrosive substance go through
Metal - anodic change
Corrosive material - cathodic change
What is a passive film
An oxide film which appears on metals such as titanium and cres which prevents corrosion
What part of a metal would corrosion normally start
The surface
What type of surface protection would be seen on aluminium alloys, steel alloys and magnesium alloys
Aluminium alloy - anodising then primed and top coated
Steel alloys - cadmium or zinc plate
Magnesium - chemical and electrochemical treatments
What is electrochemical action also known as
Galvanic action
What makes aluminium corrode very badly
Mercury
What colour is aluminium corrosion
White-grey powder
What colour is steel corrosion
Red orange rust
What colour is magnesium corrosion
Grey powder
What is filiform corrosion
- Oxygen concentration cell corrosion
- Worm like corrosion beneath paint
- Relative humidity of 78-90%
- Can be removed by glass bead blasting
What is intergranular corrosion
An attack along the grain boundaries of the material
2024 and 7075 can have inter granular if they have been improperly heat treated and in presence of electrolyte
What is exfoliation corrosion
Advanced form of intergranular corrosion
Normally in wrought products with flat type grain structure
