Corrosion Flashcards
corrosion
the destructive result of a chemical reaction between a metal and its environment
electrochemical process
most metal corrosion involves the transfer of an electronic charge in an aqueous solution
cathode
reduction
anode
oxidation
Pourbaix Diagram
shows regions of Immunity, active (corrosion) and passivity
Immune
no corrosion takes place, iron exists in metallic state (Fe)
Active (Corrosion)
where iron exists in a stable ionic state (Fe2+, Fe3+)
Passive
stable passive compounds (Fe2O3, Fe3O4)
Types of Corrosion (7)
- Uniform
- Pitting
- Crevice
- Dealloying or selective dissolution
- Galvanic
- Intergranular
- Environmentally induced
Uniform Corrosion
regular removal of metal from surface - result from corrosion cell effect without clearly defined anode and cathode
Pitting Corrosion
localised corrosion which results in formation of a pit of the surface - leads to substantial perforation of the metal in a short period of time - clearly defined cathodic and anodic area
Crevice Corrosion
attack on metal in a thin crevice at junction between two metals (or a non-metal)
- oxygen replacement difficult in crevice, making it anodic while bulk metal is cathodic
- can be caused by liquid remaining in crevice while surrounding area dries out
Dealloying or selective dissolution corrosion
one phase of an alloy selectively or preferentially attacked
Galvanic
two dissimilar metals coupled - one will preferentially corrode
- more thermodynamically stable acts as cathode (other acts as anode)
Intergranular
corrosion in or adjacent to grain boundaries of metal
