|
Case studies:
case 5.1 statue of liberty
coupling of copper skin and steel supports
built in 1886
weakening of structure by 1980
Case 5.2 sports car
brass screws and steel body
Case 5.3 aircraft
graphite grease and Mg alloy
EMF series is an orderly listing of standard
half-cell electrode potentials.
EMF Series | Galvanic Series |
absolute | relative |
quantitative | qualitative |
pure metals only | metals & alloys |
half-cell potential | corrosion potential |
standard conditions | any specified conditions |
based on thermodynamic analysis | based on thermodynamic analysis |
used for theoretical calculations | used for practical applications |
Case 5.5 helicopter
Mg component and steel bolts
Point to note:
Sacrificial anode cathodic protection is the constructive use of the galvanic corrosion.
e.g. Zn and Fe at elevated temperatures in some potable water systems
The environment plays an important role in a corrosion process.
The more active metal will become the anode of the couple in a wet corrosion cell.
The more noble metal will become the cathode of the couple in a wet corrosion cell.
The corrosion rate of the more active metal will be accelerated, while
that of the more noble metal is retarded (the cathode may
still corrode, depending upon the magnitude of the cathodic polarization
induced by coupling of the more reactive metal).
Cathode to anode area ratio:
Greater area ratio will result in a greater accelerating factor in the rate of galvanic corrosion
To reinforce learnings in this
lecture read pages 130-143
(textbook)
To prepare yourself for the next
lecture read pages 144-155
(textbook)