4. Forms of Corrosion

4.3 Galvanic Corrosion [2/4]

Mechanism

Consider two different metals such as Copper and Iron immersed in an electrolyte. As the potential of copper is +0.337 V and that of iron is -0.440 V (refer EMF Series), copper would become the cathode and iron the anode. Positive ions would flow from the iron electrode to the copper electrode and iron would continue to corrode. Copper electrode is the cathode and it would not corrode. Corrosion would continue as long as the anode and cathode continue to exist and the current flow is continuous. The further apart they are, the more the magnitude of galvanic corrosion.

Example A:

Stainless steel 410 (passive)   + Stainless steel 316 (passive)

    No galvanic corrosion
Example B:

Aluminum alloy     +    Stainless steel 316

   Pronounced galvanic corrosion
Example C:

Copper Nickel alloy    +    Brasses

   Minimal galvanic corrosion

Factors affecting Galvanic Corrosion

1. Area effect: A small anodic area to a large cathodic area, produces galvanic corrosion because of a large current density.

Example:

(a) Steel rivets in a copper sheet
(b) Copper rivets in a steel sheet
(c) Painting of corroding metal

2. Distance effect: Short paths carry most currents and long paths carry lesser currents. Moreover, the solution conductivity increases with the length of the pipe. Put the two metals as far as possible so as to allow a minimum current flow between them.

3. Environmental effect: In normal conditions, it is known that the metal with lower resistance becomes anodic & the other cathodic. However, in some environmental conditions, the case can reverse!

It is known that zinc is generally an anode when coupled with steel, but at temperatures greater than 82o C, the couple reverses and steel becomes the anode and starts to corrode.