What constitutes an anode/cathode pair?

1
What constitutes an anode/cathode pair?
In summary Anything that makes two half-cells
different gives anode cathode gt corrosion
We can call these types macro cells
2
Then what about micro cells? anode cathode
within single material operating on a micro scale
Grain boundary - high energy regionanodic wrt
bulk of grain
Differently oriented grains - some planes of
atoms dissolve faster than others
Segregation (coring)
3
Then what about micro cells? anode cathode
within single material operating on a micro scale
Segregation (coring)
4
Another type or micro cell - 2nd phase
2nd phase will tend to react differently than
matrix phase - sets up anode/cathode pair
5
Example - welds
6
Example - welds
304 stainless steel plate arc- welded to other
plates Note corrosion in HAZ
Why?
7
Remember. What makes stainless steel
stainless? tight, tenacious film of Cr2O3 on
the surface film occludes electrolytes, oxygen
8
Corrosion in stainless steel HAZ due to
"sensitization"
9
How to avoid sensitization get rid of C
10
1913 We find Ol' Harold Brearley in England
industriously testing alloy steels for gun barrels
11
Ol' Harold's records showed that the unrusted
steel sample contained 14 Cr!
The birth of stainless steels!
Moral behind every successful person, there may
be a junk pile
Stolen from Tales from Sheffield by Stan
Lessteal Cutter
Also see Brearley Steel-makers and Knotted
String (EMS Library)
12
Components of a cell anode
cathode electrolyte connection
13
Occlude electrolyte - coatings
asphalt greases, oils paints polymers ceramic
coatings "noble" metals
14
Occlude electrolyte - coatings
"noble" metals unless scratch heals, a
galvanic cell is set up
15
Cathodic protection
galvanizing Zn coating applied by
hot-dipping electroplating
16
Cathodic protection
galvanizing presence of scratch does not
promote corrosion of protected metal
but steel helps Zn corrode
17
Cathodic protection
set up cell by adding metal which is more anodic
than metal or alloy to be protected
18
Cathodic protection
example protecting ship plate