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Any corrosion which occurs at preferred site on a metal surface, for whatever reason, can be described as SELECTIVE ATTACK.
DEFECTS in metal crystal structures have electrochemical characteristics different from the rest.
Most engineering metals contain VOLUME DEFEATS:
To reveal the grain structure of a metal/alloy, polishing and etching of its sample are necessary.
The etching process is an example of grain boundary corrosion.
(Fig. 6.1)
INTERGRANULAR CORROSION occurs when a grain boundary area is preferentially attacked because of the presence of precipitates in these regions.
Grain boundaries are peferred sites for:
Two types of segregates and precipitates:
Any metal in which intermetallics or compounds are present at grain boundaries will be susceptible to intergranular stress corrosion cracking.
Austenitic stainless steels are most susceptible to intergranular corrosion.
18-8 or type 304 stainless steel: Fe18%Cr8%Ni
When C% < 0.03%, only the austenite phase is stable.
When C% > 0.03% austenite and ferrite mixed carbide (FeCr)23C6 are stable (Fig.6.2)
The proportions of carbide obtained are dependent upon the rate of cooling:
Fast cooling by water/oil quenching from > 1000oC suppresses carbide formation.
If the material is reheated within the range 600-850oC, carbide precipitation will occur at the grain boundaries.
The material is thus said to be sensitized and is in a dangerous condition - susceptible to Intergranular corrosion cracking
If the material is reheated below 600oC, the rate of diffusion of Cr is too slow for carbide precipitation to occur.
12%Cr + Fe === "stainless" steels
precipitation of carbide (FeCr)23C6 causes Cr depletion (Cr<12%) in the metal adjacent to the precipitates.
The steel is no longer "stainless".
Cr depleted zone is very anodic to the rest of the grains. Severe attack occurs adjacent to the grain boundary. Whole grains may become detached from the material
Sensitisation may occur during:
If Cr carbide nuclei pre-exist in grain boundary regions, sensitisation can occur at temperatures in the range 300-320oC.
Weld decay === Sensitisation caused by welding.
Use stainless steels with caution !!!
Case 6.1
While attempting to fold the tail pylon of a helicopter it was found that it was not possible to disengage the locking assembly because of a fractured , unstabilised stainless steel pin (Fig.6.3a).
Examination showed that the most likely cause of failure was intergranular cracking (Fig.6.3b) initiated by a network of grain boundary precipitates (Fig.6.3c)
The net removal of one element from an alloy is often referred to as dealloying.
Uniform removal does not alter the overall geometry
Dealloying leaves a porous material with reduced or no mechanical strength.
Classical problem: dezincification
Two groups of brass:
Prevention of dezincification by addition of arsenic to brass:
70/30 brass + arsenic =Admiralty brass = inhibited brass
uninhibited brass should never be specified for uses involving immersion in either fresh or sea water.
Case 6.2
The component is a 60/40 brass casting used as a connector in a water supply. The dark inner areas are dezincified and the left hand region has fractured because of the embrittlement caused by the extreme porosity.
Case 6.3
The component is part of a cast aluminum bronze sea water valve which had originally suffered wastage because of erosion, but which had not dealloyed. The component was recovered by depositing 60/40 brass along the shank and around the closing surface. The reclaimed areas have suffered dezincification and some has been broken off.
Case 6.4
Fig.6.5 shows a leaded brass bolt, 12 mm in diameter, which suffered dezincification to a depth of 3 mm in several years exposure to sea water. The fractures are a direct result of the embrittlement of the material.
60/40 Brass contains a and b phases.
%Zn in a phase < %Zn in b phase.
ECorr a = -230 mV, cathodic
ECorr b = -285 mV, anodic
b phase in brass would be attacked first:
Zn in b phase is dissolved quickly, Cu is much less affected.
Selective attack is caused by metallurgical factor of the material rather than chemistry factor of the environment. Segragations and precipitations along grain boundaries create preferential paths for the anodic and cathodic reactions. Dealloying is also caused by the metallurgical factor in that the alloying elements have wide differing electrochemical potential. The anodic constituent will be preferentially leached out of the alloy, leaving behind a porous material with no strength.
To reinforce learnings in this lecture read pages 144-155
(textbook)
To prepare yourself for the next lecture
read pages 162-172 (textbook)
