[Ferro List] Passivisation Notes

[Keith B]

Hi Chris

Your search terms did not work for me on Google, anything else to get to 
your Forum?

If passivated before splitting and deformation, even a G60 galvanize 
coat covers most of the lath area and the coat should largely convert to 
some calcium hydroxy zincate capable of later converting to a protective 
zinc carbonate layer (as carbonation attacks the remnant calcium 
hydroxide from OPC cure and drops the matrix pH).  Stucco doesn't have 
the same thickness of cover as required in conventional rebar concrete, 
and is likely more porous.  Only the slit edges are then exposed to 
rusting, even if the G60 is totally consumed, and expansive forces there 
should have far lower splitting action than would rusting of the flat 
surfaces.  You might want to look at this paper:  
http://www.saimm.co.za/events/0611corrosion/downloads/05_R%20E%20Wilmot.pdf

Even that paper gives the gospel equation for lime attack on zinc.  If a 
passivation layer is intact, at least first action is on the 
oxide/hydroxide.  No hydrogen for such reactions, and none noted by 
Bertrand in 1892 describing the preparation of the calcium hydroxy 
(hydroxo?) zincate.  See bottom of column 1 and top of column 2 here:  
http://books.google.com/books?id=5JwEAAAAYAAJ&pg=RA2-PA18&dq=calcium+zincate+hydrogen+gas&lr=&ie=ISO-8859-1 
<http://books.google.com/books?id=5JwEAAAAYAAJ&pg=RA2-PA18&dq=calcium+zincate+hydrogen+gas&lr=&ie=ISO-8859-1>

As to why we don't see more about hydrogen evolution on the net, I 
suspect it doesn't happen to a significant degree except in uncommon 
circumstances.  In Tom's case, he may have been unlucky enough to have 
inadvertently hit a combination of just about all the factors causing it 
short of stating with bright zinc.  That said, it's a bit unnerving to 
hear that engineering cement parts reinforced with galvanized fibers 
suffer significant volumetric increase...

kb