[Ferro List] Passivisation Notes

[Keith B]

Way to go Chris, and thanks for making it available.  Yes indeed, that's 
the smoking gun.  Regarding hydrogen evolution, it adds new material and 
I still don't think we have the whole tale, but I think we've now got 
the reason for the disaster to Tom's mortar.  Bottom of the top 
paragraph, right column, first page, zinc compounds can act as a set 
retarder.  For kibitzers, the last two sentences there read:

"A second possible cause is chemical and involves the solution of zinc 
and the manner in which it has concentrated near certain surfaces.  
Since zinc in certain forms is a set retarder, the concrete at these 
certain surfaces may not have become hard when the member is stripped."

I've never heard of zinc as a set retarder before, but one doesn't 
usually need much of a retarder to make a big difference.  If Tom's 
layup was white rusted rather than carbonate passivated, forcing 
abrasive mud through several layers of galvanizing would be pretty 
certain to mix retardsnt deeply into the mortar that got through the mesh. 

Regarding the classic equation for hydrogen evolution with lime attack 
on plain zinc, I'm fascinated by the observation that black iron rebar 
tied (presumably thus electrically bonded) to galvanized evolved bubles 
of hydrogen along the length of the black iron.    ..? ...?     
Whatever's going on there isn't that simple classic equation.

kb


Christopher Glasspool wrote:
> Mud-Daubers,
> One interesting short paper that I came across in the pre-caster's commercial library is a pdf you can look up but I can't appear to send. Search the phrase: Strand slippage with Galvanized Reinforcement, and you will come up with the first search item being entitled OPEN FORUM. This 1.5 page article touches upon Natural Passivisation, differing and average chromium levels in cement, and a ASTM A767-97 Parts 5.3, 5.3.1, and 5.3.2 for Zinc coated Steel Bars for Concrete Reinforcement (which doesn't apply to mesh).
> This ASTM A767-97 is what I have been looking for in the way of a smoking gun.