[Ferro List] Passivisation Notes

[Christopher Glasspool]

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. The fact that it exists says somebody at some time had a problem with hydrogen bubbles great enough to put this standard in front of the manufacturers, and users of coated rebar, but why not metal lath? I guessed the answer but confirmed it with Gary Maylon the Lath expert. Metal Lath has a G60 coating before it is slit and expanded, and the coating is only .5 mil thickness. There appears to be not enough Zinc to create a hydrogen bubble problem in metal lath. Metal lath is manufactured for the stucco industry, and they know that galvanizing is gone in seven to eight years after installment. This is due to the cathodic event that happens slowly and passivisation, and the chloride content of the sand, and the chloride ingress due to the slight breathability of Stucco.
It appears the Natural Passivisation does, or can occur, but why bother with fostering it, if contractors don't have a history of having problems with stucco lath? For me there is two reasons; first,  ferrocement is held to a higher quality standard than stucco, and second the possibility of a ratio affect exists in my opinion, in that the difference between stucco and ferrocement is the amount of reinforcement involved. It is possible with that the increased amount of zinc in the ferrocement sandwich compared to what Stucco hasin it, that this might trigger an affect that wouldn't be in stucco. I don't know, but until I'm confident of doing differently I will Naturally Passivate the metal lath.
Alan's point is well taken, if you use a product not meant for reinforcement the alloys used might not be correct. When I read the manufacturer's reports on galvanized rebar they sometimes referred to the brittlement of the underlying steel, apparently the passivisation process when in wet paste not only affects the outsideof the reinforcement but what they brought up was this embritlement problem that can weaken the inside steel. At this point I am feeling wary of galvanized steel not meant for mortar immersion. Not only are there pages of ASTMs that are intended just for Metal Lath, but the other galvanized products like the 2" mesh used for panelized home construction have to have an ICBO (International Congress Building Officials) license in the U.S.A. Which will specify the manufacturer to engineer for these sort of problems. 
Why isn't the hydrogen bubble formation in wet paste more prevalent and exampled on the Internet, and in construction manuals etc., because most of the time enough natural Chromium exists in the cement and in the aggregate to have passivated the Zinc. Why isn't the problem seen in ferrocement? Same reason plus most people probably use expanded lath or chicken wire. I would bet that chicken wire has only the smallest amount of zinc also. 
- chris


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