[Ferro List] Part 3/Tom's Crud/Passivisation

[Keith B]

Thanks Tom, you've done me a service.   I've been increasingly out of 
things since Spring 2006, with large periods completely offline.  I 
didn't know Google had a working book search up yet.  Lots of stuff to 
be found there.  Following your instructions, I visited your reference ( 
results list 1) and went on to others noting Bertrand's 1892 preparation 
of alkaline earth zincates ( results list 2) and a 6th IREM paper on 
galvanized steel fibres ( results list 2).   Your reference basically 
restates the common knowledge but, interestingly, uses the term "calcium 
hydroxo zincate".   We know about hydrogen production and weakened 
matrix adjacent galvanizing.  Bertrand started his preparation by mixing 
zinc oxide into lime water.  No hydrogen production noted, and shouldn't 
be any.  Sulphate radicals appeared to be necessary for 
crystallization?..?  Empirical formula looks strange, but atom-wise it 
matches later thought, your reference perhaps having it right.  The IREM 
paper underlines the potential of galvanizing and notes the unsettled 
question of improvement re. bond strength (by pullout resistance).  
Again, little new but 2004 thinking. 

Little of that touches the gross matrix denaturing which you documented, 
but I particularly note the last material in the IREM paper.  It appears 
that galvanized fibre reinforced cement articles have been observe to 
swell during cure.  Is that cumulative expansion of the passivation 
layers or more general effect on the matrix?  Might be a smoking gun there.

I was previously concentrating on whether "natural passivation" was 
real, its parameters and behavior in mortar slurry.  It's certainly 
real, and apparently a hydrated basic carbonate.  An alternative one has 
a different structure and does not protect (white rust).  A pH between 
6.5 and 8.0 is used for protective formation, plus hard water suggesting 
calcium involvement.  A pH over 9.0 is destructive, i.e. any mortar.  
What happens though to a surface so passivated?  Does the coating act 
like Bertrand's zinc oxide, producing the zincate without hydrogen 
evolution, or do we still get to fresh zinc and follow the usually 
quoted equation for that?  Is the exact nature a "natural passivating 
coat" the determining factor between demonstrated excellent FC results 
with glavanizing and equally dramatic lousy ones?  Most, perhaps all, of 
the reported cases, both ways, can be explained by the presumption that 
the bad "white rust" basic carbonate gives calcium hydroxide direct 
access to fresh zinc, with which it then rapidly and preferentially 
reacts. 

Since others are working on the tests you performed, I've been mulling 
different possibilities before deciding what material to ask you for.  
Currently, I'm thinking a mesh sample which I can test for cracking or 
porosity of the zinc coat, before and after flexure and distress 
simulating emplacement and mudding, using metallurgical dye, if I still 
have some, or if I can find a substitute.  A second sample for a similar 
appearing test to what's been done but with carefully made mortar as an 
initial layer and cover, per Martin Iorns to minimize emplacement stress 
and related complications, with different treatment on quarters of the 
same mesh.  A third test would need enough samples to match the number 
of layers used in the tank, again laid up per Iorns. The detail of the 
last would reflect results from the first two, and a parallel experiment 
to investigate sub-optimal mix, w/c ratio and emplacement technique.

I don't have a description of the layup and layering you used, nor, 
assuming that you plastered from one side, observations you made of the 
condition of the reverse when you stripped the form.   While the focus 
is on galvanizing, the most alarming observations relate to denatured 
matrix rather than just poor adhesion for the mesh.  One must then 
inquire into other factors which might have been causative or 
contributive. 

Unfortunately, there's no cement left to determine whether you got a bad 
batch, but a sample of mortar made with it - which was not exposed to 
galvanizing - would be helpful.  Also helpful, if available, would be a 
sample of the sand actually used for microscopic observation.

Working mortar through multiple mesh layers can cause problems.  
(There's something similar when mixing epscrete due to the eps being 
strongly hydrophobic.  If surfactant and other maneuvers aren't used, 
the beads filter solids, almost clear water falling to the bottom, 
wrecking uniformity of aggregate consist and w/c ratio.)  The equivalent 
for your tank is potentially a complex of likely interacting effects. 
Coarser aggregate may have been sieved to leave the front coarser than 
the back, water separated to give higher w/c ratio at the back with a 
possible a vertical drainage channel.  Difficulty working mortar may 
have led to gaging that raised the w/c ratio, as could the time 
consuming process when "wetting out" the mesh was difficult.  There's 
the method and regimen for maintaining proper curing conditions.   All 
could interact with a process which necessarily scoured the galvanizing, 
shifting and mixing any "crud" into the inside matrix.  (here a rdlevant 
observation would be whether there was obvious difference to the matrix 
of the inside and outside matrix layers.   Note that the excellent 
results observed with Martin Iorns' samples reflect a construction 
method where none of the above was a factor - regardless of the starting 
condition of the galvanizing.  anything you logged or can remember 
regarding your work procedure, time, temperature, conditions of cure 
would be appreciated.

White rust and passivation are discussed here, can't find the .pdf 
version at the moment:
http://www.idswater.com/Common/Paper/Paper_2/White%20Rust.htm
Good general paper on the subject:
http://www.saimm.co.za/events/0611corrosion/downloads/05_R%20E%20Wilmot.pdf

kb

tom rapenske wrote:
>   ...I'm really surprised you and Keith missed this:
>   Go to Google and type      calcium zincate hydrogen gas     in the search box.
>   Scroll down to  Corrosion of Reinforcing steel in Concrete: A Symposium  by D.E. Tonini and J.M. Gaidis. This will a Google Books Result.
>   Then scroll down to a paper on page 142 entitled: Passivation of Galvanized Reinforcement by Inhibitor Anions by D.J.H. Corderoy and H. Herzog.
>   You can't fail to note " hydrogen gas in a spongy concrete area around the zinc" on first few pages. ..
>