On 7/29/2017 7:57 PM, Nightjar wrote:
On 29-Jul-17 2:41 PM, newshound wrote:
...
Sounds like a plausible idea. The other thing you might think about is
ways to improve the keying. For example you could drill some holes in
the face to be repaired, insert plastic wallplugs, and then put in
some screws protruding a certain amount from the surface. I think I
would use stainless steel screws....

I suspect that stainless steel would not have enough oxygen reaching it
to keep the protective surface coating intact. That would mean it will
rust just as quickly as mild steel. I would use a non-ferrous metal
bonded to the existing stonework with epoxy resin.


I don't think I agree. In an alkaline environment there is nothing
present that is going to degrade the protective oxide, so why do you
need any additional oxygen. In any case, in a "breathable" mortar
there's always going to be some water (and hence oxygen) present by
diffusion from the outside.

Excuse me if I digress, but I did have one interesting instance in the
past where austenitic stainless steel was corroding at an alarming rate
in an alkaline environment. My current house has thick rubble-filled
random limestone walls built straight on the underlying limestone. And I
have a 100 metre hill rising within a kilometer behind the house. The
previous owner, towards the end of his life, dealt with the chronic
rising damp problem by putting (untreated) studding on the end wall,
stapling thick polyethylene sheet over this, then plasterboarding and
plastering. (The exterior surface was cement rendered with the
traditional bell-cast above a few inches of exposed stonework).

As a result, when we moved in the dry rot had reached many of the first
floor joists, as well as taking out the staircase in one corner and the
window frame in the other.

Being in those days totally dependent on a commercial mortgage, I had no
choice but to follow the advice of an approved "damp specialist". They
ripped out the studding and plasterboard, replaced the staircase, window
frames and doors, put in an electro osmotic damp proof course, and
re-rendered inside with waterproof sand/cement rendering, topped with
gypsum plaster. (They did use Sirapite on another wall).

This of course sealed in all the rising damp that was forced up by the
fact that the normal water table was at least half way up the wall. In
due course, the gypsum plaster blew out. Also, the new radiator fell off
the wall when the screws rusted through.

At this point I thought I would be clever. I got four lengths of 8 mm
austenitic (18-8) stainless steel studding about 15 inches long, set
these into the wall, and hung the radiator brackets on them. (At this
time I had small children, no money, and was just living with the rising
damp).

After a while I decided I needed to sort the damp properly, so I moved
the radiator to a better location, and stripped off all the interior
rendering to allow the wall to dry out properly. (The exterior rendering
was on the site boundary and faced a garage forecourt, I didn't really
want to tackle that wall). I was interested to find very substantial
corrosion on the "stainless" studding that had been holding up the
radiators. But by that time, I had also discovered that 90% of the metal
in standard galvanised electrical back boxes had also vanished on
another really wet wall. On checking the polarity of my "electro
osmotic" damp proof course (which was made from titanium wire) I decided
that the iron loss in both the mild and the stainless steel had actually
been caused by the electrochemistry because the back boxes, and also the
radiator and hence the brackets and supporting studs were well earthed.
Reverse electroplating, if you like. Provided the wall was wet (and
therefore conductive) enough, current from the protection system was
just converting any earthed metal into ions.

With all the rendering stripped, I found after a few months that I got a
little bit of salt formation on the bottom couple of feet, but the rest
of the wall remained dry. So I made the top of the wall a "feature", had
it properly pointed by an expert, and fitted well ventilated
wainscotting over the bottom three feet or so. And that has completely
cured my rising damp.

I have a few photos documenting this work. I do wonder, from time to
time, whether I ought to submit something to the DIY Wiki.

Going back to Colin's original suggestion of non-ferrous metal for the
OP's problem, I will confess to wondering whether to suggest brass
screws rather than stainless. The trouble is, readily available brass
screws will just be simple copper/zinc. Perhaps you need the 1% of tin
that is in "Admiralty brass" to improve the corrosion resistance. Or
perhaps you need something moving further into the gun-metals and
bronzes: not necessarily so easy to find, short of going to chandlers
with many pound notes.

I still think that the readily available A2 stainless should work.
Disclaimer: I'm not a proper chemist and happy to be corrected.