On 31/07/2017 21:15, newshound wrote:
On 7/31/2017 8:45 PM, Vir Campestris wrote:
On 29/07/2017 21:05, newshound wrote:
In any case, in a "breathable" mortar there's always going to be some
water (and hence oxygen) present by diffusion from the outside.

Water doesn't count. There have been cases where the stainless steel
keelbolts on boats have rusted through - and when the keel falls off
that kind of boat they'll be upside down in seconds.

Andy

Well, you say "rusted through". My guess is that keel bolts are highly
stressed in extreme conditions. With the chloride in seawater,
unsuitable stainless steels experiences stress corrosion cracking. They
will therefore fracture, rather than "rust through". Whereas in the sort
of case we are describing for the coping stone repair, the stress levels
will be very small compared to the tensile strength of the steel, and
SCC is not a problem.

Disclaimer: I'm not a metallurgist either, I'm a physicist. But I have
spent a career convincing engineers that I know more chemistry than
them, chemists that I know more metallurgy, metallurgists that I know
more physics, and physicists that I know more chemistry, engineering,
and metallurgy! Oh, and I can bluff anyone except a mathematician about
statistics.

I have no first hand experience. Lots of links, such as this

http://www.cruisingworld.com/how/keep-eye-keel-bolts

suggest that only some sorts of stainless will survive bilgewater, which
is often low in oxygen.

Andy