Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 513

um. The chromium combines with oxygen on the surface of the metal to create chromium
oxide, which forms a stable layer over the steel, protecting it from corrosion. In this way,
stainless steel works in a similar way to galvanized steel. The main difference between
stainless and galvanized steels, however, is the way in which the protective surface works:
if the stable surface layer on stainless steel is scratched, cut or disturbed, the exposed chro-
mium quickly combines with oxygen to produce a new layer of oxides, protecting the
newly exposed steel.
It has been suggested that higher-quality galvanized steel products could be sufficiently
resistant to corrosion to be used within hempcrete, and certainly there is a common-sense
argument that all metal fixings within a building corrode during their lifetime, but rarely
to the extent that the structural integrity of the building is affected. However, fixings for
buildings are generally made as cheaply as possible, and are therefore rarely produced us-
ing hot-dip galvanizing. Also, hot-dip galvanizing has proven unsuitable for screws and
bolts smaller than M10, since the thickness of the coating fills too much of the thread. If
using a nail gun, the use of either hot-dip galvanized or stainless-steel nails is acceptable.
In tests carried out by the authors, galvanized electricity back boxes placed in a hempcrete
wall for only 2 weeks showed a very high level of corrosion when they were removed.
Our current recommendation, therefore, is that without further research being carried out,
or the emergence of a guaranteed product, galvanized steel structural fixings should not be
used within hempcrete. Although electrical back boxes are not structural, if their potential
corrosion is a concern then PVC back boxes could be used instead.