Cor-ten steel sculpture

It appears that my recounting of a successful use of aqueous HCl to
renew a rust film on weathering steel has disturbed John Scott. I
believe that his concern is due to confusion between a one-time
exposure to a corrodent and continual exposure to a corrodent.

No one can dispute that under some climatic conditions the continual
exposure of weathering steel to chlorides can prevent the formation
of a protective rust layer. I have seen many sad cases of this
condition on the coasts of the continental US, in Hawaii, in South
Africa, and under leaking bridge joints in the road-salt belt of the
US, and have provided consultation in regards to marine exposures in
Japan, Europe, and South America. (Please note that in certain
coastal environments weathering steel in well-designed applications
performs satisfactorily.)  However a one time exposure to chloride
is a very different situation. My experience and that of other
investigators is that without a continuous resupply of chloride or
other corrodent from the environment, a chloride or other
artificially produced rust will over time revert to the rust film
characteristic of weathering steel of that location and exposure
condition. Consequently I believe that in general a one-time
exposure to HCl or other chloride solution will not produce a
permanent degradation to the rust layer on weathering steel, and its
use may be justified to produce a quick and relatively uniform oxide
layer.

In regards to Mr. Scott's detailed explanation of the weathering
steel rust layer formation and behavior over time, I am skeptical of
it or any other detailed description of weathering steel rusting
based on the crystallographic identity of rust components.  I have
seen too many explanations of rust formation based on one analysis
technique (e.g. X-ray diffraction, Mossbauer spectroscopy, Raman
spectroscopy, electron diffraction, optical microscopy) that
contradict explanations based on other techniques. The problem is
that, as I understand John Scott to agree, the rust on weathering
steel or other steels is stratified into layers of different
morphology and crystallography and that much of the rust is so
microcrystalline it appears amorphous to many techniques. Also, it
is the case that different exposure climates and exposure conditions
produce weathering steel rusts that show different crystallographic
identity at any one time of exposure, and over time.

In any case, I agree that conservators should consult John Scott's
publications, and others on the subject. In my experience the more
information or opinions that are available to anyone making a
materials preservation decision, the more likely a satisfactory
outcome.

Patrick Gallagher
Materials Preservation

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                  Conservation DistList Instance 12:12
                   Distributed: Friday, July 24, 1998
                       Message Id: cdl-12-12-005
                                  ***