Corrosion

Alex Flinn <alex.flinn [at] armouries__org__uk> writes

>Various iron alloy objects in our collection have developed
>particularly aggressive and unusual patches of corrosion. The
>corrosion forms a pattern of concentric circles that appear to
>radiate out from very definite central points (visually resembling
>the pattern formed when a bacterial culture is grown on a petri dish
>or that of growth rings in a tree trunk).

Alex your posting raises more questions that answers and I think
that sharing the question that come to mind might be instructive.

What you described sounds like very aggressive localized corrosion
cell phenomenon. The ring structures that layer in depth can either
be the result of etching and dissolution of the ferrous ions due to
a solute (such as in tide-rings left by coffee stains) or a
corrosion pit morphology that results from differential attack to the
microstructure (as is sometimes seen in archaeological irons).

My first question about this is how large are the corrosion cells
and is there a pattern in their location of the various objects.
Historic musket and rifle barrels, for instance, almost always
exhibit localized corrosion and pitting on their top surfaces that
have been exposed to dust and air, while the undersides of the
barrels are almost always in near original condition, being
protected by the stock. In archaeological iron this is also somewhat
true with one side becoming more anodic and the other more cathodic
in the burial environment. So, if you have this sort of overall
corrosion pattern I would expect to look for some sort of
environmental link as the genesis. It could be something as simple as
RH and dust exposure over time or even an event such as when an
inappropriate cleaning product used in an historic house or museum
was sprayed on the metal (in which case an even more defined pattern
might be produced).

Are all of these object stored of exhibited in the same room? Or
have they, at some point in their history, been associated together
in the same area? If so have they been located near windows,
ducts, or under water or steam pipes? A look into registrarial
records can tell you if this history is present, if so, they there
is a good likelihood that some sort of "event" environmental or
otherwise affected these objects as a group.

I would also look into whether these objects have received previous
conservation treatment and if so is there a common pattern there in
terms of methods and materials that were used on them? Sometimes if
the same name is on all the treatment reports that can give you a
clue as well! (yes, I have seen this!)

If you want to understand the corrosion cells more fully I would
suggest that you use metallographic replicating tape. It is simply a
thin sheet of acetate, wetted and softened in acetone, and then
applied onto the area under investigation. You press down on the top
side with the eraser end of a pencil or some other appropriate tool,
to push the acetate firmly into the surface and air bubbles out. You
allow it to dry and harden and then you can place the acetate into a
microscope, SEM, or even an XRF to look at the details of the
corrosion cell and also to get qualitative information on the
composition of the corrosion particulate captured in the acetate.
This is a non-destructive technique used in the metallographic
industry to enable examination of in-situ objects like pipes and
machinery that cannot be sectioned and removed. You just have to be
careful that the acetone is not affecting a coating that may be on
the surface or an adjacent material on the object. I once used this
method to lift the impression of fingerprints from a mummy that were
mineralized on the surface of a Moche culture copper-alloy axe--the
researcher could even count and measure the pores under the
microscope.

So these are my thoughts on this. It would be more helpful to see a
photograph or photomicrograph of the corrosion cells taken in a
direct strong light.

If you have access to a corrosion scientist then that would be your
best resource into characterizing the corrosion phenomenon of which
you are concerned.

David Harvey
Artifacts
2930 South Birch Street
Denver, CO 80222
303-300-5257


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                  Conservation DistList Instance 16:43
                Distributed: Wednesday, January 15, 2003
                       Message Id: cdl-16-43-002
                                  ***