Wet cast iron

Neil Mahrer <neil.mahrer [at] jerseyheritage__org> writes

>A 19th century steam and water mill is being restored here in
>Jersey.  One feature that has been exposed in the renovation is the
>housing for a large flywheel that was set with its axle at floor
>level in one of the rooms.  The depression into which the 4 meter
>flywheel originally sat was lined with rivetted cast iron sheets to
>keep the ground water out.  This iron structure has been completely
>submerged in fresh water for about a century and has now been pumped
>out and cleaned (it still however stands in waterlogged ground). The
>iron is in very good condition but damp patches are appearing at
>some of its rivetted joins.  I am hoping for some advice about what
>should now be done to protect the structure which will now stay in
>situ, dry and on display.  I was unsure whether painting the iron on
>the exposed dry side might do more harm than good.  The structure
>cannot be extracted to allow a coating to be applied to the wet
>side.  Any help would be gratefully received.

Attempting to paint the inside of any structure which has water
penetrating from the other side is almost impossible.  Can you bore
into the ground around the pit and pump the water level down without
causing subsidence and cracking of surrounding structures?

You report the cast iron sheets as in good condition presumably on
the basis of inspection of the dried surface.  What is the
waterlogged ground like?  Is it clay and impermeable or is it
bog-like or sandy with organic materials?  If it has significant
organic materials, I would be concerned about corrosion of the
outside just below the water table and just below the ground level

As far as painting a cast iron surface is concerned, I expect that
you have had some loss of the internal surface due to graphitisation
(galvanic corrosion of the iron in contact with the graphite flakes
in the cast iron) in the fresh water and that unless you remove all
of the black, friable layer left after this corrosion, any paint
will not adhere.

Graham Sussex
Corrosion and Materials Consultant
Melbourne
Australia


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                  Conservation DistList Instance 22:4
                   Distributed: Tuesday, July 8, 2008
                        Message Id: cdl-22-4-007
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