Stainless steel does rust, and corrode. Anyone who has owned a sailboat can
attest to that statement. Stainless steel works because the chromium in the
alloy combined with presence of oxygen in the air forms a passivation layer
of chromium (III) oxide. This layer is too thin to be visible, which
accounts for the nice shiny appearance of stainless. Anything that disrupts
that layer, or shields the stainless from direct contact with oxygen (buried
underground) disrupts that reaction. This accounts for the rust stains that
seep out from under stainless steel cleats, and the corrosion in a nice
shiny new stainless steel turnbuckle that has spent a year or so tightly
wrapped in rigging tape. Stainless steel pipe would probably last a lot
longer than either black steel or galvanized pipe in this application, but
not forever.
Actually in the environment described, polyethylene sleeved, cement lined
ductile iron would last the longest. No exposed metal to rust on the inside
and no exposed metal to corrode on the outside. The only question I would
have since I haven't looked into it is the sealing properties of the gaskets
under air verses water pressure.
.Just random thoughts brought on from following this thread.
Richard L. Mote
Designer
Rowe Sprinkler Systems, Inc
P.S. Nickel, molybdenum and vanadium also contribute to passivation and may
be present in varying quantities some grades of stainless steel.
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