Mack-- just a side technical note to say that "erosion corrosion" can occur
in high purity water via the action of the fluid flow only. This happens for
instance in condenser pipes where the velocity of the flow is increased due
to a specific shape and creates rounded holes, grooves etc. Just to give you
an idea, this phenomenon is also known to occur on carbon steel by steam
containing entrained water droplets. The other consequence of this type of
corrosion is that it removes protective films and can sometime inhibits the
formation of concretions on marine archeological artifacts. The entrainment
of solid particles is definitely an aggravating factor. Haven't we seen at
least one bronze cannon sliced in two by the sandblasting effect of the
current on the Zuytdorp site or cast iron features cut through on the H.L
Hunley? I guess my point is that "erosion corrosion" is a combination of
mechanical (erosion) and chemical (corrosion).
Cheers
Paul
Paul Mardikian
Senior Conservator
H.L Hunley Project
Warren Lasch Conservation Center
1250 Supply Street Bldg. 255
North Charleston, SC 29405
USA
T (843) 743-4865
F (843) 744-1480
www.hunley.org
"Iron rusts from disuse; water loses its purity from stagnation ... even so
does inaction sap the vigour of the mind." Leonardo da Vinci
----- Original Message -----
From: "Mack McCarthy" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, February 09, 2007 12:07 AM
Subject: Sheathing... Forgot to add this.. Of interest also erosion ofcopper
sheeting in service
Ex Ship's fastenings work: ... Modern analyses have shown, for
example, that while pure copper sheathing releases copper as its toxic
constituents into the water and thereby serves to deter marine growth, it is
also prone to erosion from water-borne debris. While copper has as "fair
erosion resistance" up to 0.9 meters per second (about two knots), alloys
are less susceptible, though there was a problem with what is now called
"de-zincification" as will be seen. Later it was found that the addition of
tin to the product, which then appeared as "Naval brass" (62 percent copper,
37 percent zinc, and 1 percent tin), and "Admiralty brass" (a 70:29:1
alloy), allowed the product to withstand twice that rate of water flow.13
The term "composition metal" resurfaces in this period.14 In
James P. Delgado's analysis of a wreck believed to be that of the famous
Mary Celeste (186185) appears a reference to the American Shipmasters'
Association requiring outer planking to be "fastened with composition or
copper to load lines in addition to the ordinary tree-nail fastenings."
Delgado also quotes the September edition of the New York Sun which
describes a "composition metal that in appearance resembles brass."15
Muntz metal, Yellow metal, Naval brass, and Admiralty brass were
forms that persisted into modern times and with the addition of silicon,
aluminum, iron, and manganese lead to the development of the "high tensile
brasses." Some of these came to be used as fastenings, as will be seen in
the section on modern shipbuilding.
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