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localized corrosion than the 300 series
austenitics. On the negative side, these materials
still present some degree of difficulty in pro-
cessing and welding; post weld heat treatment
may be required. Ferrite content must be
controlled to avoid its transformation to sigma, a
hard brittle phase.
3. 3. Super austenitic stainless steels
These materials keep the basic austenitic
structure with higher contents of chrome and
molybdenum with nitrogen. Nickel must also be
increased to offset the ferrite forming effect of
Cr and Mo. Drawbacks: their substantially higher
cost, processing difficulties and weldability
problems.
3.4. Specialized alloys
Alloy 885, a patented material, is an alloy
developed with a corrosion resistance equal to or
better than most duplex alloys, approaching that
of the super austenitic stainless steels and, at the
same time, possessing the ease of casting and
welding of the 300 series austenitics.
4. Alloy 885. A stainless steel casting alloy for
pumps in seawater applications
4.1. Localized corrosion
There has been a considerable amount of data
published regarding the metallurgical variables
that affect the localized corrosion behavior
(pitting and crevice corrosion). Chrome, molyb-
denum and nitrogen have a beneficial effect on
the pitting and crevice corrosion resistance.
Researchers suggest the use of the materials
PREN, Pitting Resistance Equivalent Number, to
evaluate the corrosion resistance.
PREN = Cr(%) + 3.3 x Mo(%) + 16 x N(%)
A technical paper presented at the NACE
(National Association of Corrosion Engineers)
convention in 1988 by T.J. Glover indicates that
a PREN of 38 is sufficient to guarantee corrosion
resistance of a stainless steel to seawater ex-
posure.
Another parameter used for evaluating the
corrosion resistance of materials is the Crevice
Factor. The formula developed from actual
crevice corrosion testing performed in the labo-
ratory, reads as shown below.
CF = Cr(%) + 3 × Mo(%) + 15 × N(%)
Experimental data shows that if an alloy has a
minimum crevice factor of 35, the material will
not crevice corrode in an aggressive acid chloride
environment test.
The Critical Crevice Temperature (CCT) of a
material is yet another parameter used to indicate
its corrosion resistance. It is the temperature of
an acid chloride solution at which corrosion is
first observed. The higher the CCT, the greater
the corrosion resistance the alloy will exhibit.
4.2. Stress corrosion
In addition to ferrite content, temperature and
oxygen content, the stress corrosion resistance of
the austenitic alloys is also a function of the