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 大型高压泵列车英文文献和翻译(3):http://www.youerw.com/fanyi/lunwen_13158.html