The Ferritic Solution - Euro Inox

group 1 ferritics are best suited to non-severe conditions,
such as inside the home (where the material is either not
exposed to water contact or gets regularly wiped dry) or
outdoors in contexts where some superficial corrosion is
acceptable. in such contexts, this group of ferritics has a
longer life than carbon steel,.
group 2 grades are effective in contexts involving
intermittent contact with water, in non-severe conditions.
group 3 grades are suitable for similar contexts to those
appropriate for group 2 grades, but are easier to weld.
group 4 ferritics are more corrosion resistant than type
304 and are suitable for a wide variety of uses.
group 5 includes, for example, grades with a very high
chromium content of around 29% cr, plus 4% Mo, which
makes them as corrosion resistant in seawater as titanium
metal.
“Nickel plays no role in resistance
to pitting corrosion.”
Storage tank, in grade 444, Brazil.
The pre FacTor
the “pre” or pitting resistance equivalent number is a
measure of the relative pitting corrosion resistance of a
stainless steel grade in a chloride-containing environment.
the higher a grade’s pre value, the more corrosion resistant
that grade will be.
35
30
25
20
15
0
the pre comparison table shows at a glance that for every
austenitic grade there is a ferritic grade with comparable
corrosion resistance.
in the commonly used shortened form of the pre formula
pre=%cr+3.3%Mo, molybdenum (Mo) is expressed as
being 3.3 times more effective than chromium against
pitting corrosion. however, chromium is always essential
for providing the basic corrosion resistance. Molybdenum
cannot replace this “base” amount of chromium in stainless
steels, but can be used to boost corrosion resistance.
nickel content is not considered in the formula, since
in most applications it plays no role in resistance to pitting
corrosion.
AustENitiC/fERRitiC PRE CoMPARisoN
PREN = %Cr = 3,3 x % Mo + 16 x %N
avoIdIng corroSIon
Stainless steel’s “passive”
layer (see p. 59) needs
oxygen to remain intact. an
accumulation of deposits can
deprive the steel of oxygen
at critical points, which
could lead to corrosion.
propagation of corrosion
may lead to eventual rupture
of the part.
317lN 446/447
316
304
444
436
430
409
AUSTENITIC FERRITIC
439
SEA wATER 20°C
CoASTAl ENv. 20°C
PURE wATER
Barbecue and trolley, in grade 430, Italy.