corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
Ferritic steel grades
are stable in price
Release of
nickel into food
Mechanical
strength of
nickel alloys
Another benefit to using ferritic steel is its relatively low and very stable price
which does not depend upon the high and volatile price of nickel (see Figure
4.2 on page 43). The price of the ferritic steel is therefore much more stable
than for the austenitic which naturally sparked a certain interest around
2006 and 07. As the price of nickel subsequently fell sharply between 2008
and 2009, interest fell correspondingly, but at the time of publication it is
slowly on its way up again. This does not, however, change the fact that in
the ferritic world there is a much poorer range of steel qualities and supply
forms than for the austenites. Especially the stabilised, ferritic steel (e.g.
4509 and 4521) are currently difficult to find in any other form but thin
steel sheets (Section 9.1 on page 169).
Apart from the price, the ferrites have the psychological advantage that the
risk of release of nickel from nickel-free steel is zero which makes the ferritic
steel particularly suitable for food-related applications. 4016 is especially
popular for many catering applications both in the UK and Italy, and the
weldable 4509 is used widely for both interiors and exteriors of refrigerators,
cf. Figure 2.4 on page 18. For further information about food-grade stainless
steel please see Section 6.9.1.
Note also that three nickel-based alloys have crept their way into Table 6.1:
Inconel 600, Inconel 625 and Hastelloy C-276. Nickel alloys can be treated
in parallel with the stainless steel when it comes to pitting corrosion, and
increased resistance to corrosion is achieved, of course, by adding as much
chromium and molybdenum to the steel as possible. Unfortunately, this
needs to be balanced by an equivalent quantity of nickel (see Figure 3.2 on
page 26), and at some point so much nickel will have been added that iron (Fe)
simply becomes a minority constituent and we get a nickel alloy instead. It
is therefore reasonable to regard the high corrosion-resistant nickel alloys
as natural extensions of stainless steel.
At the other end of Table 6.1 it is interesting that Inconel 600, despite at least
72% Ni, falls way down the table. In the case of simple pitting corrosion,
Inconel 600 simply performs poorer than standard 4301 which is because
the alloy only contains 14% Cr. The 72% Ni do not help much in the creation
of the passive layer, but there is nothing odd about that. Inconel 600 and
its relative Inconel 601 (21-25 Cr, 58-63 Ni) were developed for their good
mechanical strength at high temperatures and excellent resistance towards
corrosive flue gases, but they are no good under oxidising conditions at
room temperature. A bit like a Ferrari which is brilliant on a racetrack,
but performs poorly on a bumpy gravel road. A 4WD Land Cruiser has
the opposite strengths and weaknesses - and the same goes for metals. All
alloys have their strengths and limitations, and the trick is always to find
the combination of properties that best meet the needs of each individual
project.
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