Chemical Engineering July 2022 - 31

Feature Report
Materials of Construction:
Stainless Steel Versus
Nickel Alloys
Nickel alloys have long been the default choice for demanding applications in the
chemical process industries (CPI). A new grade of stainless steel offers an alternative
Rodrigo
Signorelli and
Jan Li
Outokumpu
IN BRIEF
STAINLESS STEEL
BACKGROUND
THE PASSIVE LAYER IS
THE KEY
CATEGORIES OF
STAINLESS STEEL
RANKING STAINLESS
STEELS
COMPARING S32654 TO
OTHER ALLOYS
STAINLESS STEEL
VERSUS TITANIUM
PROVEN IN THE FIELD
D
emanding chemical engineering
applications, such as plate heat
exchangers (PHEs; Figure 1), tubing,
pipes and pumps, call for materials
that offer exceptional performance
in extremely corrosive environments. That's
why many design engineers regard nickel
alloys as their " go-to " choice. A popular
selection is Alloy 625 (UNS N06625). This
is a nickel-chromium-molybdenum superalloy
that possesses both high strength
and resistance to elevated temperatures.
The alloy is particularly resistant to pitting
and crevice corrosion as well as to severely
corrosive environments.
However, for all the performance benefits
that nickel alloys bring, there is also a significant
downside. Because as well as being
expensive, nickel is subject to extremely
volatile pricing. For example, current geopolitical
concerns have seen nickel prices
fluctuate by a factor of around three in less
than a year. The strategic importance of
nickel for the battery sector is also likely to
act as a further driver for prices over the
next two decades.
These factors are driving design engineers
to broaden their scope to consider alternatives
to nickel alloys. A leading contender is
ASTM UNS S32654 (EN 1.4652), an austenitic
stainless steel grade known as the most
corrosion-resistant in the world.
By switching to S32654, designers can
take advantage of a stainless steel that offers
comparable performance to Alloy 625,
with a typical cost reduction of 30 to 40%.
It also comes with the advantage of much
more stable pricing.
Stainless steel background
In 1913. the English metallurgist Harry Brearley
discovered that the addition of chromium
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2022
in combination with a relatively low carbon
content dramatically increased the corrosion
resistance of steel. He called his 12.8%
chromium (Cr) and 0.24 % carbon (C) containing
martensitic alloy " rust-less steel " and
introduced it to the cutlery industry in his
hometown Sheffield, U.K.
At the same time, the company Krupp in
Germany discovered that steels with high
Cr or Cr-Ni could be stored for months in
humid and aggressive environments without
rusting. This revolutionary material had
hygienic-surface properties and was able to
withstand aggressive foodstuff, such as vinegar
or lemon juice. Stainless steel quickly
became the preferred material not only for
cutlery and tableware but also in the foodprocessing
industry, because of its hygienicsurface
properties.
Today,
there are hundreds of
different
types of stainless steels available for all kinds
of applications.
The passive layer is the key
Stainless steels are not fundamentally noble
materials in the same way as gold or platinum,
which are more or less inert to most
environments. Instead, stainless steels deFIGURE
1. For harsh, corrosive applications, selecting the best
material of construction is important for equipment, such as
heat exchanger plates
31
http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2022

Table of Contents for the Digital Edition of Chemical Engineering July 2022
