Chemical Engineering January 2017 - 28

Overcoming Corrosive
Processes with HighAlloyed
Stainless Steels
Advanced high-alloyed stainless steels with austenitic properties can address corrosion
concerns in challenging phosphoric and nitric acid processes
Daniel Gullberg
Sandvik Materials
Technology
IN BRIEF
SELECTING
ANTICORROSIVE
MATERIALS
PHOSPHORIC ACID
PROCESSES
GRAPHITE VERSUS
METALLIC TUBES
THE EFFECTS OF HIGH
ALLOYS
ENVIRONMENTAL
CONTAMINANTS
SMALLER TUBE
DIMENSIONS
NITRIC ACID PROCESSES
CHROMIUM FOR NITRIC
ACID SERVICE
TESTING ALLOYING
ELEMENTS
RECOGNIZING HIGHCORROSION
SITUATIONS
CLOSING THOUGHTS
T
he growing worldwide
demand for fertilizer has
created a significant increase
in
production
levels.
As a consequence, operators
face mounting pressure
to keep up with these trends,
and this challenge is especially
evident in the greater expectations
that are being placed on
materials for critical tubes and
pipes in heat exchangers. Heat
exchangers are essential to
fertilizer applications. Not only
must they ensure efficient heat
transfer from one medium to
another while minimizing resistance
to fluid flow through the
tubes, the heat exchangers
should also remain inexpensive
in terms of both design
and maintenance.
This is where issues of corroWater
out
Steam in
Condensates
Steam out
Pump
Filter acid Water in
Concentrated acid
FIGURE 1. Forced-circulation evaporation is typically used in phosphoric-acid
heaters for acid concentration
sion set in, made worse by the fact that such
problems may vary considerably from plant to
plant depending on local service conditions.
For these reasons, many materials traditionally
favored by the industry - such as standard
AISI 316L, 317L or 304L grades of stainless
steel, depending on the application - may
exhibit insufficient performance in acidic fertilizer
processes. In order to solve these problems,
it is extremely important to select the
correct material for any given application. This
article explains how advanced high-alloyed
materials can offer an alternative to traditional
material grades.
Selecting anticorrosive materials
Across all fertilizer applications, there are two
primary considerations when choosing a re28
placement
tube or pipe material - corrosion
resistance and the presence of contaminants
in the process stream.
First and foremost, processors must identify
a material grade with an optimal content
of chemical elements that promote good
corrosion resistance.
It
is
also important
that levels of contaminants in the production
environment are minimized, in order to
avoid the formation of harmful segregations
in the material's grain boundaries, which can
initiate corrosion.
Phosphoric acid processes
It is estimated that around 90% of the phosphoric
acid (H3PO4) manufactured worldwide
is used for fertilizer production. Phosphoric
acid is most commonly produced by
extraction from rock phosphate (phosphoCHEMICAL
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Chemical Engineering January 2017

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