Unseen corrosion plays a major role in the operational
efficiency of process plants. In refining and petrochemical processes, greater
demands are placed on construction materials for key processing
equipment. The ability to withstand corrosion at elevated
temperatures requires special consideration, especially for
material selection throughout the plant. New guidelines improve
choosing corrosion-resistant materials for heat exchangers.
Better materials can provide optimal equipment service life,
reduce maintenance, mitigate contamination
by corrosion products and minimize heat loss due to
Harsh operating environments
In refineries and process plants, greater operating demands
are required on the construction materials used in
capital equipment. The ability to withstand corrosion at
elevated temperatures involves special consideration for
materials and metallurgy to be used throughout the plant. It is
not just to the materials suitability in the process,
but, more importantly, it is the materials ability to
resist corrosion and perform efficiently and effectively. A
well-considered material selection process will provide for
optimal equipment service life, reduce maintenance spending and
Refineries now handle high-sulfur-content crude oils. The
utilization rates are higher and are geared to find more yield
per barrel of oil processed. The need for corrosion-resistant
materials is vital to eliminate equipment failure and unit
downtime. Throughout the refining process, non-hydrocarbon
compounds and additives can build up within process streams and
can be the root cause for extensive corrosion problems.
Buildup of such deposits, in or on the tubes of heat
exchangers, can be sourced from the process side due to
tenacious hydrocarbons, process slurries, or even ammonium
chloride deposits in the crude unit overhead condensers.
Equally important, contaminants can be sourced from cooling
water, which may contain sand or sediment. Such sediments can
build up due to low flowrates in horizontally mounted heat
exchangers. All can have a detrimental effect on a refinerys efficiency and heat
Better construction materials for
Applying corrosion-resistant materials not only eliminates
unscheduled plant shutdowns, but it also reduces the risks from
costly lost production and expensive emergency maintenance and repair. Attention
should also be given to the formation of crevice corrosion
beneath such deposits at temperatures below the critical
pitting temperature (CPT) of the material.
Carbon steel (CS) is extremely vulnerable to corrosion, and
austenitic stainless steel (SS), widely used in heat-exchanger
tubing, can become susceptible to stress corrosion cracking
(SCC), particularly in chloride-bearing environments.
Research shows that these materials are highly susceptible
to corrosion at the elevated operating temperatures found in
refineries. Gradually, the industry is recognizing the
advantages of duplex SS. It can offer the optimum combination
of corrosion resistance, mechanical properties and excellent
fabrication capabilities. The cumulative benefit of such an
approach is genuine cost advantages. Link this with its
compatibility with other alloys during the fabrication process
and duplex SS is an ideal material, not only for new equipment
but also for the retubing of existing heat exchangers,
replacing CS and even austenitic SS.
For example, new lean duplex SS can offer high strength with
a yield strength twice that of ASTM 316L, along with low
thermal expansion, very good weldability and
physical properties that provide design advantages, as well as
ease of fabrication and toughness.1 It can offer
technical benefits and design advantages of the material,
together with the cost advantages over conventional SS and
Chemical composition. Due to low nickel
(Ni) content, lean duplex SS has a two-phase microstructure
with approximately 50% ferrite.1 A high 23% chromium
(Cr), compensating for the absence of molybdenum (Mo), can
provide high resistance to corrosion, as listed in
Table 1. The nitrogen content further
increases the materials strength, improving weldability
and pitting corrosion resistance.
Mechanical properties. Direct comparisons between the
mechanical properties of the material, austenitic SS and CS are
summarized in Table 2. These clearly
demonstrate the high yield strength of the material, along with
high tensile strength and hardness properties.
The impact strength of the material at various temperatures,
in both welded and unwelded conditions, is illustrated in
Its toughness throughout the temperature range makes it far
more suitable than CS, which normally has ductile to brittle
transitions in the range 0°C to 80°C (32°F to
1. strength Charpy-V for duplex SS
and CS. Specimen size 10 mm x 10 mm
(0.40 in. x 0.40 in.).
Low thermal expansion. It is the low
thermal expansion of duplex SS that offers significant design
advantages. As shown in Fig. 2, it is much
lower than austenitic SS and very close to that of CS. When
used in tubular heat exchangers, whether a new build or to
replace existing CS tubes, the low thermal expansion of the
duplex SS is a favorable option to use with other alloys.
Corrosion resistance. Even in acid solutions,
the lean duplex SS has better resistance to corrosion than ASTM
304L, owing to its very high Cr content. In fact, it is even
better than ASTM 316L in most acid environments. This is
demonstrated by the isocorrosion curves, as shown in
Fig. 3. This figure illustrates the corrosion
rate in formic acid of just 0.1 mm/yr (4 mpy).
3. Isocorrosion diagram showing
materials in formic acid.
Again, the high Cr content of the material also offers good
resistance to general corrosion, pitting and crevice corrosion,
exceeding that of austenitic grades, as shown in Fig.
3. Duplex SS offers excellent resistance to SCC in
aqueous solutions. The lean duplex SS is suitable for use in
temperatures around 140°C (284°F) without risk of SCC.
By comparison, ASTM 304L and ASTM 316L should only be used in
operating temperatures below 60°C (140°F), as shown in
4. Critical pitting temperature
in neutral chloride solutions
determination at 300 mV, SCE).
For many projects, cost is the primary
priority. However, the ability of a material to fully meet the
application requirements is likewise a major consideration for
Strength of the material is a significant
factor. For example, selecting a duplex grade, such as
a lean duplex SS, despite a higher price per kg, can prove to
be the most economical solution.1 This is because
the wall thickness of the tubes subjected to internal pressure
or tensile loads is directly related to the material strength.
As thinner wall tubes can be specified, the cost of the duplex
material can be around 35% lower. This should be compared to
the cost of tubes of other material grades, which would require
a thicker wall to achieve the same strength, as summarized in
Table 3. There are also associated savings to
be achieved on transport, installation, welding, etc., when
specifying the lighter thinner walled duplex grade tubes. For
many applications, the material offers economical solutions
because of the high corrosion resistance, especially to SCC.
The use of computerized life cycle cost calculations, using all
relevant data, including maintenance, investment costs,
service life, inflation, etc., can clearly demonstrate the
savings that are possible.
Better design can save money
Within refineries, along with hydrocarbons, there are
chlorides, ammonia compounds, hydrogen sulfides, carbon
dioxide, various acids and water. Such mixtures can result in
the premature failure of many construction materials, such as
copper-based alloys to steels, as well as different types of
austenitic SS, due to corrosion. In such environments, duplex
SS can provide excellent resistance to corrosion attack as
recorded in extensive laboratory testing as well as in
successful documented installations in process plants and
refineries worldwide. The ease of fabrication, cost savings and
the durability of duplex SS can offer significant advantages
not only for new equipment, but also when retubing existing
heat exchangers. HP
5. SCC resistance in oxygen bearing
(~8 ppm) neutral chloride (Cl) solutions.
Test time of 1,000 hr. Load ≥ yield
at testing temperature.
6. Cut-away of horizontal heat
exchanger, illustrating flow direction.
1 Lean duplex SS, such as Sandvik SAF 2304,
offers high strength with a yield strength twice that of ASTM
316L. Sandvik and Sandvik SAF 2304 are trademarks owned by
Sandvik Intellectual Property AB.
Eduardo Perea is the global business
developer for tube chemicals for Sandvik Materials Technology, and is based in
Singapore. He is a metallurgist engineer and graduated
from Faculdade de Engenharia Industrial in Brazil. In
2004, Mr. Perea joined the company as a trainee and was
later promoted to technical marketing and sales
engineer. In 2008, he relocated to Sweden and joined
the global technical marketing, high-temperature
products group. In 2011, he was promoted to regional
sales manager for tube before assuming his present role