Revisiting the Selection of Stainless Steel in Water and Wastewater Treatment Environments: Part 1

Metals such as bronze, copper and iron have been used for
thousands of years by man for both peaceful and non-peaceful purposes. One of
the most useful purposes for metal is the production of steel.

 

Steel is defined as iron with a small amount of carbon
(usually less than 0.5 percent) and other elements in small quantities. Grades
of steel are determined by the amounts of non-ferrous components such as
carbon, chromium, nickel, molybdenum, copper and titanium present. Depending on
the specific content and manufacturing process, steels can be formed for a
range of properties including the following.

 

               Overcome
brittleness

               Cohesive
strength

               Corrosion
resistance

               Tensile
strength

               Fatigue
protection

               Ductility

               Hardness

               Malleability

               Shear
strength

               Torsional
strength

               Electrical
conductivity

               Thermal
conductivity

               Thermal
expansion (or resistance)

               Magnetic
properties

               Heat
treatability

 

The selection of the best material for a specific
application is complicated but very important to the success of the project.
Steel has many uses and, therefore, is one of the largest industries worldwide
today. Advantages of steel include

               High
strength,

               Uniformity,

               Elasticity under high stress,

               Ductility,

               Adaptations for connections and manufacture,

               Fatigue
strength and toughness,

               Resistance
to heat deformation, and

               No
ultraviolet light degradation.

 

Disadvantages of steel use include

 

               Maintenance
costs due to susceptibility to corrosion by air, water and microbiological
elements, and

               Fatigue
if subjected to stress reversals (pressure on, then off).

 

The most critical drawback in using steel is corrosion.
Common types of corrosion in steel include galvanic, dezincification, pitting,
crevice, intergranular, stress, cracking, erosion and microbiologically induced
corrosion (MIC). A number of solutions have been pursued to address problems
with corrosion. The development of stainless steel is one answer.

There are more than 40,000 steel grades manufactured,
depending on heat treatment, chemical composition and application. Steel can be
divided into several groups: carbon, alloy, tool, high temperature, stainless,
super-stainless, and high alloy with chromium, nickel and cobalt alloys
included.1 Stainless steel typically is chosen over painted carbon steel and
galvanized steel to reduce the maintenance and replacement costs associated
with less corrosion-resistant metals.2

Economical production of stainless steel has occurred since
the middle of the nineteenth century. The chemistry, mineralogy and metallurgy
of steel is very important in the proper manufacture of steel. The American
Society for Testing Materials (ASTM) and American Petroleum Institute (API)
specify exact maximum percentages of carbon, manganese, silicon, etc. in their
steel standards. Of concern here are the grades of stainless steel that are
commonly specified for water treatment and membrane systems. The commonly
available commercial types of stainless steel include

 

               Martensitic
Grades (Cr 12–17 percent, high carbon),

               Ferritic
Grades (Cr 18–30 percent, lower carbon),

               Austenitic
Grades (304 and 316, including low carbon forms (L), Cr 18–30 percent,
2–6 percent Ni),

               Duplex
Grades (higher corrosion resistance than Austenitic grades),

               Precipitation
Hardening Grades,

               High
Performance Grades (6 Mo), and

               Nickel
Rich Grades.

 

Duplex and austenitic are the two grades most commonly used
in water treatment applications.3 The difference between them is the
crystalline structure of the metal (face or block centered—Figure 1).
Nickel is an austenitic stabilizer. This means the addition of nickel to
iron-based alloys promotes a change in the crystal structure of stainless steel
from body centered to face-centered cubics. This change improves corrosion
resistance.

 

Stainless Steel Used in Membrane Facilities

Types 304, 304L, 316 and 316L are austenitic stainless steel
with 18 percent chromium and 8 percent nickel. Types 304L and 316L are low
carbon steels designed to be resistant to intergranular corrosion after welding
without requiring further heat treatment. Removal of the oxide film (heat tint)
and the thin, chromium-depleted layer just beneath the heat tint that occurs in
areas adjacent to the welds when welding is performed is required. Unless
removed, these areas of the pipe will corrode very easily. Removal of the heat
tint and the thin, chromium depleted layer just underneath will restore the
integrity of the stainless steel.

The duplex stainless steels are a family that combines good
corrosion resistance with high strength and ease of fabrication. The early
grades of duplex stainless steels were developed more than 60 years ago in
Sweden. Duplex stainless steels have the physical properties of austenitic and
ferritic stainless steels but tend to be closer to ferritic and carbon steel.
They have an improved resistance to chloride pitting and crevice corrosion
because of the combination of molybdenum, chromium and nitrogen.4 Duplex steel
was developed to reduce stress corrosion cracking problems that existed in the
early high-carbon austenitic stainless steels.4 Duplex stainless steel can have
a range of ferrite to austenite from 30–70 percent but typically have a
microstructure of equal proportions of ferrite and austenite. A stable duplex
structure must form. Development of inter-metallic phases at high temperatures
must be avoided during the manufacturing process to ensure corrosion
resistance. When formed in large quantities and under conditions where the
chromium depleted areas do not have time to repair themselves during annealing,
the precipitates may adversely affect corrosion resistance.

A minimum of 10.5 percent chromium is necessary to form a
passive film that is sufficient to protect stainless steel against mild
atmospheric corrosion. Corrosion resistance increases with increasing chromium
content. Chromium is a ferrite-former, meaning that the addition of chromium
stabilizes the body-centered cubic structure of iron. Iron has two unpaired
electrons in its outer orbital. Chromium shares electrons to complete the
pairing (and create a stable form) of electrons. Higher chromium content
requires more nickel to form an austenitic structure. The first generation
duplex stainless steel provided good performance but had limitations in the
welded areas. The welds have a low toughness as a result of excessive ferrite
and lower corrosion resistance than the base metal so such steel use was
restricted to unwelded applications.4

Second generation duplex stainless steel usually is
comprised of at least 22 percent chromium. Molybdenum is added to support the
chromium in providing corrosion resistance. When the chromium content is at
least 18 percent, the addition of molybdenum makes the steel much more
corrosion resistant to chloride pitting. Molybdenum is a ferrite-former but
also has the tendency to form detrimental intermetallic phases. Therefore, it
is restricted to less than 7.5 percent. Ferritic stainless steels contain
little or no nickel, while duplex stainless steels contain at least 4 percent
nickel.4

In 1968, argon oxygen decarburization was developed, making
the addition of nitrogen as an alloying element possible. This process makes
field welds more corrosion resistant. It also reduces the rate at which the
detrimental inter-metallic phases form. Since then, the super duplex grades
have been developed to maximize corrosion resistance.4

The use of nitrogen as an alloying element in stainless
steel allows the chromium nitrides to be present on ferrite-ferrite grain
boundaries, on austenite-ferrite boundaries and in heat-affected zones of
welds. Nitrogen also

 

               increases
resistance to pitting and crevice corrosion while increasing strength and
toughness;

               raises
the temperature at which austenite is formed and allows for accelerated
cooling; and

               reduces
the rate at which detrimental inter-metallic phases form.

 

Table 1 outlines the chemical composition of various grades
of stainless steel. Experience has shown that for optimum corrosion resistance,
and to avoid inter-metallic phases, the chromium, nitrogen and molybdenum
should be kept to 22–23 percent, 0.14–0.2 percent and 2.5–3.5
percent, respectively. Duplex stainless steels have much higher strength than
austenitic grades while exhibiting good ductility and toughness. All duplex
stainless steel have chloride stress corrosion cracking resistance that is much
greater than the 300 series austenitics. Super duplex stainless steel has
25–26 percent chromium, with higher levels of nitrogen and molybdenum.4

 

Problems

Stainless steel has provided good service in many
applications. However, use of the appropriate grade is important to the
ultimate success of the steel. Corrosion should be an expected by-product of
any steel use, including stainless steel. The question is, how much corrosion
is acceptable in the intended application?

Corrosion is the deterioration of a metal or alloy as the
result of exposure to and reaction with its environment. It is an
electrochemical process, like a battery, with an anode and a cathode reaction,
and ions migrating through an external circuit, even in single metal steel. Positive
electrolytes migrate through the liquid electrolyte toward the cathode, whereas
negative ions are attracted to the anode. The corrosion reaction cannot occur
without a simultaneous cathodic reaction. The typical reactions are

 

Fe0 Æ Fe2+ + 2e–

 

2H+ + 2e– Æ H2O

 

2H+ + 1/2O2 + 2e– Æ H2O

 

H2O + 1/2O2 + 2e– Æ 2OH–

 

The first equation is anodic and the rest are cathodic.
Corrosion of stainless steels is dependent almost entirely on chlorides,
sulfates and dissolved oxygen content. Chlorides also can be adsorbed or
chemisorbed in films on the metal surfaces (silicates, aluminates, iron oxides
and calcareous deposits). In potable water, which may be soft (Hardness =
0–75 ppm), moderate (Hardness = 75–150 ppm) or hard (Hardness =
150–300 ppm), the calcareous deposits can affect how stainless surfaces
react to chlorides, regardless of bulk water chemistry.3 There are several
corrosion types that are common to various grades of stainless steel.

 

               Crevice
corrosion is most likely to occur on the outside of the pipes used for sludge,
Vitaulic couplings in membrane facilities and buried pipe in damp soils. The
tightness of the crevice will affect the speed with which crevice corrosion
occurs.3 Corrosion occurs in tight, stationary crevices in the presence of oxygen
and chlorides greater than 1,000 mg/L. The initial step in crevice corrosion is
depletion of oxygen in the crevice solution. The dominant mechanism is the
presence of bacteria that will accelerate initiation of localized attack.5 In
membrane plants, crevice corrosion often will be noted in the J-bends at the
Vitaulic couplings. Type 304 will pit 1/4* in 2 years. Type 316 is
half as likely to have crevice corrosion as Type 304, but is still considered
very susceptible to crevice corrosion. Crevice gaps smaller than 0.4 µm
will initiate crevice corrosion.3 If the sulfate/chloride ratio is 3 or better,
there is less chance of crevice attack, but this ratio normally is not present
in water treatment applications.3 Second generation Duplex grades including
Zeron 100 and 2507 show significantly better resistance to chloride pitting and
crevice corrosion.3

 

               Chloride
pitting—When the chlorides exceed 50 mg/L, 304 stainless steel should be
used instead of carbon steels. Where the chlorides exceed 200 mg/L, 316L would
be preferred.2 Manufacturers often recommend a chloride ceiling for Type 316L
of 500 mg/L. This level is below all brackish water sources. Chloride induced
pitting is another problem resolved by metal selection. Inappropriate metal
selection can lead to serious damage to the pipe, as well as microbial
colonization.

 

               Pinhole
leaks—Often with microbiological components, pinhole leaks will occur as
a result of the field welding process when the heat-affected zone is not
properly removed in lower grade stainless steels. Heat tint removal is a
tedious field process, but failures can occur quickly. At the pinholes, slimy
spots that appear to be microbiological in nature, often are found. MIC is a
phenomenon that occurs as a result of prolonged exposure of the pipe wall to
raw water that contains microbes. These growths are permanent in nature and
must be controlled. Figure 2 shows critical pitting and critical crevice
corrosion temperatures of various grades of stainless steel. 316L is at the
lower range, well within the expected temperature of the raw and finished water
supplies to most treatment facilities.

 

               External
corrosion in the form of rust stains often is found in multiple locations on
the exterior of the pipe. Bands of rust streaks on the exterior of the pipes
generally result from the inappropriate use of an unpadded steel sling on the
crane to lift the pipes into place. They generally are cosmetic in nature.

 

               Stagnant
water always presents a risk for the occurrence of corrosion and
microbiological activity. The ability to completely drain the pipeline during
periods of non-use is important as static water encourages microbial activity.6
A minimum velocity of 3 fps is required to prevent corrosion.5 Conversely,
there is no adverse effect of velocity. Stainless steel performs better at high
velocities that replenish dissolved oxygen and minimize the possibility of
deposits unless the water has a severe scaling potential. Piping using
austenitic stainless steel has withstood more that 100 fps in seawater with no
evidence of erosion corrosion.3

 

               Heat
tints near welded areas have been shown to make the welds susceptible to
biocorrosion.5 Mill pickling is used to control surface corrosion on stainless
steel pipe. In the pickling process, scale is removed from prior annealing
treatments that can cause a depleted chromium layer at the annealing or inclusion
points.2 This is the same problem that occurs with field welds. The pickling
process provides a passivated, corrosion resistant surface as a result of the
thin oxide film.2

 

               Operations
issues such as scaling, over chlorination, poor fabrication and MIC can
initiate corrosion in 316L stainless steel. 3

 

The appropriate choice of stainless steel grade and
accompanying appurtenances, plus the appropriate construction and treatment
techniques should allow the metal to provide a reasonably long life by
resolving most of these corrosion issues. However, the microbiological problem
can only be controlled, and where biological problems can reasonably be
anticipated such as wells and membrane plants higher grade stainless steels
should be specified when steel strength is required.

 

Microbiological Concerns

Due to the lack of corrosion products on its surface,
corrosion resistant stainless steel can provide a good substratum for
biofouling. Biofilm formation and structure are more easily observed by electron
microscopy.5 When the stainless steel surface is passivated, the biological
mechanisms become dominant.5 Pitting and crevice corrosion can be the immediate
result of stainless steel in contact with aqueous solutions containing
microorganisms. Microbiologically induced corrosion is not in itself a
corrosion form; instead, corrosion is initiated and/or aggravated by
microorganisms. The involvement of microorganisms in metal corrosion has led to
the question of how the biological agents affect the corrosion process and how
they may modify the electrochemical nature of the reaction.5 Both aerobic and
anaerobic bacteria can cause corrosion in stainless steel.3

It should not be a surprise to anyone in the water industry
that organisms that can cause microbiologically induced corrosion are present
in raw water supplies, regardless of the source. The average dimensions of
bacteria, fungi and yeast involved in corrosion processes are small (measured
in micrometers). This allows the microorganisms to colonize inaccessible areas
such as the interior of crevices and pits where they can avoid the shear of
fluid velocity. The small size also facilitates the rapid and easy dispersion
of microbial cells.5

The typical agents for microbiological fouling include iron
bacteria, sulfate-reducing and slime-producing organisms, although fungi and
algae that typically occur only in surface water sources also may exist. Some
of these organisms are pathogens and/or opportunistic pathogens yet do not fall
under the group of bacterial species most commonly analyzed (coliform bacteria
and the subset of fecal coliform bacteria). A problem occurs when enough of
these orgnisms come together and form a slime matrix or biofilm.

Active participation of microorganisms in the corrosion
process introduces several inherent features, the most relevant of which is the
formation of biofilms at the metal/solution interface.5 There are six steps in
the formation of a biofilm.

 

1.             Free
floating cells are transported in the bulk water.

2.             Cell
absorption on the surface (that can be controlled by fluid shear forces).

3.             Permanent
attachment after a critical residence time.

4.             Adsorption
of nutrients by cells.

5.             Growth
of biofilms.

6.             Shedding
of biofilm to colonize other parts of the pipe.5

 

Generally, a two-stage sorption model is suggested for
modeling bacterial attachment. In such models, the bacteria are rapidly
attracted to the surface, but held only weakly. While the first stage is
reversible, the attachment in the second phase is irreversible. The second
phase includes synthesis of extracellular polymetric glue, a mechanism of cell
metabolism. However, the study of pseudomonas aeruginosa has indicated that the
second stage occurs almost immediately.7

Biofilms affect the interaction between metal surfaces and
the environment, not only in biodeterioration processes such as corrosion but
also in several biological processes applied to materials recovery and
handling. These adhesion processes are mediated by extracellular polymetric
substances of a polysaccharide nature.5 The bacteria in biofilms bind together
in a sticky web of tangled polysaccharide fibers that connect cells and anchor
them to the surface and each other. If the cells of a different species form
colonies in the same area, an aggregate will form.5 Within this aggregate,
aerobic and anaerobic bacteria thrive along side each other sharing water
passageways that deliver nutrients and remove wastes like a circulatory system
and may interact on an intimate metabolic level.8

The advantages to forming a microbial consortium include
enzymatic interactions, concentration and exchange of nutrients, resistance to
velocity currents and maintenance of a stable biofilm.5 Chemistry of the water
can facilitate microbiological colonization and reduce the time for the
permanent attachment of the biofilm.5 The mass of the extracellular gel may
exceed the mass of the microorganisms, which leads to a macroscopic appearance
of the biofilm.5 Direct measurement of the biofilms is restricted by the
thickness of the biofilm, the diffusion limitations of concentration profiles
and heterogeneous nature of the biofilm. Biofilms are 95 percent water and are
very porous.

The synergistic effects of microorganisms such as growth
rates and metabolisms and high surface to volume ratios allow them to be active
in the pipe environment, resulting in corrosion rates 1,000 to 100,000 times
greater than when these factors are absent.5 The concentration of dissolved
oxygen decreases to zero at a distance of approximately 180 µm from the
metal surface. The depleted oxygen layer beneath the biofilm creates an
environment suitable for the growth of harmful, anaerobic bacteria and creates
differential anoxic cells that lead to pipe corrosion. The implication is that
the bottom of the biofilm is anaerobic, and corrosive sulfate reducing bacteria
can proliferate regardless of dissolved oxygen in the flowing water. Acid-
(acetic acid) producing organisms may accumulate enough acid within the biofilm
to reduce the pH dramatically, thereby liberating metallic ions.5

Bacteria that can attach to inanimate surfaces are important
not only as agents of biofouling but also as contamination sources for any
material that affects the surface.7 Bacteria can cause corrosion directly by their
metabolic processes, forming specific chemical species such as ammonia,
hydrogen sulfide, dissolved sulfate, ferric or manganic chlorides.3 Anaerobic
bacteria in biofilms reduce sulfur to hydrogen sulfide, increasing the
susceptibility to pitting, while aerobic bacteria can corrode metals by
oxidation. Biofilms act to protect the bacteria from the shearing effect of
turbulent flow. However, periodic sloughing occurs when the biofilm becomes too
thick, releasing bacteria into the treatment systems.

Frederick Bloetscher, Ph.D., P.E., is with Public Utility Management and Planning Services, Inc., Hollywood, Fla. Phone 954-925-3492; Fax 954-925-2692; e-mail h2o_man@bellsouth.net. Richard J. Bullock is with Weir Materials and Foundries, Ponte Vedra Beach, Fla. Phone 904-285-8039; Fax 904-285-8043; e-mail r.j.bullock@att.net. Robert E. Fergen, P.E., is with Hazen and Sawyer, P.C., Raleigh, N.C. Phone 919-833-7152; Fax 919-833-1828; e-mail refergen@hazenandsawyer.com. Gerhardt M. Witt, P.G., is with Gerhardt M. Witt & Associates, Inc., West Palm Beach, Fla. Phone 561-642-9923; Fax 561-642-3327; e-mail wittassoc@aol.com. Gary D. Fries, P.E., is with Boyle Engineering Corporation, Orlando, Fla. Phone 407-425-1100; Fax 407-422-3866; e-mail gfries@boyleengineering.com.

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