Part 1 focused on the problems and microbiological concerns
of using certain types of stainless steel in specific applications.
Part 2 explained microbial influences on stainless steel and
described tests to be performed before the selection process.
Hollywood, Florida
Since the late 1960s, more than 1,600 treatment facilities
have included stainless steel for aeration, digester gas and sludge piping.2 In
RO plants, seawater is filtered and pumped at high pressure through cells
containing the membrane that remove the contaminants.13 In desalination, the
most corrosive fluid is aerated water. The most important corrosion mechanisms
for stainless steel in crevice corrosion for parent metal and pitting is the
weld.13 Seawater usually is chlorinated to prevent biofouling, but chlorine has
downstream impacts on the membranes. The resistance to crevice corrosion in
chlorinated seawater at temperatures between 4 and 30° C is important.
As part of a program of creating diversity in its options,
the City of Hollywood, Fla., pursued the use of a brackish aquifer (i.e.,
Floridan aquifer is an artesian confined supply) to supplement its withdrawals
from the Biscayne aquifer (a shallow water table aquifer). The intent was to
have multiple sources of water available in the event of drought, restrictions
or contamination. The City drilled a test/production well into the brackish
Floridan Aquifer System. Investigative studies found that the well had TDS
levels of 3,400 mg/L and chlorides of 2,200 mg/L, making the existing lime
softening treatment impossible. However, the well had artesian pressure at the
ground surface and sufficient quantities of water were available, meaning that
the brackish water could be easily treated by low-pressure reverse osmosis.
The City’s operations staff and consultant determined
that the piping for the brackish supply needed to be run above ground in the
water plant due to subsurface congestion. As a result, the water plant staff
and consultant deemed that Type 316L stainless steel would protect the pipe
from the salts anticipated from the brackish water supply and that the material
would be resistant to external corrosion so pipe maintenance would be
minimized.
Type 316L stainless steel was selected over HDPE and ductile
iron options for the raw water lines. While the chemical resistance of HDPE is
similar to stainless steel and they have similar weights, the potential for
degradation by sunlight was the reason it was rejected. Reduced maintenance (no
need for painting), improved chemical and corrosion resistance and the ability
to use thin walled, lightweight sections were the reasons stainless steel was
chosen over ductile iron. Ductile iron requires larger and more numerous piers
(support foundation) due to weight and short laying lengths that would take
longer and cost more to erect than the stainless steel, and special linings
would be needed to prevent iron related fouling (caused by iron bacteria and
iron precipitation) from affecting the membrane treatment process.6 Bacterial
analysis of the raw water revealed the presence of iron bacteria (Gallionella
ferruginia and Sphaerotilus natans) and slime forming bacteria (Pseudomonas
species and others).
Over 1,100 feet of 36?, 316L stainless steel,
3?8?-thick pipe was installed (see Figure 5). The thinner walled
pipe was used due to cost considerations and the fact that the raw water
pressure would be small. Soon after installation and hydro-testing of the 316L
stainless steel plant and piping system, external corrosion began to occur in
the form of rust stains, scratches and pinhole leaks (see Figure 6). Within a
year of installation, more than 20 leaks developed in the pipe. The leaks
occurred along the pipe section as well as near couplings and at the edges of
the pipes. In addition, similar activity was noted on the City’s reverse
osmosis skids. Most of the corrosion at the skids was present in the joints
where exposure to stagnant 3,400 mg/L TDS water (2,200 mg/L of chlorides)
occurred at the O-rings and in the J-bends. Investigation of the phenomena
revealed that there were several causes of the leaks, all associated with
corrosion cells caused by both anodic action and microbiological activity.
• The
heat tint was not removed as specified or in conformance with standard practice
in any of the circumferential field welds, inside or outside of the pipe. This
resulted in sites for preferential anodic dissolution in the pipe that
attracted both cathodic and microbiological corrosion (See Figure 6).
• Faint
heat tint was detected in the internal, longitudinal welds. It had not been
removed adequately.
• The
complete draining of the pipeline after hydro-testing did not occur due to the
failure to install drains by the contractor during construction, creating a
long-term stagnant water condition. The result was that the high chloride
content of the water and the naturally occurring and man-induced bacteria in
the water were able to attack the pipe interior at the point where the oxygen
content (interface) was significant (see Figure 7).
• The
microbial activity was not considered a major issue when specifying the pipe,
but extensive biofilm formation had occurred, especially at the edges of the
pipes and crevices (see Figures 8–10).
Perhaps most importantly, the chlorides were too high for
use of 316L stainless steel for the reverse osmosis portion of the plant. So, a
more durable grade steel should have been specified based on current knowledge.
The result is crevice corrosion in the J-bends (see Figures 11–13).
Interestingly, this recommendation was made by steel manufacturers, which was
surprising to the City and its engineer since the use of 316L stainless steel
had been the industry standard for reverse osmosis systems in Florida up to
that point.
The following actions were advised to correct the problem.
• Flare
out all pits and pinholes and fill them with an epoxy paste material suitable
for potable water service.
• Flush
the piping, and do not store high chloride water for any period of time in the
pipeline. Replacement with raw water that is potentially bacteria-laden is not
the appropriate solution, nor is chlorine laden potable water.
• Remove
all heat tint, both inside and outside of the pipe.
• Sterilize
the pipe to reduce the microbial problem. Unfortunately, once the bacteria have
attached to the pipe, they cannot be permanently removed, only controlled.
• When
the pipe is replaced, use a more appropriate grade of stainless steel.
In addition to controlling the bacteria by replacing a
number of wells (the groundwater was the source of the bacteria), the following
tasks were implemented.
• Provide
a routine disinfection program for water supply wells and raw water
transmission pipes.
• Periodically
sample and analyze wells and composite raw water for microbial growth.
• Use
non-ferric materials (ABS, PVC and/or fiberglass) in the design of water supply
wells and final casing strings, and use non-organic based drilling muds and
proper disinfection procedures during drilling of wells.
• Use
bronze (non-ferric and not red bronze) well pumps and motor housing for
submersible motors. Fiberglass and/or PVC column pipe material is suitable for
withstanding high concentrations of chlorine disinfection solutions such as
calcium hypochlorite/sodium hypochlorite.
• Use
non-ferric metals and plastics in piping appurtenances (valves, meters, nuts,
bolts and screws).
The price for the repairs is $80,000, plus an annual upkeep
cost. Lining of the pipe (a $200,000 option) also is being considered. The
lining would bond to the existing pipe, thereby reducing the long-term
potential for deterioration. Together, the repair amount is nearly half the
cost of the initial installation of the above ground pipe.
Hindsight
If the City were to start over, a more appropriate material
would have been selected. One such system that would have eliminated most of
the problems would be the use of a higher grade, super-duplex stainless steel
such as Zeron 100, 2205 or 2507. Corrosion resistance is the key at moderate temperatures.
Corrosion rates are lower in second generation duplex and super duplex
stainless steels than standard austenitic stainless steel as measured by
critical pitting temperature and critical temperature for crevice corrosion.
The critical pitting temperature is the temperature above which pitting
corrosion will initiate and propagate to a visible extent within 24 hours. This
tends to be higher with the super duplex stainless steel. The critical
temperature for crevice corrosion CCT also tends to be higher for super-duplex
stainless steels.4
Zeron 100 currently is being tested by the City in-situ as
J-bends. Reviewing the current situation, Zeron 100 would have reduced
corrosion damage because it is more resistant to pitting and crevice corrosion
than other alloys or duplex stainless steels, particularly at higher
temperatures.13 The higher chromium, molybdenum and nitrogen content provides
good resistance to chloride-induced localized corrosion in aqueous environments
and crevice corrosion. Most duplex stainless steels are far superior to Type
316L stainless steel.4
Conclusions
Stainless steel has been used where higher strength, high
pressure and corrosion resistance are important. Older duplex stainless steel
(typically 316L) has provided good performance in membrane plants. However, it
has had limitations in the welded condition. This is due to the excessive
ferrite that occurs in the weld metal, making corrosion more likely in brackish
facilities where crevice corrosion becomes evident and, in some cases, where
microbiological activity is present. Higher grade stainless steel should be
considered in plants when the chlorides are higher than 1,000 mg/L,
temperatures are high (greater than 15° C) or where substantial
microbiological activity exists.
2205, 2507 and Zeron 100 are steel grades that have two
additional benefits in reverse osmosis plants. The thickness of the castings
for the high pressure pumps can be reduced as a result of higher strength of
the steel, and thinner walled pipe for the feed water can be used as a result
of its corrosion resistance. Smaller pipes are connected with Vitaulic rubber
seals in a groove that can be rolled onto Zeron 100 10S piping.13
Welding, especially field welding, also creates considerable
problems. Use of nitrogen alloying in duplex stainless steels such as the 2205,
2507 and Zeron 100 (a registered trade name of Weir Materials and Foundries)
and the 6 Mo austenitics makes the heat zone corrosion resistance similar to
that of the base metal, without the need to remove the tinting. The cost of
higher-grade steel is greater, but the overall cost increase is small compared
with the cost to repair 316L systems. Higher grade stainless steel should be
considered when the chlorides are higher than 1000 mg/L, temperatures are high
(greater than 15° C) or where substantial microbacteriological activity
exists.
For many reverse osmosis applications, the use of higher
grade stainless steel is more appropriate than the current standard of 316L
stainless steel. As the City of Hollywood found out, this is especially true
with regard to slow moving water and crevice corrosion (J-bends) where 316L appears to be an inappropriate choice.
Higher grades of nitrogen alloyed duplex stainless steels have been used
successfully in reverse osmosis and oil applications in the North Sea, Spain
and the Spanish islands, the Philippines, Europe and the Middle East. However,
these grades of steel have not seen much use in the United States.
Despite the initial cost of these higher grade stainless
steels (perhaps 30 percent), their use does not increase costs prohibitively.
Accurate life cycle cost development is important, but life cycle cost analyses
indicate that for these conditions the higher grade stainless steels are more
cost effective. Higher grades of stainless steel are manufactured at factories
world-wide, so the availability and delivery should not differ significantly
from that of 316L stainless steel. Biological resistance to standing water
requires further scrutiny, but a recent NACE paper indicates good microbial
resistance in the Zeron 100 applications.
Based on the City of Hollywood’s experience, the use
of 316L stainless steel should be evaluated carefully due to the potential for
problems in the erection and construction of water treatment facilities that
will be in contact with high chloride water and/or other corrosive chemistries.
As with many membrane facilities, much of the stainless steel is exposed (not
buried), which subjected it to atmospheric as well as water quality problems.
Therefore, unless the quality control of the raw and reject water (chemical,
physical and microbial) can be assured, 316L stainless steel may not be the
appropriate material for engineers to specify. Other steels are available and
plastics may be more appropriate in situations where the pipe will not be
exposed to sunlight (which adversely affects plastic pipe and certain coatings)
or high pressures. For high pressure, high chloride applications, the use of
higher-grade stainless steel is required.
For a list of references, go to our website at
www.waterinfocenter.com.
