Guidelines for the use of Stainless Steel in Municipal Waste Water Treatment Plants
By Arthur H. Tuthill
Nickel Institute Technical Series #10076
Stainless steel piping has served as the standard material of construction for municipal waste water
treatment plants (WWTP's) built in the United States over the past 25 years. A typical plant is shown in
Figure 1.
Since the late 1960's over 1600 municipal WWTP's have been built using stainless steel aeration, digester gas
and sludge transfer piping, as well as stainless steel slide gates, valves, tanks, screens, handrails and
other equipment. Stainless steel was selected originally over galvanized and painted carbon steel in order to
reduce the higher maintenance and replacement costs associated with these less corrosion resistant materials.
Overall experience has been good to excellent (1). This article reviews the suitability and performance of
the several grades of stainless steels that have been used or considered for waste water treatment plants and
identifies guidelines that will assist the user to obtain the best results from the stainless steel selected
for these plants.
The austenitic grades used in waste water treatment plants have 16-20% Cr, 8-14% Ni and the Mo containing
grades, 2-3% Mo. Table I shows the common name, the UNS number, and the British, German and Swedish
designations for the wrought and cast grades. Table II shows the substantially higher ASME design allowable
stresses for stainless steels compared to those for carbon steel and copper.
The low carbon grades, Types 304L and 316L, are used for welded construction. These low carbon grades are
designed to be resistant to intergranular corrosion after welding without further heat treatment. The older
higher carbon content grades have slightly higher strength and are used primarily for pump shafts and valve
stems where welding is not involved and the higher strength can be used to advantage in the design. The Mo
containing grades are more resistant to localized corrosion and are preferred for more aggressive conditions
or simply for greater insurance against unusual conditions which may exist from time to time. The cast
equivalent grades have comparable corrosion resistance and comparable mechanical properties.
Figure 2 Typical flow diagram for a waste water treatment plant.
Table III WWTP environments
Table IV Corrosion rates (mpy) in WWTP environments
WWTP
Environments
Figure 2 shows a typical flow diagram for a waste water treatment plant, showing the locations where
corrosion test specimens were exposed to actual plant conditions for different periods of time. These tests
were run in different plants during the 1960's and early 1970's with the results being analyzed and reported
by the International Nickel Company (Inco).(2)
Table V MWWTP corrosion tests - mpy (mm/yr) Location 6: activated sludges in dewatering and
vacuum filter
Tables III, IV and V summarize data for Type 304, Type 316 and 1010 carbon steel from the
INCO test rack exposures. The plant location for each test is identified by the numeral that corresponds to
those shown in the flow diagram. Corrosion rates for carbon steel varied from 3 mpy (0.075mm/yr) in the raw
incoming sewage to 16 mpy (0.4mm/yr) in the clear effluent. The greater the oxygen, the greater was the
corrosion for carbon steel (corrosion rates are in direct proportion to the amount of oxygen reaching the
surface of the carbon steel). The corrosion rates of carbon steel in all locations are high enough to require
continual maintenance and early replacement. Galvanizing and coatings may extend the life of carbon steel
piping systems for several additional years, but they still do not offer an optimum solution to long term
protection in WWTP environments. These coatings can become damaged or sacrificial in nature.
Stainless steel Types 304/304L and 316/316L are the principal materials used for bar screens, grit removers,
weirs, bolting, slide gates and aeration basin, digester and sludge piping in municipal waste water treatment
plants today. For the past 20 years or more, these materials have become standard materials of construction,
based upon the field testing undertaken by International Nickel. Reference to Tables III, IV and V show
weight loss corrosion rates for Types 304 and 316 stainless steels were <0.1 mpy, except in one activated
sludge exposure reported in Table V, which will be discussed later. A corrosion rate of <0.1 mpy indicates
that material would be expected to have a useful life well in excess of 20 years.
Evidence of crevice corrosion initiation was reported on some of the stainless steel specimens. This type of
corrosion can occur in localized areas, where "shielded" or "creviced" areas exist, e.g. at bolted joints, at
incomplete welds or beneath adherent sludge deposits. To minimize this type of attack, crevices should be
eliminated wherever possible, all weld joints fully penetrated and periodic cleaning scheduled to remove
adherent deposits. In the case of aeration basins, deposit removal is usually accomplished when they are
taken out of service for odour control cleanings. These precautions have resulted in long term, continuing
service with no significant problems. Agitation and oxygenation in the aeration basin during operation
further benefit the performance of stainless steels.
Atmospheric Corrosion Resistance
As chromium is added to steel, corrosion resistance increases. The amount of chromium needed to make
stainless steels "stainless" can be defined by the weight loss of steels with varying amounts of chromium
(3). Faulrings's data indicates that the weight loss for steels with 12.5% and higher chromium contents is
quite low after 25 years exposure to marine atmospheres. However, the straight chromium grades of stainless
steel develop a continuous surface layer of adherent rust, which makes them unsuitable for most atmospheric
applications where appearance is important. By contrast, the austenitic, nickel containing grades, Types 304
and 316, are more resistant, retaining their bright appearance in atmospheric exposures for many years.
Hydrogen sulfide gas is generated in the digesters and throughout much of a WWTP, contributing to the
corrosion that occurs in copper alloys, aluminum and carbon steel. According to De Renzo, the corrosion rate
of Type 304 and Type 316 in moist H2S is <.0025 mmy(<0.1 mpy), indicating a negligible corrosion rate
(4). Type 304L is the standard material used for digester gas piping and has performed well.
Although there is no intentional exposure to moist chlorine vapours, several cases of general pitting of the
outside diameter of piping in confined locations with poor ventilation near chlorine storage areas have been
reported. Both Types 304L and 316L stainless steel are subject to general pitting in confined spaces where
moist chlorine vapours could collect. These occurrences were remedied by appropriate ventilation design
changes.
Chlorine - Ozone
Chlorine is a common addition. Ozone is an increasingly common alternative oxidizing agent which can be used
separately or in conjunction with chlorine.
Stainless steels are generally resistant to chlorine at the concentrations normally encountered in waste
water treatment plants. Table VI gives test specimen exposure data for type 1010 steel, cast iron and Types
304 and 316 stainless steel in chlorinated fresh waters with typical residual chlorine concentrations(5).
Both 304L and 316L exhibit excellent corrosion resistance in inland waters with up to 2 ppm residual
chlorine, whereas corrosion rates for carbon steel and cast iron are doubled. For continuous exposure to 3-5
ppm residual chlorine, the data indicate that Types 304/304L become prone to crevice corrosion and Type 316L
would be a more conservative choice. Stainless steels are not suitable for chlorine injection systems which
may contain fifty to several hundred ppm chlorine.
Type 316 has been the preferred material of construction for ozone generators, although background and data
are lacking as to why Type 316L was originally selected over Type 304L.
Other Chemical Additives
Ferrous sulfate is a chemical that is frequently added in WWTP's: De Renzo also gives the corrosion rate of
Types 304 and 316 in ferrous sulfate, contaminated with sulfuric acid at pH 1 - 2.5 (spent pickle liquor), as
<0.8 mpy(<0.02 mm/yr) which is indicative of excellent resistance. In WWTPs sulfuric acid is not
normally present, and consequently, the corrosion rate for stainless steel in ferrous sulfate environments
without acid would be considerably less, and again negligible.
Ferric chloride is sometimes used for flocculation in the conditioning tank where sludge is further
concentrated and dewatered for incineration and disposal. Types 304 and 316 specimens installed in activated
sludge location 6 (Table III), where there were 250-300 ppm ferric chloride present, suffered both pitting
and crevice corrosion. The data in Table V indicates Type 316 is slightly more resistant to ferric chloride
than Type 304 and might represent a more conservative choice downstream of ferric chloride injection points,
where significant concentrations of ferric chloride may be present. However, despite the adverse test data,
Type 304 piping is normally used to handle flowing sludge.
Figure 3 Type 409 stainless steel Rotating Biological Contractor (RCB) disassembeled with MIC attack on arms.
Microbiological Influenced Corrosion
The area near welds and welds in the standard grades of stainless steels are occasionally subject to
microbiological influenced corrosion (MIC) in stagnant and slowly moving waters. One example of MIC attack
occurred on Type 409 (11% chromium) stainless steel frames of experimental rotating biological contractors
(RBC's) in one WWTP. Figure 3 shows an RBC unit being disassembled. Figure 4 shows a MIC mound more or less
centered on the weld of the frame. The RBC's, with their stainless steel frames slowly rotating in and out of
the sludge at about 2 RPM, create conditions almost ideal for MIC. Other instances of MIC in WWTP's have not
been encountered.
The unusually low incidence of crevice corrosion and MIC in municipal waste water treatment plants may well
be due to several of the following factors favouring good performance:
- Removal of heat tint scale by pickling or mechanical cleaning on the exterior of the aeration piping
exposed to the sludge in aeration basins.
- Smooth contoured and crevices free circumferential butt welds of piping exposed to sludge. (Note: Removal
of heat tint (6) and the absence of crevices have been found to significantly enhance resistance to MIC and
other forms of localized corrosion)
- Aeration and agitation of the sludge reduces the tendency of the sludge to adhere to the surface of the
piping.
- Through wash down cleaning of the sludge from the outside diameter of the piping when the aeration basin
is taken out of service.
Figure 4 MIC mound.
Figure 5 Aeration piping sludge tank.
General Experience
Most, if not all, of the stainless steel piping installed in the older WWTP's, built in the 1960's and 1970's, as well as those built later, are still in service. A typical application is shown in Figure 5 where stainless steel aeration piping is used in a sludge aeration basin. Stainless steel has performed better in actual service than might have been predicted from the test specimen exposure data summarized earlier, especially where good housekeeping practices have been employed.
A few instances of under-deposit corrosion have occurred on the outside of submerged aeration piping. In these instances, the corrosion occurred under small adherent residues that had not been completely removed in the normal cleaning of the aeration basins.
As noted in the test rack exposures, corrosion occurred on specimens in activated sludge, where ferric chloride was being added. However, stainless steel sludge piping has generally performed well in handling activated sludge environments, except in one instance where crevice corrosion was recently reported in a sludge line in an open recess at an O-ring type mechanical joint and in an elbow in the same line. Sludge became packed in the open recess and led to severe crevice corrosion around the O-ring. Low flow and stagnant conditions apparently persisted long enough to allow corrosion to initiate under the crud that was packed into this open recess in the piping. To minimize under-deposit corrosion, stagnant and low flow conditions should be avoided in sludge lines. Free flowing conditions will assist in reducing deposit formation.
Crevice corrosion can also occur in some soils. External corrosion of buried stainless steel is dependent upon soil analysis and soil resistivity. Soils differ in their corrosiveness depending upon the wetness, pH, aeration, stray currents and surface drainage. The potential for crevice corrosion of buried stainless steel piping tends to increase as the moisture content of soils increases and is greatest in swamps, under river crossings, and in wet soils. Based on tests run in Japan, stainless steels have performed well in a variety of soils and especially soils with high resistivity(7). The Japanese have had great success with buried stainless steel connectors bringing potable water from the submain in the street to the dwellings in the city of Tokyo. Almost half a million connectors are now in service with no known corrosion problems since installation began on a major scale in 1980. For wet soils and critical piping, stainless steel pipes can also be cathodically protected, as is standard practice for buried piping.
Table VI Corrosion behaviour of 1010 steel, cast iron and Stainless steel in chlorinated fresh waters
Table VII Comparison of stainless steel piping specifications
Procurement And Specifications
Most of the problems that have been encountered with stainless steel piping in WWTP's arise in procurement, design, fabrication and erection. The common specifications for welded austenitic stainless steel piping and fittings are shown in Table VII.
ASTM specifications A312 (pipe) and A403 (fittings) define the welded austenitic stainless steel pipe and fittings developed for use in aggressive corrosion environments, typical of the chemical and other process industries. Most of the common stainless steel alloys are covered by these specifications in both the low carbon and regular grades, with sizes ranging from 3 mm (l/8") to 0.75 m (30") in diameter. ASTM specification A312 requires a final solution anneal and removal of heat tint scale.
ASTM specifications A778 (pipe) and A774 (fittings) were developed for less severe services where the low carbon and stabilized grades are normally used in the as welded condition. (Only low carbon and stabilized grades are included in this specification.) The size ranges covered by these specifications are from 75 mm (3") to 1.22 m (48") in diameter. Solution annealing after welding is not required by these specifications since only the low carbon and stabilized grades of stainless steel are covered. Although removal of heat tint scale is left optional, it should be removed and be specified in procurement documents in order to avoid potential corrosion problems that have occurred on unpickled pipe with heat scale remaining on its surface.
Design And Fabrication
ASTM specifications do not cover circumferential butt welds required for shop or field fabrication of the piping. The user, or the engineering firm handling design and construction, must develop in-house welding specifications. It is important that these specifications include:
l) A requirement for full penetration and smooth ID circumferential welds.
2) Use of inert gas backup on the inside of the piping during welding to minimize oxidation and heat tint.
3) The use of a matching composition or higher Mo content filler metal.
4) Protection of the piping with well secured protective end caps to minimize contamination during shipment and clean indoor or outdoor storage.
5) Removal of heat tint scale from the OD of aeration basin piping.
The circumferential welds in those areas exposed to the handling of sludge - the OD of the aeration piping and the ID of the digestor and sludge piping, should be free of crevices and fully penetrated.
Aeration piping is subject to vibration in service. The S-N curve for type 304 stainless steel (8) is shown in Figure 6. The design and fabrication of structures is as important, possibly more important, than the endurance limit. Smooth transitions, allowing for the full weld penetration of the structures cross-sectional wall, are desired. Most codes prohibit making the connector weld flush with the connector, where incomplete weld penetration of the wall cross-section will result in crevices and stress raisers. These areas also represent the weakest point of the structure, where vibrational stresses tend to concentrate. Figure 7 details a good transition joint, along with the use of supporting gussets.
Figure 6 S-N curves for two grades of stainless steel, types 304 and 310 stainless steel: effect of direction on S-N.
Figure 7 Transition with supporting gussetts, preventing corrosion due to vibration.
Pickling
Pickling is important to the successful performance of stainless steel WWTP piping. It ensures good corrosion resistance for aeration piping handling aeration basin sludges. Pickling is effective in removing heat tint scale, embedded iron and other pit initiating defects from the metal surfaces. Although most shops take care to minimize contact with carbon steel layout tables, platens, lifting chain, iron can still become embedded in the surface. Also during cutting and welding the adjacent metal is heated into a range where heavy brown or black oxide films (heat tint scale) form. Fabricators have a choice; they can ship as fabricated material and hope that customers may not object to staining or streaking from iron and other surface contamination arising from fabrication, or they can pickle and restore the surface to its most corrosion resistant condition. This can be done in vats or by the application of pastes and mild acid solutions to the contaminated areas, followed by a thorough neutralizing wash to remove the excess acid and pastes from the metal surfaces.
In aqueous environments where stainless steel has been selected and used for its corrosion resistance, there is new evidence, based upon a study undertaken by the University of Tennessee (9), to support the need to remove heat tint scale. Figure 8 shows the polarization curves for as-welded Type 304L (with heat tint) and as-welded and pickled Type 304L (heat scale removed). The curve for the pickled condition, with heat scale removed, shows the normal passive (protective) behaviour associated with the austenitic stainless steel materials. The vertical section of the polarization curve defines the wide passive range characteristic of stainless steels. The lack of a similar passive range for the heat affected zone (HAZ) of as-welded samples, with heat tint present, indicates that the HAZ has little passivity and that localized corrosion is likely in aggressive environments.
Cost Comparisons
Stainless steels offer a very cost effective approach for the design of light weight components and piping systems. Although the initial capital costs may be greater than for other material systems, stainless steel provides long term durability, lower overall maintenance costs and cost savings over the long term. In a European case study (10) undertaken by the European Stainless Steel Development and Information Group (EURO INOX) on an Italian WWTP, life cycle costs for stainless steel over carbon steel showed break-even after just 10 years of operation, despite the initial installation cost being approximately 25% more expensive for the stainless steel construction. This analysis took in the cost of capital, maintenance and repair and only considered a conservative estimate of 10% weight savings when designing with stainless steel. Stainless steel equipment included mechanical screens, travelling bridges and handrail assemblies, where galvanized steel is often used.
Figure 8 Polarization data showing effects of heat tint on corrosion resistance.
Conclusions
Types 304L and 316L stainless steel has been used extensively and very successfully for piping and a wide variety of other applications in waste water treatment plants. Stainless steel offers higher design strengths, thereby allowing lighter weight construction, and exhibits very low corrosion characteristics, especially if good housekeeping practices are maintained at the operating plant.
1. The ASTM specifications A312 and A778 are generally adequate for the procurement of pipe, provided that heat tint oxide scale removal is specified when ordering to ASTM A778.
2. In house specifications must be developed for circumferential butt welds, as these are not covered by ASTM specifications. These should include requirements for full penetration, rather than "commercial quality" welds.
3. Designing full penetration joints with smooth design contours and smooth weld bead profiles, rather than using fillet welds at branch connections, will optimize performance against vibration.
4. For waters where chlorides exceed 200 ppm (mg/l) or where residual chlorine exceeds 2 ppm, Type 316L is preferred to 304L.
5. For sludge service, maintaining flows above 2 ft/sec and avoiding recessed and creviced areas that are sites for sludge accumulation is essential for best performance.
6. Stainless steel 304L and 316L have been used for aeration, digester gas and sludge transfer piping in 1600 municipal waste water treatment plants built in the US since the late 1960's.
Acknowledgement
The authors would like to give credit and acknowledge the support provided by the Nickel Institute, in providing access to data and for their permission to publish and present actual field test results.
References
1) A. H. Tuthill, "Stainless Steel in Waste Water Treatment Plants", Water, Engineering & Management,, pp 31-34, July 1990.
2) "Metal and Alloy Performance in Municipal Waste Water Treatment Comparing Effects of Oxygen and Ozone", The International Nickel Company, Inc internal document.
3) G. M. Faulring, "Composition and Structure of Rust Layers and Corrosion Rate of Chromium Steels Exposed to a Marine Atmosphere", Journal of Materials, 7 (4), 1972.
4) D. J. De Renzo, Corrosion Resistant Handbook, 4th Ed., Noyes Data Corporation, Park Ridge, N.J., 1986.
5) A. H. Tuthill, "Stainless Steel Piping", AWWA Journal, July 1994.
6) J. Kearns and G. E. Moller, "Reducing Heat Tint Effects on the Corrosion Resistance of Stainless Steels" Material Performance, May 1994.
7) "Piping Manual for Stainless Steel Pipes for Buildings", Japanese Stainless Steel Association & Nickel Institute, publication No. 12 008.
8) Metals Handbook, 9th Ed., ASM International, Volume 3, p32, 1980.
9) I. Ping, R. A. Buchanan, C. D. Lundin, A. H. Tuthill, and R. E. Avery R.E., "Laboratory Studies of Microbially Influenced Corrosion of Stainless Steel Weldments in Simulated Tennesee Fresh Water Systems". Paper #40, International Conference on Microbially Influenced Corrosion, New Orleans, LA May 8-10, 1995.
10) "Life Cycle Costing and Stainless Steels", European Stainless Steel Development and Information Group.
The material presented in this publication has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice.
The Nickel Institute, its members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein.
