Stainless Steels for Bioprocessing Selection, Surface Preparation, and Cleaning

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1 2-11 Tuthill-Brunkow.qxd B I O P 11/24/ :41 AM R O C E S S Page 46 TECHNICAL Stainless Steels for Bioprocessing Selection, Surface Preparation, and Cleaning Arthur H. Tuthill and Roger Brunkow C ontamination of biotechnology products by the materials of construction used for vessels, piping, and other components cannot be tolerated and limits the types of materials that can be used. The biotechnology industry requires materials of construction that are readily fabricated withstand repeated batch-tobatch cleaning and sterilization treatments have long (10 20 year) service lives remain cleanable and sterilizable over their service lives will not contaminate products being processed, and are available at a reasonable cost. Properly processed and fabricated stainless steels meet these criteria and have become the principal materials of construction used in biotechnology and related industries. PRODUCT FOCUS: ALL BIOTECH AND PRODUCTS PROCESS FOCUS: BIOPROCESSING, MANUFACTURING, ASEPTIC PROCESSING, EQUIPMENT FABRICATION AND SPECIFICATIONS WHO SHOULD READ: PROCESS ENGINEERS, MANUFACTURING AND MAINTENANCE PERSONNEL KEYWORDS: STAINLESS STEELS, AUSTENITIC, CORROSION, ROUGE, ELECTROPOLISH, ASTM SPECIFICATIONS, ASEPTIC PROCESSING LEVEL: INTERMEDIATE 46 BioProcess International DECEMBER 2004 A water-for-injection system at Althea Technologies in La Jolla, CA (LEAH ROSIN) GRADES OF STAINLESS STEELS NICKEL-BASE ALLOYS The principal material of construction used by the biotechnology and related industries is Type 316L, Unified Numbering System (UNS) S31603: the basic molybdenum-containing austenitic stainless steel, with 16.7 ksi ASME design allowable stress (ksi = kips per square inch, or 1000 psi). The term austenitic describes a facecentered cubic crystal structure that is tough and ductile especially in the heat-affected zone of welds. The austenitic grades are designated as the 300 grades of stainless steel in the UNS. Type 316L (S31603) austenitic stainless steel is the basic material of construction used by the biotechnology industry, supplemented by the use of duplex grades when better resistance to stress corrosion cracking is needed or higher strength is required (as in centrifuges). The 6% molybdenumcontaining austenitic stainless steels are used when a process is more corrosive than can be handled by Type 316L. Nickel-base austenitic alloy C276 (N10276) is used for the most corrosive services. The ferritic grades are designated as the 400 grades of stainless steel in the UNS. Ferritic describes a bodycentered cubic crystal structure that is stronger but less ductile than the face-centered austenitic structure and has a harder, much less ductile heat-affected zone when welded. The ferritic, 400 series grades are

2 2-11 Tuthill-Brunkow.qxd 11/24/ :41 AM Page 47 used primarily for pins, springs, and other such devices that must be stronger. Duplex stainless steel 2205: Duplex stainless steels are half austenitic and half ferritic, stronger and more corrosion resistant than the common austenitic grades. Type 2205 (S32550), with 25.7 ksi designallowable stress, 54% stronger than type 316L, is representative of the higher strength, more corrosion resistant duplex grades used for centrifuges and larger vessels; its higher strength permits thinner wall thicknesses. 254SMO (S31254) and AL-6XN (N08367), austenitic grades with 6% molybdenum, which also have higher design strengths of 23.9 ksi and 27.1 ksi respectively, are the principal grades the biotechnology industry uses when greater corrosion-resistant grades are needed. As mentioned earlier, when maximum corrosion resistance is required, nickel-base alloy C276 (N10276), which also has a ductile austenitic structure, is the grade usually chosen. Table 1 shows the composition of these wrought austenitic and duplex grades, and Table 2 presents their mechanical properties. Although smaller pumps and valves can be, and are increasingly, made from wrought austenitic stainless steel bar stock, larger pumps and valves are made from the cast austenitic grades. CF-3M (J92800) corresponding to Type 316L is the main cast grade used. CF-7M (N08007) is also a widely used cast austenitic grade resistant to stress corrosion cracking. CD-4MCu (J93370) is a cast higher strength duplex grade. CK-3MCuN (corresponding to wrought austenitic alloy S254SMO), CK- 3MN (corresponding to wrought austenitic alloy NO8367), and CW- 2M (corresponding to austenitic nickel-base alloy C276, N10276) are more corrosion-resistant cast grades. The composition and mechanical properties of cast austenitic and cast duplex alloys are similar to those of the corresponding wrought alloys, except that silicon is increased from 1.0 to 1.5% to improve castability. Table 1: Principal elements (percentages) of common wrought stainless steels; the balance is Fe, which is not commonly given. Name % UNS % C % Cr % Ni % Mo % N 316L S S SMO S AL-6XN N C-276* N * 2.5 Co, 3.8W, 1.0 Mn, and 0.08 Si Table 2: Mechanical properties of wrought stainless steels Tensile Strength Yield Strength Elongation Name UNS ksi MPa ksi MPa % 316L S S SMO S AL-6XN N C-276 N PROCUREMENT AND FABRICATION STANDARDS The American Society for Testing and Materials (ASTM) has developed and continues to update specifications for stainless steel as it is produced (in sheet, plate, pipe, bar, tubing, and castings) and used for fabrication into pipes, vessels, and other equipment. Principal ASTM specifications used by fabricators for purchasing stainless steels for bioprocessing follow. Within most specifications are provisions especially useful to bioprocessing equipment suppliers and users. These are noted. Plate, Sheet, and Strip for Vessels: ASTM A240, Specification for Heat- Resisting Chromium and Chromium Nickel Stainless Steel Plate Sheet and Strip for Pressure Vessels, has seven variations of the 2 3% molybdenum grade of Type 316 stainless steel. Basic S31603 is the standard grade used for bioprocessing vessels and piping. The Type 316LN (S31653 grade) with N is sometimes specified for the somewhat better corrosion resistance provided by nitrogen. Surface Condition: ASTM A480, Standard Specification for General Requirements for Flat Rolled Stainless Steel Plate Sheet and Strip, is a companion specification to A240, with detailed information on surface condition. Included in A480 is the important distinction between sheet and plate. Stainless steel sheet and plate can be ordered hot-rolled in any thickness, or if less than 3 /16-inch thick, stainless steel sheet and strip can be ordered cold-rolled. Cold rolled is preferred to hot rolled for its much better surface finish. When wall thickness is 3 /16-inch or greater, it is often necessary to improve the rougher hot-rolled surface finish by polishing as described in A480. The standard grades of stainless steel have a maximum 2% Mn to combine with S, forming MnS particles that are strung out in the direction of rolling as elongated inclusions. Although not harmful in many applications, those MnS inclusions form multiple defect sites in electropolished sheet, plate, and bar stock. It is nearly impossible to electropolish the cross section of bar stock where multiple ends of these stringers are showing. The biotechnology industry can eliminate the numerous defect sites created by MnS stringers by specifying readily available lowsulfur bar sheet and plate. Low-sulfur sheet and plate for vessels ( % S) is readily DECEMBER 2004 BioProcess International 47

3 2-11 Tuthill-Brunkow.qxd 11/24/ :41 AM Page 48 available today, but low-sulfur, lowinclusion stock must be specified in orders for sheet and plate for vessel construction. Even cleaner, VIM (vacuum inducted melted) and VAR (vacuum arc remelted) with minimal inclusions are also available and increasingly used when the cleanest and best quality surface finish is desired. Low-sulfur and cleaner stainless steels greatly reduce the numerous defect sites that occur in the surface when the standard grades of stainless steels are electropolished. The electronics industry has virtually standardized on low-sulfur stainless steels to ensure the best surface finish and best resistance to rouging for vessels and piping. Pipe and Tube: ASTM A270, Standard Specification for Seamless and Welded Austenitic Stainless Sanitary Tubing, includes various mechanical and electropolish surface finishes, low-sulfur stainless steel, and chemical cleaning options needed for, and frequently specified by the biotechnology industry. Cold-rolled stock needs to be specified, because hot-rolled stock is slightly cheaper and frequently supplied unless otherwise specified. Low-sulfur tubing is readily available by specifying procurement to Supplementary Requirement S2.1.1, which restricts S to 0.005% 0.017%. If pipe is ordered to ASTM A312 or A778 specifications (commonly used in other industries), the low-sulfur grade must be specified as an extra requirement. Tubing for Heat Exchangers: ASTM A269, Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service, covers heat-exchanger tubing ¼-inch and larger and inch wall thickness and heavier. Bar: ASTM A276, Specification for Stainless Steel Bars and Shapes, is used for procuring the large number of small-diameter wrought valves and fittings used in lieu of cast material. The cut ends of Type 316L bars expose numerous MnS stringers, making those ends almost impossible to electropolish. For the large number of small diameter fittings and valves to be made from bar, low-sulfur bar with minimal MnS inclusions can be ordered as SCQ (semiconductor-quality) bar. SCQ grade bar has % S. Even lower sulfur bar is available on special order for the best electropolish finish. TERMS USED IN THIS ARTICLE Austenite: Metallurgical term describing a face-centered cubic atomic structure, that is, a cubic structure with an atom at each corner of a cube and an atom in the middle of each face of the cube Annealing: In reference to stainless steels refers to heating to high temperatures and quenching in water. Annealing of carbon and low-alloy steels refers to heating to high temperatures and slow cooling. Chelating: Chemical term describing treatment of stainless steel with citric acid plus EDTA (ethylene-diamine-tetraacetic acid). Cold-rolled: Metallurgical term describing steel products that are rolled down in thickness while cold. Cold-rolled products have a smooth finish. Detritus: Loose material resulting from wear (grinding and polishing) or disintegration. Duplex: Metallurgical term describing an atomic structure that is half austenite, half ferrite. Electropolish: Chemical term describing the corrosion of a thin surface layer when a metal is made the anode in a wellcontrolled electrochemical circuit. Ferrite: Metallurgical term describing a body-centered cubic atomic structure. The term ferritic describes a crystal structure with an atom at each corner of a cube and an atom in the center of the cube, hence a body-centered cubic structure Ground and polished (G&P): Metallurgical term describing a surface that has been ground smooth and bright with an abrasive-filled polishing wheel or stone to a 120, 180, 360 grit or other finish. Heat-affected zone: Metallurgical term referring to the usually discolored zones along either side of a weld and on the side opposite, where the metallurgical structure has been altered by the heat of welding, and a heavier oxide film has formed. Hot-rolled: Metallurgical term describing steel products that are rolled down in thickness while heated to high temperatures that produce red colors. MnS stringers: Manganese combines with sulfur, forming manganese sulfide particles that are strung out and elongated in the direction of rolling. Multiple strings of manganese sulfide stringers are strung out in the direction of rolling. In any cross section, there are multiple cut ends of these stringers. Nickel-base alloy: An alloy with 50% or more nickel. Passivate: Stainless steels are naturally passive (not actively corroding) and resistant to corrosion as the term stainless implies. Hence, it is somewhat misleading to passivate an already passive material. The term passivate generally refers to a cleaning treatment, usually in nitric acid, widely used for cleaning stainless steels after fabrication Pickling: Pickling of stainless steel refers to a treatment in a nitric-hydroflouric acid bath. The bath actually corrodes a thin layer from the surface, allowing a clean new oxide film to reform. RMS: Root mean square is the term used in describing the smoothness/roughness of a metal surface. It is found by taking the root mean square of all the various high to low points of the surface profile. Rouge: Rouge is a reddish oxide of iron, but the term is used in biotechnology and related industries to refer to the black and variegated colored deposits found on stainless steel vessels. Scale: The term used to refer to a heavy oxide layer that develops when stainless steel is heated to high temperatures in a furnace or in the heat-affected zone of welds. 48 BioProcess International DECEMBER 2004

4 2-11 Tuthill-Brunkow.qxd 11/24/ :41 AM Page 50 CLEANING FABRICATED EQUIPMENT ASTM A380, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Equipment and Systems, and ASTM A967, Standard Specification for Chemical Passivation of Stainless Steel Parts, are widely used to cover final cleaning of fabricated and machined equipment. Both recognize that the widely used nitric acid passivation treatments, though useful, cannot be relied on for complete removal of embedded iron. Pickling in nitric- HF per Table A2.1 part 1 of ASTM A380 or electropolishing per ASTM A967 are necessary for complete removal of heat tint and embedded iron. Chelating treatments per A380 are very effective in removing embedded iron, but they are ineffective in removing heat tint scale from welds. AUSTENITIC STAINLESS STEEL PUMPS AND VALVES Castings for pumps, valves, and other cast components are normally procured to ASTM A743, Castings, Iron-Chromium, Iron-Chromium- Nickel, Corrosion Resistant, for General Application, which does not require postweld heat treatment after minor weld repairs. Because postweld heat treatment after all weld repairs is necessary to reduce rouging, it is better to procure cast components to ASTM A744, Castings, Iron-Chromium-Nickel, for Severe Service, which requires postweld heat treatment after all weld repairs have been made. Neither A743 nor A744 requires removal of scale formed during weld repairs and heat treatment. Scale removal is necessary for bioprocessing and must be specified as an extra requirement. OTHER STANDARDS ASME has developed a very useful Bioprocessing Equipment Standard (BPE 2002) for this industry, which covers additional requirements beyond those of the ASTM standards. In addition to materials specifications, the BPE standard covers the design, fabrication, 50 BioProcess International DECEMBER 2004 inspection, certification, cleaning, sterility, and surface finish of piping and vessels. Unfortunately, its present version allows ground and polished finishes, which are major sources of rouge. Fortunately, a proposal for inclusion of an electropolished finish in lieu of a ground and polished finish is under consideration for BPE Standards and may soon be adopted. Dairy Industry: 3-A Sanitary Standards is widely used by the dairy and other industries concerned with surface condition and cleanliness. Many equipment suppliers for bioprocessing supply the dairy industry as well and are quite familiar with the cleanliness requirements of this standard. Industrial Piping: ASME B31.3, Process Piping, is used by a number of industries for procurement of piping systems and piping components. It covers mechanical property requirements, but not the cleanliness required by the bioprocessing industry. Federal Food and Drug Regulation: Appendix 4 of the Pharmaceutical GMP Annex of the US FDA deals with cleanliness required for handling food and drugs. ISPE: The series of Baseline pharmaceutical engineering guides also deal with cleanliness requirements. CORROSION RESISTANCE CONSIDERATIONS Stainless steels owe their corrosion resistance to a thin, tough, adherent, and protective ironchromium-nickel oxide film enhanced by the addition of molybdenum to the chromiumnickel grades of stainless steels. The basic Type 316L grade with 2 3% molybdenum (especially the lowsulfur version) has been and is likely to continue to be the principal material of construction used by the bioprocessing industry. Metallurgists and engineers have become increasingly aware of the role that the normal ironchromium-nickel oxide film plays in protecting stainless steel from corrosion and also the role it plays in preventing release of ions into the product being handled. The thin film that protects stainless steel from further corrosion is breached wherever manganese sulfide particles, present in most stainless steel, penetrate the surface. This protective film can also be breached in tight stationary crevices (crevice corrosion), under adherent deposits (under deposit crevice corrosion), and in some aggressive media by pitting corrosion on boldly exposed surfaces (1, 2). Finally, the protective film can be destroyed in some low ph and especially aggressive media, such as HCl, where the film provides little resistance to further corrosion. For most bioprocessing applications, the media are not aggressive enough to require upgrading from Type 316L, but they are often aggressive enough to take advantage of defects in the protective film. Reducing MnS defects in the film by specifying the low-sulfur grade of Type 316L and strengthening the protective film by electropolishing makes Type 316L even more useful to the biotechnology industry. These steps have greatly reduced rouge formation and made electropolished low-sulfur Type 316L almost standard for the electronics industry. The bioprocessing industry has done an excellent job of minimizing crevices that would otherwise lead to both crevice corrosion and batchto-batch contamination. Unless baffles are required, the interiors of vessels are smooth, clean, and free of crevices. When baffles are required, baffle-to-vessel-wall welds should be continuous with as smooth a weld contour as possible. For bioprocessing, special piping joints can minimize the crevices common at standard flanged joints. Welds are smooth, clean, and flush with the surface. The clean-in-place (CIP) procedures the industry uses to clean vessels and piping between batches remove residues and deposits that might otherwise lead to under-deposit crevice corrosion. Film defects are of primary concern to the bioprocessing

5 2-11 Tuthill-Brunkow.qxd 11/24/ :41 AM Page 52 industry. The most common film defects in the protective film protecting stainless steel are the common MnS inclusions present in most grades of stainless steel. As mentioned earlier, Mn is added to stainless steel to combine with S and is present as MnS stringers oriented in the direction of rolling and as multiple inclusions in the surface. Each MnS particle in the surface is a defect site in the thin protective film where crevice corrosion and/or pitting may more easily occur. Lowsulfur stainless steels with minimal MnS inclusions are available today. However, the low-sulfur version must be specified in procurement as outlined above. For best performance, low-sulfur sheet, plate, strip, tubing, and bar are almost mandatory for the bioprocessing industry, as indeed they already are for the electronics industry. ROUGE Rouge is an iron oxide film that forms on the surface of stainless steel with the industrial use of distilled and high-purity water. It is a form of superficial corrosion that goes unnoticed and unreported in most industries, but it is a major problem wherever exceptional cleanliness is required. Rouge has four principal sources: cast stainless steels used for pump impellers and valves, vapor compression stills, ground and polished surfaces, and chromium stainless steel components used in pins, discs, seals, snap rings, and other components requiring greater strength than can be offered by austenitic stainless steels. Rouge manifests in two ways: as a dark deposit that wipes off on a clean white rag, and/or as a multihued deposit that adheres tightly and will not rub off on a white rag. Tverberg identified three types of rouge (3): Rouge generated from external sources cavitation (pump impellers), erosion, foreign sources, carbon steel or low alloy nuts, bolts, springs, snap rings, and so on 52 BioProcess International DECEMBER 2004 Rouge from in situ oxidation of stainless steel as from mechanically polished and nonchemically passivated surfaces; such surfaces release significant iron into the process environment Black oxide from surfaces in high-temperature service. Reduction of Rouge: Cast stainless steels are a primary source of rouge. Smaller valves and fittings can be and are increasingly produced from low-sulfur SCQ bar stock to reduce the MnS inclusions present in most castings. Cast pump casings, impellers, and valves can be ordered to ASTM A744 instead of A743. A744 requires solution-annealing heat treatment after all foundry weld repairs, making such castings less vulnerable to localized corrosion and rouge formation at weld repair sites. Neither A743 nor A744 requires annealing after weld repairs, leaving such castings more vulnerable to localized corrosion and maximizing rouge formation. Specifying pickling or electropolishing after the final solution annealing step will remove scale formed during annealing. Pickling or electropolishing will leave the castings in their most corrosion-resistant condition and reduce rouge formation from cast stainless steels. Vapor Compression Stills: Requiring pickling or electropolishing of vaporcompression still components after fabrication will leave the surfaces within these stills in their most resistant condition. Some manufacturers do a thorough pickling or electropolishing of all still components after fabrication. Ground and Polished Surfaces: The ground and polished (G&P) surfaces, though bright and clean to the eye, contain heavily coldworked, highly distorted grains loaded with silica, grinding residue, and detritus, all of which contribute to rouge formation. The oxide film on G&P surfaces is thinner, weaker, and more easily penetrated, again contributing to greater rouge formation. Prohibiting G&P finishes except at welds and specifying electropolishing as discussed below will eliminate almost all of the rouge that forms on, or comes from ground and polished surfaces (4, 5). Series 400 Chromium Stainless Steel Components: Specifying duplex stainless steel and nickel-base alloys such as alloy 625 and C276 for pins, discs, snap rings, and other items requiring higher strength will eliminate series 400 ferritic chromium stainless steels. Such steels are less corrosion resistant and are quite susceptible to rouge formation. Rouge Transfer: One main and troubling aspect of rouge formation is its frequent appearance downstream of its origin. For example, cast stainless steel pump impellers are known to be a primary source of rouge. However, the impellers themselves are often bright and shiny, with no visible rouge when examined. The rouge from the impellers is deposited in piping and vessels downstream. Removal of rouge that deposits downstream from where it is formed does little or nothing to reduce further rouge formation or deposits. ELECTROPOLISHING Electropolishing is very useful in removing the surface layer of distorted grains and embedded iron and detritus contamination characteristic of G&P and as fabricated stainless steel surfaces (6). The process corrodes the peaks of the surface profile, opens its detritus-filled valleys, and smooths the surface. In terms of RMS, electropolishing reduces the surface profile by about half. For example, a surface with a good RMS 20 profile will have an even better RMS 10 profile after electropolishing. Specifying removal of at least inch from the surface by electropolishing will eliminate widely specified but ineffective flash electropolishing. Flash electropolishing removes little if any of the distorted, detritus-filled surface layer. Specifying removal of no more than inch from the surface will prevent removal of excessive material and keep metal

6 2-11 Tuthill-Brunkow.qxd 11/24/ :41 AM Page 53 loss within reasonable limits. Care must be taken when machined components are electropolished to allow for inches of metal loss for closely dimensioned components. Another useful criterion is to electropolish until grain boundaries disappear, which normally occurs within the inch metal-removal range cited above. Industries that rely on electropolishing have found that when done properly, it removes embedded iron, detritus, distorted grains, and embedded grinding compound from the surface, leaving a surface clean and in its most durable and corrosion-resistant condition. The common nitric acid passivation treatment removes iron from the film and increases the Cr/Fe ratio, but it does not remove distorted grains or all of the iron often embedded in the surface. Citric acid: EDTA chelating treatments, as described in ASTM A380, are increasingly common. They effectively remove iron embedded in the surface, but they do not remove the thicker oxide film formed during welding. The thicker, readily visible oxide film left by welding must be removed mechanically by pickling or electropolishing (7). RECOMMENDATIONS To minimize rouge, specify electropolished surfaces for piping and vessels and prohibit G&P surfaces that are a major source of rouge formation. Electropolishing standard grades of Type 316L, although a major improvement over G&P surfaces, can be improved further by specifying low-sulfur sheet, strip, and plate for pipe and vessels. Specifying removal of at least inch and no more than inch by electropolishing and specifying low-sulfur Type 316L will result in smooth, more uniform, almost featureless surfaces of RMS that have excellent resistance to rouge formation. For the large number of valves, fittings, and small components made from bar, specify low-sulfur (SCQ) TAKE-HOME POINTS Low-sulfur, electropolished Type 316L can be repeatedly cleaned and sterilized to provide the biotechnology industry with its most useful material of construction. In the low-sulfur, electropolished condition, Type 316L is quite resistant to rouge formation. Rouge formation from cast components can be greatly reduced by the use of wrought materials wherever possible, thereby minimizing the use of cast stainless steels. When cast stainless steels must be used, procurement to ASTM A744 instead of A743 and requiring pickling or electropolishing after solution annealing will greatly reduce rouge formation from pumps, valves, and other components that must be made from castings. Rouge formation can be greatly reduced by specifying electropolished rather than ground and polished (G&P) surfaces for vessels and piping. Rouge formation in piping can be reduced by specifying cold-rolled instead of hot-rolled material for forming into piping when the wall thickness is less than 3 /16 inch. Rouge formation can be reduced by specifying appropriate higher alloyed stainless steels and nickel base alloys for pins, seals, snap rings, and other similar components and prohibiting the use of ferritic chromium stainless steels (Series 400 stainless steels) for such components. Duplex alloys are used for centrifuges and increasingly for vessels, for which their higher strength permits thinner walls that offset much of the higher material cost. 6% molybdenum stainless steels and nickel base alloys are increasingly used when corrosion resistance better than that of Type 316L is needed. bar stock to obtain a good electropolish finish on the cross section of components made from bar. Electropolished SCQ bar will provide improved resistance to rouging compared with bar procured to ASTM A276. For components that must be made from castings, specify procurement to ASTM A744 in lieu of A743. As an additional requirement, specify pickling or electropolishing after solution annealing to remove scale formed during solution annealing. Procurement to A744 followed by pickling or electropolishing will minimize rouge discharge from cast components. REFERENCES 1 Kain RK, Tuthill AH, Hoxie EC. The Resistance of Types 304 and 316 Crevice Corrosion in Natural Waters. J. of Materials for Energy Systems 5(4) 1984: Tuthill AH. Usage and Performance of Nickel Containing Stainless Steels in Natural Waters and Brines. Materials Performance, July 1988: Tverberg JC, Ledden JA. Rouging of Stainless Steel in High Purity Water Systems. Presented at Preparing for Changing Paradigms in High Purity Water, October 1999 (San Francisco, CA). Institute for International Research: New York, NY; 4 Collins SR. Stainless Steel for Semiconductor Applications. Presented at 39th Mechanical Working and Steel Processing Conference, October 1997 (Warendale, PA). The Iron and Steel Society (now the Association for Iron and Steel Technology): Warendale, PA; 5 Tverberg JC. Conditioning of Stainless Steel Surfaces for Better Performance. Stainless Steel World, April Electropolishing: A User s Guide to Applications, Quality Standards and Specifications. Delstar: Houston, TX, June Tuthill AH, Avery RE. Stainless Steel Surface Treatments. Encyclopedia of Chemical Processing and Design, 54. Marcel Dekker: New York, NY 1998: Corresponding author Arthur H. Tuthill is a corrosion consultant, 33 Glade Court, Altavista, VA 24517, , fax , MrTurtil@aol.com. Roger Brunkow is president of the Society of Bioprocessing Professionals, Indianapolis, IN. DECEMBER 2004 BioProcess International 53