Stainless Steel Ronald H

Home , Mill finish

Characteristics of Food Contact Surface Materials: Stainless Steel Ronald H. Schmidt,1* Daniel J. Erickson,2 Steven Sims3 and Philip Wolff4 1Food Science and Human Nutrition Dept., University of Florida, P.O. 110370, Gainesville, FL 32611, USA; 2Harold Wainness and Associates, 2460 – 1st Ave. East, North St. Paul, MN 55109, USA; 3U.S. Dept. of Health and Human Services, Food and Drug Administration, Office of Food Safety, Center for Food Safety and Applied Nutrition, 5100 Paint Branch Pkwy., College Park, MD 20740-3835, USA; 4U.S. Dept. of Agriculture, Agricultural Marketing Service, Dairy Grading Branch, 1400 Independence Ave., SW, Washington, D.C. 20250-2030, USA

summary This technical review article describes the properties and characteristics of stainless steel in sanitary design when used as a food contact surface, particularly when compliance with the requirements of standards promulgated by 3-A Sanitary Standards, Inc. is intended. It discusses the general characteristics of stainless steels, including ferrite, austenite, cementite, and martensite. It discusses stainless steel categories and grades used in industrial applications and their properties, including both non-magnetic types such as superaustenitic and magnetic types such as ferric and martenistic as well as duplex stainless steel and precipitation hardened stainless steel. Specific emphasis is given to those categories and grades used in food contact applications. The article contains a discussion on the general steps in stainless steel manufacturing, including melting/casting, forming, heat treatment/annealing, descaling, cutting, and finishing, and describes common types of acceptable surface modification and finishing of stainless steel. In addition to the topics listed above, the article describes common types and causes of corrosion of stainless steel, including uniform, pitting, crevice, stress cracking, galvanic, contact, and biologically and microbiologically influenced corrosion. Finally the article discusses methods of preventing corrosion, including surface maintenance and cleaning and a process referred to as “passivation.” This article contains micrographs showing, at varying levels of magnification, chemical corrosion and some common surface finishes, including number 4 (150 grit), 2B, and 2D finishes, as well as photographs showing unacceptable product contact surface finishes of welds in stainless steel tubing.

A peer-reviewed article

*Author for correspondence: Phone: +1 352.367.2877; Fax: +1 352.392.9467 E-mail: [email protected]

574 FOOD PROTECTION TRENDS | OCTOBER 2012 INTRODUCTION sitional distribution and chemical com- While there is considerable variation binations of iron and carbon. For exam- with regard to properties within each of From a sanitary design perspective, ple, ferrite (predominately α- iron and the stainless steel categories, their general food contact surfaces should be smooth, δ- iron) is relatively soft, while austenite properties are summarized in Table 1. impervious, free of cracks and crev- (predominately γ- iron) is much harder, The general classes of stainless steel ices, non-porous, non-absorbent, non- and cementite (predominately Fe3C) is alloys can be further subdivided accord- contaminating, non-reactive, corrosion extremely hard. Martensite has inter- ing to composition, common AISI grade, resistant, durable, and cleanable (24). mediate Fe-C levels and mixed chemical crystal structure, specific properties, and Further, materials used in food contact species and forms. In general, the crys- applications. A comparison of proper- surfaces must be non-toxic, and materi- talline structure of ferrite, austenite, and ties, characteristics, and applications of als containing heavy metals (e.g., lead, martesite is usually body-centered cubic, selected stainless steel grades is presented cadmium, hexivalent chromium or mer- face-centered cubic and needle-like, re- in Table 2, and the chemical alloy com- cury) or other toxic materials must be spectively. position of selected stainless steel grades avoided. Finally, these surfaces should be Stainless steel, in general, is an is presented in Table 3. fabricated, operated, and maintained in Fe-C alloy with additional alloying ele- The most common stainless steels a manner such that these criteria are not ments. These alloying elements may used in food processing and handling compromised. Stainless steel is generally include copper (Cu), chromium (Cr), equipment are made from Fe-C-Cr alloys the most preferred and most commonly molybdenum (Mo), manganese (Mn), and Fe-C-Cr-Ni alloys, with other alloy- used material in the design, construction nickel (Ni), nitrogen (N), phosphorus ing elements used to varying degrees. Cr, and fabrication of food processing equip- (P), silicon (Si), sulfur (S), selenium (Se), upon exposure to air, gives stainless steel ment and is specified in the 3-A Sanitary tungsten (Tn), and titanium (Ti). its characteristic high resistance to “stain” Standards (1) as well as in other com- Base stainless steels are generally (or corrosion) by forming a thin “passive” formulated from iron (minimum of 50% monly used food processing equipment layer of chromium (III) oxide (Cr2O3), standards throughout the world (20). by weight), carbon and chromium (min- iron oxide, and other oxides. The passive The general advantages of stainless imum 10.5% by weight), and their prop- layer protects the “active” material (iron), steel over other materials for food con- erties are dependant upon the relative which is susceptible to rust and corro- tact are as follows: concentrations of these elements. Prop- sion. Nickel (when present) provides • corrosion resistance (ranging erties of additional alloying elements will additional corrosion resistance as well from fair to outstanding); be discussed. as greater strength and structural hard- • high strength, high hardness, ness to stainless steel. However, as will high modulus; Stainless steel categories be discussed, Ni-based stainless steels are • availability of a wide range of and grades, and their properties more susceptible to one type of corro- product forms; sion, termed Stress Corrosion Cracking • relative ease of machining and More than 150 grades or types of (SCC), than are those without Ni. fabrication; and stainless steel exist. The traditional no- In general, alloys formulated at • relatively low cost. menclature to define stainless steel grades higher carbon levels are of greater struc- is the numbering system of the American A wide variety of stainless steel tural hardness and strength than those Iron and Steel Institute (AISI), a func- materials are available with widely formulated at lower carbon levels. Addi- tion now under the American Society of varying composition, surface finish, and tional carbon also provides resistance to Testing and Materials (ASTM) (2). The functional properties (2, 4, 5, 6, 7, 8, 23). oxidation and creep. However, the risk traditional AISI Stainless Steel Products In addition, the functionality of stainless of corrosion may be increased with high- Manual, currently published by the Iron steel is impacted by surface treatments carbon stainless steel materials. and Steel Society (14), provides a listing and coatings. Thus, an awareness of the In higher grade stainless steels, Mo of stainless steel grades, composition, properties of the various stainless steels and/or N (and to a lesser extent Cu) are and properties. While AISI grading sys- is required by fabricators, as well as by added to enhance the passivation layer tem has traditionally used a three-digit potential users of food equipment. through forming oxides of these ele- numbering system, newer grades allocate ments, thus providing additional corro- General characteristics 1-letter + 5 digit UNS numbers. While sion resistance. Mo (which forms molyb- of stainless steel the AISI grade designations define the denum oxide in the passivation layer) is individual grades, they are not specifi- especially effective at preventing the type From a metallurgical perspective, cations as such. Specifications used for of corrosion termed “pitting corrosion.” steel is an iron alloy composed of iron stainless steel are from the ASTM. In Enhanced machinability is attained (Fe) and carbon (C), with ferric carbide addition, international specifications are through the addition of P, S, and/or (Fe3C) as the base component. However, used to identify and distinguish special- Se. Stainless steels of excellent strength, depending upon formulation/alloying ized stainless steel products (e.g., welding strength, durability, heat resistance, and and manufacturing techniques, the iron wire). corrosion resistance (especially in acid exists as a solid solution of varied levels The two general categories of stain- environment) may be attained with the of iron and carbon of different crystalline less steel are non-magnetic and mag- addition of Ti. Because of the relatively textures (e.g., ferrite, austenite, pearlite, netic stainless steel. Through working, high economic cost, Ti-based stainless martensite, ledenburite, spheroidite, and blending and manufacturing techniques, steels are primarily used in situations cementite) as well as different strengths, additional categories of stainless steel where high acid and/or salt levels (e.g., hardnesses, and other properties. These (e.g., duplex, precipitation hardened) citrus juice, tomato products) may be properties are dependent upon compo- are also available for specific applications. encountered.

OCTOBER 2012 | FOOD PROTECTION TRENDS 575 Group Magnetic Work Corrosion Hardenable Ductility 2 High Low Weldability 1 TABLERe s1.p on gseeneral Hard epropertiesning Resis tandance stainless steel categoriesT (2,em p23)erature Temperature Rate Resistance Resistance A. Non- No Very High High Cold or work Very High Very High Very High Very High magnetic hardened (Austenitic) only B. Magnetic

1. Ferritic Yes Medium Very High No Medium High Low Low

2. Marten- Yes Medium Medium Yes – quench Low Low Low Low sitic & temper C. Duplex Yes Medium Very High No Medium Low Medium High

D. Precipitation Yes Medium Medium Hardened by Medium Low Low High Hardened low heating (aging) 1Variation between grades and formulations within each group 2The property of metal which permits it to show considerable elongation with increase in local stresses

Non-magnetic stainless steel Higher grade alloys, termed super- ity, than either austenitic or marstensitic austenitic (e.g., AL-6XN, 254SMO), alloys; however, they have superior weld- The non-magnetic stainless steels, contain higher levels of all alloying ele- ability, because of high heat resistance. primarily austenitic alloys, are generally ments (especially molybdenum). These These stainless steels are primarily used Fe-C alloys with Cr (ranging to 26%) and alloys have excellent corrosion resistance for decorative purposes (e.g., appliances, Ni (usually less than 35%). As shown in and strength for use in harsh environ- automobile and architectural trim) and Table 3, other alloying elements may be ments such as marine applications. are not commonly used in the manufac- used, depending upon the grade. Variations with regard to composi- Most of the stainless steel used in tion and properties are also seen within ture of food equipment. However, as will the fabrication of food equipment is of the stainless steel grades, depending upon be described, certain grades of ferritic the austenitic AISI 300 series. Approxi- the composition. For example, alloys stainless steel are used in the brewing mately 50% of all stainless steel pro- may be formulated at low carbon levels industry, because of their resistance to duced is 304 stainless steel, formulated at (e.g., 304L, 316L) to enhance weldabil- Stress Corrosion Cracking (SCC). 18% Cr and 8% Ni. As shown in Table ity. These alloys, however, would have Martensitic stainless steels are Fe- 3, type 316, austenitic stainless steel has lower strength than the base grade ( e.g., C-Cr alloys often used for their high higher Ni (10%) and Mo levels and is 304, 316). Conversely, higher carbon hardenability. In addition to varying generally considered a higher grade ma- levels may be used for enhanced strength amounts of the base alloying elements, terial for food contact surfaces than 304 alloys (e.g., 304H, 316H). Thus, modi- sulfur and/or selenium may be added to stainless steel because of its enhanced fied stainless steel of lower AISI grade enhance machinability of certain grades. corrosion resistance. The 3-A Sanitary may, through compositional modifica- Martensitic stainless steel grades have a Standards (1) require that food pro- tion, exhibit properties similar to those variety of industrial applications (e.g., duct contact surfaces be of stainless steel of a higher grade designation. ball bearings, races, dies, screws, bolts) that conforms to the applicable composi- as well as medical/dental applications tion ranges established by ASTM A 959 Magnetic stainless steel (e.g., surgical instruments, dental instru- or AIST Stainless Steels: Steel Products ments). Higher carbon martensitic alloys Manual for 304/304L and 316/316L These alloys are less commonly used are used for high-grade cutlery and razor or corresponding Alloy Cast Institute in food equipment applications than are blades because of their strength, polish- types or metal that, under conditions non-magnetic alloys. However, they may ability, and sharpenability. Martensitic of intended use, is at least as corrosion be used for highly specialized applica- stainless steels may also be used in a variety of food contact applications (e.g., resistant as 304 stainless steel. The 3-A tions. They are formulated from Fe-C- utensils, scoops, blades, bushings, bear- Sanitary Standards (1) restrict the use of Cr alloys, generally without Ni. The two ings, buckets). 303 stainless steel, and expressly prohibit basic categories of magnetic stainless steel alloys containing lead, leachable copper are ferritic and martensitic. Duplex stainless steel or other toxic materials. Lower grade Ferritic stainless steels are Fe-C-Cr austenitic stainless steel alloys (e.g., AISI alloys formulated at higher Cr and/or Excellent corrosion resistance and 100 and 200 Series) are generally not rec- lower C than the martensitic alloys. They high strength can be attained by the use ommended for use in food equipment. possess less strength, but higher ductil- of duplex stainless steel alloys, which

576 FOOD PROTECTION TRENDS | OCTOBER 2012 TABLE 2. general sub-classes, characteristics, and usage applications of stainless steel alloys (2, 4, 5)

General Classification Common AISI Grades Characteristics applications

A. Non-magnetic Stainless Steel 1. Austenitic chromium- 100 Series Low strength, General purpose; nickel-manganese Type 101, 102 durability, and Furniture; not 200 Series corrosion recommended for Type 201, 201 resistance food equipment 2. Austenitic chromium- 300 Series nickel Type 301, 302, Relatively higher Formed products; 303 carbon imparts low corrosion hardness and resistance limits food machinability; equipment usage; low corrosion 303 sometimes resistance used in low corrosion applications Type 304 More corrosion Most commonly resistance, less used in food strength than 303 equipment applications Type 308, 309 Higher Used as a temperature filler metal resistance than 304; Type 316 More corrosion Second most resistance than commonly used in 304, enhanced by food equipment inclusion of molybdenum Type 321 Higher Welding weldability than applications 304 3. Superaustenitic AL-6XN, High strength, Specialized Enhanced levels of all 254SMO hardness, applications; marine alloying elements, abrasion environments especially molybdenum resistance, (> 6%) resistance to pitting and other corrosion B. Magnetic Stainless Steel 1. Ferritic – chromium 405, 409, 429, High heat Decorative and iron alloys resistance, good appliances, 430, 439, 446 formability and automobile and machinability, architectural and poor corrosion appliance trim resistance

2. Marstensitic – chromium, 410, 416, 420, 431, high strength, Dental and surgical iron, carbon alloys 440 wear resistant; instruments; blades polishable bushings, buckets, ball bearings, molds and dies, utensils, cutlery

OCTOBER 2012 | FOOD PROTECTION TRENDS 577 TABLE 2. General sub-classes, characteristics, and usage applications of stainless steel alloys (2, 4, 5) (Continued)

General Classification Common AISI Grades Characteristics applications

D. Precipitation Hardening – 600 Series Excellent Specialty primarily marstensitic hardness and applications (some austenitic) by low heat strength; formed aging process

TABLE 3. general composition of selected grades of stainless steels (23)

Approximate content (% of weight) General aISI Carbon manganese Chromium Nickel molybdenum Titanium Category grade (C)1 (Mn)1 (Cr) (Ni) (Mo) (Ti)

Austenitic Stainless Steels 304 0.08 2.00 17.5 – 20.0 8 – 10.5 -- 316 0.08 2.00 16.0 – 18.0 10.0 – 14.0 2.0 – 3.0 321 0.08 2.00 17.0 – 19.0 9.0 – 12.0 -- 5.0 254SMO 0.02 1.00 18.5 – 20.5 17.5 – 18.5 6.0 – 6.5 1.9 AL-6XN 0.03 1.00 20.0 – 22.0 23.5 – 25.5 6.0 – 7.0

Ferritic Stainless Steels 405 0.08 1.0 11.5 – 14.5 ------430 0.12 1.0 16.0 – 18.0 ------446 0.20 1.5 23.0 – 27.0 ------

Martensitic Stainless Steels 410 .15 1.0 11.5 – 13.0 ------416 .15 1.25 12.0 – 14.0 -- 0.6 -- 431 .10 1.0 15.0 – 17.0 -- 1.25 – 2.0 --

possess a mixture of ferritic and auste- stainless steel, they also have a potential brewing tanks, pipelines) or for hot nitic crystal structures. In addition to the price advantage. Higher grades of du- chemical storage and/or transport. iron-carbon base, the primary alloying plex stainless steel (e.g., superduplex) elements are chromium and low levels have high chromium and molybdenum Precipitation hardened stainless of nickel. Typically, duplex stainless steel levels. Although type 2205 is the most steel exhibits considerably higher strength widely used duplex stainless steel grade, than austenitic stainless steel, with cor- other grades are available (e.g., 326, 329, This general class of stainless steel rosion resistance similar to higher grade A219, 2RE60, IC378, IC381). Potential refers to materials formed by harden- austenitic alloys, and they possess bet- food industry applications for duplex ing of stainless steel materials, usu- ter corrosion and cracking resistance. stainless steel are in situations where the ally by a low temperature aging heat Because of a nickel level typically less corrosion potential is high, such as brew- treatment to develop high tensile than half of that found in austenitic ing and fermentation (e.g., brine tanks, strength. Only certain types of stain-

578 FOOD PROTECTION TRENDS | OCTOBER 2012 TABLE 4. stainless steel finishes (2, 8, 23)

Stainless Steel Finish Category Description

Mill Finishes No. 0 Hot rolled, annealed, thicker plates No. 1 Hot rolled, annealed, passivated No. 2D Cold rolled, annealed, pickled, passivated No. 2B Same as 2D with additional roller polishing No. 2BA Bright annealed 2B

Polished Finishes No. 3 Coarse abrasive finish No. 4 Brushed finish No. 5 Satan finish No. 6 Matte finish No. 7 Reflective finish No. 8 Mirror finish No. 9 Bead blast finish No. 10 Heat-colored finish (widely varied)

less steel are hardenable by heating facturing steps. In addition, strict qual- descaling method is pickling, in which methods, the most common of which ity control must be practical to prevent a nitric-hydrofluoric acid bath is used are precipitation hardened martensitic surface etching or the presence of debris to remove the scale. Electro-cleaning, a (e.g., AISI 601 – 630) stainless steel. Fer- (e.g., tramp steel) on the surface which process by which scale is removed elec- ritic stainless steel alloys generally cannot could limit the corrosion resistance qual- trochemically, may also be used. be hardened by the precipitation harden- ity of the material. The general steps in Cutting. In this step, the mate- ing method. Although austenitic stain- the manufacture of stainless steel (29) are rial is cut into desired blank, shape, less steels are generally not readily hard- the following: size or gauge. Cutting is usually done enable by heating, certain precipitation Melting/Casting. Raw materials mechanically by use of a variety of shear- hardened austenitic grades (e.g., AISI are melted together during prolonged ing methods with specially designed heating at intense temperatures in an 650 – 653, 660 – 665) are available. knives. Flame cutting (using a flame- electric furnace. The molten steel is cast Although not hardenable by heat- fired torch) or plasma jet cutting (using into various forms (e.g., blooms, billets, ing, 300 Series austenitic stainless steel ionized gas with an electric arc) may also slabs, tubes). be used. can be hardened by a process known Forming. This step is generally as “work hardening,” or “cold work- done by a rolling process, with hot rolling ing” the material. This is accomplished being the most common method. During Finishing and surface mechanically by cold rolling down to forming, cast materials are formed into modification lighter gauges or by drawing through a various shapes ( e.g., wire, bar, sheets). die or similar device. Heat Treatment/Annealing. Stain- The final surface finish of stainless less steel is heated (or annealed) to relieve steel is critical to its properties and applications (11). Thus, finishing is an im- STAINLESS STEEL internal stresses and soften the material. The specific conditions used in this an- portant step in stainless steel manufac- MANUFACTURING nealing step will vary with the general ture. Generally, a dull finish is attained General manufacturing steps type of material (e.g., austenitic, ferritic, through hot rolling, annealing, and des- martensitic). caling (especially if pickling is used). Any The gauge and properties, as well Descaling. The annealing step surface debris which may have formed as the applicability and uses of stainless during manufacture must also be re- steel, are greatly impacted by the manu- causes a surface build-up, or scale, that must be removed. The most common moved.

OCTOBER 2012 | FOOD PROTECTION TRENDS 579 FIGURE 1. 2B Mill finish (magnifications shown at 6.3, 12, 25, and 63 X) No. 2 finish is attained by a cold rolling process. The most common in this series is the 2B finish (shown under magnification in Fig. 1), which is smoother and brighter than the 2D finish (Fig. 2). Bright Annealed (e.g., 2BA) is mechanically polished (or brushed). Because of its superior release properties for higher fat products (e.g., cheese, butter, meats), the 2B mill finish has been traditionally accepted under specific 3-A Sanitary Standards (1) and U.S. Department of Agriculture (USDA) guidelines (28) for such applications. Certain cold rolled stainless steels of the No. 2 series are used in architectural applications. Polished finishes are attained by use of a variety of techniques to “polish” stainless steel. Grinding with abrasives and/or buffing withcloth wheels may be used to achieve a reflective finish. Fur- FIGURE 2. 2D Mill finish (magnifications shown at 6.3, 12, 25 and 63 X) ther polishing with progressively finer abrasives, extensive buffing with cloth rollers, or electropolishing will result in a mirror finish. Additional finishing methods ( e.g., tumbling, dry etching/ sandblasting, wet etching by use of acid solutions, wire brushing) are used to achieve specific finishes. In some cases, a process termed “bead blasting” is used in which the surface is bombarded with glass beads. While this is an efficient process, it not usually recommended for food contact surfaces because the glass beads tend to degrade into irregular shapes during this process, leaving an irregular surface finish. The lower smoothness polished finish (No. 3) is a ground (80 – 100 grit abrasive) finish, and is not used extensively for food contact surfaces. The No. 4 ground finish (80 – 150 grit), the most commonly used general purpose stainless steel for food A wide variety of stainless steel hot rolled annealed (e.g., HRA or No. 0) contact surfaces, is shown under magni- surface finishes are available, depend- finish, since it is neither pickled nor pas- fication in Fig. 3. Buffed finishes (No. 5, ing upon the desired applications. The sivated (to be described later), does not costs of achieving different surface fin- develop the passive corrosion-resistant 6, 7, 8) are generally more expensive and ishes are varied. In general, smoother or layer and has a scaly black finish. No. thus not used extensively in the food in- more polished surfaces are more expen- 1 Finish stainless steel has a rough, dull dustry. The 3-A Sanitary Standards (1), sive than rougher surfaces. Stainless steel surface and is also not recommended for as well as USDA guidelines specify that finishes are given a number depending food contact surfaces, although it is used all surfaces, including fabricated, welded upon their characteristics and properties, in industrial applications. and soldered joints, shall be at least as with higher numbers indicating higher Mill finishes are attained by direct smooth as a No. 4 (150 grit) finish and smoothness (9, 26). The general stainless application of rollers and mechanical shall be free of pits, folds, crevices, cracks, steel finishes are described in Table 4. abrasives to flat rolled (wrought) stainless and misalignments in the final fabricated A variety of methods of surface steel sheets and are the basic finishes for form. Examples of welded surfaces that modification are used to achieve - ade all flat stainless steel. Differing finishes are not considered as smooth as a No. 4 sired surface finish (9, 26). In general, are attained with hot or cold rolling. For finish are shown in Fig. 4. hot rolled base stainless steels are not example, cold rolling on polishing roll- The Average Roughness (Ra), used in food contact applications. The ers is used to obtain a bright finish. A measured by the profile method and

580 FOOD PROTECTION TRENDS | OCTOBER 2012 FIGURE 3. Number 4 (150 grit) finish (magnifications shown are 6.3, 12, 25 assurance of the acceptability of a 2B fin- and 63 X) ish on stainless steel sheets that have not been inspected and found free of these serious flaws.

CORROSION OF STAINLESS STEEL Types of corrosion and their causes Being composed of an iron-based alloy, nearly all common grades of stainless steel are prone to corrosion by removal of the passive film through continued exposure to incompatible cleaners, abrasive cleaners, abrasive cleaning pads, or chlorine and oxidizing sanitizers. In general, stainless steel does not “rust,” a process in which the characteristic flak- ing red oxide forms on regular steel. If red discoloration occurs, it is due to contamination by iron particles on the surface (or imbedded in the surface) of stainless steel that have rusted. The 3-A FIGURE 4. Examples of surface texture or finish on the interior surface of welds Sanitary Standards (1) require that any in metal tubing that are not as smooth as a No. 4 (150 grit) finish. These flaws are stainless steel or other metal material the result of improper welding techniques: (A) Inadequate inert gas purge during used be at least as corrosion resistant as welding; (B) Over pressure of inert gas during welding; (C) Uneven distribution of 304 stainless steel under the conditions of heat during welding intended use of the stainless steel, metal, or alloy to be used. Corrosion of stainless steel is shown in Fig. 5. The general types of stainless steel corrosion (11, 27) are as follows: • Uniform Corrosion. This type of corrosion is usually associ- ated with continued exposure to dilute acid or alkaline solutions, or by acute exposure to more concentrated acid or hot alkali. General resistance to this type of corrosion is better expressed in µm, is the roughness param- the relationships between surface rough- in stainless steels formulated eter most widely used and is accepted by ness and cleanability, as well as the rela- with higher levels of Cr. Sulfur various international bodies, including tionship between surface roughness and addition, which enhances ma- the International Organization for Stan- biofilm formation, are not clearly estab- chinability, reduces the resis- dards (ISO standards) and the European lished and need further research (20, 21, tance to uniform corrosion. Hygienic Engineering Design Group 24), and many other factors may be im- • Pitting Corrosion. This type (EHEDG) (13, 15, 16, 20, 21). An R portant. According to the study by Stein- of corrosion results from more a localized destruction of the of less than 0.8 µm is specified for food er et. al. (24), a 2B mill finish on stain- passive layer and subsequent contact surfaces by the EHEDG and less steel sheets may, in performance, be corrosion of the steel alloy be- by the American Meat Institute (AMI) as cleanable as a No. 4 (150 grit) finish. low. Pitting corrosion gener- Equipment Design Task Force (10) rec- Under the 3A Sanitary Standards (1), a ally is the result of exposure to ommendations. Under 3-A Sanitary 2B finish may be now be accepted for chlorides, bromides, and other Standards(1) criteria, an R of less than a certain surfaces, provided that the stain- halides and is accelerated by 32 µin. (0.8 µm) is specified and is con- less steel sheets have been inspected and high temperature and lower sidered equivalent to a No. 4 finish. selected to be free of pits, folds, cracks, pH level. Once formed, pit- Surface roughness has been general- inclusions, and similar defects. It should ting corrosion has a tendency ly related to cleanability of stainless steel be noted, however, that, because mean Ra to continue to grow and is dif- surfaces (25), with smoother surfaces of- is not sensitive to individual extreme sur- ficult to remove. Stainless steel ten considered more cleanable. However, face flaws, it is not accepted by 3A as an grades formulated at high Cr

OCTOBER 2012 | FOOD PROTECTION TRENDS 581 FIGURE 5. Examples of chemical corrosion and the resulting pitting of polished • Contact Corrosion. This type stainless steel surfaces (two upper photos) and on interior surfaces of unpolished of corrosion occurs when small tubing (lower left) and unpolished metal tubing at a welded joint (lower right). particles of foreign matter (es- Magnification shown at 6.3 X) pecially carbon) are left on a stainless steel surface. Contact corrosion combines the elements of galvanic and pitting corrosion, in that it starts as a galvanic cell, and, if a pit occurs, will be followed by pitting corrosion. Contact corrosion is usually the result of poor fabrication techniques that allow carbon debris to remain on the final surface. In manufacture, removal of carbon debris is accomplished in the pickling and/or passivation steps. Contact corrosion is best avoided in the cutting, machining, grinding and polishing steps by use of dedicated tools specific to the steel being worked. Good manufacturing practices and specific standard operating procedures (SOPs) and Mo and/or N have higher containing chlorides can lead should be established and resistance to pitting corrosion. to stressed areas in stainless implemented in fabrication The Pitting Resistance Equiva- steel and SCC. In general, the shops and in plant equipment lent (PRE) number, used as an austenitic stainless steels are installations to prevent cross- indicator of pitting resistance most vulnerable to SCC. SCC contamination of stainless steel of stainless steel, is calculated is related to the Ni content; materials with mild steels or as: PRE = %Cr + 3.3 × %Mo thus low-Ni stainless steels other dissimilar metals. Most + 16 x %N (e.g., ferritic) are virtually im- importantly, the same grinders, • Crevice Corrosion. There is a mune. Since SCC is a common sanding or polishing tool used potential for this type of corro- problem in the brewing indus- for stainless steel should not be sion when crevices are formed try, ferritic stainless steel grades used for mild steel. during equipment fabrica- are often used in this industry • Biological and Microbiologi- tion, or as a result of improper (17, 22). Further, since SCC cally Influenced Corrosion equipment design. Crevice is related to the Ni content, (MIC). In food processing and corrosion may occur in any 316 stainless steel offers no handling facilities, inadequate crevice formed during fabrica- advantage over 304 stainless cleaning of food equipment may accelerate corrosion of the tion and/or installation (e.g., steels with regard to this type food contact surface because under gaskets, incomplete of corrosion. However, duplex of residual biological materials welds, overlapping surfaces). (e.g., 2205) and superduplex (e.g., food soil, other materi- Crevice corrosion is accelerat- stainless steels offer excellent als) on the surface as a seeding ed in equipment use situations resistance to SCC. Resistance point where corrosion can oc- which allow the loss of the pas- to SCC can be achieved by cer- cur. Microbial biofilms, espe- sive layer through prolonged or tain annealing processes and cially those formed by highly by using techniques that apply stagnant contact with reducing oxidizing bacteria, will also at- a compressive stress to the sur- materials (reducing acids). tack the surface of the stainless face (e.g., shot peening). • Stress Corrosion Cracking steel, accelerating pitting cor- • Galvanic Corrosion. Certain (SCC). Stressing stainless steel, rosion reactions. solutions can generate this either during manufacture or • More Severe Corrosion. Other during rigorous usage, can type of corrosion because of types of corrosion may be pres- result in localized pinholes or the flow of electric current, ent in industrial applications other stress areas, which then especially where two dissimilar where severely corrosive solu- become vulnerable to stress metals are in contact. Preven- tions or very high temperatures corrosion cracking (SCC). For tion of galvanic corrosion can are used. For example, Suphi- example, continued exposure be achieved by avoiding mixed de Stress Corrosion Cracking to high-temperature solutions metal fabrications. (SSSC), a type of corrosion

582 FOOD PROTECTION TRENDS | OCTOBER 2012 caused by hydrogen sulphide ing acid) at an appropriate concentration Removal of corrosion exposure, may occur on equip- and time period. It is recommended that ment used in the oil, gas, and stainless steel surfaces be passivated ini- Once formed, corrosion on stainless related industries. In industrial tially, at a defined frequency thereafter, steel can be difficult to remove. Mild cor- use involving extremely high and after any surface repair, polishing, or rosion can be removed by rigorous clean- temperatures (425–850°C) for other modification. In addition, passiva- ing or, in some cases, the re-working of a period of time, microstruc- tion of stainless steel food contact sur- surfaces. More severe corrosion usually tural breakdown (e.g., carbide faces is recommended after any surface requires more rigorous treatment, such as precipitation) may cause a repair, polishing, or working. a passivation treatment. However, severe defect termed Intergranular Certain precautions must be foll- pitting corrosion may not be removable Corrosion. owed with regard to passivating stain- by passivation, and may therefore require less steel. It is imperative that the surface more rigorous treatment (e.g., pickling Prevention of corrosion to be passivated be clean, as passivation paste) for removal. will not remove surface contaminants Selection of a type of stainless steel that result from fabrication or other resi- that is appropriate to the conditions of SUMMARY AND dues from food processing operations. intended use is critical to preventing and CONCLUSIONS Such surface contaminants will also minimizing corrosion of stainless steel impede the effectiveness of the passiva- It is clear from the foregoing dis- surfaces, particularly in high acid, high cussion that not all stainless steels are salt (brine), or high temperature envi- tion process. In the fabrication of stain- created equal. Fabricators/manufactur- ronments. Consideration also needs to less steel, diligent care must be used to ers of food processing equipment must be given to the type of cleaning chemi- assure that the surface is free of embedded cals and cleaning temperatures to which iron particles (caused by ferrous grinding consider food types, cleaning/sanitizing/ the equipment surfaces will be exposed methods), high carbon “tramp” steel, sterilization processes, and all environ- (see Tables 1 and 2). machine lubricants and other oils, cray- ments of intended use when selecting Surface maintenance and clean- on, paint, other markings, and/or shop stainless steel material types. Food pro- ing/sanitizing. To prevent the problems dirt. If not adequately removed, these cessors must be aware of the general of corrosion, it is imperative that an ade- contaminants could lead to pitting, properties and of the diversity of stainless quate preventative maintenance program rusting and crevice or crack formation. steels. If the wrong type of stainless steel is implemented for all food equipment. Additional fabrication defects that may is selected for severe use applications, it This program should include routine in- enhance corrosion include heat tint from will surely fail and cause processed food spection of food contact surfaces for signs welding, weld flux, arc strikes and spat- products to be unacceptable for market of corrosion and assurance that condi- ter. It is imperative that new equipment or human consumption. tions that may induce corrosion (e.g., delivered from the manufacturer be in- As part of food safety audits by inadequate draining of solutions) are spected and cleaned appropriately prior regulatory officials and/or third party en- being avoided. In addition, an adequate to passivation, as the surfaces may have tities, food processors/manufacturers are cleaning and sanitizing program must be an oily film or other surface residues. increasingly asked to obtain and provide in place, which includes an appropriate Prior to passivation of a stainless assurances from equipment suppliers frequency as well as validation. Adequate steel surface, it is recommended that that the equipment has met appropri- cleaning and sanitizing removes biologi- an expert be contacted for assistance. ate standards, where applicable, and to cal materials that can attack the surface Detailed procedures for cleaning/passi- ensure that food contact materials are with time and serve as a seed point for vation using nitric acid and other acids non-toxic. Alloys containing the heavy corrosion. Adequate cleaning also re- are provided under ASTM A380 (3). In metals lead, cadmium, hexivalent chrom- moves mild corrosion. However, strong general, a complete passivation process ium or mercury, must be avoided when cleaning and sanitizing chemicals, if may involve the following steps: clean- choosing food contact materials. used at improperly high concentrations ing, degreasing, inspection, passivation If appropriate standards are not or on surfaces that are not well main- (immersion or spraying) following rec- available for “one-of-a-kind” processes or tained, could increase the potential for ommendations, and neutralization /rins- non-standardized equipment, it is recom- corrosion. Extended contact with lower mended that experts be consulted prior concentrations of chlorine sanitizers and ing. Extreme care, with regard to worker to designing, manufacturing or buying similar chemicals can also increase the safety as well as environmental discharge, equipment for assurances that the equip- likelihood of corrosion of stainless steel must be exercised when using strong pas- surfaces. sivating/oxidizing chemicals. In addi- ment is cleanable, durable and safe under Passivation. The process known as tion, these chemicals, if not neutralized the conditions of intended use. Finally, passivation (18, 19) is done to maintain appropriately, will corrode non stainless strict attention should be paid to main- the passive (non-reactive) oxide film, to steel surfaces (e.g., non-product con- tenance of food processing equipment, as enhance the Cr content on the surface, tact surfaces, sewer drains and piping) well as to validating cleaning, sanitizing and to protect the active (reactive) surface and will etch or damage concrete or tile or sterilization operations, prior to plac- from corrosion. In general, this is accom- floors. Any leak or spill must be imme- ing any food processing equipment into plished by exposing the surface to a solu- diately diluted with water or neutralized service. tion of nitric acid (or other strong oxidiz- with a basic solution.

OCTOBER 2012 | FOOD PROTECTION TRENDS 583 REFERENCES 10. Bilgili, S. F. 2006. Sanitary/hygien- 21. Milledge, J. J. 2011. Cleana- ic processing equipment design. bility of stainless steel used as a 1. 3-A Sanitary Standards, Inc. Avail- Worlds Poultry Sci. J. 62(1):115–122. food contact surface. Available at: able at: www.3-a.org. Accessed 10 11. Cowan, C. T. 1977. Corrosion of http://johnmilledge.yolasite.com/. January 2012. stainless steel: How to prevent it. Accessed 22 October 2011. 2. Anonymous. 2005. Stainless steel Food Eng. Int. 2(9) 34–37. 22. Redmond, J. D. 1984. Solving brew- – Introduction to the grades and 12. Daufin, G., L. Kerherve, and ery stress corrosion cracking pro- families. AZo materials – The A J. Richard. 1977. The effect of sur- blems. Technical Quarterly, Master to Z of Materials. AZo J. Materials face finish on stainless steel on its Brewers’ Assoc. of the Americas. 21 Online. Available at: http://www. cleaning and resistance to pitting (1):1–7. azom.com/articleaspx?Article corrosion. Industries Aliment. et 23. Sourmail, T. , and H. K. D. H. Bhadeshia. ID=2873. Accessed 10 January Agricoles 95(1): 43–51. 2005. Stainless steels. University 2012. 13. Eastwood, C. A., D. L. Woodall, D. of Cambridge. Available at: http:// 3. Anonymous. 2007. Passivation of A. Timperley, G. J. Curiel, P. Peschel, www.searchsteel.com/2009/10/ stainless steel: Summary of guide- and G. Hauser. 1993. Welding stain- thong-tin-them-ve-thep-khong-gi. less steel to meet hygienic require- lines recommended by 3-A Sani- html. Accessed 22 October 2011. ments. Document 9. European tary Standards and the European 24. Schmidt, R. H., and Erickson, D. J. Hygienic Engineering Design Group Hygienic Engineering Design Group 2005. Sanitary design and construct- (EHEDG). Available at: http://www. ion of food equipment. Univ. of (EHEDG). Trends in Food Sci. Tech- ehedg.org/. Accessed 10 January Florida Coop. Ext. Serv. FSHN 0409/ nol. 18, (1, EHEDG Yearbook 2007): 2012. FS119, Available at: http://edis.ifas. S112–S115. 14. Iron and Steel Society. 2011. Stain- ufl.edu/FS119. Accessed 10 January 4. Anonymous. 2011a. Austenitic less steel products manual. Available 2012. stainless steels. Available at: http:// at: http://www.techstreet.com/info/ 25. Steiner, A. E., M. M. Maragos, and www.gowelding.com/met/auste- iss.html. Accessed 22 October R. L. Bradley. 2000. Cleanability of nitic.html. Accessed 22 October 2011. stainless steel surfaces with vari- 2011. 15. ISO. 1984. Surface roughness – ous finishes.Dairy Food Environ. San. 5. Anonymous. 2011b. Frequently terminology — Part 1; Surface and 20(4):250–266. asked questions. The Stainless Steel its parameters. ISO 4287-1, Inter- 26. Timperley, D. A., and G. G. Lawson. Information Center. Available at national Organization for Standards, 1979. Test rigs for the evaluation of http://www.ssina.com/faq/index. Geneva, Switzerland. Available at: hygiene in plant design. In R. Jowitt, html. Accessed 22 October 2011. http://www.iso.org/iso/home.html. (ed.). Hygienic design and operation 6. Anonymous. 2011c. How products Accessed 10 January 2012. of food plants, Horwood Ltd. are made–stainless steel. Available 16. ISO. 1984. Surface roughness — 27. Tuthill, A., and R. Avery. 1992. at: http://www.enotes.com/how- terminology — Part 2; Surface and Specifying stainless steel surface products-encyclopedia/stainless— its parameters. ISO 4287-2, Inter- treatments. Adv. Mater. Procs. 160 steel/html. Accessed 22October national Organization for Standards, (12):34–38. 2011. Geneva, Switzerland. Available at: 28. Tuthill, A. H., and R. A. Covert. 7. Anonymous. 2011d. Stainless steel. http://www.iso.org/iso/home.html. 2000. Stainless steels: An intro- Answers.com. Available at: http:// Accessed 10 January 2012. duction to their metallurgy and www.answers.com/topic/stainless- 17. Klang, L., and G. Gemmel.1984. corrosion resistance. Dairy Food Environ. San. 20(6):506–517. steel. Accessed 22 October 2011. How to combat stress corrosion 29. USDA. 2005. USDA guidelines for 8. Anonymous. 2011e. Stainless steels. cracking. Brewing & Distilling Int. 14, the sanitary design and fabrication Available at: http://www.materi- (5): 30–32, 34, 43. of dairy processing equipment. alsengineer.com/E-Stainless-Steel. 18. Maller, R. R. 1998a. Passivation of Available at www.ams.usda.gov. htm. Accessed 22 October 2011. stainless steel. Dairy Food Environ. San. 18(5): 294–298. Accessed 10 January 2012. 9. ASTM. 1997. Standard specifica- 19. Maller, R. R. 1998b. Passivation of 30. Werra, B. 1974. Metallurgy and tions for general requirements for stainless steel. Trends Food Sci. Tech- processing of stainless steel from flat-rolled stainless steel and heat nol. 9(1): 28–32. mill to finished product. J. Milk Food resisting steel plate, sheet, and strip. 20. Milledge, J. J., and R. Jowitt. 1980. Technol. 37(12):612–617. American Society for Testing and The cleanability of stainless steel Materials, West Conshohocken, PA. as a food contact surface. Inst. Food Sci. Technol. 13:57–62.

584 FOOD PROTECTION TRENDS | OCTOBER 2012