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To ensure the highest quality gas at the point-of-use (POU), efforts have focused on the selection of the appropriate materials for construction of the distribution system. Traditionally, 316 stainless steel has been employed in the construction of subcomponents for use in reactive gas distribution systems. The proper selection of the stainless melt can improve thecorrosion resistance in UHP gas systems. In the case of high purity semiconductor gas filter assemblies, it is important to specify the proper chemistries, grain size and inclusions2. In addition, the suitability of the material to be formed, mechanically polished, and electropolished (EP) must also be evaluated. The careful selection of the 316L stainless steel utilized in the fabrication of filter assemblies is critical, as materials of similar composition can perform very differently. Wang and coworkers noted variability in the corrosion behavior of steel alloys with bulk compositions that are virtually identical when exposed to moist HCl3. Smudde et al.4 observed that when moisture is below 1 ppmv, bromine from HBr is not incorporated beyond the native oxide of 316L stainless steel and no macroscopic degradation of the metal occurs. ​ Fine and coworkers confirmed the latter observations by investigating the effect of moisture content on the extent of HBr corrosion for 316L electropolished stainless steel. The scanning electron microscopy (SEM) and x-ray emmision spectroscopy (XES) analysis of the exposed sample coupons indicated no effect upon exposure to HBr containing less than 0.5 ppm of moisture. A moisture content of 10 ppm resulted in bromide incorporation and the onset of corrosion. The formation of corrosion pits was noted upon increasing the moisture level in the HBr to 100 ppm. A dense bromide scale was noted at a moisture concentration of 1,000 ppm. Corrosion is typically quantified by techniques such as trace gas analysis, change of surface morphology, particle shedding and leakage. Wang and coworkers estimated the lifetime of EP 316L stainless steel tubing in HCl service, containing 1 ppm of moisture, to be of the order of 2-3 years based on particle shedding due to corrosion. The latter value is in agreement with field experience. The extrapolated lifetime of EP 316L stainless steel tubing at various moisture concentrations is shown in Table I. The extrapolated lifetime is based on the time required to produce 10 particles/scf at a flow rate of 3.531 scfm (100 slpm). Table I Extrapolated Lifetime of 0.25 – in. 316L stainless steel tubing [10 particles/scf at 3.531 scfm (100 slm)]H2O (ppm)Lifetime5170 Days2425 Days12.3 Years0.54.7 Years0.211.6 Years0.123.3 Years A number of maintenance practices are recommended to eliminate the introduction of moisture into the corrosive gas distribution system7. It has been demonstrated that if adequate purge and evacuation procedures are followed to remove corrosive gases (such as HBr), EP 316L stainless steel can be exposed to moist air without diminishing the initial surface quality. However, if the purge and evacuation procedures are not followed, iron and bromine rich crystalline deposits form on the surface. In order to maintain higher purity in corrosive gas service, new materials of construction have been investigated as a possible replacement for 316L stainless steel. One of the materials being investigated is nickel as it is corrosion resistant in aggressive environments. Nickel, however, is also a reactive material, commonly used as a hydrogenation catalyst.  The thermal decomposition characteristics of active specialty gases on various metal surfaces were investigated by Prof. Ohmi and his group at Tohuku University8. The metal surfaces investigated included nickel, oxygen passivated 316L stainless steel, chromium passivated 316L stainless steel, and 316L stainless steel with an electropolished (EP) surface. The thermal decomposition of the active specialty gases was monitored with the aid of a gas chromatograph (GC) and a Fourier Transform Infrared Spectrometer (FTIR). The FTIR was utilized to monitor the specialty gas concentration exiting the test sample (0.25″ diameter, 1 m long 316L stainless steel tubing). In the case of phosphine, 100 ppm of phosphine in argon was passed through the 316L stainless steel tubing at a flow rate of 5 sccm. The nickel sample exhibited a strong catalytic effect on the phosphine decomposition (see Figure 1). The nickel surface reduced the phosphine concentration to undetectable levels at a temperature of 55°C. In contrast, the EP 316L stainless steel sample resulted in complete thermal decomposition of the phosphine gas at 260°C. The chromium passivated 316L stainless steel surface resulted in complete thermal decomposition at 370°C. 316L Stainless Steel Media and Nickel Media in Corrosive Gas Service The purpose of the SEMI document “Test Method for Evaluation of Particle Contribution from Gas System Components Exposed to Corrosive Gas Service” is to provide a test method to compare gas handling components for potential particle generation in corrosive gas service. The document is intended as a practical means of generating performance data for a group of components to be compared in a selection process. A flow chart of the sequence of exposure of gas handling components to corrosive service and the subsequent determination of the particle contribution is shown in Figure 3. The test sequence was used to compare the corrosion resistance of a gas filter assembly employing nickel media and a gas filter assembly employing 316L stainless steel media.

The continued increase in semiconductor device sophistication, and resultant decrease in line widths, require cleaner components throughout process gas delivery systems. Properly designed high purity gas filter assemblies are transparent to homogeneous constituents in the gas stream, i.e., to remove particulate contamination with no other effect on the inlet gas purity. In order to accomplish this goal, the assemblies used to house the filter devices are most often constructed of smooth, clean and electropolished 316 stainless steel. “Smooth and clean” is a relative term, requiring more explicit definition. Specification requirements are established whenever performance limits for filter assemblies are clearly understood and measurable. For example, moisture contribution and surface roughness are reasonably well defined. However, these criteria are based on measurements limited by the sensitivity of the best available instrumentation. In order to meet these criteria, the “best” available raw materials from which to fabricate components are often specified. The logic in simply picking the best material follows only if the material ultimately meets the intended specification requirements and if the filter assembly manufacturer understand which material properties are governing. wilsonpipeline carefully evaluates all raw materials used to fabricate semiconductor gas filter assemblies. There are many important characteristics of 316L stainless steel that must be considered in designing these high purity assemblies. Type 316L stainless steel is a well characterized and understood alloy. Its good corrosion resistance is a result of both careful formulation of the major alloying elements (Fe, Ni, Cr, Mn, and Mo), as well as the control of critical trace constituents (C, S, and Si). Similar to other finished components, filter assemblies utilize stainless steel which has been further worked, i.e., rolled, drawn, machined, or welded.  In these cases, it is not sufficient to merely specify material chemistry. Each intended use of the stainless steel may entail slight modification of the basic specification, but the most commonly specified requirements resulting in a fine, electropolished surface finish are: balanced 316L chemistry. homogeneous austenitic microstructure. very few inclusions. fine grain structure. Materials Comparison: Stainless Steel Bar HEAT ID# SPEC.–» N/A SA479 35439 Typical N/A VAR Spec.Typ.1 ELEMENTAL(%): Fe BAL. BAL. BAL. Cr 16-18 17.1 16-18 Ni 10-14 12.0 10-14 Mo 2-3 2.1 2-3 Mn 2* 1.7 <2 Si 1.0* 0.45 ≈0.5 C 0.03* 0.020 <0.03 S 0.03* 0.0020 ≤0.005 * = Maximum Additional Criteria GRAIN SIZE — 5 Not Specified2 INCLUSION TYPE (ASTM E-45) — NUMBER NUMBER Sulfides — 0.5 2.03 Alumina — 0 03 Silicates — 0 2.53 Oxides — 1.5 1.53 1 316L VAR does not specify (or result in) chemical composition which is of higher performance than SA479 with the noted exception of reduced sulfur. 2 Typical measured values were within a window of 5-10.  3 Not specified. Typical indicated. In order to remove microscopic voids in the melt, as well as to further purify the metal by reducing certain contaminant impurities, many high purity stainless steels undergo a secondary melting process (e.g. vacuum arc remelt, known as VAR). This process reduces sulfides, silicates, phosphates, aluminas, and other impurities. Of these impurities, sulfur is the most detrimental to the achievement of fine surface finishe (assuming the other impurities are within normally specified limits).  These trace impurities tend to disturb (“break”) the stable austenitic structure and form inclusions which are less noble than the parent metal. When the inclusions are infrequent and very small in size, this may not be a problem. However, when very fine surfaces (<10 microinch Ra) are desired, all inclusions become apparent after final mechanical polishing or electropolishing. It is for this reason that 100X microscopic inspection after fine mechanical polishing is commonly used to quantify the inclusion content. Electropolishing often accentuates the defects in the surface by removing the less noble inclusions first, via preferential dissolution. Grain structure must also be considered when choosing high purity stainless steel. Fine grained materials are preferable since they tend to be more homogeneous. As grains grow (e.g during slow cooling), there is more time for the austenite breakers to precipitate at the grain boundaries. This has the effect of transforming a clean dislocation of the structure (“healthy” grain boundary) into a site for inclusions. In addition, fine grained materials conduct electric current more uniformly. Uniform conductance is an important characteristic during the electropolishing process.

The specification of stainless steel bar (to BS970 EN 10088-3 ) and stainless steel tube plate (to BS1449) before 1983 covered two type 316 grades: a ‘low’ carbon with 0.03% max (316S12) and a ‘standard’ carbon with 0.07% max 316S16. Both had a molybdenum content in the range of 2.25-3.0 %. The 1983 and subsequent versions of the standard introduced new molybdenum content ranges of 2.0-2.5% and 2.5-3.0% for both the low and ‘standard’ carbon grades, thereby increasing the number of these grades from two to four: 316S11 low carbon, lower molybdenum range 316S13 low carbon, higher molybdenum range 316S31 standard carbon, lower molybdenum range 316S33 standard carbon, higher molybdenum range The chromium range was maintained across all of the grades at 16.5-18.5%, but the nickel range was adjusted to ‘balance’ the structure of these austenitic stainless steel. Nickel is an ‘austenite stabiliser’ and so is used to balance the combined effects of carbon (also an austenite stabiliser) and molybdenum which has the opposite effect as a ‘ferrite stabiliser’. The only equivalent for the old 316S12 is the 316S13, so that the minimum molybdenum of 2.25 is met. Similarly, the only equivalent for the old 316S16 is the 316S33. For atmospheric exposure service conditions, except in extreme marine environments, the substitution of either 316S11 for 316S12 or 316S31 for 316S16 should give satisfactory corrosion resistance, depending on design and finishing techniques. Pre 1983 specifications to BS970-1 and stainless steel tube plate (to BS1449) 316S12316S16% min% max.% min% maxC 0.03 0.07Si0.201.000.201.00Mn0.502.000.502.00Ni11.014.010.013.0Cr16.518.516.518.5Mo2.253.02.253.0S 0.030 0.030P 0.045 0.045 Post 1983 specifications to BS970-1 and stainless steel tube plate (to BS1449)316S11316S13316S31316S33% min% max% min% max% min% max% min% maxC 0.030 0.030 0.07 0.07Si 1.0 1.0 1.0 1.0Mn 2.0 2.0 2.0 2.0Ni11.014.011.514.510.513.511.014.0Cr16.518.516.518.516.518.516.518.5Mo2.002.502.503.002.002.502.503.00S 0.030 0.030 0.030 0.030P 0.045 0.045 0.045 0.045 Specification to EN 10088-2 and stainless steel bar (to EN 10088-3 ) In 1995 BS EN 10088-2 replaced BS1449 and BS EN 10088-3 replaced BS 970-1. The four 316 sub-grades have numerical designations as follows:Former BS designation316S11316S13316S31316S33Current BS EN numbers1.44041.44321.44011.4436 Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Stainless steel is selected for architectural application, for their corrosion resistance and potential aesthetic appearance. For internal applications other factors also important, including: – Risk of the fingermarking from contact with ‘human traffic’ (on wall cladding and lift panels) risk of scratching or staining from contact with glasses or drinks (bar tops) can routine cleaning be expected to maintain the surface appearance but not cause any unacceptable changes in appearance Selection of stainless steel grade With the exception of harsh internal environments often encountered in leisure and hydrotherapy pool buildings, grade selection is not a critical factor. For most building interiors intended for human occupation, either the ferritic 430 (1.4016) or austenitic 304 stainless steel can be considered. Depending on surface finish, there can be slight natural colour differences between them, which could influence the choice. The ferritic steel can have a blue tinge, whilst by comparison, the nickel containing 304 stainless steel has a slight yellow tinge. Surface finishes There is a wide range of possible finishes. Normally ‘ex-mill’ finishes, with the exception of the bright annealed, 2R finish, may not be considered as aesthetically suitable as textured, polished and coloured finishes. British Standard EN 10088-2:2005 defines these basic ‘special’ finishes in table 6. Specifying finishes to EN 10088-2. The finishes include: Patterned, Corrugated, Mechanically Polished or Brushed, Coloured Mechanically polished or brushed finishes This includes a wide range of possible finishes, including designations ‘G’, ‘J’, ‘K’ and ‘P’ of EN 10088-2. Brushed finishes may be better choice than ‘ground’ or ‘polished’ for preventing fingermarking. As the austenitic 304 stainless steel is not particularly hard, highly polished surfaces can be susceptible to damage. Where surface scratching is very likely on bar or counter tops, then a uniform (non-directional) polish may be better than directional finish. Unless contaminated with iron from normal carbon steel, however, scratches on stainless steel do not detract from theircorrosion resistance in such application areas. This is due to the ‘self-healing’ nature of the passive surface layer on the steel. Coloured finishes Colours including blue, black, bronze, charcoal, gold, green and red are available on 316 and 304 stainless steel types. These colours can be applied to mill, patterned, polished, bead blasted or etched surface finishes and offers a wide range of colour and texture. The colour does not contain dyes or pigments, but relies on light interference path difference effects in a chemically thickened surface passive layer. Generally colouring is done on tube sheet material before fabrication and so the scope for complex shapes or forms may be limited. Surface scratches are difficult to repair and so this range of finishes may not be suitable for high traffic areas and is better suited to cladding or roofing applications. Patterned finishes Cold rolled embossed, three-dimensional patterns to either one or both sides. These ‘2M’ finishes help mask scratches and marks and so are useful in high traffic areas, especially lift doors, fascias and lining panels. Tube Sheet with patterned rolled finishes, depending on the pattern depth, can exhibit improvements in stiffness for panel applications.

Stainless steel grades, such as the 304 (1.4301) or 316 (1.4401) types are generally suitable for storing and handling cold or unheated drinking town’s waters. Localised corrosion by crevice or pitting mechanisms is not usually a hazard in properly designed, fabricated and finished tanks handling clean waters of drinking quality. Hot water tanks however may be at risk from stress corrosion cracking (SCC). Stress corrosion cracking (SCC) risk factors. The factors that influence SCC attack are: temperature, chlorides, tensile strength, oxygen level For these types of applications chlorides and oxygen levels are fixed by the water chemistry, but chlorides can concentrate in splash zones or at the water /air line by evaporation. This can also be a hazard if external insulation to tanks becomes wet.Temperature should be fixed by the tank system controls, but hot spots can be a problem, especially if chlorides concentrate, as described. A design and fabrication method with as few ‘engineering crevices’ as possible is advisable as this reduces the risk of stress concentrations and also guards against crevice corrosion attack. Fully filled welded joints are preferable to seams with laps or mechanically fastened joints. Controlling residual stresses The main design and fabrication factor that can be ‘controllable’ is stress. Residual tensile stresses can be a cause of SCC failure. Relief of these stresses can be advisable where high level of residual stresses are possible or the application is critical. A range of treatments for the 304 and 316 type austenitic can be considered: – ‘sub-critical’ stress relief treatment, e.g. 450C and slow cool ‘full-anneal’ e.g. 1050-1100C and ‘quick’ cool (air) Stress relieving austenitic stainless steel The restraining effect around welds on austenitic stainless steels such as 304 or 316 types can also be a source of tensile stresses. If it is impractical to post fabricate heat treat, then control of welding parameters may help. These would include: – Careful pre-weld tacking Minimising heat input during welding by controlling welding speed Alternatives to the austenitic stainless steel to reduce the risk of scc failure Alternatives to the austenitic, which have nickel levels making them particularly susceptible to SCC are either: Ferritic stainless steel Duplex stainless steel These types have lower nickel levels and so are more resistant to SCC as a result of the ferrite phase present. (Higher nickel level alloys are also more resistant to SCC, but are not considered economically justifiable for these applications.) The ferritic grade 444 (1.4521) or duplex grades 1.4362/S32304 or 1.4462/2205 have been considered as alternative SCC resistant grades for hot water applications. Availability and cost in the UK could be factors prevailing against these types. Forming and welding differences from the austenitic stainless steel must also be borne in mind when making the steel selection. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Stainless steel is widely used in foods and beverage manufacturing and processing industries, bulk storage and transportation, preparation and presentation applications. Depending on the grade of stainless steel selected, they are suitable for most classes of food and beverage products. This artical includes an extenive section on stainless steel. Stainless steel used in food processing Most containers, pipework and food contact equipment in stainless steel is manufactured from either 304 or 316 type austenitic stainless steel. The 17% chromium ferritic stainless steel (type 430) is also used widely for such applications as splashbacks, housings and equipment enclosures, where corrosion resistance requirements are not so demanding. In addition to these non-hardenable austenitic and ferritic types higher strength ‘duplex stainless steel’ types, such as grades 1.4362 and1.4462 are useful for ‘warm’ conditions where stress corrosion cracking (SCC) can be a corrosion risk, such as in brewery sparge tanks. Hardenable martensitic type stainless steel is widely used for cutting & grinding applications, especially as knives. Is 316 type the only stainless steel that is classed as the ‘food’ grade The 316 grades (1.4401 / 1.4404) are often referred to as the food grades. S32101 (1.4162) is very similiar with 316. There is no known official classification for this and so, depending on the application, the equally common 1.4301 and 1.4016 grades may be suitable for food processing and handling, bearing in mind that in general terms the corrosion resistance ranking of grades can be taken as:  1.4401/1.4404 (316 types) > 1.4301 (304 types) > 1.4016 (430 types) Corrosion hazards to stainless steels in food processing If the grade of stainless steel is correctly specified for the application, corrosion should not be encountered. Surface finish and condition is very important to the successful application of stainless steels. Smooth surfaces not only promote good cleansibility but also reduce the risk of corrosion. The types of corrosion to which stainless steels can be susceptible are summarised below. This can be useful in identifying problems due to wrong grade selection or inappropriate use of equipment. Pitting Corrosion and Crevice Corrosion Both crevice corrosion and pitting corrosion occur most readily in aqueous chloride-containing solutions. Although attack can occur in neutral conditions, acidic conditions and increases in temperature promote pitting and crevice corrosion. Pitting corrosion is characterised by local deep pits on free surfaces. Crevice corrosion is occurs in narrow, solution-containing crevices or sharp re-entrant features in a structure. Examples of potential sites for crevice corrosion are under washers, flanges and soil deposits or growths on the stainless steel surface. Stress Corrosion Cracking ‘SCC’ is a localised form of corrosion characterised by the appearance of cracks in materials subject to both stress and a corrosive environment. It usually occurs in the presence of chlorides at temperature generally above 50�C. Intergranular Corrosion ‘IGC’ or ‘ICC’ (known in the past as ‘weld decay’) is the result of localised attack, generally in a narrow band around heat affected zones of welds. This is more likely to occur in the ‘standard’ carbon austenitics. The risk of IC attack is virtually eliminanted if the low carbon (0.030% maximum, eg 1.4307) or the ‘stabilised’ (eg 1.4541) types are selected. Cleaning of stainless steel equipment Effective cleaning is essential in maintaining the integrity of the process and in prevention of corrosion. The choice of cleaning method and the frequency of its application depends on the nature of the process, the food being processed, the deposits formed, hygiene requirements etc. The cleaning methods listed are suitable for stainless steel equipment. Water and SteamMechanical ScrubbingScouring Powder and DetergentsAlkaline SolutionsOrganic SolventsNitric Acid Disinfection of stainless steel equipment Chemical disinfectants are often more corrosive than cleaning agents and care must be exercised in their use. Hypochlorites Hypochlorites, chloramine and other disinfectants can liberate free chlorine, which can cause pitting. Sodium hypochlorite or potassium hypochlorites are often used in commercial sterilising agents. If these substances are used with stainless steel, the duration of the treatment should be kept to minimum and followed by thorough rinsing with water. At higher temperatures, chloride-containing sterilising agents should not be used with stainless steel. Milton solutions (hypochlorite & chloride) can be very aggressive to stainless steels. Tetravalent ammonium salts Tetravalent ammonium salts are much less corrosive than hypochlorites, even when halogens are present in their formulation. Iodine Compounds Iodine compounds may be used for the disinfection of stainless steel. Nitric acid Even at low concentrations, nitric acid has a strong bactericidal action and can be a low cost disinfectant for stainless steel equipment, especially in dairies and pasteurising equipment. Maintenance of food process equipment Stainless steel equipment often contains gaskets or other components that can absorb or retain fluids. These liquids may be become concentrated by evaporation and corrosion may ensue. Equipment should be disassembled occasionally for thorough cleaning. If the disassembled equipment exhibits corrosion (crevice corrosion usually), then the corroded surfaces should be cleaned. Typical applications of the various stainless steel types TypesTypical Applications420 (martensitic)Cooks and professional knives, spatulas etc430 (ferritic)Table surfaces, equipment cladding, panel (ie components requiring little formability or weldability). Used for moderately corrosive environments (e.g. vegetables, fruits, drinks, dry foods, etc).304(austenitic)Vats, bowls, pipework, machinery parts (i.e. components requiring some formability or weldability). Corrosion resistance superior to 430.316(austenitic)Components used with more corrosive foods (e.g. meat/blood, foods with moderate salt contents), which are frequently cleaned, with no stationary solids and not under excessive stress.904L 1.4539(austenitic)Used with corrosive foods (e.g. hot brine with solids that act as crevice forms, stagnant and slow moving salty foods).1.4462(duplex)Used with corrosive foods (e.g. hot brine with solids, stagnant and slow moving salty foods). Higher strength than austenitics. Good resistance to stress corrosion cracking in salt solutions at elevated temperatures.6%Mo. types (austenitic)Used with corrosive foods (e.g. hot brine with solids, which act as crevice formers, stagnant and slow moving salty foods). Good resistance to stress corrosion cracking in salt solutions at elevated temperatures. Used in steam heating and hot work circuits, hot water boilers, etc Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Stainless steel is selected for architectural applications, as with most other applications, for their corrosion resistance. This is usually the prime consideration. Environmental factors such as temperature and humidity need to be taken into account, but the location of the proposed site is the initial consideration. The Nickel Institute’s ‘Stainless Steel in Architecture, Building and Construction Guidelines for Corrosion Prevention’ publication categorizes sites as either: Rural,Urban,Industrial,Marine Definitions of sites Rural sites are defined as unpolluted, inland sites away from industrial atmospheres or discharges. Urban sites are defined as residential, commercial or light industrial areas with non-aggressive airborne pollution, typically from road traffic (exhaust fume and winter road salt spray may be issues). Industrial sites are typified by airborne pollution such as sulphur dioxide or gases released from chemical process plants, which can form potentially dangerous acid condensates. Marine sites are defined as areas where windborne sea spray or mist may be present. These contain chlorides which can also concentrate in condensates or as surface moisture evaporates. Local micro-climates and changes to the enviroment The environment cannot usually be defined precisely in these terms and it also important to bear in mind that environmental changes may occur during the design life of a proposed building ie. is the environment getting more polluted or cleaner, for any given location? Additionally ‘micro-climates’ can influence the general categorisations and may be worth investigating for any proposed site before a final stainless steel grade selection is made. Microclimates can exist in coastal locations or near chemical plant chimneys, where unexpected acid condensates can form. Sub-pisions of the ‘site-types’ should also be considered. Low temperatures and low humidity reduce the risks of corrosion and can mean that a steel grade perhaps not thought suitable for a particular site may be worth considering. Selection of stainless steel grades Selection guidelines are summarised in the table. Only the ‘common’ 304 (1.4301) and (1.4401) 316 stainless steel types are considered as candidates for most UK sites. .RuralUrbanIndustrialMarine.LMHLMHLMHLMH31633332222122130422222111X21X The ‘local’ conditions are defined as: ConditionsLLeast corrosive conditions e.g. low humidity and low temperaturesMTypical atmospheric conditions for the site typeHHarsh atmospheres, typified by persistent high humidity, high temperature or high levels of pollution The performance ratings are defined as: Performance Rating3Probably over-specified, for corrosion resistance requirements and cost2Probably the best choice for corrosion resistance and cost1Worthy of consideration if precautions are taken (i.e. good standard of surface finish and regular cleaning specified)XLikely to suffer severe corrosion This shows that the 304 (1.4301) type can be considered for most sites, except either heavily polluted industrial sites or most marine sites. In these cases the (1.4401) 316 stainless steel types should be the preferred choice. Life expectancy for stainless steels in external environments Natural rain washing of the items should be considered an advantage, as the corrosion risk from pollutants or condensates is reduced. Similarly, exposed sections are less likely to hold condensation due to the improved natural ‘ventilation’ available to the steel surfaces. Additional factors for consideration Other important factors in stainless steel selection are: – Surface finishDesignFabrication methodsAccessibility for cleaning and maintenanceMechanical properties and physical properties of stainless steels. Surface finish As a general rule, the smoother the finish, the better the corrosion resistance. Selection of polished surface finishes often requires a considerable amount of work before a final agreement is reached. This may involve having swatch samples prepared and agreed by the specifying parties. Polished finish K of BS EN 10088-2 is noted in the standard, Table 6 as being intended for external architectural applications, but is only one of many options. Highly reflective finishes may not be advisable especially for roofs, as this could be a hazard to air traffic on buildings near airports or on flight paths. Alternative dull finishes have been developed for such applications. Reflective finishes can be used to advantage however to reflect light into dark, enclosed courtyard areas of buildings. Patterned finishes are better for hiding scratches and fingermarks in ‘high traffic’ areas. Coloured finishes are also available for special aesthetic affects. Design Crevices must be avoided, as these can be sites for localised corrosion. Fabrication methods and corrosion hazards Fabrication methods that avoid crevices should be considered. Mechanical fixings can introduce crevices both at the fastener and at the lapped metal joint. Aluminium fasteners (e.g. rivets) should be avoided for securing stainless steel panels, as galvanic corrosion to the aluminium can be a problem in harsh environments. Avoid moisture traps at any mechanically fastened joints. Contact with lead or copper should not result in galvanic corrosion, but staining to stainless steel parts from the patina may be visible if rain water drains over the stainless steel. Sealants can be considered to avoid such problems. Adhesive bonding, if mechanically strong enough, usually eliminates such problems. Welds should be full seam welds, rather than intermittent fillet welds. Compatible welding consumables should be specified with full penetration weld designs, where possible. Iron contamination during storage and erection MUST be avoided. This is a common cause of unnecessary rust staining and attendant remedial post hand-over costs. Mortar cleaning (hydrochloric) acids must not be allowed to come into contact with stainless steels. Accessibility for cleaning and maintenance Periodic cleaning is advisable on stainless steel, as with most building exterior materials. The frequency will depend on local conditions and the ‘visibility’ of the steelwork. Where cleaning and maintenance is difficult or costly, e.g. on the outside of high rise buildings, then a more resistant grade selection than suggested by the tables may be appropriate. Mechanical and physical properties of stainless steels The mechanical properties of the commonly used 304 and 316 stainless steel types do not usually present a cause for concern. The thermal expansion rates of these grades however is about a third as much again as most steels. i.e. around 16 x 10-6 /C compared to around 12.2 x 10-6/C for carbon steels. Expansion joint allowances must account for this to avoid thermal buckling problems and any sealants used must be compatible.

The raw material cost for a particular component may be 20 times the cost if made from one material compared to another on a weight to weight basis. However the lifetime costs may be very similar if all of the other factors are also taken into consideration. The material cost of a mass produced investment casting item may be 80% of the final cost. The material cost of a single complicated machined item may be less than 10% of the final cost. It is not possible to provide cost comparisons between different metals to any level of accuracy. Each metal is varying in price on a day to day basis and different alloys of the same metal can have significantly different costs. A grade 7 titanium alloy costs twice as much as pure titanium (grade 1,2 or 3). Comparing costs should only be based on final installed costs. eg. for a domestic, industrial piping system a screwed steel system would cost about 40% more than a copper piping system. Example : The price of a titanium / titanium alloy products results from a number of factors: Alloying grade .some grades e.g with Pd alloying component, can significantly increase the price of the alloy. The purity of the grade… the more pure the higher the cost The test and inspection requirements; The procured quantities. The more ordered the lower the specific cost The geometry ..rolling or forging affects prices per volume or weight Demand ..e.g High defence demand for aerospace industry can result in higher metal prices Local economy.. Metal availability In year 2000 the price of titanium was about 13 000 to 43 000/tonne.. In 2002 the price of raw titanium was about to 8960/tonne. In 2005 to-date the price of titanium has varied between 6000 and 9000 /tonne Ref 2010 ..I have enclosed a chart from the metalprices webpage to illustrate to range of titanium ingot pricing over a 12 month period Table showing relative metal costs The table below can only really be used to give broad relative initial material costs.  The figures are based on a reference source originating about 2002 MaterialDensityCost/tonneRelativeCost /m3Relativekg/m3/tonne/tonne/m3/m3Carbon Steel7820550143011,0Alloy Steels78208301,516490,61,5Cast Iron72258301,515996,751,4Stainless Steel778044508,134 6218,0Aluminium alloys270022204,059941,4Copper Alloys8900555010,149 39511,5Zinc alloys710022204,015 7623,7Magnesium alloys180040007,372001,8Titanium alloys450017 00030,976 50017,4Nickel alloys890018 00032,7160 20036,8 Current Metal Prices.. November 2010 I have tried to obtain some current (Nov 2010) metal prices from various internet sources and I list them below..These sometimes differ considerably from the table above MaterialCost/tonneRelative Cost (weight)Relative Cost (volume)/tonneSteel (Billet) LME-Nov-201032111Steel (Hot Rolled Plate)-MEPS-July-20105051,61,6304 Steel (Hot Rolled Plate)-MEPS-July-20102 5367,97,9316 Steel (Hot Rolled Plate)-MEPS-July-20103 5351111Tin- LME-Nov-201015 4584845Aluminium Alloy – LME-Nov-20101 4074,.41,5Aluminium – LME-Nov-20101 4254,41,5Copper – LME -Nov-20105 27916,418,7Zinc – LME -Nov-20101 4124,44,0Nickel – LME -Nov-201014 3984451Lead – LME-Nov-20101 4144,46,4Titanium (ingot 6AL-4V) steelonthenet (11$/lb)15 7004928 Prices in dollars for 25mm round bar x 300mm long (0,00015m3 ) …November 2010 (Onlinemetals (USA)MetalPrice $ 1000/tonne 1000/m3 Relative cost (weight)               Relative cost (volume)                 Steel HR (A56)63,224,711Steel CD (12L40)7,43,930,51.231.23Alloy Steel (4130)9,995,341,21.661.66St.Stl (304L)15,348,163,22.62.56St.Stl (316L)20,6110,984,93.43.44Aluminium (2011-T3)8,5813,135,44.21.43Copper (C110)37,9117,5156,25.66.32Brass (C360)25,311,7104,33.74.22Bronze42,0919,5173,56.27.0Titanium (6AL-4VG5)107984413117.8 Comparison of raw bulk material costs above with actual metal stock prices Looking at the cost per tonne of HR steel in the form of a 1″ dia round bar 1 foot long at 3200 against the LME price for the raw material in bulk for at 321/tonne illustrates the massive diiference in the price of raw materials to the actual price of materials as supplied for initial machining processes in small quantities.

Stainless steel is probably the least understood. Selecting the right stainless steel for your application can be perplexing. Here we tell you differences between various stainless steel alloy. Hope help you to select the most appropriate material for specific application. The focus of this issue is on the stainless steel and related alloy that combine a high degree of hardness,corrosion resistance, low cost and availability. It is intended for the designer need to specify material on the drawing. Stainless Steel 52100 The alloy is homogeneous so that all of the chromium is available for corrosion resistance. The alloy melts clean and heat treats beautifully so high cycle fatigue strength is good and the alloy supports high rolling contact stress. Good quality stock is easily purchased. QC requires checking decarb in as-received stock and after heat treating checking decarb and grain size. Stainless Steel 440C It is hard. The alloy has big, blocky, primary carbides. The matrix chromium is only a little higher than H13’s so its corrosion resistance is only a little better than H13. The primary carbides make for noisy rolling contact, markedly lower rolling contact fatigue strength, fairly good compressive strength, and poor tensile properties. There are very consistent supplies of good quality. QC requires monitoring the heat treater’s results for excessive austenite grain boundary precipitation, prior austenite grain size, primary carbide particle size and retained austenite. Stainless Steel H13 Not as clean as 52100 so its rolling contact fatigue strength isn’t quite as high but its higher chromium gives pretty goodcorrosion resistance. Hard to buy good quality and hard to heat treat well. QC requires monitoring incoming steel for segregation and monitoring the heat treater’s results for austenite grain size and precipitation in the austenite grain boundaries. Stainless Steel 17-4PH This compromise has better corrosion resistance, good toughness when properly heat treated, and lower hardness. Coarse grain can be a problem. Commercial stocks seem to be good quality. QC requires monitoring the heat treater’s results for austenite grain size, precipitation in the austenite grain boundaries and through-thickness hardness. Hardness after the solutionizing heat treatment needs to be checked, which is the usual as-purchased condition for small quantities. Segregation can be a problem. Stainless Steel 420 The alloy has good corrosion resistance and pretty good hardness. However, it heat treats with a coarse grain, which makes it somewhat unpredictable and reduces the fatigue strength and rolling contact stress. It is moderately easy to purchase good quality. QC requires monitoring segregation and grain size after heat treating. Hard worked Stainless Steel 304  High hardness and strength with pretty good corrosion resistance are available in heavily cold worked type 304. It has much less toughness than annealed 304 but it is right up there with the other hard stainless steel tube. Wire, small stainless stel bars and small strip dimensions are available. Quality control includes surface finish, which can be scaly on a microscopic scale. Other hard stainless steel include: 416 for improved pitting resistance 440A and 440B for better homogeneity than 440C but still suffer from primary carbides 17-7PH for more of the same as compared to 17-4PH The growing family of fully densified powder metals gives great opportunities for combining all the properties except affordability. In a critical application, where the metal cost can be absorbed, be sure to look over what’s available. We see these fully densified powder metals in a frustratingly limited range of small stainless stel bar stock sizes. An example of compacted powder metal stainless stel bar stock is Crucible Materials Corporation’s CPM T440V which has 17% chromium along with other good things. The powder process keeps the carbides fine as long as the heat treater doesn’t mess it up during final heat treating after machining. The heat treater’s results must be monitored for carbide and grain size or the product isn’t any better than the regular ingot and strand cast mill product.

Stainless steel is defined as iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties, but fundamentally, stainless steel tubes are considered for selection as steels with corrosion resistant properties. In economic terms they can compete with higher cost engineering metals and alloys based on nickel or titanium, whilst offering a range of corrosion resisting properties suitable for a wide range of applications. They have better strength than most polymer products (GRP), are readily repairable and ‘recyclable’ at the end of their useful life. When considering stainless the most important features are: Corrosion resistance or oxidation resistanceMechanical properties & physical propertiesAvailable forming, fabrication & joining techniquesEnvironmental & material costs (including total life cycle cost) The basic approach is to select a grade with as low a cost as possible, but the required corrosion resistance. Other considerations such as strength and hardenability are secondary. Corrosion resistance Chromium (Cr) content sets stainless steel apart from other steel. The unique self-repairing ‘passive’ surface layer on the steel is due to the chromium. Commercially available grades have around 11% chromium as a minimum. These can be either ferritic or martensitic, depending on carbon range control. Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 1.4016 ferritic would be expected to be an improvement over the 410S 1.4000 types. Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 1.4021/ 1.4028 types. Chromium levels over 20% provide improved ‘aqueous’ corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade. In addition to this basic ‘rule’, nickel (Ni) widens the scope of environments that stainless steels can ‘handle’.The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally. Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%. The corrosion resistance is not simply related to nickel level however. It would be wrong to assume that a 304 (1.4301) with its 8% Ni therefore has better corrosion resistance that a 1.4462 duplex with only 5% Ni. More specific alloy additions are also made with the specific aim of enhancing corrosion resistance. These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N. Copper is also used to enhance corrosion resistance in some ‘common’, but hazardous, environments such as ‘intermediate’ concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type. Mechanical Properties and Physical Properties Basic mechanical strength increases with alloy additions, but the atomic structure differences of the various groups of stainless steels has a more important effect. Only the martensitic stainless steels are hardenable by heat treatment, like other alloy steels. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism to the martensitic types. The ferritic, austenitic and duplex types cannot be strengthened or hardened by heat treatment, but respond to varying degrees to cold working as a strengthening mechanism. Ferritic stainless steel types have useful mechanical properties at ambient temperatures, but have limited ductility, compared to the austenitics. They are not suitable for cryogenic applications and lose strength at elevated temperatures over about 600, although have been used for applications such as automotive exhaust systems very successfully. Austenitic stainless steel types, with their characteristic face centred cube ‘fcc’ atomic arrangement, have quite distinct properties. Mechanically they are more ductile and impact tough at cryogenic temperatures. The main physical property difference from the other types of stainless steel is that they are ‘non-magnetic’ i.e. have low relative magnetic permeability’s, provided they are fully softened. They also have lower thermal conductivity’s and higher thermal expansion rates than the other stainless steel types. Duplex stainless steel types, which have a ‘mixed’ structure of austenite and ferrite, share some of the properties of those types, but, fundamentally are mechanically stronger than either ferritic or austenitic types. Forming, fabrication and joining techniques Depending on their type and heat-treated condition, wrought stainless steels are formable and machinable. Stainless steels can also be cast or forged into shape. Most of the available types and grades can be joined by use of appropriate ‘thermal’ methods including soldering, brazing and welding. Austenitics are suitable for a wide range of applications involving flat product forming (pressing, drawing, stretch forming, spinning etc). Although ferritics and duplex types are also useful for these forming methods, the excellent ductility and work hardening characteristic of the austenitics make them a better choice. Formability of the austenitic types is controlled through the nickel level. The 301 (1.4310) grade which has a ‘low’ nickel content, around 7% and so work hardens when cold worked, enabling it to be use for pressed ‘stiffening’ panels. In contrast nickel levels of around 8.5% make the steel ideally suited to deep drawing operations, for example in the manufacture of stainless steel sinks. Martensitics are not readily formable, but are used extensively for blanking in the manufacture of cutting blades. Most stainless steel types can be machined by conventional methods, provided allowance is made for their strength and work hardening characteristics. Techniques involving control of feed and speed to undercut work hardening layers with good lubrication and cooling systems are usually sufficient. Where high production volume systems are employed, machining enhanced grades may be needed. In this respect, stainless steels are treated in similar ways to other alloy steels, sulphur additions being the traditional approach in grades like 303 (1.4305). Controlled cleanness types are now also available for enhanced machinability. Most stainless steels can be soldered or brazed, provided care is taken in surface preparation and fluxes are selected to avoid the natural surface oxidising properties being a problem in these thermal processes. The strength and corrosion resistance of such joints does not match the full potential of the stainless steel being joined, however. To optimise joint strength and corrosion resistance, most stainless steels can be welded using a wide range of techniques. The weldablity of the ferritic and duplex types is good, whilst the austenitic types are classed as excellent for welding. The lower carbon martensitics can be welded with care but grades such as the 17% Cr, 1% carbon, 440 types (1.4125) are not suitable for welding. Summary of the main advantages of the stainless steel types TypeExamplesAdvantagesDisadvantagesFerritic stainless steel410S,430, 446Low cost, moderate corrosion resistance & good formabilityLimited corrosion resistance, formabilty & elevated temperature strength compared to austeniticsAustenitic stainless steel304,316Widely available, good general corrosion resistance, good cryogenic toughness. Excellent formability & weldabilityWork hardening can limit formability & machinability. Limited resistance to stress corrosion crackingDuplex stainless steel1.4462Good stress corrosion cracking resistance, goodmechanical strength in annealed conditionApplication temperature range more restricted than austeniticsMartensitics420, 431Hardenable by heat treatmentCorrosion resistance compared to austenitics & formability compared to ferritics limited. Weldability limited.Precipitation hardening17/4PHHardenable by heat treatment, but with bettercorrosion resistance than martensiticsLimited availability, corrosion resistance, formability & weldability restricted compared to austenitics Other ‘Families’ of stainless steel There is a wide range of stainless steel types. Special grades with enhanced compositions have been developed and are available that minimise the short comings of any particular type. These include: –Super ferritic stainless steels Super austenitic stainless steelsDuplex stainless steelsLow carbon weldable martensiticsAustenitic precipitation hardening types

Austenitic stainless steel grades are those alloys which are commonly in use for stainless steel applications. The austenitic stainless steel grades are notmagnetic. The most common austenitic alloys are iron-chromium-nickel steel and are widely known as the 300 series. The austenitic stainless steel pipe, because of their high chromium and nickel content, are the most corrosion resistant of the stainless steel group providing unusually fine mechanical properties. They cannot be hardened by heat treatment, but can be hardened significantly by cold-working. Straight Grades The straight grades of austenitic stainless steel pipe contain a maximum of .08% carbon. There is a misconception that straight grades contain a minimum of .03% carbon, but the spec does not require this. As long as the material meets thephysical requirements of straight grade, there is no minimum carbon requirement. “L” Grades The “L” stainless steel grades are used to provide extra corrosion resistance after welding. The letter “L” after a stainless steel pipe type indicates low carbon (as in 304L). The carbon is kept to .03% or under to avoid carbide precipitation. Carbon in steel when heated to temperature in what is called the critical range 800 degrees F to 1600 degrees F precipitates out, combines with the chromium and gathers on the grain boundaries. This deprives the steel of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By controlling the amount of carbon, this is minimized. For weldability, the “L”stainless steel grades are used. You may ask why all stainless steel are not produced as “L” stainless steel grades. There are a couple of reasons: “L” stainless steel grades are more expensive Carbon, at high temperatures imparts great physical strength Frequently the mills are buying their raw material in “L” stainless steel grades, but specifying the physical properties of the straight grade to retain straight grade strength. A case of having your cake and heating it too. This results in the material being dual certified304/304L; 316/316L, etc. “H” Stainless Steel Grades The “H” stainless steel grades contain a minimum of .04% carbon and a maximum of .10% carbon and are designated by the letter “H” after the alloy. People ask for “H” stainless steel grades primarily when the material will be used at extreme temperatures as the higher carbon helps the material retain strength at extreme temperatures. You may hear the phrase “solution annealing”. This means only that the carbides which may have precipitated (or moved) to the grain boundaries are put back into solution (dispersed) into the matrix of the metal by the annealing process. “L” stainless steel grades are used where annealing after welding is impractical, such as in the field where stainless steel pipe and fittings are being welded. Type 304 The most common of austenitic stainless steel grades, containing approximately 18% chromium and 8% nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the milder chemicals. Type 316 Contains 16% to 18% chromium and 11% to 14% nickel. It also has molybdenum added to the nickel and chrome of the 304. The molybdenum is used to control pit type attack. Type 316 is used in chemical processing, the pulp and paper industry, for food and beverage processing and dispensing and in the more corrosive environments. The molybdenum must be a minimum of 2%. Type 317 Contains a higher percentage of molybdenum than 316 for highly corrosive environments. It must have a minimum of 3% “moly”. It is often used in stacks which contain scrubbers. Type 317L Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra corrosion resistance. Type 317LM Requires molybdenum content of 4.00% min. Type 317LMN Requires molybdenum content of 4.00% min. and nitrogen of .15% min. Type 321  Type 347 These types have been developed for corrosive resistance for repeated intermittent exposure to temperature above 800 degrees F. Type 321 is made by the addition of titanium and Type 347 is made by the addition of tantalum/columbium. These stainless steel grades are primarily used in the aircraft industry. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

316 types stainless steel are used widely in marine application, but their corrosion resistance in contact with seawater is limited and they cannot be considered ‘corrosion proof’ under all situations. They are susceptible to localized attack mechanisms, principally crevice and pitting corrosion. This limits the scope for the use of these steels in seawater contact. Type 304, and more especially the free machining 303 types, should not be considered suitable for seawater service. Sulphide inclusions outcropping on the surface of the 303 type are preferential pitting corrosion sites. Factors governing the corrosion resistance of 316 types in seawater The factors governing the corrosion resistance and hence suitability of the 316 types has been well documented by many workers in these fields of research. These factors work together and include Water quality Flow rates Temperature Oxygen levels Cathodic protection Seawater quality The chloride levels can vary depending on the location and influence of tides. The levels encountered in even ‘brackish’ waters are above those where crevice corrosion can be expected to be a corrosion hazard. Intermittent exposure, for example in tidal zones, has been noted as less of a corrosion risk. This may be due to the fact that the steel surfaces are effectively ‘washed’ by the changes in water levels. Water evaporation effects could however increase the corrosion risks in splash zones, if the chlorides concentrate in a damp or wet environment. It is important not to let seawater stand in contact with the steel unnecessarily. Horizontal 316 tube sections handling seawater have been noted to fail by pitting after only short periods. Free draining surfaces and the avoidance of horizontal tube runs are important to the successful use of 316 in contact with seawater. If tubing systems are hydro-tested using seawater, this must be drained and flushed immediately after the test period. Failure to do this has resulted in corrosion to 316 systems. Water flow rates ‘High’ flow rates are preferable (ie over 1 metre / second). Slow moving water can encourage biofouling, which can then result in shielding or crevice corrosion. Stagnant seawater conditions must be avoided. Increases in flow rates reduce the risk of corrosion and so applications such as pumps, can be successful applications for 316 types in seawater handling. Water temperature The crevice corrosion risk increases with temperature. Contact with heated seawater is not advisable. Ambient temperatures in northern European waters, as a guide, are around the maximum that a 316 should be expected to cope with, even if other conditions are favorable. Stress corrosion cracking is not usually a concern in the temperatures that the 316 would be used at. (Higher temperatures would probably result in crevice and pitting corrosion anyway) Water oxygen levels (deaeration) Stainless steel rely on a source of oxygen to maintain their passive condition. Aerated seawater however can be more corrosive than de-aerated seawater. It has been found that very low levels of oxygen, such as those found at sea depths of around 200 metres, make seawater less aggressive. This is associated with the slowing down of pitting corrosion rates. Cathodic protection Cathodic protection can be applied ie electrically or derived from contact with less ‘noble’ metals, including carbon steel and aluminium. Direct contact with these metals can help improve the resistance of the 316 types of stainless steel, at the expense of the other metal. Although the stainless steel can benefit, there may be a concern that the overall durability of a fabrication involving such combinations could be compromised. ‘Engineered’ crevices (surface finish and post fabrication cleaning) Crevice and the closely related pitting corrosion mechanisms are the forms of local attack that are normally responsible for the failure of the 316 types in seawater service. Any form of crevices must be avoided. These can occur through Design geometry (sharp corners or grooves)Flanged joints with gasketsMechanical fastening systems Intergranular corrosion has been detected on laboratory sensitized (heat-treated) 316 when subsequently exposure in seawater. The use of the low carbon 316L types such as grades 1.4404 or 1.4432 should avoid this additional corrosion risk in welded structures. The surface finish weld quality and finishing of the steel can be important factors in the successful use of 316 types in seawater service applications. These may be more important issues than factors such as the actual chloride concentration. Smooth, clean finishes and well-finished welded joints contribute to the corrosion resistance of the steel.