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Stainless steel react with corrosive deterioration phenomenon is known as corrosion. Common metal corrosion of intergranular corrosion, pitting corrosion, stress corrosion, fatigue, corrosion and crevice corrosion. Intergranular corrosion  Intergranular corrosion of stainless steel corrosion in a particular medium of steel along the grain boundary of a local selective corrosion occurred. If this corrosion is the metal surface, will form micro-cracks and deep inside, until the cause breakage. In certain corrosive media (such as nitric acid, phosphoric acid, sulfuric acid, lactic acid, formic acid, hydrofluoric acid and copper sulfate, etc.), will occur along the grain boundary corrosion of stainless steel. There exist in the case of stress, intergranular corrosion may develop as intergranular stress corrosion cracking. To avoid corrosion, generally use ultra-low carbon (ω (C) ≤ 0.03%) austenitic stainless steel, duplex stainless steel and containing Titanium, Nb stabilized austenitic stainless steel. Pitting corrosion  Point, also known as pitting corrosion, is the most common form of localized corrosion. It is because the metal surface to a partial role in the micro-cell, some corrosion holes, to depths of development. Presence of impurities in the stainless steel surface, dirt and bug parts to rust or damage of passive film on stainless steel used in seawater, that there will be pitting. Pitting and pitfalls of large destructive. Corrosion occurred, although the weight loss is not metal, but the anode area is very small, the anode corrosion current density flowing through a large, resulting in higher metal dissolution rate, serious damage can perforated metal equipment. Pitting also make intergranular corrosion, erosion, stress corrosion cracking and corrosion fatigue increased, in many cases the origin of these localized corrosion. To prevent the occurrence of pitting corrosion of stainless steel (pitting), should be selected with high chromium, nickel, molybdenum, nitrogen stainless steel, to improve the purity and lower stainless steel non-uniformity. Choose strong passivation material and passivation of stainless steel to prevent corrosion of effective measures point. Crevice corrosion  Under the action of the corrosive medium, stainless steel crevice corrosion, the crevice corrosion. Crevice corrosion is generally based on the shape of the gap has a different shape. Minor, the crevice corrosion in general, more serious point to sheet erosion or ulcers form.  All can cause corrosion of the media can cause crevice corrosion. If the surface of stainless steel and non-metallic inclusions in the metal (metal micro-materials, dust, dirt, sand material, marine life), or structural reasons, such as screws riveting, riveting, gaskets and other non-metallic contacts formed gap, in these cases and corrosion of stainless steel parts, if the media contact, so that crevice corrosion occurs. Especially in the Cl-of environment, crevice corrosion is most common.  Crevice corrosion of stainless steel was mainly due to acidification of the crevice solution, hypoxia caused by passive film damage. Prevention of crevice corrosion of stainless steel so strong measures are: choice of high chromium, nickel, molybdenum, nitrogen stainless steel, to improve the stability of passive film and passivation, re-passivation ability to improve the purity of stainless steel, and stainless steel is not reduced uniformity. Stress corrosion  Stress corrosion of stainless steel in the permanent tensile stress (including the external load, thermal stress and cold, hot working or welding residual stress after, etc.) and specific joint action of the corrosive medium appears brittle cracking. Local corrosion damage of stainless steel, it is the most common form of the most damaging kind of corrosion.  Characterized by stress corrosion of stainless steel corrosion cracks or faults occur, the origin point of fracture is often the corrosion pits or corrosion holes in the bottom; cracks have spread along the grain boundaries, transgranular and mixed three, usually perpendicular to the stress of the main crack direction, mostly branching; sharp crack tip, crack walls and metal surfaces are usually very minor corrosion, cracking the end of the proliferation of fast fracture has the characteristics of brittle fracture. Lead to stress corrosion of stainless steel is the most common media containing Cl-and oxygen atmosphere and industrial water, seawater, etc., in the tensile stress exceeds the critical value (including stress) and high temperature result of the role.  Cr-Ni austenitic stainless steel caused by stress corrosion common media are:  1. various chloride or a solution containing chloride.  2. water, salt water, river water, well water, water, solar terms, and marine atmosphere.  3. hydroxide, such as NaOH, KOH aqueous solution.  4. nitric acid and nitrate; HNO3 + HF and HNO3 + HCl + HF acid lotion.  5. hydrofluoric acid, fluorine-containing F-acid aqueous solution.  6. sulfate and sulfite; even more sulfuric acid; hydrogen sulfide solution.  For stress corrosion cracking, usually can be controlled through proper selection.  1. chloride in high concentration, usually choose a high nickel content stainless steel, high silicon Cr-N stainless steel and ferritic stainless steel.  2. chloride in the intergranular stress corrosion environment, the option with Ti, Nb austenitic stainless steel (by stabilizing treatment), control of ultra-low carbon or nitrogen austenitic stainless steel.  3. chlorine ions in aqueous solution, is lower than 60 ℃ when the low concentration of chloride ions medium, no enrichment or enrichment can be used 18-8,18-12-2 type austenitic stainless steel, AISI 444 ferritic stainless steel and 18-5-Mo duplex stainless steel; lower than 60 ℃ in the low concentration of chloride ions medium (concentration, enrichment), the choice of 444 ferritic stainless steeland 18-5-Mo Double phase stainless steel; in less than 60 ℃, the medium of high concentration of chloride ions (with concentration, enrichment), the choice of XM27, AISI 444 ferritic stainless steel, and ω (C) 22% ~ 25% and Mo containing duplex stainless steel ; in the 60 ~ 200 ℃ low concentration of chloride ions medium (concentration, enrichment), the choice of type 18-5 ,22-5 ,25-5 with Mo duplex stainless steel and high Cr, Mo, Ni stainless steel 904L UNS N08904 at 200 ~ 350 ℃ low concentration of chloride ions (concentration, enrichment) medium, can be used Incoloy 800 iron – nickel alloy, Inconel 69 nickel-based alloys.  4. aqueous solution containing NaOH, when ω (NaOH) ≤ 20%, non-chlorine sub-boiling temperature ≤ 120 ℃, optional or 18-12-2 type 18-8 austenitic stainless steel; when ω (NaOH) ≤ 50%, ω (NaCl) = 2.5%, NaOH aqueous solution at 85 ℃, you can use ultra-low carbon austenitic stainless steel 18-8, UNS S44629 ferritic stainless steel.  5. duplex stainless steel especially for the point of corrosion (pitting) caused by stress corrosion cracking of the occasion, such as S32750, S32205 S31803 has an excellent resistance to stress corrosion properties of duplex stainless steel. Fatigue corrosion  Corrosion fatigue of stainless steel parts in the corrosive medium and the combined effect of alternating stress corrosion occurs thereby causing part of the damaged. Characterized by fatigue, corrosion pits produced a large number of cracks, so the metal mechanical fatigue limit ceased to exist; crack mostly through the grain, generally unbranched; crack tip blunt; fracture of corrosion products covered most of the small department was brittle fracture damage.  Activity causes fatigue, corrosion media are acidic medium, chloride, containing H2S, SO2 and O2 to produce gas and other corrosive media.  In order to prevent the generation of fatigue, stainless steel should have a good pitting resistance and high strength, it should be selected with Cr, Mo high stainless steel, super austenitic stainless steel, and high chromium, molybdenum and nitrogen content of the double phase stainless steel typically has: S31254 1.4547, S31050 1.4466, S32750 and so on.  Of course, the corrosion resistance of stainless steel in the selection process, according to the actual situation, analyze the causes of corrosion and corrosion characteristics under test, summed up best and then select the good corrosion resistance, economically rational, the market has stainless steel material. 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The ‘appropriate grades’ notes are taken from the Avesta Sheffield Corrosion Handbook and are shown for general guidance only. Specific grade selection should be rechecked with corrosion tables. A link to the SST ORG web site article that describes where to find the corrosion tables on the Stainless steel tube corrosion can be found in the Related Links section of this article. The table is referenced by the common name. Common Chemical NamesCommon nameChemical nameFormulaGrade selectionalumpotassium aluminiumsulphateKAl(SO4)2304 or 316 useful at all concentrations, up to 50Caqua fortisnitric acidHNO3Selection of stainless steels for handling nitric acid (HNO3)aqua regianitric and hydrochloric acid mixtureHNO3 and HClLikely to attack most stainless steel, This mixture is used for dissolving gold.bleachsodium hypochloriteNaOClSelection of stainless steels for handling sodium hypochlorite (NaOCl) – Bleachcaustic potashpotassium hydroxideKOHsimilar selection as for sodium hydroxidecaustic sodasodium hydroxideNaOHSelection of stainless steels for handling sodium hydroxide (NaOH)chromic acidchromium trioxideCrO3304 or 316 useful up to 40%, 20Cethanol (alcohol)ethyl alcoholC2H5OH304 or 316 useful at all concentrations, up to boiling pointglycolethylene glycol (anti-freeze preparations)C2H4(OH)2304 or 316 useful at all concentrations at ambient (or lower) temperaturesgypsumcalcium sulphateCaSO4304 or 316 useful at all concentrations, up to boiling pointJavelle (javel) watersodium hypochloriteNaOClSelection of stainless steels for handling sodium hypochlorite (NaOCl) – BleachLabarraque’s solutionsodium hypochloriteNaOClSelection of stainless steels for handling sodium hypochlorite (NaOCl) – Bleachmarine acidhydrochloric acidHClSelection of stainless steels for handling hydrochloric acid (HCl)methanol (alcohol)methyl alcoholCH3OH304 or 316 useful at all concentrations, up to boiling pointmuriatic acidhydrochloric acidHClSelection of stainless steels for handling hydrochloric acid (HCl)oil of vitriolsulphuric acidH2SO4Selection of stainless steels for handling sulphuric acid (H2SO4)oleumconcentated sulphuric acidH2SO4Selection of stainless steels for handling sulphuric acid (H2SO4)rock saltsodium chlorideNaClCrevice and pitting corrosionhazard with most stainless steels, depending on conditionssalt acidhydrochloric acidHClSelection of stainless steels for handling hydrochloric acid (HCl)spirit of salthydrochloric acidHClSelection of stainless steels for handling hydrochloric acid (HCl)sulphurated hydrogenhydrogen sulphideH2SCan be a pitting or stress corrosion cracking hazard to non-molybedum alloyed stainless gradeswood acidacetic acid (mainly)CH3COOHSelection of stainless steels for handling acetic acid (CH3COOH) Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The austenitic stainless steel tubes remain tough and ductile to very low temperatures. Metals such as iron and constructional steel undergo a marked decrease in ductility at lower temperature. Common Cryogenic Temperatures:EnvironmentDegrees CDegrees FBoiling Helium– 268.6– 454Boiling Hydrogen– 252.5– 418Freezing Nitrogen– 209.9– 346Boiling Nitrogen– 195.8– 319Boiling Methane– 164.0– 265Solid Carbon Dioxide– 78.5– 108Boiling Freon 12– 29.8– 22Water as ice0.032 Effect of Temperature on Modulus of Elasticity: Stainless SteelRoom Temp. Tensile – ksiDegrees CDegrees F Room Temp.At -196øC301 Stainless Steel10625.4 x 10-628.6 x 10-6302 Stainless Steel8826.227.2304 Stainless Steel8724.426.2347 Stainless Steel9523.327.6 Mechanical Properties at Cryogenic Temperatures: Strength by Alloy Listed in ksiMaterial Grade301302304304L310S3163213470øCYield4242382835404040Tensile160120115988890100100Elong.60%50%60%58%50%68%58%60%Charpy keyhole notch in ft. lb.393437–38353229 301302304304L3105316321347-40øCYield4443403040454545Tensile18013513511594105120115Elong.40%58%55%55%70%67%55%58%Charpy keyhole notch in ft. lb.–––––––– 301302304304L3105316321347-78øCYield5058453548575050Tensile162164165138110120138136Elong.52%52%48%50%78%65%53%52%Charpy keyhole notch in ft. lb.212726–30262523 301302304304L3105316321347-168øCYield6262554062735850Tensile210210208175140160185180Elong.44%43%42%44%90%61%47%45%Charpy keyhole notch in ft. lb.5.59.5211624192219 301302304304L3105316321347-196øCYield5562574575806052Tensile275225225192151180211195Elong.30%40%38%42%93%19%22%19%Charpy keyhole notch in ft. lb.–––––––– 301302304304L3105316321347-253øCYield–62695096936865Tensile–258250220180230248230Elong.–38%27%41%71%55%34%–Charpy keyhole notch in ft. lb.–––19–––– Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Ferritic, martensitic and duplex stainless steel tend to become brittle as the temperature is reduced, in a similar way to other ferritic / martensitic steel. The austenitic stainless steel such as 304 (1.4301) and 316 (1.4401) are however ‘tough’ at cryogenic temperatures and can be classed a ‘cryogenic steels’. They can be considered suitable for sub-zero ‘ambient’ temperature sometime in service specification sub-arctic and arctic application and location (typically down to -40°C). This is the result of the ‘fcc’ (face centred cube) atomic structure of the austenite, which is the result of the nickel addition to these steel. The austenitic do not exhibit an impact ductile / brittle transition, but a progressive reduction in Charpy impact values as the temperature is lowered. There is a useful summary of low temperature data for austenitic stainless steel on the Nickel Institute. Impact toughness and impact strength measurement Impact tests e.g. Charpy, are done to assess the toughness of materials. To assess their suitability for cryogenic applications, the test is done after cooling the test piece. The Charpy impact test measures the energy absorbed in Joules when a standard 10mm square test piece (usually with a 2mm deep ‘v’ notch) is fractured by striking it in a pendulum type testing machine. The more energy absorbed, the tougher the material, and less likely it is to fail ‘catastrophically’ if subject to mechanical shocks or impacts. The impact toughness of steel varies with temperature. Ferritic and martensitic steels exhibit what is known as a ‘ductile / brittle transition’ where, over a certain temperature range, pronounced reduction in the impact toughness for a small decrease in test temperature. When plotted on a graph, the energy absorbed against temperature produces an ‘S’ curve. The mid-point on the ‘S’ is known as the ‘transition temperature’. Here the fracture failure mode changes as the temperature is lowered, from ‘ductile’, where the steel can absorb quite a lot of energy in breaking, to brittle, where only a small of amount of energy is absorbed. For this reason it is dangerous to use steels in this brittle state in structural applications, as even small shock loads can result in sudden, possible catastrophic failures. Affect of steel structure on toughness The toughness of the austenitic relies on their fcc atomic structure. The presence of either ferrite or martensite can limit the cryogenic usefulness of the austenitic stainless steel. The small levels of ferrite usually present in wrought austenitics is not usually detrimental. Cold working of austenitic stainless steels can also affect their cryogenic toughness. This is due to the progressive formation of martensite from the ‘meta-stable’ austenite. In effect this is similar to the presence of ferrite and can be controlled in the same way through compositional changes that stabilise the austenite. In addition the effects of cold work can be removed by heat treatment. Solution annealing (softening) by heating to around 1050 / 1100 °C and cooling in air, depending on section size, will completely stress relieve the structure and transform the structure back the naturally tough austenitic one. Welded areas may be at risk of brittle failure at very low temperatures, as ferrite levels in welds are higher than the surrounding wrought steel (to avoid hot cracking on solidification).　Special low ferrite level welding consumables are available for cryogenic applications and should be considered for very low, safety critical, temperature applications. Casting compositions for austenitic stainless steel also have ferrite levels higher than the corresponding wrought grades BS3100 – Steel Castings for General Engineering Purposes, requires special impact tests at -196°C for the cryogenic application grades such as 304C12LT196. Although there are no major restrictions on composition, this grade is required to meet an additional Charpy impact test requirement of 41 Joules minimum at -196°C Impact toughness of austenitic stainless steel When austenitic stainless steel are Charpy tested at -196°C the test piece is usually ductile enough not to fracture (which actually invalidates the test). Data available however quotes impact energies of over 130J for the 304 (1.4301) type. This is well within the 60-Joule minimum required in EN 10028-7 pressure vessel standard for 304 (1.4301) at -196°C. Any of the austenitic stainless steels should be suitable for applications at these temperatures. The best choices of grades for very low temperatures are those with austenite stabilising additions such as nitrogen e.g. asi n grade 304LN (1.4311). (Higher alloy grades such as 310 (1.4845) or 904L (1.4539) which derive their austenite stability from higher nickel levels could also be considered) Wrought grades with ferrite stabilising additions such as 321 (1.4541) or 347 (1.4550) may not be suitable at very low temperatures e.g. at the liquid helium boiling point of -269°C. Impact toughness of other stainless steel The ferritic, martensitic and duplex stainless steels cannot be considered as cryogenic steels. Their impact characteristics change at sub-zero temperatures in a similar way to low alloy steels. The transition temperatures will depend on composition and heat treatment. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Phosphoric Acid is also know as orthophosphoric acid and is classed as a weak acid. Austenitic stainless steel have goodcorrosion resistance to chemically pure phosphoric acid. Wet process phosphoric acid (WPA) can be aggressive. It is used as a chemical-cleaning agent for stainless steel but is not considered to be a ‘passivating’ acid. Commercially concentrated acid is around 85wt. % . The iso-corrosion diagram 0.1mm/year lines are represented for the stainless steel 304(blue) and stainless steel 316 (red) types and show that the stainless steel 304 types should be satisfactory up to acid’s boiling point to around 25% concentration. Corrosion resistance of stainless steel The austenitic stainless steel have good corrosion resistance to chemically pure phosphoric acid over a wide range of concentration and temperature. (The broken line represents the boiling point) At higher concentrations stainless steel 316 is resistant at higher temperature for any particular concentration i.e. the lines are essentially parallel. The stainless steel 316 types should be considered if chlorides are likely to be in the acid. Wet process phosphoric acid (WPA) can be aggressive towards stainless steels, depending on the range of impurities that the acid contains.  This can be of particular concern in bulk handling and transportation of raw phosphoric acid and specialist advice is needed to optimise grade selection. Chlorides, fluorides and sulphuric acid impurities increase the risk of corrosion, along with increases in temperature. The more pitting resistant steel grades should be considered when these impurities are known to be present. Uses for phosphoric acid with stainless steel Phosphoric acid is used as a chemical-cleaning agent for stainless steel. It is used in commercially available stainless steel cleaning preparations and so if used in accordance with the manufacturers / suppliers instructions will not etch or corrode the steel surface. Phosphoric acid is not considered to be a ‘passivating’ acid but the clean surface left after treatment should allow the stainless steel to naturally self passivate. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Hydrofluoric Acid is extremely aggressive and attacks most metals and glass. The approach to the selection of stainless steel is similar to that for hydrochloric acid. Commercially concentrated acid is around 40wt. %. Plastics are normally considered for handling hydrofluoric acid. The common stainless steel types, 304 stainless steel and 316 stainless steel should be considered non-resistant to hydrofluoric acid at any concentration and temperature. Corrosion resistance of stainless steel The iso-corrosion diagram 0.1 mm/year lines are represented for stainless steel 316 types (red) and a 6% molybdenum austenitic types (green) Higher grades of stainless steel can have limited resistance, up to around 2% maximum at ambient temperatures, but may suffer local attack, mainly as crevice and pitting corrosion, even at such low concentrations. Uses for hydrofluoric acid with stainless steel Hydrofluoric acid is used, along with nitric acid, in stainless steel pickling solution and paste preparations. Its main function is to loosen and help remove scale deposits. Incorrect use can be the cause of pitting problems if the solutions or pastes are left in contact with the stainless steel too long. It is very important that the manufacturers / suppliers instructions are followed to avoid damage to stainless steel products when using products containing hydrofluoric acid. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Sulphur dioxide dissolves readily in water, which is then classed as a weak reducing acid (sulphurous acid H2SO3). Theoxidation of sulphur dioxide can be assisted by chlorine. Sulphuric acids and hydrochloric acids can be formed in aqueous (water) systems. As a dry or liquefied gas, sulphur dioxide does not tend to be aggressive towards stainless steel. Grades with more than 18 to 20% chromium should be resistant to dry sulphur dioxide. Corrosion risks to stainless steel A 20% concentration of sulphur dioxide dissolved in water at 20 degC can be expected to give a corrosion rate between 0.1 and 1.0 mm/year on stainless steel 304 types, but a rate below 0.1mm/year on stainless steel 316 types and hence is ‘mildly’ corrosive. If exposed to the air sulphuric acid (H2SO4) is formed from the sulphurous acid and so sulphur dioxide dissolved in water can be hazardous, where sulphuric acid has been allowed to form. In damp vapours where oxidising conditions exist sulphur dioxide can be aggressive, depending on the dew points of acid and water at the service temperature under consideration. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Citric acid is a weak organic acid, found in fruits such as lemons (citrus) Either the stainless steel 304 or stainless steel 316 types can be considered for most storage and handling applications. Citric acid is also be used forcleaning stainless steel and passivating stainless steel. The low carbon types (304L stainless steel or 316L stainless steel) may be needed for temperature above around 60 degC to avoid any risk of intergranular attack in weld heat affected zones. Corrosion resistance of stainless steel The iso-corrosion diagram 0.1mm/year lines show that either the stainless steel 304 (blue) or stainless steel 316 (red) types can be selected for normal storage & handling applications. (The chain line represents the solubility and the broken line the boiling point) The stainless steel 304 types should be adequate for most applications. At temperatures above 80 degC the stainless steel 316, or preferably the stainless steel 316L types, should be considered. In common with most acid handling applications, chloride contamination may be a cause of pitting corrosion and so in these cases more pitting resistant grades may need to be considered. Uses for citric acid with stainless steel Citric acid can be used for cleaning & passivating stainless steel, as an alternative to nitric acid. Solution strengths of 5-10% citric acid are used for passivation treatments.

Chlorine readily forms chlorides when in contact with gases such as methane, hydrogen sulphide and ammonia. Hydrochloric acid HCl can also be formed by these reactions. Chlorine dissolves readily in water forming hydrochloric and hypochlorous (HOCl) acids, which is very corrosive mixture. Chlorine can also assist in the oxidation of dissolved gasses, such as sulphur dioxide (SO2), forming sulphuric andhydrochloric acid in water. It is these oxidising properties that make chlorine an aggressive component in waters. Corrosion resistance of stainless steel Chlorine in contact with water and as a dissolved gas, sometimes found in water treatment applications, is potentially aggressive to stainless steel. Localised crevice & pitting corrosion attack is a hazard in water and stress corrosion cracking (SCC) can be an additional hazard in damp chlorine gas, if the temperature is high enough. Condensates formed over chlorinated water in storage tanks have been known to result in staining or pitting to stainless steels. Improvements to ventilation in such situations should help reduce the risk of attack. Chlorine as a sterlizing or sanitising agent When using chlorine as a sterilizer or sanitiser in contact with type 316 stainless steel items, a maximum of 15-20 ppm (mg/lt) ‘free’ chlorine is suggested, for maximum times of 24 hours, followed by a thorough chlorine free water flush. As with any additions, thorough dilution around the injection point is important to avoid localised ‘over-concentration’ problems. Residual chlorine levels in waters of 2ppm maximum for 304 stainless steel and 5ppm for 316 stainless steel types should not normally be considered a crevice corrosion hazard. Chlorine dioxide as a sanitiser in contact with stainless steel Chlorine dioxide (ClO2), occurs naturally as a gas, but is normally dissolved in water, as the gas is highly explosive. Although a powerful oxidiser, unlike chlorine it does not breakdown to release chlorides. The chlorine and oxygen work together tending to form chlorites in the oxidation process. Although the chlorite can break down to form chlorides, it is a weaker oxidising agent than the chlorine dioxide and so can be expected to be less of a hazard to stainless steel when used as a water sanitiser. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Hydrochloric acid is classed as reducing acid and lacks the oxidising properties that stainless steel need to maintain their ‘passive’ corrosion resistant surface layer. The acid is formed as a product of the pitting / crevice corrosion mechanisms in stainless steel and so when hydrochloric acid is present in any ‘external environment’ corrosion is promoted. Commercially concentrated acid is around 37wt. %. The common stainless steel types, 304 stainless steel and 316 stainless steel should be considered non-resistant to hydrochloric acid at any concentration and temperature. ​ Corrosion resistance of stainless steel The iso-corrosion diagram 0.1mm/year lines are represented for stainless steel 316 types (red) and a 6% molybdenum austenitic type (green). Higher grades of stainless steel can have limited resistance, up to around 3% maximum at ambient temperatures, but may suffer local attack mainly as crevice and pitting corrosion, even at such low concentrations. The steep curves for 316 and the higher alloyed grades on the iso-corrosion diagram illustrate their very limited resistance. Any additional chlorides or chlorine in the acid can be expected to make attack more severe. Nickel based alloys, rather than stainless steels, should be considered for handling hydrochloric acid. Contact between stainless steel and building mortar cleaners The use of building mortar cleaners that contain hydrochloric acid can result in staining and pitting to nearby stainless steel items. Architectural metalwork and kitchen equipment has been reported with such problems resulting from either splashes or from the fumes given off from the acid. Commercially available cleaning acids are around 16% hydrochloric acid. Using these either ‘neat’ or as a one to one (50%) dilution, which may be recommended by manufacturers / suppliers of these cleaners, makes these solutions extremely aggressive to most stainless steel grades. It is advisable not to use such cleaners anywhere near stainless steel items. What can be done if building mortar cleaners come into contact with stainless steels? If damage has occurred it is usually evident as brown staining. Provided visible pits have not been formed it may be possible to renovate the surface by routine cleaning methods. If pitting has occurred then this must be bottomed out with abrasives (grinding). This is necessary to avoid further staining or corrosion in these areas. If this is not acceptable then the affected parts may have to be replaced. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Ammonia at normal atmospheric temperature and pressure is a gas. It can be stored under pressure as a liquid or below it’s atmospheric boiling point of -34 degC. It has been assumed that there is no corrosion risk to stainless steel that are normally considered for the storage and handling of bulk ammonia (ie stainless steel 304 or stainless steel 316 types) although there does not appear to be any published data to substantiate this. The general corrosion resistance of the 304 types should be adequate for installations at most sites, but for coastal or marine sites stainless steel 316 should be considered if the outer-casing or parts are exposed. Corrosion resistance and cryogenic toughness of stainless steel. The main issue is the cryogenic toughness of stainless steels at around -40 degC, compared to carbon steel, which can also be considered for ammonia service under these conditions. The austenitic stainless steel are impact tough at ‘cryogenic’ temperatures. Selection of stainless steel for cryogenic application Even when tested at the temperature of boiling liquid nitrogen (-196 degC) unless they have been severely cold worked (ie contain martensite) or have high levels of ferrite, the austenitic stainless steels are ‘tough’ and are accepted as suitable cryogenic materials. Risk of corrosion to insulated storage tanks If external tank insulation is proposed, then painting or a coating of aluminium foil on the outside face of a stainless steel 304 type can be considered. This is particularly important if there is a risk of the insulation becoming wet and absorbing chlorides. In a marine environments, where normally a type stainless steel 316 would be the more normal choice of grade, this can reduce the material selection costs. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Sodium Hydroxide (Caustic Soda) is a strong base. It is used in metal degreasing and cleaning processes in a wide range of industry applications. Stainless steel types 304 and 316 can be considered resistant below 80 degC, up to the limit of solubility. Both 304 and 316 stainless steel types are resistant to a wide range of concentration and temperature. Below 80 degC they can be considered resistant to any concentration of sodium hydroxide, up to the limit of solubility. There can be a risk ofstress corrosion cracking (SCC) attack at higher temperatures, which is common to both the 304 and 316 types. Corrosion resistance of stainless steel The iso-corrosion diagram 0.1mm/year lines for 304 and 316 types coincide (purple). (The broken line represents the boiling point) This should not be an issue if service temperatures are limited to a 95 degC maximum. Risk of stress corrosion cracking attack ‘Caustic stress corrosion cracking’ occurs at higher temperatures than chloride stress corrosion cracking (which can occur at temperatures as low as 60 degC). The area of risk in sodium hydroxide is shown on the iso-corrosion diagram by the area bounded by a green line. Risk of pitting attack by chlorides Chlorides should not pose as great a pitting and crevice corrosion attack threat in sodium hydroxide as they do in acid solutions. The high pH values of the ‘basic’ sodium hydroxide helps arrest the normal mechanisms of attack. Care may be necessary when selecting 304 types for sodium hydroxide cleaning systems or tanks, where ‘carry-over’ of chlorides could occur from prior treatment stages. It may be better to consider 316 or the 316L types if this could occur. The 316L type may be a marginally better choice where the steel may have been sensitised in the heat-affected zone (HAZ) of welds and post weld softening / stress relief is not practical. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)