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Site location is the initial consideration in selection of stainless steels for external applications. These are classified as rural, urban, industrial or marine. Micro-climates can influence the steel type selection. Other factors that influence the selection are surface finish, design, fabrication methods, accessibility for cleaning and maintenance and mechanical and physical properties of the steels. The main steel types 304 and 316 only are considered for UK external applications. (102)
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The appeal of stainless steels for internal applications is based on their corrosion resistance and wide range of finishes and textures available. For most building interiors intended for human occupation, either the ferritic 430, (1.4016), type or the austenitic 304, (1.4301), type are suitable. Grade, surface finish, fabrication and routine cleaning procedures are all important considerations. Fingermarking can be avoided by correct surface treatments. (101)
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Ferritic, martensitic and duplex stainless steels tend to become brittle as the temperature is reduced, in a similar way to other ferritic / martensitic steels. The austenitics stainless steels such as 304, (1.4301) and 316, (1.4401), are however ‘tough’ at cryogenic temperatures and can be classed a ‘cryogenic steels’. The toughness of the austenitics relies on their fcc atomic structure. Ferrite or martensite phases in austenitic weld metal or castings can affect the suitability for cryogenic applications. Charpy impact tests are done to assess the toughness of materials.
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Acetic acid is a weak reducing acid. It is used in plastics manufacture and is a constituent of foods as vinegar. Ferritic stainless steels such as the 430 type can be considered but normally the 304 types are used for most applications, including handling and storage. Acetic anhydride, (CH3CO)2O can be aggressive to either 304 or 316 types in the absence of any water and in the presence of chlorides. Peracetic acid CH3C(O)OOH, (peroxyacetic acid), should be safe with stainless steels. Vinyl acetate C4H6O2 may be considered with the 316 types for ambient temperature storage applications.
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The austenitic stainless steels are impact tough at ‘cryogenic’ temperatures compared to carbon steels and so are suitable for handling and storage of liquid ammonia at temperatures around -40 degC. The general corrosion resistance of the 304 types should be adequate for installations at most sites, but for coastal or marine sites 316 types should be considered if the outer-casings or parts are exposed.
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Dry chlorine gas should not attack stainless steels. Damp gas or chlorine dissolved in water can be a corrosion hazard. Corrosion can take the form of localised crevice and pitting corrosion. Stress corrosion cracking, (SCC), can be an additional hazard in damp chlorine gas, if the temperature is high enough.
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Citric acid is a weak organic acid, found in fruits such as lemons, (citrus). Either the 304 or 316 stainless steel types can be considered for most storage and handling applications. Citric acid is also be used for cleaning and passivating stainless steels.
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Hydrochloric acid lacks the oxidising properties that stainless steels need to maintain their ‘passive’ corrosion resistant surface layer. Stainless steel have limited resistance. Building mortar cleaners that contain hydrochloric acid can result in staining and pitting to nearby stainless steel items.
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Hydrofluoric acid is extremely aggressive. It is used, along with nitric acid, in stainless steel pickling solution and paste preparations. The 304 and 316 types should be considered non-resistant. Higher grades of stainless steel can have limited resistance.
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Nitric acid is strongly oxidising and promotes the resistance of stainless steel to corrosion. Generally stainless steels are resistant to corrosion in nitric acid. Nitric acid is used in the chemical ‘passivation’ of stainless steels. The 304 types are preferable to 316 types. Localised attack at grain boundaries can occur in hot concentrated nitric acid.
