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Phosphoric, (orthophosphoric acid), is a weak acid. Austenitic stainless steels have good corrosion resistance to chemically pure phosphoric acid. Wet process phosphoric acid, (WPA), can be aggressive.
Sodium Hydroxide, (Caustic Soda), is a strong base, used widely for cleaning metals. Stainless steels types 304 and 316 can be considered resistant below 80 degC, up to the limit of solubility. There can be a risk of stress corrosion cracking (SCC) attack at higher temperatures.
Sodium Hypochlorite is widely used as a sanitiser in water systems and is the main constituent of household bleach, at around 5.25 %. It is aggressive to stainless steels. Pitting or crevice corrosion can occur on most stainless steel grades. Pitting corrosion has been reported from household bleach spills on stainless steel, (304 type), sinks in domestic environments. There is an additional risk of stress corrosion cracking, (SCC), at higher temperatures.
Dry or liquefied sulphur dioxide gas should not be aggressive towards stainless steels. When dissolved in water it forms sulphurous acid H2SO3 and is mildly corrosive. Oxidation of the acid to sulphuric acid can be a corrosion risk to stainless steels.
Sulphuric acid is oxidising when concentrated but is reducing at low and ‘intermediate’ concentrations. Most stainless steel types are resistant at either low or high concentrations, but not at intermediate concentrations. Corrosion resistance of stainless steels also depends on aeration and oxidising conditions and the velocity of flow in pipework systems. ‘Battery acid’ is sulphuric acid with a weight percentage concentration of over 35 % (specific gravity of 1.28).
The special compositions, non-metallic inclusion, ferrite and grain size requirements of these steel grades, D and E specified for surgical implants are outlined. Commercially produced 316 type steels cannot be used for surgical implants as the available composition and micro-cleanness information is very unlikely to meet the requirements of BS 7252-1 ISO 5832-1.
Steel grades A, (410, 1.4006), B, (1.4021), C, (1.4028), D, (1.4034), E, (1.4116), F , G, H, I, K, (1.4122), R, L, (1.4105), M, (304, 1.4301), N, (303, 1.4305), O, (301, 1.4310), and P, (316, 1.4401) of BS EN ISO 7153-1:2001, (BS 5194-1:1991), are shown. The surgical and dental applications for these grades are outlined. These include cutting and non-cutting instruments and fitting parts and assemblies. (Body implants are NOT covered here.) Corrosion resistance, corrosion testing and the affects on these surgical steels of sterilisation practices are covered.
Stainless steels are widely used in food and beverage manufacturing and processing industries for manufacture, bulk storage and transportation, preparation and presentation applications. The ‘316’ types are often referred to as the ‘food’ grades. The 304 and 430 types are suitable for food processing and handling, depending on the corrosion resistance required in the particular application. There is no known official classification, or restriction for stainless steels for food industry applications. Duplex types are used in more aggressive environments where there may be a risk of stress corrosion cracking.
Stainless steel grades, such as the 304 or 316 types are generally suitable for storing and handling cold or unheated drinking, (town’s), waters. Hot water tanks however may be at risk from stress corrosion cracking, (SCC), Austenitic types such as 304 or 316 can be useful and are used, but in extreme cases the more ‘SCC’ resistant duplex stainless steels should be considered.
The benefits of stainless steels are given together with guidelines for selection in different chloride levels, good design and fabrication practices, which together will promote optimum corrosion performance.