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In certain aggressive environments some grades of stainless steel will be susceptible to localised attack. Six corrosion mechanisms are described in this article, namely pitting corrosion, crevice corrosion, bimetallic, (galvanic), corrosion, stress corrosion cracking, (SCC), general, (uniform), corrosion and intergranular, (IGC), sometimes known as intercrystalline or IC, or weld decay attack.
This article provides a link to the Outokumpu web-site.This provides a web based version of the tables published as the ‘Outokumpu Stainless Corrosion Handbook’. The chemical compatibility of a range of stainless steel types with a large range of acids, bases, compounds, some foodstuffs and liquid metals can be assessed using these tables. Tables include acetic acid, acetone, aluminium chloride and sulphate, ammonium bisulphite, bromide, carbonate, fluoride and chloride, benzene, beryllium chloride, boron trichloride, bromine, calcium chloride, carbon tetrachloride, chloric acid, chlorine, chlorine dioxide, chlorobenzene, chloroform, chromic acid (chromium trioxide), cobalt sulphate, copper acetate and cyanide, ether, ethyl alcohol, ethylene bromide, fluorine, formaldehyde, formic acid, glucose, glycerine, glycol, hydrobromic acid, hydrogen chloride gas, hydrogen iodide and peroxide, hydrogen peroxide, iodine, iron (ferrous and ferric) chloride, iron nitrate, lactic acid, lead nitrate, lithium chloride, magnesium sulphate, malic acid, mercury, methyl alcohol, methylene chloride, nickel chloride, nitrous acid, oxalic acid, perchloric acid, phenol, potassium bisulphate, chlorate and dichromate, silver bromide, sodium bicarbonate, chlorate, citrate, hydroxide, hypochlorite, perborate, perchlorate, phosphate, silicate, thiosulphate, stannic (tin) chloride, sulphamic acid, sulphur, sulphurous acid, sugar, tannic acid (tannin), tartaric acid, textile dyes, toluene, trichloroethylene, urea, urine, vinegar, xylene, zinc and zirconium oxychloride. This data was originally compiled jointly in Sweden by technical specialists at Avesta and Sandvik.
Life expectancy is estimated from pitting depth measurements made on exposed test samples. The results depend on steel grade, environment and surface finish. Staining from micro pitting may result in rejection of the steel on aesthetic grounds, long before pitting has perforated it. Steel types 430, (ferritic), 304 and 316, (austenitic) are considered. (104)
Stainless steels do not have an intrinsic ‘fire rating’. Tests to assess fire resistance are performed on specific fabrications under precise conditions to BS476 parts 20, 21, (load-bearing elements), and 22, (non-load-bearing elements). Fire tests results on some specific products demonstrate the good fire resisting properties of stainless steels in building and ship bulkhead applications. (186)
The main factor in the selection process for stainless steels is corrosion resistance. Careful consideration of the application should be done to enable a choice of grade with suitable corrosion resistance whilst keeping costs to an economic minimum. Other considerations such as mechanical properties, (strength and toughness), physical properties, (magnetic permeability), and forming, fabrication and joining methods available should be secondary. (91)
Stainless steel is a great material to work with. As with all materials you need to know how to treat it correctly for maximum cost effectiveness. This article summarises the typical pitfalls and remedies.
This article has a listing of terms often specifically associated with stainless steels, their processing and use. Terms listed include, active, annealing, austenite, austenitising, bright annealing, cathodic protection, chlorides, (halides), cold and hot working, corrosion, creep, deep drawing, duplex, fatigue, (endurance), ferrite, martensite, normalising, passive, passivation, permeability, pH, pickling, pinch pass, pitting, precipitation hardening, scaling temperature, sensitisation, stabilisation, stress relieving, stretch forming and tempering
Appropriate grade selection is a balance between attaining adequate corrosion resistance, whilst minimising cost. Useful aids to material selection include the Outokumpu Stainless Corrosion Handbook and the Nickel Institute’s Crevice Corrosion Engineering Guide.
A colour chart is shown for guidance on exposure temperatures on 1.4301, (304), type. The factors affecting the degree or depth of colours formed are outlined. These include steel composition, atmosphere, time and surface finish.
The corrosion resistance of stainless steel arises from a passive, chromium-rich, oxide film that forms naturally on the surface of the steel. Although extremely thin at 1-5 nanometres, (i.e. 1-5 x 10-9 metres), thick, this protective film is strongly adherent, and chemically stable (i.e. passive) under conditions which provide sufficient oxygen to the surface. The affects of alloying elements nickel, molybdenum and nitrogen on localised forms of corrosion attack is discussed. The mechanisms include crevice corrosion, pitting corrosion, Intercrystalline, (or intergranular), corrosion, (ICC), stress corrosion cracking, (SCC) and galvanic, (bi-metallic), corrosion, and are often associated with chlorides or acid conditions.