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Stainless steels can be susceptible to certain localised corrosion mechanisms, namely crevice corrosion, pitting, intercrystalline corrosion, stress corrosion cracking and bimetallic (galvanic) corrosion. Localised corrosion is often associated wuth chloride ions in aqueous environments. Corrosion resistance relies on a good supply of oxygen. Higher levels of chromium, nickel, molybdenum and nitrogen increase resistance to localised corrosion.
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.
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)
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, normalizing, passive, passivation, permeability, pH, pickling, pinch pass, pitting, precipitation hardening, scaling temperature, sensitisation, stabilisation, stress relieving, stretch forming and tempering
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, Intercystalline (or intergranular) corrosion (ICC), stress corrosion cracking (SCC) and galvanic (bi-metallic) corrosion and are often associated with chlorides or acid conditions.
Stainless steels naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen. Passivation treatments are also sometimes specified for finishing stainless steel fabrications. Passivation normally involves using nitric acid. Citric acid treatments can also be considered. Recommended practices from ASTM A380, A967 and BS EN 2516 are shown.
Crevice corrosion along with stress corrosion cracking, is one of the major corrosion hazards to stainless steels. Careful selection of the steel grade can help reduce the risk of attack, but a brief review of the mechanism and factors that promote crevice corrosion should help reduce the risk of attack. Shielding corrosion beneath surface deposits works in a similar way to crevice corrosion.
Grades 1.4301 (304) 1.4401(316) and 430 (1.4016) are compared for food contact applications. A list of typical applications for martensitic 1.4028 (420) and 1.4116 ferritic 1.4016 (430) austenitic 1.4301 (304) 1.4401 (316) and 1.4539 (904L) austenitic 1.4362 and 1.4462 (2205) duplex and 1.4547 (254SMO) superaustenitic types is presented. Corrosion hazards in food applications ie pitting crevice and stress corrosion cracking are discussed. Suitable cleaning and disinfection systems should avoid the use of hypochlorite or chloride solutions.
Localised corrosion mechanisms pitting, crevice and stress corrosion cracking are mentioned, but normally stainless steels are considered “inert” in supply (or town’s) waters. The affect of chloride levels, temperature, oxygen levels, flow rates and bacterial oxidants ie chlorine on the resistance of stainless steels in waters is discussed. Crevice corrosion should be rare at chloride levels below 200 and 1000 ppm (mg/lt) respectively for 304 (1.4301 / 1.4307) and 316 (1.4401 / 1.4404) types. Water chloride ranges for duplex 1.4462 (2205) super austenitic 254SMO (1.4547) and super duplex types SAF2507 (1.4410) and Zeron 100 (1.4501) are also shown.
The corrosion resistance of stainless steels is derived from the alloying element chromium. A chromium-rich oxide film forms naturally on the surface of the steel. If damaged, the film will normally repair itself. In this condition the steel is in the passive state. If the film is destroyed the surface is in the active state.