Category: Crevice Corrosion
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.
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.
The 316 types are used widely in marine applications, but their corrosion resistance in contact with seawater is limited. They cannot be considered 'corrosion proof' under all situations. These grades are susceptible to crevice and pitting corrosion, which limits there use in seawater applications. The affects of water chloride levels, flow rates, temperature and oxygen levels are noted and cathodic protection that can be derived from contact with less noble metals such as carbon steels and aluminium. The 304, and more especially the free machining 303 types, should not be considered for seawater service.
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.
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.
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.