Category: Pitting Corrosion & PRE Numbers
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.
Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing stainless steels, using their chemical compositions. The formulae are based mainly on chromium, molybdenum and nitrogen contents. Grades with a PREN of 40 or more are known as 'super' austenitics or duplex types, depending to which basic family they belong. A table of calculated PREN values compares some of the ferritic, austenitic and duplex steel grades.
Grade 316Ti is a 316 type stainless steel, stabilised with titanium to reduce the risk of intergranular corrosion (ICC). The 316L 1.4404 or 1.4432 grades can be considered as alternative choices. Under most conditions 316Ti and 316L are interchangeable, but the elevated temperature strength, corrosion resistance, machinability, cold-formability and polishing characteristics can affect the final choice of grade.
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.
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)
Rust staining can occur and has been reported as anything from a slight brown 'bloom' on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items. Iron contamination can be costly to remedy, and is avoidable. The ferroxyl test can be used to detect 'free' iron contamination. (108)
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.
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.
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.