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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.
Pitting and stress corrosion can result from moist thermal insulation where chlorides are present. This information sheet provides background information on the sources of chlorides within such insulation materials and describes two corrosion prevention methods. Paints (e.g. a high temperature silicone type) or 0.06 mm thick aluminium foil can be used as barriers between insulation layers and stainless steel. BS5970, BS5422 are cited in this information sheet.
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 NACE MR0175 standard for sulfide ( sulphide ) stress corrosion cracking resistant metallic materials for oilfield equipment is now also available as an ISO standard 15156. This article summarises the requirements for all types of stainless steel austenitic , ferritic, duplex, martensitic and precipitation hardening stainless steels which are covered by the standard. Mill softened material will normally be found to comply with the requirements.
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 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.
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.
This paper was originally published in The Structural Engineer (Volume 82 Issue 9 4th May 2004) and reviews structural failures in swimming pool buildings, attributed to stress corrosion cracking of stainless steel and the progress made since it was identified where the risks of failure can occur. Pool building environments are reviewed in terms of water temperature, disinfection systems (chlorine), humidity and condensation. Prevention of SCC (stress corrosion cracking) failure is the joint responsibility of the designer, structural engineer and pool operator. Grade selection to avoid SCC involves considering grades such as 1.4547 1.4529 1.4565 or 1.4507 rather than the 1.4301 (304) and 1.4401 / 1.4404 (316) types, which are more suited to non stressed, non safety critical or load bearing applications.
Guidelines for the grade selection, design, fabrication, maintenance, cleaning and inspection of stainless steels items for use in swimming pool buildings. Avoidance of stress corrosion cracking (SCC) of safety-critical, load-bearing components. (57)