Introduction
In pure water at normal (ambient) temperatures, stainless steels can be considered "inert". Except for chemically purified waters there are various levels of anions, including chlorides, which can be aggressive to stainless steels under certain circumstances. General corrosion over large areas of the surface is not usually encountered on stainless steels, due to formation and maintenance of an inert "passive" surface layer, but localised corrosion can occur due to the breakdown of the passive layer by chlorides at discrete sites.
There are three main types of localised corrosion mechanisms that can affect stainless steels :
- Pitting corrosion
Crevice corrosion
Stress corrosion cracking
• Pitting corrosion tends to be associated with free surfaces of stainless steel. This form of localised attack can be severe and lead to rapid perforation of thin sheets or tubes. Attack is concentrated at microscopic defects in the protective passive oxide film (for example at the site of inclusions at the surface of the steel).
• Crevice corrosion is a form of attack based on "engineering" sites rather than the "metallurgical" sites responsible for pitting. It tends to occur more readily at sharp changes in section or shielded areas and so is potentially more hazardous.
• Stress corrosion cracking normally occurs at temperatures over 60°C This condition must be accompanied by tensile stresses and chlorides to enable this form of attack.
The main factors that promote corrosion in waters are:
- Chloride level
Temperature
Oxygen level
Water flow rates
Presence of bacterial oxidants
Chloride levels A limit of 200 ppm (mg/litre) maximum has been set for chlorides in drinking (potable) waters by the EU. Levels in the UK tend to be below 100ppm and at "ambient" UK water supply storage and distribution temperatures these chloride levels should not be a crevice or pitting corrosion hazard to stainless steel types 304 (1.4301 / 1.4307) or 316 (1.4401 / 1.4404), which are commonly used in these applications.
The accepted view is that crevice corrosion is rare below:
- 200 ppm for 304 (1.4301 / 1.4307)
1000 ppm for 316 (1.4401 / 1.4404)
In situations where exposure times are short and there is some "rinsing", as in the case of coastal sewage outfall installations, where high tide chloride levels can reach 1000- 2000ppm levels for short periods of time, the 316 (1.4401 / 1.4404) types can be a satisfactory and economic choice. Well maintained water flow rates and oxygen levels will help these steels maintain their corrosion resistance in these applications.
Care must be taken in design however to ensure that chloride levels do not increase by evaporation at liquid surfaces or in splash zones.
Chloride concentration under water scales can also be a corrosion hazard.
Between 1000 and 3600ppm duplex grade 1.4462 (2205) can be considered. Above these levels "super duplex" grades, such as 1.4410 (SAF2507) or 1.4501 (Zeron 100) or super-austenitics, such as 1.4547(254SMO) or 1.4529(1925hMo) are appropriate. These can also be used in seawater applications, where chloride levels are of the order of 26,000ppm.
Temperature The risk of crevice & pitting corrosion attack increases with temperature. At the temperatures and chloride levels normally encountered in water supply & treatment applications, these forms of attack are not usually relevant.
These corrosion mechanisms, along with stress corrosion cracking, are only usually of concern in applications such as water boilers and heating systems, particularly in situations where chlorides can concentrate. Lower temperature environments such as swimming pool building atmospheres, which are particularly aggressive to stainless steels, can also be a SCC hazard concern.
Source: http://www.bssa.org.uk/
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