Introduction
Stainless steel grades, such as the 304 or 316 types are generally suitable for storing and handling cold or unheated drinking (town's) waters.
Localised corrosion by crevice or pitting mechanisms is not usually a hazard in properly designed, fabricated and finished tanks handling clean waters of drinking quality.
Hot water tanks however may be at risk from stress corrosion cracking (SCC)
Stress corrosion cracking (SCC) risk factors
The factors that influence SCC attack are: -
- temperature
- chlorides
- tensile stresses
- oxygen level
For these types of applications chlorides and oxygen levels are fixed by the water chemistry, but chlorides can concentrate in splash zones or at the water /air line by evaporation. This can also be a hazard if external insulation to tanks becomes wet.
Temperatures should be fixed by the tank system controls, but hot spots can be a problem, especially if chlorides concentrate, as described.
A design and fabrication method with as few 'engineering crevices' as possible is advisable as this reduces the risk of stress concentrations and also guards against crevice corrosion attack.
Fully filled welded joints are preferable to seams with laps or mechanically fastened joints.
Controlling residual stresses
The main design and fabrication factor that can be 'controllable' is stress. Residual tensile stresses can be a cause of SCC failure.
Relief of these stresses can be advisable where high levels of residual stresses are possible or the application is critical.
A range of treatments for the 304 and 316 type austenitics can be considered: -
- 'sub-critical' stress relief treatment, e.g. 450°C and slow cool
- 'full-anneal' e.g. 1050-1100°C and 'quick' cool (air)
stress relieving austenitic stainless steels
The restraining effect around welds on austenitic stainless steels such as 304 or 316 types can also be a source of tensile stresses. If it is impractical to post fabricate heat treat, then control of welding parameters may help.
These would include: -
- Careful pre-weld tacking
- Minimising heat input during welding by controlling welding speed
Alternatives to the austenitic stainless steels to reduce the risk of SCC failure
Alternatives to the austenitics, which have nickel levels making them particularly susceptible to SCC are either: -
- Ferritic stainless steels
- Duplex stainless steels
These types have lower nickel levels and so are more resistant to SCC as a result of the ferrite phase present. (Higher nickel level alloys are also more resistant to SCC, but are not considered economically justifiable for these applications.)
The ferritic grade 444 (1.4521) and particularly the lean duplex grades 1.4162 and 1.4362 are routinely used as SCC resistant grades for hot water tank applications.
Forming and welding differences from the austenitics must also be borne in mind when making the steel selection.
Source: http://www.bssa.org.uk/
Comments