Nerves of stainless steel
Consultant Carol Powell reports on tackling corrossion in stainles steelStainless steel is common in our domestic lives and is associated with hygiene and ease of cleaning - in food processing, kitchens and hospitals. It is also associated with high-tech modern design in architecture.
However, while it is a relative newcomer to the water industry in the UK, its use has been increasing over the last decade, see Table 1, for the following reasons:
- corrosion resistance to a wide range of waters,
- no coatings required,
- tolerance of high flow rates,
- good strength and ductility,
- lightweight design, easy fabrication and transport,
- very low metal leaching levels into water,
- full recyclability.
The challenge to water engineers therefore becomes to recognise and achieve all these benefits that stainless steel has to offer by suitable grade selection and design.Grade Selection
An initial perception is perhaps there is just one material called stainless steel that rigorously resists corrosion under all exacting conditions. With closer scrutiny, the situation is seen to be more complex. For example, there are more than 100 different stainless steels to choose from with differing strengths and levels of corrosion resistance.
There are only three predominant stainless steels used in the water industry today and they are the type 304L and 316L grades, and duplex alloy 2205. Although we know from the behaviour of stainless steels in our daily lives that they have a high degree of resilience, in some instances they can corrode.
For more severe conditions, so-called super stainless steel grades are available. A protective, invisible oxide film on the surface ensures general corrosion or thinning in waters is negligible. The presence of chlorides in waters can penetrate the film locally under certain conditions and their level is an easily measurable first indicator of whether corrosion might occur.
The guidelines given in Table 2 may be used for waters at ambient temperatures with pH levels above six, typical of most waters. The values given are chloride levels below which experience has taught us localised corrosion by pitting and at crevices has been found to be rare.
Where severe conditions might occur - for example, lower pH, high temperatures, low flow and other conditions where there is a risk higher chlorides might concentrate locally or just for conservatism - upper chloride limits in the region of 50ppm for 304L and 250ppm for 316L can be preferred. Alternatively, if the stainless steel is cathodically protected, the waters are de-aerated or there is only transient exposure to these chloride levels then the values in the Table 2 can be relaxed. Type 316L stainless steel offers a greater corrosion resistance than 304L due to a 2-3% molybdenum content, table 3. For aggressive waters and exposures to tidal environments, higher alloyed superduplex stainless steel (austenitic-ferritic) grades and the 6% molybdenum superaustenitic stainless materials may be required. Duplex alloys have higher strengths and this can be used to advantage to further decrease wall thicknesses and weight, if necessary, Table 4. Under external atmospheric conditions, 304L and 316L can retain their bright appearance for many years when fabricated and finished to suitable standards, particularly when any surface deposits that build up are removed by a periodic wash down with tap water. In marine or chloride bearing atmospheres the 316L grade is recommended where maximum life and good appearance are required.
With excellent corrosion-erosion characteristics in high flows of water (to more than 30m/s), stainless steel can easily handle changes of cross-section, high aeration, pumping turbulence and high velocities. Stainless steel systems require no corrosion allow-ance or coatings and can be designed using thin walls. Also, as higher flow velocities can be accommodated, smaller cross-sectional piping sizes can be used for the same mass flow rate that would be permissible with conventional materials.
Stainless steels are at their best in flowing conditions and extended stagnant conditions should be avoided where possible. It is important to drain and dry stainless steel water lines and vessels promptly after hydrotesting (which is often used to check the integrity of systems after construction) if the equipment is not going into service directly.
Alternatively, maintaining regular flushing of the system should limit any potential problems. This is particularly so in handling raw waters, where bacteria and water stream sediments can settle out and initiate corrosion attack and therefore potable waters should be used for testing rather than raw waters.Chemical Treatments
Stainless steel provides a material with extremely low corrosion rates in a very wide range of waters. Unlike the traditional construction materials, water chemistry control is not required to prevent corrosion attack, although bactericide treatments will still be necessary as for all potable water streams. Care must be taken, however, when using aggressive chemicals such as ferric chloride (added for flocculation purposes) and chlorine compounds to ensure they are added centrally into the process stream for good dispersion. Concentrated forms of these chemicals directed at or down the side of stainless steel piping or equipment can result in localised attack.
Ozone is a powerful oxidant and type 316L stainless steel is a standard material used in ozone generation and for the handling of the ozonated water streams. Materials used for the treatment, storage and distribution of drinking water must not introduce any contamination above the levels permitted by the relevant legislation. The stainless steel grades likely to be used in these applications have been tested in different countries. These tests have shown the leaching of metallic elements is at a level consistently below those allowed by the regulations. For example, the rig tests carried out as part of a European pre-normative research project gave leaching values for chromium and nickel which were less than 5% of those allowed by the European Drinking Water Directive.
This work is now forming the basis for tests that will be used to assess the suitability of construction materials to be used in contact with drinking water under the European Approval Scheme currently being developed.Fabrication
Optimum performance from stainless steels, as with all other materials, relies on attention to fabrication practices. Much good work has been carried out by stainless steel organisations and others internationally to raise awareness of the importance and to provide support and information. These aspects are mainly aimed at avoiding crevices that may cause problems during service. The more important include:
- thorough degreasing is required before welding,
- marks from oil, crayons, sealant, sticky deposits (including stick-on labels), slag, arc strikes, weld spatter should be removed,
- tooling, blasting and grinding operations that can leave embedded iron should be avoided (for example, use stainless steel not carbon steel brushes) or if unavoidable, removed,
- welds should be full penetration,
- inert gas back purging is necessary during welding to minimise heat tint, where formed, surface oxides and heat tint should be removed.
Embedded iron on stainless steel surfaces causes rust marks to form which are often mistaken for the stainless steel itself corroding. Pickling, either by immersion or applied in paste form, removes embedded iron and a thin surface layer of metal that may contain surface defects.
The clean metal surface naturally passivates itself by reforming the oxide film straight away on exposure to air. This is the most corrosion resistant state. If pickling is not practical, the embedded iron can be removed mechanically.
Acceptable methods include the use of medium to fine grit abrasives such as clean flapper wheels, flexible disks, blasting with clean and iron-free abrasives, such as glass beads or by electropolishing. All these methods are also acceptable for removing heat tint. Experience with stainless steels here and abroad has been good - long service lives can be achieved when particular consideration is given to good design, fabrication and operational practices. Guidance is available to all in the Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply available from the www.bssa.org. uk website. Compliance to this is a requirement for DWI approval. The WRc publication, A Water Industry Information and Guidance Note. IGN 4-25-02, Applications for Stainless Steel in the Water Industry, also contains comprehensive information. Such information allows water engineers to rise to the challenge and make full use of stainless steel