As proved by the increasing number of stainless steel applications in the potable water sector, some of which will be briefly described in the last section of this article, the use of stainless steel is growing in this specific emerging sector. The article describes the “state of the art” in this sector particularly focussing on stainless steel grades used in contact with potable water, its corrosion resistance and hygienic properties, laws and standards.
 Stainless steels and the potable water cycle: the situation nowadays
Unlike other materials, metallic and not metallic, which have a highly specific area of application, stainless steel can definitely be adopted for components used in all the individual stages of the integrated cycle of drinking water, from collection to end distribution and domestic taps. This latter concept has been summed up briefly in table 1.
The types of stainless steel most used to date in the water cycle are those of the austenitic chromium-nickel series, in particular AISI 304 and 304L (EN 1.4301 and 1.4306), or chromium-nickel-molybdenum, in particular AISI 316 and 316L (EN 1.4401 and 1.4404), in the formats which are readily available on the market, although there are also several applications with duplex stainless steels, type 2205 (EN 1.4462) and ferritic stainless steels, type 444 (EN 1.4521). Table 2 gives the chemical composition of these most commonly used austenitic types.
 The choice of one or the other type depends on various factors, which must also be taken into account in order to find the right type of alloy according to the application. The basic parameters are: concentration of corrosive agents (in this case the potable water), in particular chlorine and fluorine ions, working temperature, pH, velocity of the fluid on the walls of the stainless steel. It should not be forgotten however that other factors also influence the triggering of corrosion such as surface finish, connection with other materials, design criteria. For example a careful design which eliminates possible risks due to aggressive deposits will be an added guarantee of the efficiency of the system.
Corrosion resistance
Usually, regardless of the application sector, stainless steels are used for their corrosion resistance properties, high durability and absence of maintenance. The general basic parameters which may influence the corrosion resistance have been already discussed. For this specific application (potable water), the European Directive 98/83/CE, implemented in Italy with the Decree n. 31 2 February 2001, establishes the  chemical composition of what is commonly named “potable water”. Therefore in the absence of a chemical analysis of the water, in order to establish the right grade to use for the application it is suggested to turn to the chemical composition of the potable water specified into the European Directive, paying particular attention to chlorine and chloride. Furthermore to reduce bacteria in the potable water plants, the chlorination is usually done. If the rate of chlorination is high, potable water may become aggressive. Anyway disinfection generally does not last enough time to cause corrosion. Chlorination has not to be underestimated otherwise severe corrosion problems may occur.
Usually, the austenitic stainless steel grades, known as the 300 series, demonstrate the best behaviour against corrosion, in particular chromium-nickel-molybdenum alloy steel, due to a highly resistant passive film. Accordingly stainless steels type 304/304L or 316/316L are the standard choice. This is not a mandatory choice; different grades may be used as alternative, such as ferritic stainless steels, particularly type 444 (EN 1.4521), and such as duplex stainless steels (type 2205 – EN 1.4462). Focussing our attention on type 444, we point out that both DVGW, Deutsche Vereinigung des Gas und Wasserfaches (the German technical association for gas and water) and both SVGW, Schweizerischer Vereindes Gas und Wasserfaches (the Swiss technical association for gas and water) have recently established in their respective countries that, as a result of several tests, this stainless steel grade is suitable for potable water applications because it has corrosion resistance properties very similar to those of AISI 316.
In order to minimise the onset of corrosion on stainless steel, precautions should also be taken during manufacture and installation. First of all any form of contamination, for example ferrous, which could occur during storage or due to machining with tools previously used on carbon steel, should be avoided. Contaminated stainless steel is definitely more prone to corrosion.
Joints welded with filler material must be made with an electrode compatible with the base metal, while the connecting part of mechanical joints, for example bolts, must be in stainless steel or a material of the same quality (e.g. monel). In this way unfortunate processes of corrosion caused by galvanic coupling will be avoided. Surfaces can be decontaminated by pickling products and inhibitors, appropriately measured and applied. For cleaning, detergents without a chlorine base can be used. In general soap and water or water with added soda will represent excellent products for cleaning stainless steel. Steam will also be an optimal sanitising product, again controlling the composition of the water base.
 Hygiene, laws and standards
The hygiene of a material can in general be defined as the combination of a series of aspects which can be summed up as follows:
-) Resistance to corrosion, in turn represented by:
- inertia in relation to substances with which the material comes into contact, so as to avoid release of its basic elements which alter the toxicological or taste and smell properties;
- resistance to the action of detergents, solvents, sanitising substances and disinfectants, so as to enable actions for removing even the smallest traces of deposits, dirt and bacterial contamination;
-) lack of any type of protective coating which, when it splinters, gets worn or cracks or in any way deteriorates, creates surface gaps which are transformed into receptacles for germs and dirt. These gaps can become the location for the start of corrosion or lead to the uncovering of a base material which could be toxic;
-) compact surface without pores: the surface must not absorb particles of any origin, which subsequently alter the product with which they come into contact;
 -) high resistance to impact and to mechanical stress in general: chipping and cracks would become fertile ground for germs;
-) resistance to thermal shock: during the cycle of use the sudden changes in temperature must not create breakage or cracks for the reasons already mentioned;
-) high bacteria removability: in the cycles of cleaning and sanitising of equipment and systems, whose surfaces are regularly contaminated by colonies of bacteria, it must be possible to restore all their original qualities. Bacteria removability must also be ensured throughout the life cycle;
-) low bacteria retention potential.
All the types of stainless steel, varyingly combined, offer an excellent answer to all these requirements. For example the figure 1 shows the trend in bacteria retention potential according to the number of washes for surfaces, in various types of material, used.
 With regard to laws and standards, as confirmation of this fact, we should remember that, in Italy, there is a list of appropriate stainless steel types in the Decree of 21 March 1973 which lays down “Regulations on the hygiene of packaging, containers and tools intended to come into contact with substances for food use or with substances for personal use”. This list numbers, with the relevant updates, around thirty types of stainless steels; definitely among those most commonly used are AISI 304 and 316 (EN 1.4301 and 1.4401) and also low carbon grades, respectively 304L and 316L. The same decree states, under Art. 37, the limits of specific migration for the items in stainless steel intended for prolonged or brief contact with food substances. These limits are fixed on the basis of official tests and are both for Chromium trivalent (Cr III) and for Nickel (Ni) 0,1 ppm max. In any case these limits are much higher than the actual values found in practice. The important thing is, rightly, the safeguarding of consumers.
Furthermore in Italy the Decree Law no. 174 of 6 April 2004 (Regulations concerning materials and objects to be used in permanent plants for the collection, treatment, feeding and distribution of water for human consumption) recently came into effect, this is a specific decree about materials suitable for coming into contact with potable water; it states that all stainless steels identified in the 21.3.1973 Ministerial  Decree as suitable for contact with foodstuffs (with any restrictions stated therein) may also be used in production of items destined to be in contact with potable water.
On 27 October 2004 the European Parliament issued a regulation (n. 1935/2004) on materials and articles intended to come into contact with food which aims at standardizing the specific laws of member states and which repeals Directives 80/590/EEC and 89/109/EEC. In Europe together with the 1935/2004 EC regulation there are other laws and standards which give specific directives about the use of stainless steel in contact with potable water. They are:
- EN 10312: Welded stainless steel tubes for the conveyance of aqueous liquids including water for human consumption – Technical delivery conditions
- DVGW W 541: Rohre aus nichtrostenden Stählen und Titan für die Trinkwasser-Installation; Anforderungen und Prüfungen
- ANSI/NSF 61: Drinking water system components – Health effects
- BS 4127: Light gauge stainless steel tubes, primarily for water applications
- UNE 19049-1: Tubos de acero inoxidable para instalaciones interiores de agua frìa y caliente
- JIS G 3448: Light gauge stainless steel tubes for ordinary piping
- DWI (Drinking Water Inspectorate) Application 56.4.477: Operational guidelines and code of practice for stainless steel products in drinking water supply
- French Decree 13 January 1976 (citato come riferimento nel documento : « Arrêté du 29 mai 1997 relatif aux matériaux et objets utilisés dans le installations fixes de production, de traitement et de distribution d’eau destinée à la consommation humaine-Journal officiel du 1er juin 1997 »: Journal Officiel de la République Française – Matériaux au contact des denrées alimentaires – Edition mise à jour au 4 juin 1997 - Arrête du 13 Janvier 1976 relatif aux matériaux et objets en acier inoxydable au contact des denrées alimentaires (Journal officiel du 31 janvier 1976)
- NF A 36-711: Aciers inoxydables destinés à entrer au contact des denrées, produits et boissons pour l’alimentation de l’homme et des animaux
- Kiwa BRL-K762/02 : Beoordelingsrichtlijn - voor het Kiwa-productcertificaat voor naadloze en gelaste roestvast stalen buizen voor waterinstallaties
- NBR 14863: Reservatòrio de aço inoxidàvel para àgua potàvel
In addition to the Ministry of Health specific migration tests, it should however be noted that other tests, performed in various European laboratories, gave successful results. Table 3 lists a collection of these experiences.
To prove the growing interest of the potable water sector towards the use of stainless steel we shortly report some applications done during last years.
 The growth start up
The growth of the stainless steel use in the potable water sector started up more than 10 years ago. In 1997, the town of Venaria Reale (TO) redeveloped about 550 m of an old potable water pipeline made in cast iron (600 mm diameter) with a new pipeline made in 304 stainless steel grade, thickness 2,5 mm. Three years later, in 2000, also the town of Padova redeveloped 235 m of a pipeline using stainless steel grade 304, thickness 3 mm. In the same year the town of Livigno (SO) has made a water collection tank with the capacity of 2.000 m3 (figure 2). Grade used was AISI 304. The realization of the tank was based on the assembly of prefabricated panels already cladded with stainless steel. Panels were welded together on site. Stainless steel has been chosen on the base of a Life Cycle Cost with a period life of 50 years.
Nus aqueduct (Valle d’Aosta)
During the flood of 14 and 15 October 2000, the town of Nus lost part of its water network, including springs and water tanks. The new design provides for the use of stainless steel pipes inside the two new water tanks, each having a capacity of about 150 m3. All the parts placed inside in the water tanks are made of grade 304 (figure 3); pipes, ladders, conduits for electric cables, etc. The pipes, with major diameters varying among 150, 100, 80, and 50, are 3 mm thick. About 3.000–3.500 kg of stainless steel were used.
Como Lake
Among the most important applications of the last years there is certainly the purification plant of the Como lake. Built in 2002, the plant has doubled the old conditioning capacity increasing it up to 16 million cubic metres, equal to 600 litres per second. For the realization of the plant it has been used about 49 tonnes of stainless steel, grades 304 and 316, this last one for the lines conveying ozone and chemical additives. Total pipe length is 985 m (figures 4 and 5) with diameters DN 400/550/700 and thicknesses 3 and 4 mm.
 Bascapè aqueduct (Pavia)
The aqueduct network in Bascapè, in the province of Pavia, is drawn from an aquifer through a well located in the township. A water purification system was necessary; the system is capable of treating 72 m3/h of water, enough to satisfy users’ current and future requirements. Also for this application the grade chosen for the piping was 304. The pipes reach an overall length of 115 m (figures 6 and 7).
Lodrino (Canton Ticino – Svizzera)
The village of Lodrino, in the Ticino canton, draws the water for its water system from a torrent close by which, because it is in the mountains, also carries solid elements (leaves, branches, etc.). It is thus necessary that the water be purified from foreign bodies. The pre-existing treatment plant in carbon steel, which was very run-down (figure 8), has been rebuilt (figures 9 and 10) using stainless steel grades 304 and 304L. The filtering plant (figure 11) is made up of a stainless steel plate, which supports the weight of the sand while it is being washed through the injection of both air and water together.
Aqueduct in Valtellina
On 2005 the town of Valdisotto (SO) has ended the works for the realization of two collection tanks for the aqueduct (figure 12). The tanks capacities are respectively of 2.000 m3 and 500 m3. Stainless steel has been chosen for its low maintenance and hygienic properties. The tanks (figure 13) were fabricated using the same system adopted for the tanks of the town of Livigno: prefabricated panels (figure 14), composed of 2 parallel slabs of concrete with a thickness of 5 cm, connected to each other by means of electro-welded latticework incorporated into the casting. The surface of the slab which will enter into contact with the water is covered with a sheet of AISI 304 stainless steel, 1,5 mm thick (figure 15).
 Domestic taps
While the use of stainless steel for industrial taps has been widespread for some time now, it should be  remembered that stainless steel has also appeared in the production of domestic models (figures 16 and 17), for both bathrooms and kitchens. The increasingly restrictive demands concerning quality of water have forced some producers to seek an alternative material to that traditionally used (chromium-plated brass). With stainless steel it is therefore possible to combine design needs with those arising from current laws. Another aspect found in the interviews is that relating to durability of properties. The cleaning cycles gradually wear down the coating of traditional taps, revealing the classic golden yellow colour of the material beneath. In large developments, such as hotels, a solution which eliminates the aforesaid problem is therefore very welcome. Production starts with bar or pipe, cold worked and machined, but there are also examples of taps obtained by casting.
Conclusions
Stainless steel resists to corrosion induced by aggressive agents or by chlorination treatments. It has hygienic properties, a high cleanability and low maintenance. These features make stainless steel very suitable for potable water applications. Furthermore there are laws, standards and directives, both Italian and International, which clearly states that stainless steel is suitable for contact with potable water and nowadays a lot of tests have been done on stainless steel which confirm its suitability in the water cycle, starting from the collection and ending with the distribution, domestic taps included.
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By P. Viganò, V. Boneschi (Centro Inox)
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