Water and its effects.
The use of untreated water (= tap water) for the technical application described herein is not advisable, since this type of water does not possess the necessary level of purity in most cases. For this reason, tap water must be treated appropriately in order to be used as feed water for the sterilization chamber and/or the accompanying steam generator in technical systems.
相关标准中规定了必要的水质，其中DIN EN 285标准目前适用于消毒器的水质领域(见表B1)。在实际应用中，自来水通过单一混合床离子交换和/或涉及膜技术（反渗透）的系统进行处理（实际淡化/脱盐化）。在某些情况下，还可以使用额外的电化学脱盐工艺。在每种情况下，目标是以有目标和控制的方式降低自来水中的离子或矿物质含量，水的特定电导率通常被用作水质的重要指标。
The necessary water quality is specified in the relevant standards, with the DIN EN 285 standard being currently applicable to the area of water quality for sterilizers (see Table B1). In practice, tap water is treated (for actual desalination / demineralisation) by means of a single mixed-bed ion exchange and/or in systems involving membrane techniques (reverse osmosis). In some instances, additional electrochemical post-demineralisation processes may also be used. In each case, the goal is to reduce the ion or mineral content of the tap water in a targeted and controlled manner.
Water constituents and treatment
Though one will occasionally encounter re- ferences to 'deionised water' in practice in the context of technical water treatment for steam generation, it should be noted that 'deionised water' is not a defined normative term. In the binding Ph. Eur. and USP compendiums, 'deionised water' is defined as 'aqua purificata'. As a result, each individual is free to interpret the meaning of the terms 'deionised water' or 'partially deionised water' for his/her own field of activity. From a technical standpoint, this state of affairs is highly unsatisfactory, especially when one considers that it would appear to be extre- mely important to have on hand at all times a precise definition of the standardised water quality required for specific applications, and also to be able to take the appropriate steps to monitor compliance with the required/defined quality limits for the water constituents.
Particularly where the use of mixed-bed ion exchanges in water treatment is concerned, both a continuous monitoring of the specific electrical conductance of the demineralised water and testing for the silicic acid content in treated water are important, since the latter (or in analytically correct terms, the silicate content) is not covered by the typical measurement for specific electrical con- ductance.
A required—and achieved—specific elect- rical conductance value of 5 µS/cm therefore does not automatically equal a permissible silicate content (of less than 1 mg/l). The amount of silicate or silicic acid can only be determined using chemical me- thods, not by measuring electrical conductance. Specifically where standardised mixed-bed ion exchanges are used, one encounters the reoccurring problem of the silicic acid / silicates breaking through the ion exchanges. This is why the silicic acid / silicate concentrations which are ultimately present in the treated water are frequently cited as being the (partial) cause of the formation of yellowish-brown to violet-blue discolourations found on stainless steel surfaces.
Additional information is available from the Red Brochure published by the German Instrument Reprocessing Workgroup AKI ('Instrument Reprocessing, Reprocessing of Instruments to Retain Value') at www.a-k-i- .org.
Another element that can in some cases be found in inadmissibly high concentrations in treated water following an incomplete or improper water treatment process (e.g. during water softening) are chloride ions. Even where small amounts on the ppm scale are present as the stainless steel surface dries following thesterilization process, this is enough to cause a critical accumulation on the surfaces. At low chloride concentrations, austenitic stainless steel alloys with low pitting resistance equivalent numbers in particular, such as 1.4301, 1.4404, 1.4571 or other similar alloys, exhibit a susceptibility to what is known as chloride-induced (local) pitting and to stress corrosion cracking.
Figure 3: Chloride-induced pitting
Another important factor for water treatment is the actual oxygen content in the water. At room temperature, natural water will always contain a certain amount of dissolved atmospheric oxygen. This oxygen content can be greatly reduced by the processes applied during water treatment – in particular, through heating.
However, because oxygen—in addition to other conditions—is critical to the formation of passive layers and also in regard to maintaining these layers on stainless steel surfaces, the aspect of oxygen availability is extremely important for any other considerations. (see also Chapter 2) Disruptions in the passive layer, which is rich in chromium oxide, inevitably cause the material to lose its chemically inert surfac e properties and, as a direct result, lead to the possible appearance of corrosion effects under critical environmental conditions.