Hot-dip galvanised components

For hot-dip galvanised components (holders, supports, handrails etc.) in waste water treatment plants that lie above the level of the waste water, it can be expected that there is an increase in the protective effect by the duplex system by approximately 50 to 200%, i.e. a four-fold protective effect, compared to the protective effect of the individual system components [43]. 

However, below the level of the water, hot-dip galvanised components are only regarded as resistant in exceptional cases [43]. This behaviour is attributed to the numerous waste water parameters [39]. The existing good practical experience is therefore often difficult to transfer to other systems and can only be generalised in a limited manner. This applies e.g. to the good experience with hot-dip galvanised components described in various publications [44—46]. 

Corrosion damage of hot-dip galvanised components in oxygen-poor waste waters with pH values between approximately 3.5 and 5.0 is reported in [47]. The reason for the damage is attributed to the low oxygen content under the layers of dirt and sewer slime. Because the oxygen content is too low, the conditions for the formation of protective layers of basic zinc carbonates deteriorate. 

Examples for the use of hot-dip galvanised components for waste water engineering are described in [75, 76]. 

Hot-dip galvanised steel components are used in sewer technology as drain gratings, covers and as dirt pans for balconies, roofs, and cellars in road and estate drainage. It is also used as buckets for drains in roads [40]. A zinc layer of at least 400 g/m is required for these components [41]. 

The combination of hot-dip galvanisation and coating is a synergetic system, i.e. the overall protective effect is greater than the sum of the protective effects of the individual components of the system [42]. This system is defined as a duplex system. 

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