Mixed installation and corrosion: the flow rule, material pairings and what always works

What a mixed installation is – and why it is state of the art

When different metallic materials are used in one continuous drinking-water installation, this is called a mixed installation. It is not prohibited but the normal case: the codes of practice (DIN 1988-200, EN 12502) presuppose it and require the designer to assess the materials for corrosion chemistry. It becomes a problem at one point only – where a less noble material lies downstream of a more noble one.

The mechanism is contact corrosion: oxygen-rich drinking water picks up metal ions as it flows through more noble components – with copper these are copper ions – and deposits them downstream on less noble surfaces. On galvanised steel these deposits form local cells; the zinc and, beneath it, the iron dissolve until the wall is destroyed. Viega calls this copper-induced pitting in its planning manual; the less noble materials in the drinking-water field are above all galvanised ferrous materials. In the reverse case – less noble first, then more noble – the risk is markedly reduced.

The flow rule in practice

Less noble first, then more noble. Galvanised steel (an existing riser, a water-meter bracket, a galvanised distribution line) belongs upstream of copper and gunmetal in the direction of flow, never downstream. A copper press system such as Viega Profipress or SANHA-Press may therefore lie downstream of a galvanised line, but a galvanised line must not lie downstream of Profipress. If the building situation forces the order, the manufacturers state that a component of gunmetal or silicon bronze with a length of one pipe diameter (a non-ferrous valve, an adapter) usually serves as the separation.

Stainless steel is neutral. Stainless-steel system components (Viega Sanpress Inox, Sanpress with stainless-steel pipe, SANHA NiroSan) may, according to the manufacturers, be combined with other materials irrespective of the flow direction – stainless steel releases no ions that cause damage downstream. That is why stainless steel is the material of choice when existing and new pipework meet in an unclear order.

Unalloyed and galvanised steel stays out. The steel systems of the press world – Viega Megapress and Temponox, SANHA-Therm and Heavy Steel Press, also Prestabo – are intended for closed heating and cooling circuits and industrial plant and are not approved for drinking water or other open systems: the constant ingress of oxygen would corrode the system from inside. The manufacturers therefore mark pipes and connectors with the symbol “not for drinking-water installations”. Within a closed circuit the flow rule plays no role, because the oxygen is consumed after filling – there steel, copper and stainless steel may lie in any order.

The materials in detail

Copper and gunmetal

Copper system pipe to EN 1057 with press connectors of copper or gunmetal (Viega Profipress, SANHA-Press, PURAPRESS of lead-free silicon bronze) is proven in drinking water but must observe the flow rule: never upstream of galvanised ferrous materials. Copper is also sensitive to soft, acidic waters – the limits of use to DIN 50930-6 (pH, hardness) apply irrespective of the joining system.

Stainless steel (1.4401, 1.4404, 1.4521)

Stainless-steel system pipe with connectors of stainless steel or gunmetal (Viega Sanpress Inox, Sanpress, SANHA NiroSan with at least 2.3 % molybdenum in material 1.4404 according to Sanha) is uncritical in drinking water in any order. Its weak point is outside: chlorides from the medium (swimming-pool water, chlorinated systems) and from the surroundings (chloride-containing insulation, screed additives, spray water with de-icing salt, cleaning agents) can trigger pitting and stress-corrosion cracking. Manufacturers therefore demand low-chloride insulation and dry installation; for moisture-prone areas there are dedicated versions (at Sanha the NiroTherm system for the screed).

Unalloyed and galvanised steel

Thick-walled steel pipe (Megapress, Heavy Steel Press), galvanised system pipe (Temponox, SANHA-Therm, Prestabo): closed circuits only, no drinking-water, rainwater or service-water lines. With galvanised pipes the zinc layer is the corrosion protection – manufacturers stipulate that only the components belonging to the system are used, because foreign fittings can damage the protective layer at the pipe end.

Brass

Brass system components (adapters, threaded parts) must, like stainless steel, be protected from high chloride concentrations, here because of the risk of dezincification; for drinking water the low-lead, dezincification-resistant materials of the German UBA evaluation basis apply. At Sanha lead-free silicon bronze has been standard for adapter and threaded fittings since 2009.

Multilayer and plastic pipe

Multilayer composite pipe (Geberit Mepla, FlowFit, Viega Raxofix) and polybutene pipe (GF iFIT) take no part in contact corrosion; only the metallic connectors and adapters (gunmetal, brass, PPSU) are critical. Multilayer pipe electrically separates a copper floor from the steel riser – one reason why it so often forms the bridge in refurbishment.

External corrosion: chlorides, moisture, insulation

Most damage to stainless-steel and steel lines does not arise in the medium but from outside: under damp insulation, in the screed, in shafts with condensate, under de-icing salt. Three rules that all system manufacturers carry in their planning documents: first, low-chloride insulation materials (observe the manufacturers' statements on freedom from chlorides) and no insulation that retains moisture; second, no installation in screed without a protective conduit or a system expressly approved for it; third, no cleaning or disinfection agents containing chlorides on unprotected surfaces. Galvanised steel additionally needs an intact zinc layer – damaged insulation that lets water reach the pipe wall is the most frequent cause of external corrosion in heating circuits.

Transitions between systems – they are always possible

From our practice (experience value): the transition between two systems can always be made – if in doubt via a threaded joint or a flange. Every press system carries adapters with female or male thread, every steel-pipe system flange adapters; with these, copper can be connected to stainless steel, multilayer pipe to steel and existing to new pipework. The flow rule decides only the order, and a gunmetal or silicon-bronze adapter takes over the separation at the critical point.

What to check at the transition: material and order (flow rule), sealing element and medium (see Seals and media), profile and tool of both systems (see Press profiles and tools), and whether the existing pipe is a pipe that a press system releases at all – the pipe dimension tables show the released press systems for each size. For malleable iron fittings, stainless-steel threaded fittings and flanges the availability sentence applies: The standard names dimensions. What is listed here, we carry – from stock or to order; how quickly a size is available we clarify on request.

Further reading

Tools of this section: System finder (which system suits medium, pipe and size) · Joining methods compared · Cost comparison calculator · Press systems overview.

The cross-cutting topics: Press profiles and tools · Seals and media · Leak testing and SC-Contur · Zeta values of press connectors