Classic copper

Copper is a precious metal that has influenced the development of human technology and culture for millennia. Since it is easy to form and harden, (or anneal), copper and its alloys remained the only relevant metal for tools, cult objects and many everyday implements for over 5000 years.

Table 1: Overview of copper materials that have no longer been included on the European positive list of materials since 2011 and will no longer be permitted in the future from 2024 due to the reduction of the permissible lead content in drinking water.

To this end, a European initiative was launched back in 2011 by the Federal Republic of Germany, the Kingdom of the Netherlands, the United Kingdom of Great Britain and Northern Ireland, the Republic of France and later also Denmark under the name 4MS, which published a positive list of safe materials in contact with drinking water. This list has now been adopted into European law and further tightened. In order to create a regulated transition, limit values have been introduced for lead in drinking water, for example. These were successively lowered and the number of known copper materials in contact with drinking water was continuously restricted. As a result, new copper materials have been developed for drinking water applications and fittings made from these materials have been manufactured and installed - in one case since 2009 - and this trend is continuing. When developing new alloys (and subsequently products), technical and physical properties must be taken into account in addition to hygiene and health considerations in order to achieve the desired longevity, for example.

The most important alloying elements [Dies 1967]

The main common alloy components include lead, antimony, arsenic, phosphorus and silicon as well as tin, manganese and nickel. The last three form solid solutions and have a strength-enhancing effect. Tin also improves machinability.

Technically, lead was used in copper-zinc materials primarily because of its positive effect on machining. Lead has no major influence on the mechanical properties. The addition of lead reduces the risk of stress corrosion cracking.

Antimony has a positive effect on dezincification resistance and thus improves the corrosion resistance of the corresponding alloy. However, it is difficult to cold form and leads to embrittlement of the material. As it is difficult to remove from the recycling cycle or return to it in a meaningful way, this would result in the recycled copper materials becoming brittle in the long term.

Arsenic is mainly added to copper alloys to improve corrosion resistance. A proportion of 0.02-0.06 % has a positive effect on dezincification. The same applies to phosphorus (in combination with or without arsenic).

Silicon is used to increase the strength and hardness of the alloy as well as the tarnish resistance. Oxidation and zinc evaporation are reduced.

Phosphorus has a positive effect on dezincification resistance, even at very low levels. The optimum range is 0.02-0.06 %.

Like lead, antimony and arsenic are already under legislative scrutiny due to health concerns. The reason for this is their toxicity. Their use is unsafe in the long term, as the limit values are constantly being tightened here too (0.010 mg/l arsenic, 0.005 mg/l antimony).

Development and evaluation of new copper materials

In the past, so-called binary alloys (CuZn, two-material alloys, binary brass) were used. These are characterized by the fact that, depending on the zinc content and further processing (heat treatment), they can have the beta phase (and others) in addition to the alpha material phase. Numerous multi-material alloys have been developed to improve certain properties such as corrosion and dezincification resistance, as chip breakers or for better processability. These include, for example, complex materials such as CC499K (gunmetal, CuSn5Zn5Pb2) or CW602N (CuZn36Pb2As).

In addition to hygiene requirements from EN 15664, physical and technical requirements for materials in contact with drinking water are also essential for assessing the suitability of these new materials for drinking water installations. There are several Danish and international standards that address the issues of resistance and corrosion and provide clear recommendations for action to ensure compliance with legal requirements. The most important of these are:

• EN 12502-2: 2005 Protection of metallic materials against corrosion – Guidance on the assessment of corrosion likelihood in water distribution and storage systems Part 2: Influencing factors for copper and copper alloys
• EN 1254-7: 2021: Copper and copper alloys – plumbing fittings – Part 7: Press fittings for use with metallic tubes
• EN 806-4: 2010: Specifications for installations inside buildings conveying water for human consumption – Part 4: Installation
• ISO 6509-2: 2017: Corrosion of metals and alloys — Determination of dezincification resistance of copper alloys with zinc

EN 12502-2 describes seven forms of corrosion: Uniform, Pitting, Selective, Bimetallic, Erosion, Stress and Fatigue. Every piping material, whether steel, plastic or copper, has its own strengths and weaknesses. It is crucial to note that not only the chemical composition but also the processing of the materials has a significant influence on their resistance. Cast materials with the same chemical composition have a slightly poorer corrosion resistance than the comparable wrought material due to surface pores or segregations [GDM 1997]. Much more important, however, is the heat treatment, which leads to different phase proportions in the microstructure of the same alloy with the same composition, see Figure 2. Even minor alloy adjustments within the standardized materials lead to different material structures and thus to different physical properties. This is why there are different brand names for the same material that complies with the standard, such as silicon bronze, cuphin or ecobrass, which are all slightly different variants of CW724R. Figure 2 shows an example of one material and two states. The left image shows approx. 11 % kappa phase and the right image approx. 2 %.

Figure 2: Two cross-sections of CW724R with different phase fractions. The left image shows approx. 11 % kappa phase and the right image approx. 2 %.

As this has such a great influence, a test for selective corrosion has been developed. ISO 6509 makes the dezincification resistance of copper-zinc alloys comparable and therefore assessable. It should be noted that a chemically identical material may or may not be resistant to dezincification depending on processing and heat treatment.

EN 806-4 identifies the issue of bimetallic corrosion of stainless steel pipes with fittings made of copper and copper alloys as unproblematic, see Table 1. It is therefore not surprising that there is no Danish or European test standard for this. Instead, the combination of stainless steel pipes with fittings made of copper and copper alloys is mandatory for all test waters in the hygiene test in accordance with EN 15664. This combination is generally recognized.

Table 2: Suitability of different fitting/valve materials for combination with different pipe materials.

Suitable alloys are resistant to dezincification

According to the requirements of ISO 6509, the technically conceivable copper-zinc alloys that are harmless to health can be divided into dezincification-resistant and limited dezincification-resistant alloys. The latter group is only suitable for drinking water installations to a limited extent, mainly for technical and physical reasons (higher susceptibility to corrosion), but can often be used for other applications such as heating or gases, as no oxygen or water is introduced into these circuits and therefore no corrosion processes occur, or only to a very limited extent.

The material CW724R (chemical designation CuZn21Si3P, silicon bronze) is very well suited for drinking water without any restrictions. In addition to an alpha phase, CuZn21Si3P usually has a gamma and a kappa phase. The beta phase, which is particularly susceptible to dezincification, is thus avoided. This is also reflected in the consistently very good results of the dezincification test in accordance with ISO 6509. The market launch was preceded by extensive basic material tests, which had already proven the suitability for drinking water in theory and in field tests. CW724R has also been on the market in Germany and numerous other countries since 2009, meaning that most practical experience is available with this material. No exceedances of limit values for lead or other ECHA-listed substances have been reported or proven in the entire period. In addition, silicon and phosphorus have an extremely positive influence on corrosion resistance. Alongside arsenic and antimony, phosphorus is considered a dezincification inhibitor [EN 12502] and replaces arsenic in hygienic materials, which, like lead and antimony, is considered to be of concern. The positive wear properties of intermetallic silicon phases prevent erosion. There is also a very practical consideration in favor of this material: fittings made from this alloy can be connected to both copper pipes and stainless steel pipes.

The positive experience with CuZn21Si3P since 2009 led to the introduction of CuSi4Zn9MnP (CC246E), a second silicon-containing CuZn multi-material alloy for fitting production, in 2018. The established trade name "silicon bronze" was already known and was adopted for this alloy. Although the term silicon bronze was introduced by the piping systems manufacturer for CW724R and CC246E has a slightly different composition, both have a particularly positive characteristic: With a silicon content of more than 2 %, they are insensitive to stress corrosion cracking [EN 1254-7]. CuSi4Zn9MnP also contains manganese. However, there is little or no information on the long-term effects of manganese in combination with silicon, zinc and phosphorus. It is also not known to what extent products made of CuSi4Zn9MnP in contact with drinking water have been used on the market to date. The consistently positive experience with the established silicon bronze CuZn21Si3P since 2009 suggests that this alloy is also suitable.

In 2022, another complex alloy CuSn4Zn2PS [still without CC designation] with the main alloying elements Zn and Sn will follow, which can also be used without restriction for drinking water applications. This relies on phosphorus and sulphur instead of lead. As with the two silicon-containing alloys, phosphorus serves as a dezincification inhibitor, among other things. Sulphur is mainly used for good machinability, as it forms a brittle Cu2S phase with copper, which reduces the chip length and thus improves machinability. Initial scientific findings were presented at the "Copper alloys 2022" [Haake 2022] specialist congress. The artificial ageing tests with chlorine only show superficial material attacks. Furthermore, according to the author, field tests in Germany over a period of four years did not result in any major damage.

Another material with unrestricted suitability for drinking water is currently still under development, but is already on the new positive list: CuZn35Sn1P (CW727R) uses tin and phosphorus as an inhibitor of the much-noticed selective corrosion. Extensive tests are still pending here; the experience gained from the development of materials already on the market will be incorporated.

Table 3 shows an overview of the newly introduced hygienic copper-zinc alloys that have been developed for drinking water applications instead of the lead-containing copper-zinc materials.

Table 3: Overview of the market launch of new lead-free copper materials for drinking water applications.

Alloys with limited suitability

In addition to these four copper materials, which can be used without restriction for drinking water applications, there are others that can only be used to a limited extent in technical and physical terms. For example, a material was introduced in 2023 to replace CW617 (CuZn40Pb2). The material CuZn41Mg, known as EZEEE, relies on an alloy of magnesium instead of lead. As these alloys should not be used in drinking water installations, they are not considered further here.

Conclusion

In summary, the strict regulations for materials that come into contact with drinking water, in particular the updated EU Drinking Water Directive 2020/2184, but also influences such as ROHS, ECHA and REACH, have had a significant impact on the use of copper alloys in drinking water installations. The continuous tightening of the permissible lead content and the review of other harmful substances have led to the development of safer alternatives over the years.

Efforts such as the 4MS initiative have led to the creation of the ECHA positive list of safe materials that ensure a regulated transition away from hazardous copper alloys (and others). The development of new alloys has been driven by the need to meet not only hygiene and health standards, but also technical and physical requirements for durability and corrosion resistance.

The suitability of modern copper materials for drinking water is ensured by the addition of hygienically and health-safe elements such as silicon, phosphorus or tin.

Ongoing research and development work has produced promising alternatives such as CuSn4Zn2PS and CuZn35Sn1P, which utilise phosphorus and tin to effectively prevent selective corrosion. Although these materials are still under development, they have great potential for widespread use in the future. Although these materials are still under development, they have great potential to complement the already established material CW724R and qualify for future widespread use.

Overall, the switch to lead-free copper materials for drinking water applications reflects a commitment to public health and environmental sustainability. With continued innovation and compliance with regulatory standards, the prospects for safe and reliable drinking water installations using copper alloys remain very promising.

About the author

Dr. Philipp Skoda taught and researched materials and joining technology at Esslingen University of Applied Sciences, focusing on copper materials. He later moved to the renowned DKI Copper Institute and finally to SANHA®, a leading manufacturer of piping systems. There he is responsible for the area of innovation, which includes future topics such as hydrogen and new sustainable copper materials.

Sources

ECHA 2024: EU Drinking Water Directive 2020/2184

UBA 2024: Approval and Harmonization – 4MS Initiative | Umweltbundesamt

Dies 1967: Kurt Dies: Copper and copper alloys in technology; 1967

EN 15664: 2014 Influence of metallic materials on water intended for human consumption – Dynamic bench test for the assessment of the release of metals –

EN 12502-2: 2005 Protection of metallic materials against corrosion - Guidance on the assessment of corrosion likelihood in water distribution and storage systems Part 2: Influencing factors for copper and copper alloys

ENEN 1254-7: 2021: Copper and copper alloys - plumbing fittings - Part 7: Press fittings for use with metallic tubes

EN 806-4: 2010: Specifications for installations inside buildings conveying water for human consumption - Part 4: Installation

ISO 6509-2: 2017: Corrosion of metals and alloys - Determination of dezincification resistance of copper alloys with zinc

Haake 2022: Haake, M. (1); Hansen, A. (2): CuSn4Zn2PS – Lead Free Gunmetal for drinking water applications; Copper Alloys 2022 – Düsseldorf, 22. - 23. November 2022

GDM 1997: Gesamtverband Deutscher Metallgießer GDM, Deutsches Kupferinstitut DKI, Verein Deutscher Gießereifachleute VDG: Guß aus Kupfer und Kupferlegierungen -Technische Richtlinien-; 1997