Colglo Blog

Plumbing and Heating - joining copper pipe to pipes of dissimilar materials

Posted by Collister & Glover on Aug 16, 2018 3:33:17 PM
Collister & Glover

Many plumbing and heating installations make use of copper tube and fittings for water and gas services. However, in the past, many other materials were used, and still are used to manufacture or join pipes, tubes and fittings. Sometimes work on these mixed metal pipe joints is unavoidable. This post shows different methods that can be used to join copper pipe to other dissimilar materials.



Regulations for Drinking Water UK, EU and WHO standards

Currently in the UK all drinking water, whether from public supplies or other sources, has to meet standards laid down in the EU Drinking Water Directive (98/83/EC). Each EU member state then translates the requirements of the directive into its own local laws.

UK regulations follow the EU directive, but some regulations are stricter than those defined by the directive. In the UK legislation and regulations are specific to each of the four devolved administrations:

There are exceptions - connecting pipes made from dissimilar metals

Galvanic Corrosion: The regulations require that tubes and fittings made from different types of metals should not be connected directly together. The only exceptions are where galvanic corrosion is unlikely, or where prevention measures are in place to prevent it.

What is Galvanic Corrosion?

Galvanic corrosion occurs when two dissimilar metals come into electrical contact with a conductive electrolyte. This can be water within the pipe, rainwater or groundwater.

This process called electrolysis causes a metal atom to be oxidized, it leaves its bulk metal. The atom then loses one or more electrons which allows it to be transferred to another site. The site where the metal atoms lose electrons is called the anode, and the site where the electrons are transferred to, is called the cathode.

This process in effect creates an electric cell where the water acts as the electrolyte. The tiny current flow created will, over time, cause one metal to corrode.

For heating systems and drinking water we are looking at flowing water. Flowing water in a pipe will not be a problem if the adjoining metals are close to each other on the Galvanic Series, metals such as copper and tin, as this closeness reduces the risks of galvanic corrosion.  The Galvanic Series is also referred to as a Galvanic Scale.


Minimising Corrosion

So, the problem of corrosion occurs when the joining metals are further apart on the galvanic scale, the further apart they are, the stronger the corroding effect will be on the more active metal.

To minimise galvanic corrosion in the pipe, we can use metals that are not dissimilar (this is where corrosion is unlikely) or prevent dissimilar metals form becoming electrically connected by water (using prevention measures).

Copper connections to lead pipe

The water regulations specifically prohibit the introduction of lead for potable water fittings.

Any lead pipe system requiring repair would be best removed and replaced completely with copper. This is not always practical. Where a joint has to be made to lead pipe then a "lead-lock" type compression joint can be used.

Image 1. Lead to copper compression joint

Lead to copper compression joint

The compression joint shown above utilises a large rubber compression ring/seal to mitigate problems that occur when using lead pipe, primarily the pipes softness and varying diameter.

How to connect copper pipe to an existing lead pipe

  • If possible find a straight section of lead pipe and cut it square.
  • Any identification strip needs to be removed before fitting and tightening the joint.
  • Any underground copper pipe should be EN 1057 (previously Table Y) thick wall half hard soft coiled copper. If possible, this should also be plastic coated. Either 'CR' marked Type 'B' compression joints or EN 1254 Capillary joints can be used.
  • Above ground installations can be EN 1057 (previously Table X) thin wall half hard pipe, and either Type ‘A compression joints or ’EN 1254 capillary compression joints can be used.

Joints that can be used to Connect Copper to Threaded Galvanised and Plain Steel Pipe

Below are examples of male – female capillary and compression fittings that can also be used for threaded galvanised or plain steel pipe.  All can be bought with male and female BS pipe threaded ends.  

For the joints 2.a, 2.b, and 2.c below these should only be assembled with either BS approved sealing paste or a 0.2mm PTFE tape. (PTFE tape approved to BS EN 751-3 should be used for gas services.) 

A 50% overlap must be left when wrapping the joint.

How to disconnect and re-assemble pipe joints of dissimilar metals

Sometimes joints need to be disassembled, so which of the three choices available below are suitable?

Well, if regular disconnection of a joint is needed then a faced union joint is best, see Image 2a.

Image 2a. A Union Joint can be disassembled

A Union Joint can be disassembled

Image 2b. A compression joint will allow only occasional disassembly or disconnection.

A compression joint will allow only occasional disconnection.

if a capillary joint with a threaded end is used to connect directly to the steel tube, (Image 2c) it will be difficult to dismantle or remake the threaded joint.

Image 2.c Male & Female Joints. A Plain Connector

Male & Female Joints. A Plain Connector

Joining Copper Pipe to Stainless Steel Pipe

Stainless steel is difficult to solder, usually phosphoric acid-based flux is used. Manufacturer instructions should strictly be followed and the assembled pipe and joint should be thoroughly flushed out internally and washed off on the outside of the tube after jointing.

Because such care required for stainless steel soldering, compression joints similar to those above are the preferred method of jointing. Stainless steel pipe to EN 10312 can be jointed using EN 1254 capillary or compression fittings.

Connecting copper pipe to plastic pipes

Plastic pipe can easily be connected to copper. Polythene, Polyethylene and Polybutylene can all be joined using non-manipulative type 'A' compression joints with metal liners to support the tube wall as in Image 3.

Image 3.  Type ‘A’ Compression Joint


To assemble the above, select the correct type and size of fitting and liner for the plastic pipe to be joined. Liners are often colour coded by paint marking on the flange.

If the work needs to be buried underground, Blue Polyethylene tube (MDPE) to EN 12201 can be laid, it can be jointed with EN 1254 compression fittings.

Joining Copper Pipes to PVC (Polyvinyl Chloride)

If you are connecting copper to un-plasticised PVC or chlorinated PVC you will need to solvent weld a threaded end pipe fitting to the PVC pipe a suitable compression joint can then be used.

Do not use any oil based jointing compound on plastic joints as the oil can damage the plastic. The joint becomes soft and fails. PTFE tape, fibre washers or rubber rings can be used to form a seal on plastic materials.


Good Practice when joining pipes together

It is possible to prevent corrosion by remembering the three conditions that promote corrosion and implementing good jointing practice.

For galvanic corrosion to occur there are three conditions which must be met

  • Condition 1. Metals must be far apart on the galvanic series
  • Condition 2. The metals must be in electrical contact
  • Condition 3. The metal junction must be bridged by an electrolyte

Whatever type of joint is to be made the following can all be regarded as good practice:

  • use the minimum quantity of jointing material necessary to produce good quality joints;
  • keep jointing materials clean and free from contamination;
  • remove any cutting oil if used, as well as protective coatings and clean the surfaces to be jointed;
  • prevent the entry of surplus materials into the tube bore;
  • on completion of the joint, remove all excess materials and flush thoroughly.
  • hemp must not be used for drinking water applications, as it promotes the growth of microbes and over time can contaminate the water supply.

If you require any further advice please get in touch.

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