Will Using Dissimilar Plumbing Pipe Metals Accelerate ...

What types of metals are inside your plumbing system? Plumbing pipes can be manufactured out of iron, cast iron, stainless steel, galvanized pipe, brass and copper. The unfortunate truth is that all of these metals don&#;t work well together, and when they are placed together in a plumbing system without the proper precautions, they can cause galvanic corrosion, which occurs when dissimilar metals are used in the same plumbing system within close proximity, usually touching.

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Types of Metal Used for Plumbing Pipes

There are three commonly used metals for potable water plumbing pipes, including stainless steel, copper, and brass. These metals are used because of their durability and long lifespans.

• Brass &#; Up to 45 years
• Cast Iron &#; 100+ years
• Copper &#; 50+ years
• Galvanized Steel &#; 20 years on average

It&#;s important to note that mixing these metals is not generally advised due to the potential for sudden pipe corrosion and discolored water. The reason is the molecular makeup of dissimilar metals. Metal is typically classified into two categories, including Noble and base. Noble metals do not like to share their electrons, while base metals share their electrons easily. When you pair a Noble metal with a Base metal, the Base metal gives its electrons to the Noble metal, resulting in corrosion of the Base metal. If the corrosion becomes severe enough, it can lead to premature failure of the pipe.

Order of Dissimilar Metals Used in Plumbing Pipes

When we talk about plumbing pipes, we are talking about only a few different metals with copper, brass and stainless steel being the most common. According to a list by APPMFG, noble to base plumbing pipe metals in order include:

• Stainless Steel
• Copper
• Brass
• Lead
• Zinc &#; used to coat galvanized steel pipes

Stainless Steel and Copper Plumbing Pipes Within the Same System

By looking at the list of common plumbing materials, you would think that copper and stainless steel could easily be connected together and cause minimal problems in the system. Unfortunately, this is not the case. While stainless steel is a common plumbing material for water mains and fittings because it has an exceptionally long lifespan and is resistant to corrosion, it&#;s not a good idea to directly connect stainless steel to copper. This is because stainless steel and copper are actually dissimilar metals.

When two dissimilar metals are directly connected together, the lesser of the two metals experience galvanic corrosion, which on a molecular level means that one metal readily gives up its electrons and one metal holds onto their electrons. When stainless steel is directly connected to copper, copper galvanic corrosion occurs. Though, there is some debate as to whether the copper corrodes or the stainless steel. According to Copper.org, the stainless steel plumbing pipes corrode. According to PHCP Pros, the copper corrodes because the stainless steel is a more noble metal when compared to the copper. While the metal that corrodes more readily may be up for debate, the fact that one metal will corrode faster than the other is not up for debate.

Stainless Steel Plumbing Pipes and Brass Plumbing Pipes Within the Same System

Stainless steel pipes and brass pipes and fittings work together better than stainless steel and copper. However, the Chicago plumber installing the stainless steel and brass pipes and fittings needs to pay attention to which types of stainless steel he or she is using. This is because connecting stainless steel plumbing pipes to brass fittings and pipes is risky. Unless the plumber knows the specific types of stainless steel and brass that work well together, connecting them should be avoided. The reason is because only certain types of stainless steel and brass can be used together. If the wrong types are used together, the system will experience galvanic corrosion.

The Reason Some Stainless Steels and Brasses Do not Work Together

The metals should have no more than a .15V to .25V on the anodic index or metals that are extremely close together as listed on the index. In the case of stainless steel (410) and brass, they are seven metals apart, which means they would be likely to corrode if connected directly. When it comes to stainless steel (301, 304 and 310) and brass, they are only two metals apart, which means they can be safely connected together with minimal risk of galvanic corrosion.

Conditions that Lead to Galvanic Corrosion

Three conditions must be met in order for galvanic corrosion to occur, according to Corrosion Doctors.

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• Two or more dissimilar metals must be located within the system.
• There must be electrical contact between the dissimilar metals.
• An electrolyte must be present. (Water, moisture, and humidity can be considered electrolytes under the right conditions.)

How to Prevent Copper Galvanic Corrosion

• Only use similar metals within the plumbing system and double-check that they are similar by referencing the anodic index.
• Never use dissimilar metals when installing the plumbing system, including fasteners, hangers, bolts, screws, etc. Even a screw in a dissimilar metal can cause accelerated corrosion around the screw.
• If dissimilar metals must be used within a plumbing system, connect the two pipes with dielectric connections, which will prevent electrical contact between the two dissimilar metals.

Plumbing Pipe Inspections with Nu Flow

If you are uncertain as to whether you have dissimilar metals in your plumbing system, or if you are experiencing potable water that is discolored or stains fixtures, it is imperative that you schedule a plumbing pipe assessment. These assessments include a camera inspection of the insides of your plumbing pipes. This can help identify the types of materials used in your system, the amount of useful life left in the pipes and if there is any corrosion, including galvanic corrosion, or damage to your plumbing pipes that need to be immediately repaired in order to prevent leaks and pipe blowouts. Our building pipe assessment also includes a recommendation for or against plumbing pipelining, which can help stop future corrosion as well as current and future pinhole leaks.

EB Industries is often asked how we join dissimilar metals together with precision welding. The most commonly requested combination is copper to stainless steel since both metals have unique material qualities which make them very different, yet great when combined. Copper has very high thermal and electrical conductivity, but it is fairly soft and malleable.  Stainless steel has good hardness and high tensile strength, but it is not a great conductor. Also stainless steel is usually a less expensive material than copper. With the right experience and technology, these two unique metals can be cost effectively welded together for a high strength leak tight joint. The photo attached to this article shows a weld section of a 0.020&#; thick 316L stainless steel tube electron beam welded to an OFC copper plug. As can be seen, amazing results can be obtained with the proper design and the right welding technology.

When designing the weld joint, the alloys of the stainless steel and copper should be considered first.  The most common stainless alloys used in this type of application include 304, 304L, and 316L.  316 is not a recommended material as its higher carbon content makes it difficult to weld in general, let alone in a dissimilar metal joint.  Preferred alloys of copper include the class of the oxygen-free copper (OFC) and oxygen-free high thermal conductivity (OFHC) alloys. These alloys typically have 99.95% or better copper purity with very low levels of oxygen and other chemical elements.

The alloys listed in this article are by no means the only alloys that can be joined. However, as different elements are added to the weld pool, the metallurgy of the joint can change drastically making the welding more challenging.  For example, brass, which is made up of a combination copper and zinc, has significant challenges welding as the zinc boils out of the weld pool leaving voids and stress cracks. Sometimes these problems can be overcome by applying different welding technology, thermal treatment before and after welding, and the use of filler material. For any dissimilar material joint, thorough weld development and testing by a qualified and experienced precision welder or welding job shop, such as EB Industries, is highly recommended.

An important aspect of welding stainless steel to copper is the physical design of the joint. Problems occur if the joint is designed where the weld energy comes through the copper first. Copper, being a great conductor of heat, tends to require a significant amount of welding energy before it melts. The high amount of energy causes two problems. First, the extra energy causes a larger than necessary heat affected zone and thermal distortions. Second, as the energy comes through the copper into the stainless steel, its high energy basically disintegrates the stainless steel causing holes and blow outs. A much better design would be to have the weld energy come through the stainless first and then into the copper.  The advantage of this design is the right amount of heat can be applied to fuse the stainless to the copper. Any extra energy is quickly dissipated into the copper, much like a heat sink.  The design gives a much more forgiving weld, and ultimately uses much less energy to fuse the parts.

Electron beam welding is the preferred welding process for welding copper to stainless steel. The main reason for this involves the fact that EBW is a great process for welding copper in general, which is the more challenging of the two metals to weld. The electron beam has a very high energy density, which is not significantly impacted by the thermal conductivity of the copper, nor copper&#;s high reflectivity. The energy of the electrons quickly penetrates the surface of the copper resulting in a very narrow and deep weld pool. This pool can then be easily controlled to the required penetration depth.

Laser beam welding is generally thought to not be a good welding process for joining copper to stainless steel. However, lasers have advanced in the last few years to the point where good results are regularly obtained. The primary limitation with laser beam welding stems from the fact that copper is highly reflective to laser beam light.  For a CO₂ laser, almost 99 percent of the incident light energy is reflected away. Lasers with shorter wavelengths such as Nd:YAG perform much better. Additionally, there are tricks that can be done with pulsing the laser to break the surface reflectivity.

Overall, electron beam welding welds copper to stainless steel better than laser beam welding. However, in applications with smaller joints and less weld penetration required, laser beam welding can provide excellent results.

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