Posted on January 14, 2020
Will a ductile iron coupling on stainless steel pipe cause dissimilar metal corrosion?
Strength, corrosion resistance, and low cost of maintenance are all reasons that mechanical building services projects may call for the use of stainless steel pipe. But how should stainless steel pipe be joined together? And do I need to worry about dissimilar metal corrosion if I choose a grooved ductile iron coupling? No. In this blog post, we will explain how dissimilar metal corrosion occurs and why choosing a grooved mechanical pipe joining solution is ideal for joining stainless steel piping systems.
What is Dissimilar Metal Corrosion?
Dissimilar Metal Corrosion is the electrochemical process that damages metal by reducing its strength and thickness. Dissimilar metal corrosion is also known as “Galvanic Corrosion” or “Bimetallic Corrosion.”
Three things that need to be present for galvanic corrosion to occur:
- Two dissimilar metals
- Return path
What is a dissimilar metal?
To understand dissimilar metals, you need to understand the Galvanic Series, which determines the nobility, or resistance to chemical reactions, of metals.
The Galvanic Series Chart lists metals in the order of their nobility. The metal that is more noble, and higher on the Galvanic Series chart, is called the cathode. The metal that is less noble, and lower on the Galvanic Series chart, is called the anode. Cathodic metals have a higher relative potential, or volts, than the anodic metals.
In dissimilar metal corrosion, the most severe attacks occur between metals that have greater differences in relative potentials. For example, titanium and aluminum would have a far greater or severe attack, in a dissimilar metal situation, than copper and brass would. This is due to the fact that titanium and aluminum have a great difference in relative potentials. (Which is illustrated by their distance from one another on the Galvanic Series chart.)
What is an electrolyte as it relates to metal corrosion?
To understand how and why the “attacks” occur between dissimilar metals, we will look at the flow of electrons from one metal to another.
All metals have specific relative electrical potentials. When metals of different electrical potentials are in contact in a corrosive setting, a low energy electrical current flows from the metal having a higher position in the Galvanic Series. As previously mentioned, more noble metals are cathodic; metals that are less noble are anodic. The anode metal is less likely to corrode relative to the other metal that it is in contact with.
How to create a path for metal corrosion
When two dissimilar metals come into contact in a corrosive setting, the anodic metal undergoes galvanic corrosion while the cathodic metal maintains galvanic protection. This corrosion is created because the electrolyte has a path to travel.
The image below is a representation of two dissimilar metals that are not joined properly, that have water flowing through them.
In this instance, the anodic metal is on the left. This metal is less noble, and the electrons are traveling from this metal to the cathodic metal through the pathway via the water. In other words, the water is transporting the electrolytes from the anodic metal to the cathodic metal. Depending on the metal and the setting, the anodic metal will eventually corrode away in time. The dissimiliar metal corrosion occurs because the two pipes were not joined together properly.
There are many instances where dissimilar metals are in contact with one another, and no corrosion occurs. An every day example would be when you carry a copper penny and a nickel in your pocket. Using the Galvanic Series chart we see that copper is more anodic, or active, than nickel. We have two dissimilar metals that are in contact with one another; however, neither of the coins experience corrosion. That is because we are missing an electrolyte and a path.
The same concept of the nickel and penny your pocket applies to Victaulic products in a building services project. Some projects call for stainless steel pipes. In these projects, stainless steel is most often being utilized for its strength, corrosion resistance, and low cost of maintenance. Despite being dissimilar metals, ductile iron couplings can be the best overall choice for joining stainless steel pipes.
The use of Victaulic ductile iron couplings on stainless steel pipe is common practice when pipe material selection is based on compatibility with the fluid media and where the risk of external corrosion is low. Due to the design of the coupling housing and elastomer gasket, the ductile iron couplings will never contact the internal fluid media, because the pressure-responsive gasket completely isolates the fluid media within the interior of the pipe and gasket.
Victaulic has many years of jobsite experience with galvanized ductile iron couplings installed on stainless steel systems without the report of galvanic corrosion between the pipe and couplings.
What happens when a corrosive setting is introduced?
Thinking back to the nickel and penny in your pocket, if by chance you jump into a swimming pool with the coins still in your pocket, then dissimilar metal corrosion will occur. This is because the three requirements, as previously discussed, are present in this situation:
- Dissimilar Metals: Nickel and Copper
- Electrolyte: Water
- Pathway: Electrons Travel via the Water
Can I use stainless steel couplings on stainless steel pipe?
Yes, you can use stainless steel couplings on stainless steel pipe; however, it can be costly and may not be necessary. Some projects will specify stainless steel piping because of the external environment surrounding the piping system. While the fluid media is isolated from contact with the coupling housings by the gasket, the pipe joint must be protected from exterior water.
Situations where exterior moisture can build-up and where the dissimilar metals are in contact include:
- pipe sweating
- buried applications
In these situations, dissimilar metal corrosion will occur causing the galvanized ductile iron housings to show signs of surface corrosion similar to any ferrous metal when subjected to moist environments. If stainless steel piping has been chosen because of aggressive external environmental conditions surrounding the piping system, then the system engineer of record must evaluate the suitability of painted vs galvanized couplings. Or for extremely corrosive external exposure, the system engineer may wish to use stainless steel couplings.
Can ductile iron couplings be used on copper tube?
Yes. Victaulic has a long history (since 1988) of providing ductile iron couplings for use on copper tube.
The Victaulic copper connection system provides a fast, easy, clean, reliable method for joining roll grooved copper tubing with no weld/solder flame, cutting oils or metal chips. Victaulic copper connection products are permitted for use on aboveground or underground potable water systems by national organizations such as International Association of Plumbers and Mechanical Officials (IAPMO) and International Plumbing Code (IPC).
Copper connection products have been successfully used in buried service applications for many years. Soil conditions in buried pipeline services must be examined to determine what corrosive effect they will have on the system components, including the coupling housings. The system designer has a responsibility to incorporate the appropriate protective coatings on the couplings and flange adapters to protect from galvanic corrosion and aggressive soil conditions such as ground water and soil acidity/alkalinity.
Victaulic couplings’ benefits address the problems of multiple negative environments. In addition to being a solution for dissimilar metals, Victaulic couplings also can provide noise and vibration control for HVAC systems. Click here to learn more.
Dissimilar metals can be connected by both rigid and flexible couplings. But how are these couplings actually different? Click here to learn more about this history, differences, and applications of Victaulic flexible and rigid couplings.