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Two different phenomena can cause metal pipe walls to deteriorate or wear away — erosion and corrosion. Erosion is the wearing away of the metal’s surface by high-velocity water molecules. Corrosion is the deterioration or degradation of metal by chemical or electrochemical reactions between the metal and its environment or between dissimilar metals.
A few years ago, I was asked to investigate a hospital’s plumbing system because it was experiencing “green” water. During my investigation, I discovered that instead of installing galvanized steel water mains on the large-diameter pipe, a change was made to install Schedule 10 stainless-steel water mains.
The copper branch piping in direct contact with the stainless steel was less noble than the stainless-steel water mains. The copper was acting as the sacrificial anode and corroding to protect the stainless-steel water mains. This corrosion caused the appearance of green water at various sinks because of the high copper levels.
Dielectric unions or dielectric waterways should have been used to electrically separate the dissimilar metals to interrupt the electrical path and minimize corrosion. I asked the contractor why he didn’t install dielectric unions and he said they weren’t needed if neither material will rust. I told him it is covered in the code, and I had to explain the galvanic series of metals and why, in this case, the copper was corroding instead of the galvanized steel that he was familiar with and why dielectric unions were still needed.
Corrosion results from a flow of direct current through an electrolyte from one location on a metal surface to another location on a metal surface. An electrolyte is an ionized material, such as water or wet soil, capable of conducting an electric current.
Environmental conditions, such as soil conditions or water content, may affect the performance of a piping material, especially when dissimilar metals are connected. Metal pipe for the conveyance of liquid and gas, either below ground or aboveground, can encounter external and internal conditions that will cause corrosion of the pipe walls on the inside and outside.
Outdoors, aboveground pipe encounters weather and atmospheric conditions; underground pipe encounters soil conditions and water. Internal conditions in the pipe, or the fluid conveyed inside the pipe, can also serve as an electrolyte, and may affect the corrosion rate and performance of a piping material.
Protection of piping from corrosion is addressed in the Uniform Plumbing Code (UPC), the International Plumbing Code (IPC) and the International Residential Code (IRC). Simply put, if pipe or fittings are of dissimilar metal, the joints where there is metal-to-metal contact need dielectric connections conforming to the American Society of Sanitary Engineering (ASSE) standard ASSE 1079, Performance Requirements for Dielectric Pipe Unions.
2021 Uniform Plumbing Code
With respect to corrosion control, the 2021 UPC requires joining dissimilar metals in sections 605.15 and 605.16, and dielectric unions to conform to ASSE 1079. Protectively coated pipe or tubing, mentioned in section 312.5, is generally used only on underground piping. The relevant sections of the 2021 UPC are:
312.4 Corrosion, Erosion and Mechanical Damage.
Piping subject to corrosion, erosion, or mechanical damage shall be protected in an approved manner.
312.5 Protectively Coated Pipe.
Protectively coated pipe or tubing shall be inspected or tested, and a visible void, damage or imperfection to the pipe coating shall be repaired in an approved manner.
605.15 Dielectric Unions.
Dielectric unions, where installed at points of connection where there is a dissimilarity of metals, shall be in accordance with ASSE 1079.
605.16 Joints Between Various Materials.
Joints between various materials shall be installed in accordance with the manufacturer’s installation instructions and shall comply with Section 605.16.1 through 605.16.3
605.16.1 Copper or Copper Alloy Pipe and Tubing to Threaded Joints
Joints between copper and copper alloy pipe or tubing to threaded pipe shall be made using copper alloy adapter, copper alloy nipple [minimum 6 inches (152 mm)], dielectric fitting or dielectric union in accordance with ASSE 1079. The joint between the copper or copper alloy pipe or tubing and the fitting shall be soldered, brazed, flared or press-connect joint and the connection between the threaded pipe and the fitting shall be made with a standard pipe size threaded joint.
605.16.2 Plastic Pipe to Other Materials
Where connecting plastic pipe to other types of piping, approved types of adapter or transition fittings designed for the specific transition intended shall be used.
605.16.3 Stainless Steel to Other Materials
Where connecting stainless steel pipe to other types of piping, mechanical joints of the compression type, dielectric fitting or dielectric union in accordance with ASSE 1079 and designed for the specific transition intended shall be used.
2021 International Plumbing Code
With respect to corrosion control, the 2021 IPC requires joining dissimilar metals in section 605.23 and its subsections, and dielectric unions to conform to ASSE 1079. The relevant sections of the 2021 IPC are:
305.1 Protection Against Contact
Metallic piping, except for cast iron, ductile iron and galvanized steel, shall not be placed in direct contact with steel framing members, concrete or cinder walls and floors, or other masonry. Metallic piping shall not be placed in direct contact with corrosive soil. Where sheathing is used to prevent direct contact, the sheathing material thickness shall be not less than 0.008 inch (8 mil) (0.203 mm) and the sheathing shall be made of plastic. Where sheathing protects piping that penetrates concrete or masonry walls or floors, the sheathing shall be installed in a manner that allows movement of the piping within the sheathing.
605.1 Soil and Ground Water.
The installation of a water service or water distribution pipe shall be prohibited in soil and ground water contaminated with solvents, fuels, organic compounds or other detrimental materials causing permeation, corrosion, degradation or structural failure of the piping material. Where detrimental conditions are suspected, a chemical analysis of the soil and ground water conditions shall be required to ascertain the acceptability of the water service or water distribution piping material for the specific installation. Where detrimental conditions exist, approved alternative materials or routing shall be required.
605.23 Joints Between Different Materials.
Joints between different piping materials shall be made with a mechanical joint of the compression or mechanical-sealing type, or shall be made in accordance with Section 605.23.1, 605.23.2 or 605.23.3. Connectors or adapters shall have an elastomeric seal conforming to ASTM F477. Joints shall be installed in accordance with the manufacturer's instructions.
605.23.1 Copper or Copper-Alloy Tubing to Galvanized Steel Pipe.
Joints between copper pipe or tubing and galvanized steel pipe shall be made with a copper-alloy or dielectric fitting or a dielectric union conforming to ASSE 1079. The copper tubing shall be soldered to the fitting in an approved manner, and the fitting shall be screwed to the threaded pipe
605.23.2 Plastic Pipe or Tubing to Other Piping Material.
Joints between different types of plastic pipe or between plastic pipe and other piping material shall be made with approved adapters or transition fittings.
605.23.3 Stainless Steel.
Joints between stainless steel and different piping materials shall be made with a mechanical joint of the compression or mechanical sealing type or a dielectric fitting or a dielectric union conforming to ASSE 1079.
2021 International Residential Code
The 2021 IRC addresses protection of metal pipes from steel framing members or corrosive soils in Plumbing Part VII, Chapter 26, section P2603, Protection Against Corrosion, which is similar to the 2021 IPC section 305.1, Protection Against Contact, but there is no mention of connections between dissimilar metals in the piping system. The relevant section of the 2021 IRC is:
P2603.3 Protection Against Corrosion
Metallic piping, except for cast iron, ductile iron and galvanized steel, shall not be placed in direct contact with steel framing members, concrete or masonry. Metallic piping shall not be placed in direct contact with corrosive soil. Where sheathing is used to prevent direct contact, the sheathing material thickness shall be not less than 0.008 inch (8 mil) (0.203 mm) and shall be made of plastic. Where sheathing protects piping that penetrates concrete or masonry walls or floors, the sheathing shall be installed in a manner that allows movement of the piping within the sheathing.
The Corrosion Cell
A corrosion cell is similar to a dry cell battery. For a corrosion cell to occur, four elements must be present:
1. An electrolyte;
2. An anode;
3. A cathode;
4. A return circuit.
A corrosion cell can occur when two different metals are exposed to an electrolyte and the current flows from one type of metal (the anode, or the less noble metal) through the electrolyte to the other type of metal (the cathode, or the more noble metal), and then returns to the anode through the return circuit. The potential (voltage) difference between dissimilar metals is an electromotive force causing a movement of electrical current through a conductor.
Corrosion occurs when the current leaves the metal and enters the electrolyte; the point where the current leaves the metal is the anode. The point where the current enters the metal is the cathode; no corrosion occurs at the cathode.
Using a sacrificial anode to protect a pipe or tank (cathode) is referred to as cathodic protection. Storage, tank-type water heaters use a form of cathodic protection by installing a magnesium or aluminum (a less expensive option) anode rod into the water heater tank (see Figures 1 and 2) The anode rod is in contact with the steel tank at the tank nozzle threads.
The anode rod (magnesium or aluminum) is a less noble metal than the steel tank (cathode), so the anode rod is a sacrificial metal that will corrode before the steel tank starts to corrode. (Note: Because of the dissolved metals in hot water, this is why “they” say you should cook with cold water.) The maintenance of a storage, tank-type water heater should include checking and replacing the anode rod to extend the life of the steel tank.
Electromotive Series of Metals
Dissimilar metals, when coupled together in a wet environment, will corrode according to Faraday’s law of induction. The electrical voltage potential for each metal is shown in Table 1 (from the Plumbing Engineering Design Handbook, Chapter 8), which shows the electromotive force series of metals listed in their electromotive force order, from the least noble metal at the top to the most noble metal at the bottom.
This list is in order of the standard electrode potentials, with positive potentials (greater than 1) for elements that are cathodic to a standard hydrogen electrode and negative potentials (less than 1) for elements that are anodic to a standard hydrogen electrode.
In most cases, any metal in this series will corrode to protect the more noble metal. There are exceptions to this rule because of the effect of ion concentrations in a solution and because of the different environments found in some installations. This exception usually applies to metals close together in the series, which may suffer reversals of potential. Metals far apart in the electromotive series will behave as expected.
The galvanic series of metals in Table 1 exemplifies why most sacrificial anodes in storage tank-type water heaters are typically made of magnesium.
Table 1. Electromotive Force Series of Metals
Metal Potential of Metal
Magnesium (galvomag alloy)a 1.75
Magnesium (H-I alloy)a 1.55
Cast iron 0.68
Carbon steel 0.68
Stainless-steel type 430 (17% Cr)b 0.64
Ni-resist cast iron (20% Ni) 0.61
Stainless-steel type 304 (18% Cr, 8% Ni)b 0.60
Stainless-steel type 410 (13% Cr)b 0.59
Ni-resist cast iron (30% Ni) 0.56
Ni-resist cast iron (20% Ni+Cu) 0.53
Naval rolled brass 0.47
Yellow brass 0.43
Red brass 0.40
Admiralty brass 0.36
90:10 Cu-Ni+ (0.8% Fe) 0.35
70:30 Cu-Ni+ (0.06% Fe) 0.34
70:30 Cu-Ni+ (0.47% Fe) 0.32
Stainless-steel type 430 (17% Cr)b 0.29
Stainless-steel type 316 (18% Cr, 12% Ni, 3% Mo)b 0.25
Stainless-steel type 410 (13% Cr)b 0.22
Titanium (commercial) 0.22
Titanium (high purity) 0.20
Stainless-steel type 304 (18% Cr, 8% Ni)b 0.15
Hastelloy C 0.15
Stainless-steel type 316 (18% Cr, 12% Ni, 3% Mo)b 0.12
Note: The lower the number, the more noble the metal. These numbers are based on potential measurements in sea water, velocity of flow 13 feet/second (3.96 meters/second), temperature 77 F (25 C).
a Based on data provided by the Dow Chemical Co.
b The stainless steels, as a class, exhibited erratic potentials depending on the incidence of pitting and corrosion in the crevices formed around the specimen supports. The values listed represent the extremes observed and, due to their erratic nature, should not be considered as establishing an invariable potential relation among the alloys covered.
Other factors affecting corrosion of metal pipe:
• Oxygen and dissolved carbon dioxide or other gases can contribute to corrosion of domestic water piping systems.
• High-dissolved solids such as salts and sulfates can contribute to chemical or bio-chemical corrosion.
• Higher water temperatures can decrease the biological rate of growth and increase chemical corrosion.
No specific language in the model codes addresses velocity limitations for circulated hot water in hot water supply and return piping for erosion control; however, all codes require following water supply fixture units for sizing, manufacturers’ recommendations and good engineering practices.
Erosion, or what is sometimes referred to as “erosion corrosion,” is the increase in the rate of wear on a metal pipe from abrasive or erosive effects of high-velocity water flow in a pipe. It typically occurs in high-temperature copper piping but can occur in other materials if the velocity is high enough and flows are circulated.
Noticeable grooves and rounded holes are indicators of erosion; usually they are smooth and have a turbulent pitting, directional or flowing patterns. Copper pipe erosion is typically noticeable when no green patina develops inside. Erosion corrosion is caused by high-velocity, turbulent water flow and can be more severe with suspended solids, which is like sand-blasting the internal walls of a pipe.
Erosion is often localized in areas where water changes direction, where there is an increase in velocity associated with turbulence, and following obstructions or components producing a concentrated nozzle flow.
The pipe walls in these areas wear thin and begin to leak. If the high-velocity condition is not addressed as soon as possible, it could cause major damage to the entire circulated piping system.
Cavitation (damage due to the formation and collapse of bubbles in high-velocity turbines, pump impellers or propellers, etc.) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough.
High-velocity erosion is a common source of water piping system problems in circulated hot water systems where the pump is oversized, or where the system is not balanced properly.
High Water Temperature and Copper Pipe Erosion
According to the Copper Development Association (CDA), high temperatures in copper piping can cause erosion at flow velocities in excess of 5 feet/second (fps) for hot water system temperatures up to 140 F. As the temperature goes up, the maximum recommended flow velocity goes down. The CDA recommends a maximum flow velocity of 2 fps to 3 fps for water over 140 F.
Over the years, I developed a velocity chart for domestic hot water systems (a little more developed than the CDA recommendations). I would suggest using the following domestic hot water flow velocities in copper plumbing systems to prevent erosion of the copper pipe walls:
Copper Piping: Maximum Velocity Chart
Based Upon Temperature
Temperature Maximum Velocity
in Feet/Second (fps)1
Cold water maximum velocity 8 fps
Hot water up to 140 F 5 fps
Hot water above 140 F up to 150 F 4 fps
Hot water above 150 F up to 160 F 3 fps
Hot water above 160 F 2 fps
1 These are recommended maximum velocities for control of erosion in copper piping systems. Lower velocities can be achieved by increasing pipe sizes during design or reducing flow in circulated systems.
In my next column, I will introduce you to John H. Fitzgerald III, a corrosion expert and my mentor. He once invited me to work on a perplexing problem in a Chicago high-rise apartment building, where pump impellers were disintegrating within weeks, requiring frequent changeouts. The solution was in knowing the difference between corrosion and erosion, and the nature of water flows through circulated plumbing systems.
Sadly, I recently read that John passed away in March 2018; I am sure he will be dearly missed by all who knew him, including me. The field of corrosion engineering is much better today because of his contributions, passion and educating the people he met. John’s memory and teachings live on in his writings and in his students’ and colleagues’ memories, and with readers of this column.
This column and the next are dedicated to the memory of John H Fitzgerald III.