Plumbing Design for Success: Understanding and Preventing Copper Corrosion

Learn some methods to prevent different types of copper corrosion that can result from the reduced use of water in unused buildings due to the COVID-19 pandemic.

We live in a time of change. Plumbing engineers need to be aware that system operating parameters are changing, and rules of thumb from 20 years ago likely are no longer applicable. Some examples of the seismic changes are as follows:

  • Lower water usage by plumbing fixtures
  • Increased water temperature to help mitigate the risk of waterborne pathogen amplifications
  • Water purveyors switching from chlorine to chloramine (or vice versa) or modifying other water quality criteria, which impacts building water (or HVAC) systems
  • Increase in technology such as onsite water reuse in components and design approaches
  • Building water usage probabilities completely changing due to more people working from home

These changes all will have an impact on the products and materials used in a building.

Copper tubing (type K, L, or M) has been utilized consistently for many plumbing installations in the last century. It is often considered the default material of choice by many plumbing engineers for a variety of reasons. Indeed, copper tubing has the ability to deliver clean, safe, and trustworthy drinking water to building occupants. However, just like any other man-made material, copper has limitations in the realms of physics, chemistry, and microbiology. To make the best decision for a specific project, engineers need to be aware of these constraints.

This article focuses on copper and its corrosion potential related to the changing water environment due to the reduced use of water and highlights several forms of corrosion that can impact drinking water quality (i.e., through metals leaching) and copper plumbing systems (i.e., through mechanical failure). It also provides an overview of blue water (a form of pitting corrosion) that has been found to occur in an increasing number of new plumbing installations in recent years. Several strategies are provided to reduce the probability of blue water, other forms of corrosion, and microbiological growth by addressing water quality and water stagnation.

Common Types of Corrosion

Several types of corrosion might occur in plumbing systems beyond those that might be specific to copper. The common types of corrosion are:

  • Uniform: Most common and characterized by a general dissolving of the metal wall
  • Pitting: A localized breakdown of a “protective” film or layer of corrosion products
  • Galvanic: Occurs when two dissimilar metals are in contact with an electrolyte
  • Concentration cell: Differences in concentration of a solution such as oxygen or metal ion concentrations
  • Impingement: Result of turbulent fluid at high velocity
  • Stress corrosion cracking: The placement of highly stressed parts in a corrosive environment
  • Erosion corrosion: A combination of a mechanical wearing and electrochemical corrosion process.

Pitting corrosion and erosion corrosion are the more common causes of pinhole leaks in copper systems. Galvanic corrosion doesn’t normally cause copper failure, but it can cause failure of other metals used in a copper system. Uniform copper corrosion is unavoidable but can be exacerbated by certain water chemistry conditions.


Blue Water

A challenging form of pitting corrosion condition referred to as “blue water” deserves special mention. Blue water can occur when the transition to a stable patina is prevented or delayed. The condition results in cloudy blue or green-colored water at the tap and is associated with newly installed copper distribution systems that have experienced stagnation and/or low disinfection residual concentrations. The condition may occur after a period of months to several years. Water discoloration is a result of loosely adhered copper corrosion by-products formed inside the piping that are sheared off and suspended by water flow.

Findings from prior research, paired with a seemingly inconsistent distribution of blue water case reports, indicate complex dynamics, relying on multiple contributing factors that precipitate the condition for which practical preventative and restorative measures have not yet been universally established. In addition to a characteristic low disinfection residual, the occurrence of blue water has been associated with supply water characterized by low bicarbonate, alkalinity, and hardness, and in some cases high pH. In many cases, biofilms containing sulfate-reducing bacteria (SRB), elevated organic carbon concentrations, as well as sediment and debris have been reported to be present in water systems exhibiting blue water, indicating a microbiological component.

Preventing Corrosion in Copper

Copper is a highly versatile material with high corrosion resistance under most normal operational conditions. Copper’s resistance to corrosion relies on precipitation of a layer of corrosion by-products at the pipe surface to form a protective surface (i.e., a patina) that slows further corrosion. The rate of corrosion and type of by-products formed depend on the conditions to which the piping is subjected.

Some types of corrosion that are specific to copper can be prevented with proper specification, installation, and inspection.

Blue Water: Ensuring Adequate Disinfection Residual

Outside of water treatment, the most promising preventive measures for blue water align with general recommendations for improving water quality and managing health risks related to building water systems: ensuring that an adequate disinfection residual is routinely brought into contact with all system piping. This relies on two main factors: the disinfectant’s stability (how long the residual lasts in the water before being consumed) and water ages within the system (residence times of the water throughout the potable water system). Disinfectant stability is most influenced by the type of disinfectant used (e.g., free chlorine will react and be consumed more rapidly than chloramine); water temperatures (e.g., higher temperatures increase the rate of disinfection reactions, thereby reducing residual disinfectant); water chemistry (e.g., waters with lower buffering may increase reactivity due to pH gradients and corrosion); the presence of reactants such as suspended solids; foreign matter in the system; and the system materials themselves.

Since blue water corrosion is associated with water stagnation and low disinfection residual concentrations, it can also be linked with a higher risk of microbiological growth. The following strategies can be used to lower the probability of blue water corrosion and drinking water health concerns associated with microbiological growth:

  1. Determine if the supply water may pose a corrosion risk, and if so, decide how to mitigate based on building water use and operator constraints.
  2. Ensure that the potable water system design considers baseline disinfectant concentrations that are anticipated, accounting for seasonality and usage variations.
  3. Revisit plumbing design strategies to reduce stagnation. Design plumbing systems that optimize water quality by minimizing the volume of retained water and interior surface area of the potable water plumbing system.
  4. Eliminate extended periods of stagnation in new piping during and following installation (i.e., manual or automated flushing may be required between pressure testing, disinfection events, and prior to full occupancy).
  5. Where necessary, prevent suspended solids from entering the potable water system and remove foreign matter such as organic matter, debris, pipe dope, and flux that may have entered the system during construction. Such substances can directly impact corrosion, increase disinfectant demand, and create a habitat for bacteria.
  6. Design systems to allow for comprehensive flushing and velocities sufficient to mobilize deposits and introduce fresh water to all parts of the system under low use, while avoiding excessive noise, water hammer, erosion corrosion, etc.
  7. Install flow and water quality monitoring equipment to determine whether conditions are conducive to corrosion and/or microbiological growth.

Pitting Corrosion: Solder-Flux Induced Corrosion

A necessary component for making soldered joints, solder fluxes serve multiple purposes including cleaning the microscopic fissures (interstices) in the copper joint surfaces and protecting those surfaces from oxidizing during the soldering process. Ideally, as solder fills the joint, the molten flux would be pushed out of the joint to the external surfaces of the piping system. More often, poor soldering technique forces the molten flux from the joint to the inside of the piping, where it cools and collects. Once this occurs, the flux-covered surface is prevented from forming protective oxides or other scales and becomes more susceptible to pitting corrosion.

This can be prevented by specifying that only fluxes meeting ASTM B813, which requires flux residues to be flushed from the system with cold water, be used and that installation methods follow the guidelines of ASTM B828 to improve soldering technique. Both of these are code-mandated provisions. Once non-B813 soldering fluxes are allowed to collect and cool within a piping system, those residues can persist for years even with continual water flow.

This issue can also be addressed by specifying mechanical (press-connect or other) joints instead of soldered joints.

Erosion Corrosion: Unreamed Tube End

Erosion corrosion can be caused by poor system design in which water velocity limitations are not observed, the piping is undersized for the water demand, or pump capacity is oversized. A far more common cause of erosion-corrosion in copper piping systems arises from a simple failure to follow proper installation procedures. When copper tube is cut, a small burr can form on both the inside and outside diameter of the tube end. When inserted into a fitting, the interior burr creates a sharp, abrupt disruption to water flow that can initiate erosion-corrosion immediately downstream of the unreamed tube end, which will proceed to cause further damage along the tube run and eventually perforation of the tube wall and a leak.

This failure mechanism is easily diagnosed upon visual examination of the inner tube and fitting surfaces where damage results in U- or horseshoe-shaped pits with the closed end of the U pointing upstream. Since the damage occurs immediately downstream of the unreamed tube end, this sharp edge remains and is easily identifiable. While all plumbing codes mandate reaming all pipe and tube ends regardless of material, this step is often skipped. To further reinforce the necessity of this step, specifications should indicate that all tube ends be reamed in accordance with the guidelines of ASTM B828.

Erosion Corrosion: Velocity and Temperature Considerations

One of the often-forgotten considerations when utilizing copper piping is the temperature and velocity nexus. Many plumbing engineers can quickly point out that the cold water velocity limit for copper is about 8 feet per second (fps), while the hot water velocity limit for copper is about 4 to 5 fps. This is to prevent erosion corrosion. However, the hot water velocity limit of 4 to 5 fps is based on a maximum temperature of 140°F. Per the Copper Tube Handbook, once the fluid temperature exceeds 140°F, the velocity needs to slow down to 2 to 3 fps. This is needed as the hot water becomes more aggressive the hotter it becomes. What this means is that to slow the velocity of water in a given pipe, either (a) the flow rate must reduce or (b) the pipe must get larger. Either way the heat loss of the piping is affected.

Galvanic Corrosion: Dissimilar Metal Corrosion

While not a common cause of copper failure since most other materials used in piping systems are more anodic than copper, the connection of dissimilar metals to copper within a piping system can cause corrosion and failure of those metals. In this case, copper should be considered broadly as the family of copper alloys (there are more than 700 registered copper alloys in the U.S.) such as brasses, bronzes, copper-nickels, etc., as they mostly have similar galvanic (electromotive force) potentials. When copper is connected with other metals (e.g., steel, galvanized steel, stainless steel, aluminum) in the piping system conveying water, a galvanic cell is created, causing slow degradation of the more anodic metal (copper is the cathode), which can eventually lead to failure of the other metal component.

The corrosion rate and time to failure are determined by the difference in the galvanic potential between the two metals, the strength of the electrolyte, and the surface area/masses of the material. To avoid this consider:

  • Electrolyte strength: Potable water is a weak but effective electrolyte, so failure can occur in drinking water systems. Water in closed-loop systems is a poor electrolyte, so direct connection between the metals in heating, cooling, and fire sprinkler systems generally are not an issue.
  • Anode/cathode mass or surface area ratio: Since corrosion rate is determined by current flux density, a small anode area (e.g., a steel valve) in relation to a large cathode area (copper piping system) increases current density and accelerates corrosion. A small cathode (e.g., brass valve) in relation to a large anode (steel piping system) limits current density and suppresses corrosion rate.
  • Distance effect: As the distance between the anode and cathode increases, flux density decreases. This is important in deciding on separation techniques. In using a dielectric coupling to prevent corrosion, the two metals are only separated by a thin dielectric component (plastic sleeve/washer), which slows the corrosion rate but as conductive surface deposits build can become ineffective in stronger galvanic pairs or electrolytes. Dielectric nipples provide further separation and protection.

Uniform Corrosion: Water Chemistry

Research has shown relationships between water chemistries, copper corrosion rates, and corrosion mechanisms. For example, supply waters with low pH and high total alkalinity/dissolved inorganic carbon (DIC) have been found more likely to be associated with higher uniform corrosion rates and copper concentrations (i.e., cuprosolvency), while waters with high pH and low total alkalinity/DIC have been found more likely to be associated with copper pitting corrosion and lower copper concentrations (Lytle, et al., 2018). Therefore, adjustment of water chemistry and/or adding corrosion inhibitors (e.g., orthophosphate) may provide effective solutions to reducing corrosion and copper concentrations in many conditions.

Corrosion mechanisms can be very complicated, and there are other relationships between water chemistry and corrosion that are not discussed here. Therefore, the treatment of copper corrosion must be informed by thorough investigation based on multiple system and water quality factors.

Uniform Corrosion: Chlorine and Chloramine Disinfectant

Due to attempts to mitigate Legionella pneumophila and other waterborne pathogens in domestic water systems, free chlorine and chloramines are frequently being utilized. Free chlorine and chloramines are oxidizing agents that have been found to reduce the concentrations of many waterborne pathogens, including Legionella pneumophila. However, one of the negative effects of using these oxidizing chemicals is that they both cause corrosion of copper piping.

Some studies have shown that chloramine is not as corrosive as free chlorine under certain water quality conditions, but the potential for corrosion resulting from chlorine/chloramine is dependent on water quality. For example, in 1985, Gordon Treweek tracked the corrosion rate of both free chlorine and chloramine for various commonly used materials in plumbing systems (“Pilot-Plant Simulation of Corrosion in Domestic Pipe Materials,” AWWA Journal). Extrapolating the values from this study shows that 1 part per million (ppm) of free chlorine or chloramine could reduce the lifespan of copper piping to three to five years (instead of 50+ years) under the conditions of that report.

Corrosion inhibitors may help in preventing corrosion; however, these can be pH dependent and may have other unintended consequences. Also worth noting is that chloramine has its own set of specific concerns (e.g., nitrification, operational chlorine/ammonia ratios, non-regulated disinfection by-products) compared to free chlorine. Plumbing engineers need to learn about these variables and then balance them to come up with holistic solutions for clients.

For More Information

The reader is directed to the following references to obtain additional and more detailed information on the subject of copper corrosion:

  1. ASTM B828: Standard Practice for Making Capillary Joints by Soldering of Copper and Copper Alloy Tube and Fittings
  2. Plumbing Engineering Design Handbook, Volume 1: Fundamentals of Plumbing Engineering. Chapter 8: Corrosion, American Society of Plumbing Engineers
  3. Conditions Contributing to Underground Copper Corrosion,” Journal AWWA
  4. Copper-Tube Corrosion in Domestic Water Systems,” Boiler Systems Engineering
  5. Copper Tube Handbook, Copper Development Association
  6. Corrosion by Potable Waters in Building Systems,” Materials Performance
  7. Dissimilar Metals in Contact
  8. NACE Corrosion Engineer’s Reference Guide, Fourth Edition
  9. Shreir’s Corrosion, First Edition
  10. A Model for Estimating the Impact of Orthophosphate on Copper in Water,” Journal AWWA
  11. Effect of pH, DIC, Orthophosphate and Sulfate on Drinking Water Cuprosolvency, U.S. EPA
  12. The Blue Water Phenomenon,” Journal AWWA
  13. Pitting Corrosion of Copper in Waters with High pH and Low Alkalinity,” Journal AWWA
  14. NSF/ANSI/CAN 61-2020: Drinking Water System Components—Health Effects, Informative Annex 6 (gives some guidance on water qualities that have been shown to be of relatively more risk of having high levels of dissolved copper when newly installed)

About the Authors

This article was put together by members of the AWWA Premise Plumbing Committee (Copper-in-Water Subcommittee). The lead contributors from that committee for this article are Christoph Lohr, PE, CPD (IAPMO); Robert Fields, PE (STV Inc.); Jim Kendzel, MPH (ASA); and Andy Kireta (CDA Inc.). The authors thank the countless others who had significant contributions and are not listed above.

The opinions expressed in this article are those of the authors and not the American Society of Plumbing Engineers.

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