Corrosion Plumbing Piping

Understanding the electromotive series of metals

Plumbing design professionals and design/build contractors traditionally have designed plumbing and piping systems containing fluids to meet the material requirements in the plumbing codes. Recently, stainless steel has been added as a new material to plumbing codes for domestic water distribution piping systems. Stainless steel has traditionally been a more expensive piping material, and did not gain much support until Schedule 10 stainless steel was used in lieu of copper for larger projects with a cost advantage. The new material provides a better choice than galvanized steel, which has been prone to corrosion and water quality issues. Schedule 10 stainless steel is also much lighter and easier to install than Schedule 40 galvanized steel piping, which provides a savings for labor and installation costs.  

I once inspected a building where the contractor installed stainless steel water mains without installing dielectric unions between the stainless steel and the copper. The contractor did not think copper and stainless steel were dissimilar metals, and that stainless steel and copper are corrosion-resistant materials, and so a dielectric union or dielectric waterway was not required. 

When a decision is made to use stainless steel water mains, it is very important to understand the electromotive series of metals. When choosing any two dissimilar metals, one will act as an anode and corrode to sacrifice itself to the other more noble metal. Each metal has an electric potential, and the farther these two metals are from each other on the chart of the electromotive series of metals, the more aggressive the corrosion rate can be for the less noble material.    

Recent developments in the construction of buildings have been to seek newer and less expensive materials for domestic water distribution piping, such as stainless steel. Previously, large diameter water mains in a domestic water distribution system were copper or galvanized piping. When there is any change in pipe material, there should be a dielectric union and a dielectric waterway between the dissimilar metals to minimize or eliminate corrosion.    

Design professionals and contractors need to understand the consequences of a substitution of pipe materials. In this case, the change was from copper or galvanized water mains to stainless steel mains. This caused the copper piping branches to go from neutral with copper mains to becoming the anode with the stainless steel water mains. The water started to turn green. Testing revealed high levels of copper, zinc and magnesium. The copper pipe and brass valves and fittings were acting as the anode and corroding to sacrifice themselves to the stainless steel pipe, which is a more noble metal.   

Specification Section Division 220500, Requirements for Common Work Results for Plumbing

The paragraph on dielectric fittings describes dielectric fittings as a combination fitting of copper alloy and ferrous materials with threaded, solder joint, plain or weld-neck end connections that match piping system materials. It calls for an insulating material suitable for the system fluid, pressure and temperature application. It also covers factory fabricated dielectric unions and sets the minimum working pressure at 180 F. The section also covers pressure and temperature requirements for dielectric flanges with insulating sleeves for the bolts and insulating gaskets.

Most model codes also address dielectric connections when connecting dissimilar metals.

Corrosion is very common in water piping systems and can be caused by the following conditions:

Low pH — Acidic water, with a low pH (below 7 on the pH scale), can cause corrosion. The lower the pH is, the more aggressive the water can be. Water with a higher pH can increase the electrical conductivity of the water allowing galvanic corrosion to occur if dissimilar metals are directly connected. Generally, the ideal pH is slightly above 7.

High velocity erosion corrosion — Erosion corrosion is the increase in the rate of metal deterioration from abrasive or erosion effects from high velocity water flow in the pipe. Erosion can be identified by grooves and rounded holes, which usually are smooth and have a turbulent pitting, directional or flowing pattern. Erosion corrosion is caused by high water flow velocities and can be increased with suspended solids which can act like sand blasting. 

Erosion is often localized at areas where water changes direction, and there is an increase in velocity associated with turbulence, following obstructions or a concentrated nozzle flow. Erosion damage usually shows up at elbows and near balancing valves, where there is an increased flow velocity and the pipe walls 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 the system is not balanced properly.   

High water temperature — High water temperature can increase biological rate of growth and chemical corrosion.  

According to the Copper Development Association, high temperatures in copper piping can cause erosion at flow velocities in excess of 5 feet per second (fps) velocity for hot water system temperatures up to 140 F. As the temperature goes up, the maximum recommended flow velocity goes down. The Copper Development Association recommends a maximum flow velocity of 2-3 fps for water over 140 F. 

Over the years, I have developed a velocity chart for domestic hot water systems that is a little more developed than the recommendations in the Copper Development Association. 

I would suggest using the following domestic hot water flow velocities in copper plumbing systems to prevent erosion of the copper pipe walls:

a. Cold water maximum velocity = 8 fps
b. Hot water up to 140 F maximum velocity = 5 fps 
c. Hot water above 140 F up to 150 F maximum velocity = 4 fps 
d. Hot water above 150 F up to 160 F maximum velocity = 3 fps 
e. Hot water above 160 F maximum velocity = 2 fps 

Oxygen and dissolved carbon dioxide or other gasses— Oxygen and dissolved CO2 or other gasses can contribute to corrosion of domestic water piping systems.  

High dissolved solids— High dissolved solids, such as salts and sulfates, can also contribute to chemical or bio-chemical corrosion. 

Chloride to sulfate mass ratio (CMSR) — If the mass ratio or chloride to sulfate mass ratio (CMSR) of chloride to sulfate is greater than 0.2 but less than 0.5, there is a slightly elevated concern, but if the CMSR is greater than 0.5 and the alkalinity of the water is less than 50 mg of calcium carbonate (CaCO3) per liter (L) there should cause concern, as the by-products of water treatment chemicals can be corrosive to the plumbing system. 

Corrosion related bacteria — Corrosion-related to microbiologically-induced corrosion can occur if there is determined bacteria high-standard plate counts. Electrochemical corrosion can result in pinhole leaks, isolated corrosion, aesthetic water quality problems presence of suspended solids, such as sand, sediment, corrosion by-products, and rust, can aid in physical corrosion and damage and facilitate chemical and biochemical corrosion. This did not appear to be an issue in this system. 

Types of corrosion — The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons: dissimilar piping materials, impurities in metal castings, localized stresses, metal grain size or composition differences, discontinuities on the surface and differences in the local environment (e.g., temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling or surface oxidation is usually a more serious problem than equipment failure.

Localized corrosion — Localized corrosion occurs when the anodic sites remain stationary and is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (e.g., dezincification), galvanic corrosion, crevice or under-deposit corrosion, inter-granular corrosion, stress corrosion cracking and microbiologically-influenced corrosion. Another form of corrosion, which cannot be accurately categorized as either uniform or localized, is erosion corrosion caused by high velocity turbulence near elbows or valves.

Pitting — Pitting is one a destructive form of corrosion, and also one of the most difficult to predict in laboratory tests. Pitting occurs when anodic and cathodic sites become stationary due to large differences in surface conditions. Pitting can occur from holidays in coating surfaces. It is generally promoted by low-velocity or stagnant conditions (e.g., shell-side cooling) and by the presence of chloride ions. Once a pit is formed, the solution inside it is isolated from the bulk environment and becomes increasingly corrosive with time. The high corrosion rate in the pit produces an excess of positively charged metal cations, which attract chloride anions. In addition, hydrolysis produces Hydrogen Ions (H+) ions. The increase in acidity and concentration within the pit promotes even higher corrosion rates, and the process becomes self-sustaining. Corrosion inhibitors can be used to control pitting, but they must be approved for domestic water service and applied correctly.

Selective leaching/dezincification — Selective leaching is the corrosion of one element of an alloy. The most common example in building water systems is dezincification, which is the selective removal of zinc from copper-zinc (brass) alloys. The conditions that promote the pitting of steel also promote the pitting of brass, which in building water systems usually occurs by dezincification. Dezincification is common in brass valves and fittings that are of yellow brass (with filler metals of zinc, aluminum, arsenic, antimony, phosphorus and other filler metals in excess of 15 percent in the molten brass mixture). Dezincification is common in cheap imported cast brass fittings of yellow brass. The codes have recently changed to stop using the term “brass;" they now refer to brass as “copper allows.” Low pH conditions and high free chlorine residuals are particularly aggressive in producing dezincification. Under the microscope, yellow brass that has experienced dezincification looks like a sponge. Yellow brass becomes brittle as the filler metals corrode away. Water hammer or pressure surges that cause pipe movements can cause yellow brass pipe fittings or valves to fracture and cause leaks or flooding. The dezincification resistance varies with the alloy. The lower the zinc and other filler metals in the finished product, the better the alloy will be with resistance to dezincification corrosion. 

Galvanic corrosion — Galvanic corrosion occurs when two dissimilar metals are in contact with each other and in contact with a water solution that allows electrical current flow between the two dissimilar metals. (see figure #1) The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between the two metals. This difference increases as the distance between the metals in the galvanic series of metals increases. The list of metals below shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in a liquid solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. Using this concept, corrosion engineers have used sacrificial anodes made of magnesium, with copper leads welded to underground metal structures, to protect underground metal structure from corrosion. The magnesium anode will corrode first. The anodes can be checked and replaced for continuous corrosion protection. 

The list below shows the galvanic series of metals and alloys. The higher the metal is on this list, the noble the metal will be. The greater the distance between the two metals, the greater the electrical potential between the two dissimilar metals will be and the greater the corrosion rate will be for the less noble metal. When two metals from the list below are connected together in a piping system, and they have an electrically conductive fluid like municipal water in contact with both metals, there will be a current flow through the fluid from the less noble material (Positive charge) to the more noble material (Negative charge). Where the current leaves the less noble metal, corrosion will occur at the point of current leaving the less noble material. The current is usually greater, and the corrosion is greater closer to the contact point of the two metals. This is called a corrosion battery cell. It is very common when two dissimilar metals are in contact with each other and there is a fluid that allows current to flow between the two metals. When two dissimilar metals are connected together in a piping system and in the presence of an electrolyte like water, it allows an electrical current to flow between the two different metals. The greater the difference in the electrical potential number of the two metals, the greater the corrosion rate will be for the less noble metal. Current will flow from the metal with the higher number to the metal with the lower number causing corrosion to occur near the joint between the two dissimilar metals. It did not appear to me that there was a significant galvanic corrosion problem. The pipes I examined were clearly eroded away by high velocity domestic hot water.

Example: In the list on page 00 if galvanized pipe (steel pipe with a Zinc Coating) is connected to copper, the galvanized (Zinc) pipe will corrode to sacrifice to the copper. If Copper is connected to Stainless steel Type 304, then the copper will corrode to sacrifice to the stainless steel. 

Galvanic corrosion can be controlled by the use of sacrificial anodes, isolation of the metals from electrical continuance, protective plastic coatings or epoxy coatings or with corrosion inhibitors. Proper placement of sacrificial anodes is a precise science.

The most serious form of galvanic corrosion occurs in plumbing or HVAC piping systems that contain both copper and steel alloys and are filled with water (an electrolyte). A di-electric unions or di-electric waterways or both are often used to interrupt the flow of electricity between the two dissimilar metals in an electrolyte to prevent the flow of current which causes galvanic corrosion.

Oversized hot water recirculation pumps often cause high velocity hot water to circulate through the piping, which causes erosion of the pipe wall and water leaks. (See Appendix “A” Pipe Sizing Chart - Pipe Size, Friction loss/100 ft, Velocity FPS, Flow GPM)

Stainless steel piping for your next project, but it is important to make sure the copper and stainless steel are isolated to prevent corrosion of the copper pipes and brass valves. 

Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. Visit

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