Galvanic Corrosion

Galvanic corrosion results when two different metals are electrically connected and surrounded by the same electrolyte.  This is the electrochemical basis for the lead acid battery.  The material with the highest electrochemical energy level would experience corrosion, called the anode, and the other material, called the cathode, would not experience corrosion. 

In the water industry, galvanic corrosion could be expected from connections between ferrous materials such as, steel and ductile iron, and copper, stainless steel and brass. In this corrosion cell the ferrous materials would corrode relative to the other materials. This relationship is normally depicted in a table referred to as the galvanic series. 

The following table lists the common piping and construction materials in order of galvanic activity:

This chart indicates that for two dissimilar metals being electrically connected, the one towards the most active end would be anodic or experience corrosion, and the one towards the least active end would be cathodic or not experience corrosion.  For the corrosion cell to establish, both materials need to be in a common electrolyte and electrically connected.  Common examples would be a copper service connected to a ferrous metal main in soil and stainless steel pipe repair clamp with alloy steel bolts. In these examples, the copper and stainless steel would be cathodic, and the ferrous materials anodic and experience corrosion.

This is the fundamental concept of cathodic protection. Cathodic protection causes certain electrochemical reactions to occur on the structure that is being protected against corrosion.  The protected structure becomes cathodic relative to the anode material.  For sacrificial anode cathodic protection, this is typically effected through the use of magnesium anodes on steel or ductile iron pipe.  Cathodic protection will be presented in more detail later in this paper.

A condition that is similar to the corrosion exposure that results from dissimilar metals, arises when new pipe materials are installed within old piping systems.  Because the old materials have developed a corrosion layer or film, their corrosion activity has slowed.  The new materials are more active, as there is no corrosion film present, and therefore when connected to the old pipe they can experience corrosion. 

Stray Current

Stray current corrosion results from sources outside the influence of the pipe and it’s environment.  To cause stray current on a pipeline, the current must flow onto the pipe at one location and then flow off the pipe at another location.  Where the stray current leaves the affected pipe, corrosion will occur.  Common sources of stray current in the water industry are primarily rapid transit systems and transmission pipeline operations that employ impressed current cathodic protection. 

Current flow in a pipeline which provides a continuous metallic path to the power neutral ground grid will not cause corrosion damage.  If the current discharges from the pipeline into the environment, it will cause corrosion loss at the rate of 20 pounds of metal loss in one year per ampere of discharge.  Therefore, a current discharge of 0.5 amperes for 1 year would remove 10 pounds of metal.  At a density of 450 lbs/ft³ of cast iron for example, this equates to 38 in³ of metal removed in one year.  This type of corrosion manifests itself on the pipe surface by a clean, sharp, pocked surface.

The primary source of stray current in large metropolitan areas comes from the operation of rapid transit systems.  Those that operate on direct current, DC, with the running rail or track functioning as the negative return are capable of generating stray current.  Because these systems operate at 700 to 1,000 volts, and trains draw upwards of 15,000 amperes when accelerating, the possibility of small amounts of stray current flowing into the ground is clear.  Stray current patterns from this type of operation are dynamic, they constantly fluctuate.

The other source of stray current comes from the operation of impressed current cathodic protection systems.  Impressed current cathodic protection is utilized to protect long pipelines, poorly coated structures, above ground storage tank farms and other type of structures.  This type of protection is required where sacrificial anode cathodic protection would not provide adequate protection.  As these are DC current systems, stray current can be generated from their operation. Stray current patterns from this type of operation are static, they do not fluctuate over time unless adjusted.

Stray current corrosion exposure can be very localized within the piping network.  Because of the variable magnitude of the stray current, corrosion loss can be far greater than any of the other corrosion mechanisms acting on the pipe.

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