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TECHNICAL BULLETIN
Review Date: 7/11/2014 Original Date: 7/11/2014 Document ID: NTB-0714-01
Dielectric Products Relative to Electrolysis and Galvanic Corrosion
Dielectrics – unions, flanges, bolt packages, etc. - are often prescribed by some as a remedy to the corrosion attack that is associated with a combined use of dissimilar metals in a piping system. They are intended to provide an insulating barrier (a break in the electrical circuit), separating dissimilar metals within an electrolyte, thereby mitigating the damaging effects of both galvanic and/or electrolytic corrosion. While mitigating, they may not fully prevent galvanic corrosion from occurring.
Galvanic corrosion or dissimilar metal corrosion is an electrochemical process that is created through the electrical interaction of two different metals under the influence of a conductive media (i.e. an electrolyte). An electrolytic cell, much like a battery, is generated by these dissimilar metals using water as the electrolyte. The electrical charge, developed within the electrolytic cell, drives a preferential attack on the more electrically active metal with the water acting as the recipient of the discarded metal ions. Such galvanic attack is often encountered in service where iron or steel components are installed, and later corrode, in a largely copper piping system.
The basic driving force feeding galvanic corrosion is the electrical potential difference between the two dissimilar metals. All metals possess electrical potentials. The Electrochemical Series (Galvanic) Chart (see below) denotes a relative measure of a metal’s tendency to become active within a given electrolyte. The greater the distance in chart ranking between the relative metals – the greater the driving force for the more active metal to be given up via the electrolyte. The most obvious way to eliminate the driving force behind galvanic corrosion is to use metals that are in close proximity with respect to their electrical potential.
Galvanic corrosion is sometimes wrongly called electrolysis. Electrolysis is driven by current applied from outside the system rather than generated from within the system. As an example, stray current in copper piping can promote electrolytic corrosion pitting through electrolysis. Such stray current corrosion (electrolysis) issues are addressed through proper design and grounding of the entire system.
If the use of dissimilar metals in plumbing cannot always be avoided, then galvanic corrosion attack can only be reduced through proper system design. A proper design calls for there to be a high ratio of surface area for the active metal versus the more noble metal, minimizing the corrosive effect of the galvanic cell.
The nature of both the internal and external environments should also be considered with bimetallic piping systems. Internally, the water conveyed within the pipe can act as the electrolyte; while externally, any moisture in the surrounding
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environment (condensate, wet soil, etc.) can envelop both metals, completing the electrolytic cell. These wet electrical conductive pathways can bridge the insulting barrier afforded by the dielectric product. The very nature of such environments can help to negate any insulating advantage claimed by the dielectric. Water can cause corrosive attack on the piping and create surface deposits and scales on waterway surfaces which further diminish the protective barrier.
The above considerations are expanded to include mechanical damage to the dielectric surface during installation when a coating, plating or gasket is considered as a permanent dielectric barrier. Such products may offer up their own satisfactory corrosion resistance to the environment but only afford conditional protection with respect to galvanic or electrolytic corrosion.
Electrochemical Series (Galvanic) Chart The following is an abbreviated listing of the electromotive force series with regard to commonly used piping materials, from Cathode (most noble/passive material) to Anode (least noble/active material) metals:
GOLD – Fixtures, Faucets, Plating PLATINUM SILVER – Brazing alloys, Silver-bearing solders TITANIUM – Condenser tubes MONEL (67% Ni - 33% Cu) – Specialty piping & equipment CUPRO-NICKEL – Condensers, Marine, Nuclear COPPER – Pressure, DWV, Gases, Air, Refrigeration, etc. BRASS (85/15 - Red) – Cast fittings, Valves BRASS (70/30 - Yellow) – Gas-cocks, Fittings, Connectors LEAD – Solder, Pipe, Sheet, Coating, Lining TIN – Solders, Coating, Lining CAST IRON – Pressure, DWV WROUGHT IRON – Pressure MILD STEEL – Fire Protection ALUMINUM – Refrigeration, Irrigation, some Solar GALVANIZED STEEL – Pressure, DWV ZINC – Coatings, Linings, some Fittings MAGNESIUM – Water Heater Anodes, cathodic protection for pipelines
For answers to additional questions regarding this or any other technical issue involving NIBCO products, please call toll-free:
NIBCO Technical Services at 1-888-446-4226
NIBCO Technical Services • Phone: 1.888.446.4226 • Fax: 1.888.336.4226 2