The pros and cons of galvanic coupling
Cathodic protection is a method of reducing the rate of corrosion damage to a metal surface by supplying it with electrons from an external source, effectively forcing it to become the cathodic (passive) element of a galvanic cell.
When metal corrodes in an aqueous solution, the metal anode is oxidised to form metal ions (typically ), releasing electrons in the process. These electrons are consumed in the cathodic reaction, which is dependent on the nature of the electrolyte. In neutral or basic solutions oxygenated water is reduced to hydroxyl ions (). In acid solutions the electrons combine with hydrogen ions to form hydrogen (H2) gas. As one option to prevent such oxidation, electrons can be supplied to the metal surface from an external source (impressed current cathodic protection), or through a galvanic coupling (galvanic protection). In galvanic protection a more active material, (i.e. one with a more negative potential in the galvanic series for the specific environment) is connected electrically to the material/component to be protected. If the material combination is selected correctly (i.e. the galvanic potential of the material differs by more than approximately 250 mV), this piece of material will be anodic and oxidise sacrificially, supplying electrons to the system, preventing the oxidation of the material/component being protected until the sacrificial anode is completely oxidised. This is the mechanism of the corrosion protection afforded by the zinc layer on galvanized components or by zinc/magnesium anodes used on ships in which the zinc/magnesium corrodes sacrificially, protecting the underlying steel.
Although galvanic coupling can be used to significant effect, galvanic couplings can also be highly detrimental when components manufactured from different metals are placed in an aqueous environment (e.g. steel and aluminium components in contact in moist sea air). Electroless nickel coatings are often used to prevent corrosion of steel and provide an attractive corrosion resistant coating. However, if the electroless nickel coating is damaged the cathodic potential of the steel is such that the steel in the damaged region acts to protect the nickel coating, causing rapid degradation of the steel. Rapid corrosion also occurs when electrical potential is incorrectly applied. A classic example occurs when power is supplied to a boat while docked in a harbour, such as while charging a battery or powering appliances. If not properly connected, current entering the vessel will naturally be drawn to ground, conveniently accessible via the water, exiting through the hull and/or propellers. Another classic example involves the implementation of impressed current cathodic protection in components/tanks, resulting in the stray current corrosion of nearby metal structures. This frequently occurs in industrial applications, where nearby underground pipes are corroded rapidly after the implementation of impressed current systems.
A more obscure, but frequently used application of cathodic protection, is the use of tin based coatings applied to copper tube-fin heat exchangers. This coating affords good corrosion resistance, without having significant adverse effects on the heat transfer characteristics of the exchanger. In a recent failure analysis conducted on a liquid cooled heat exchanger, used as an intercooler for pressurized fuel-gas in a large engine, the progressive removal of the tube-fins tin coating, due to the condensation of corrosives present in the fuel, resulted in severe pitting corrosion damage to the base copper material. The more active (anodic) tin material initially provided a significant degree of cathodic protection, by effectively forming localised galvanic cells between itself and the more passive (cathodic) exposed copper surface. Continual exposure and associated removal of the tin reduced the degree of available protection, resulting in eventual pitting corrosion attack and complete structural degradation.
The application of galvanic protection principals through the use of impressed current systems, or as a result of inherent potential variations of material in contact, is a powerful and effective tool for corrosion protection. However, inadvertent coupling of dissimilar materials can have highly detrimental consequences and can lead to accelerated component failure.
Published in Technical Tips by Origen Engineering Solutions on 1 July 2017