One of the eight types of corrosion is galvanic corrosion. This occurs when two dissimilar metals are in contact, particularly when exposed to a corrosive medium. One of the two metals will have a greater corrosion potential than the other, and it will corrode sacrificially while protecting the other metal. The combination basically creates an “electricity” flow, with the one metal being the anode and the other becoming the cathode. The anode will sacrifice itself for the cathode.
In a controlled way, this type of corrosion is used as a corrosion preventive in galvanized steel. Galvanized steel can be different grades of steel – usually flat-rolled – that is coated with zinc. The zinc is the anode in this galvanic “cell,” resulting in protection of the steel. A coating of paint can also protect the steel, but if the paint coating is compromised, the steel will corrode (rust). With galvanized steel, a scratch can occur through the coating, but as long as some coating remains, it will continue to protect the steel. Aluminum can also be used as a galvanic protector for steel.
Hot-dip galvanizing is one way to produce galvanized steel. It has been used effectively for more than 150 years. In fact, two French chemists are credited with process firsts. In 1742, Paul Jacques Malouin described the process of coating iron by dipping it in molten zinc. In 1837, Stanislas Sorel obtained a patent for a zinc cleaning/coating process.
Hot-dip galvanizing is a coating process that results when steel is passed through a molten bath of zinc at a temperature of around 860°F (460°C). This process can be performed on steel strip on a continuous line. When exposed to the atmosphere, the pure zinc reacts with oxygen to form zinc oxide, which reacts further with CO2 to form a dull-gray zinc carbonate. The resulting steel is used for automotive body panels, roofing and walling, guardrails and heating/cooling ducts. A common application of galvanized metal is a pail or washbasin. The pattern on zinc-coated steel is often referred to as spangle.
For applications requiring a thinner layer of zinc, such as surfaces where paint will be applied, a more modern galvanizing technique is typically used. One of these techniques is electrogalvanizing, which deposits a thinner and better-bonded layer of zinc by electroplating in an aqueous electrolyte.
Interestingly, our familiar heat-treatment process can affect the corrosion rate of the steel by this same galvanic process. When a steel is quenched from the hardening temperature, it is a single phase: martensite. Upon tempering, carbides are precipitated, resulting in numerous galvanic microcells and a corrosion-rate increase. If tempering occurs at higher temperatures, the carbides coalesce, causing a reduction in the number of galvanic cells. As a result, the corrosion rate decreases. Similarly, when an age-hardenable aluminum alloy is solution hardened, the corrosion rate is increased upon formation of the second phase.
The sacrificial, dissimilar-metal process discussed can be used in ways other than coating of steel with a more-sacrificial metal. Underground pipes are often protected by connecting sacrificial anode plates of magnesium. When the magnesium anode is consumed, it can be replaced. Similarly, the use of a magnesium bar often protects water tanks. Ships can be protected from corrosion by attaching zinc plates to the hulls. All of these examples result in the corrosion of the sacrificial anode and the protection of the cathode.
An example of galvanic corrosion in action was found on the Statue of Liberty when regular maintenance revealed corrosion of the wrought-iron support where it was attached to the copper skin. Because this corrosion was anticipated when the statue was built in the 1880s, a shellac insulation was used between the two metals. This coating failed in some locations, resulting in the rusting of the iron supports. Chrome-plated bumpers, of bygone days, are another example of this phenomenon. When the plating would crack or chip, the steel/iron substrate would quickly rust because it became the sacrificial anode.
Be advised: Depending on the service conditions and the amount of coating present, the zinc will eventually be consumed, and the metal will start to degrade.