Lasagna Cell - Preventing Galvanic Corrosion

Preventing Galvanic Corrosion

There are several ways of reducing and preventing this form of corrosion.

One way is to electrically insulate the two metals from each other. Unless they are in electrical contact, there can be no galvanic couple set up. This can be done using plastic or another insulator to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminium and steel parts. Use of absorbent washers that may retain fluid is often counter-productive. Piping can be isolated with a spool of pipe made of plastic materials or made of metal material internally coated or lined. It is important that the spool has a minimum length of approx 500 mm to be effective.

Another way is to keep the metals dry or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy, and allowing them to dry.

Coating the two materials or if it is not possible to coat both, the coating shall be applied to the more noble, the material with higher potential. This is necessary because if the coating is applied only on the more active material, in case of damage of the coating there will be a large cathode area and a very small anode area, and for the area effect the corrosion rate will be very high.

It is also possible to choose metals that have similar potentials. The more closely matched the individual potentials, the lesser the potential difference and hence the lesser the galvanic current. Using the same metal for all construction is the most precise way of matching potentials.

Electroplating or other plating can also help. This tends to use more noble metals that resist corrosion better. Chrome, nickel, silver and gold can all be used. Galvanizing with zinc protects the steel base metal by sacrifical action.

Cathodic protection uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Metals commonly used for sacrificial anodes include zinc, magnesium, and aluminium. This is commonplace in water heaters. Failure to regularly replace sacrificial anodes in water heaters severely diminishes the lifetime of the tank.

Finally, an electrical power supply may be connected to oppose the corrosive galvanic current. (see impressed current cathodic protection)

For example, consider a system is composed of 316 SS (a 300 series stainless steel; it is a very noble alloy meaning it is quite resistant to corrosion and has a high potential) and a mild steel (a very active metal with lower potential). The mild steel will corrode in the presence of an electrolyte such as salt water. If a sacrificial anode is used (such as a zinc alloy, aluminium alloy, or magnesium), these anodes will corrode, protecting the other metals. This is a common practice in the marine industry to protect ship equipment. Boats and vessels that are in salt water use either zinc alloy or aluminium alloy. If boats are only in fresh water, a magnesium alloy is used. Magnesium has one of the highest galvanic potentials of any metal. If it is used in a salt water application on a steel or aluminium hull boat, hydrogen bubbles will form under the paint, causing blistering and peeling.

Metal boats connected to a mains shore line will normally have to have the hull connected to earth for safety reasons. However the end of that earth connection is likely to be a copper rod buried within the marina, resulting in a steel-copper "battery" of about 0.5V. For such cases the use of a galvanic isolator is essential - typically 2 diodes in series, preventing any current flow while the applied voltage is less than 1.4V (i.e. 0.7V per diode), but allowing a full flow in case of an earth fault. It has been noted that there will still be a very minor leak through the diodes which may result in slightly faster corrosion than normal.