Preventing Decay: Galvanic Corrosion and Anodes Quiz

In a galvanic cell, the metal with the lower or more negative reduction potential is more easily oxidized. Iron has a much lower reduction potential than copper, meaning it loses electrons more readily. Therefore, iron acts as the anode and corrodes, while the copper remains protected as the cathode.

This is a common misconception. Electrons flow through the metallic path or the physical electrical connection from the anode to the cathode. The electrolyte facilitates the movement of ions to complete the circuit and maintain charge neutrality, but it does not conduct free electrons.

A sacrificial anode must be more active than the metal it is protecting. This means it must have a higher oxidation potential or a more negative reduction potential. By being more active, it ensures that it will be the site of oxidation, giving up its electrons to the pipe.

Zinc and Magnesium are both higher on the activity series than iron. They are relatively inexpensive and effectively protect steel by corroding in its place. Gold and Platinum are noble metals with very high reduction potentials; they would actually accelerate the corrosion of iron if connected to it.

Cathodic protection works by making the structural metal the cathode of an electrochemical cell. By supplying the metal with a constant stream of electrons from a sacrificial anode, the metal is prevented from losing its own electrons, thus stopping the oxidation process that leads to rust.

For galvanic corrosion to occur, there must be both an ionic path through the electrolyte and a conductive path for electrons to travel between the two metals. If the metals are electrically insulated from each other, the circuit is broken and galvanic corrosion cannot proceed between the two materials.

Aluminum has a significantly higher oxidation potential than Zinc, meaning it is more eager to oxidize. While both can protect iron, aluminum provides a stronger driving force for protection. However, in practice, Zinc is often used in saltwater because Aluminum can form a passive oxide layer that slows down its effectiveness.

When two dissimilar metals meet, the more active metal (anode) will show rapid material loss, often in the form of deep pits or holes. At the same time, bulky corrosion products like rust or white powder (zinc oxide) will accumulate around the site of the chemical reaction.

In the galvanic series, aluminum is much more active than stainless steel. When they are in contact in seawater (an electrolyte), the large aluminum plate acts as the anode to the small stainless steel bolts. This leads to the rapid electrochemical destruction of the aluminum around the bolt holes.

The area effect is critical in corrosion. A small anode connected to a large cathode is a recipe for disaster because the large cathode can consume electrons very quickly, forcing the small anode to oxidize at an extreme rate to keep up, leading to rapid failure of the component.

The SHE is the universal reference point used to determine the relative activity of all other metals. By assigning it a potential of zero volts, scientists can rank metals on a scale to predict which ones will act as anodes or cathodes when paired together in an electrochemical cell.

To stop galvanic corrosion, you must break the circuit. Plastic washers provide electrical insulation, while painting both surfaces increases resistance. Using metals with similar potentials (like Tin and Lead) minimizes the driving force or voltage that causes the corrosion to occur in the first place.

A sacrificial anode only provides protection as long as it is physically present and electrically connected. Once the anode has completely oxidized away, the structural metal no longer has a source of replacement electrons and will begin to oxidize according to its own environmental conditions.

Magnesium has a much higher driving voltage (more negative potential) than Zinc. In fresh water, which has low conductivity, this extra push is necessary to ensure the protective current reaches the entire surface of the structure being protected, whereas Zinc might not provide enough current.

Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. This is achieved by either connecting it to a more active sacrificial metal or by using an external direct current power source to supply electrons.