Question

In 1973 the wreckage of the Civil War ironclad USS Monitor was discovered near Cape Hatteras, North Carolina. [The Monitor and the CSS Virginia (formerly the USS Merrimack) fought the first battle between iron-armored ships.] In 1987 investigations were begun to see if the ship could be salvaged. It was reported in Time (June 22, 1987) that scientists were considering adding sacrificial anodes of zinc to the rapidly corroding metal hull of the Monitor. Describe how attaching zinc to the hull would protect the Monitor from further corrosion.

Short answer

Attaching zinc sacrificial anodes to the iron hull of the USS Monitor would protect it from further corrosion by creating an electrochemical cell. Zinc, being more reactive than iron, will corrode preferentially, losing electrons and becoming zinc ions. This diverts the corrosive process away from the iron hull, preserving its structural integrity and preventing further damage.

Step-by-step solution

Understanding Corrosion

Corrosion is an electrochemical process that occurs when a metal reacts with an electrolyte, such as seawater. In the case of the USS Monitor, the iron in the hull reacts with the seawater, causing the formation of iron oxide (rust) and weakening the hull.

Understanding Sacrificial Anodes

A sacrificial anode is a piece of metal that is more reactive than the metal it is designed to protect. It is connected to the metal it is protecting, and its purpose is to be consumed through corrosion, thus diverting the corrosive process away from the protected metal. In this exercise, zinc is proposed as the sacrificial anode to be attached to the USS Monitor's iron hull.

The Galvanic Series

The Galvanic Series is a list of metals arranged in order of their reactivity with an electrolyte. A metal higher up on the series will corrode in preference to a metal lower down on the list when they are in contact. In this case, zinc is higher on the Galvanic Series than iron, which means that the zinc would corrode before the iron when placed in seawater.

Connecting the Zinc Anode to the Iron Hull

By attaching zinc anodes to the iron hull of the USS Monitor, an electrochemical cell is created. The zinc anodes act as the anode (negative electrode), and the iron hull acts as the cathode (positive electrode). When a current flows in this cell, zinc atoms lose electrons and become zinc ions, thus corroding, while iron does not corrode.

Consequences of Adding Zinc Anodes

When zinc anodes are attached to the iron hull of the USS Monitor and placed in seawater, the following process occurs: 1. The zinc corrodes preferentially, losing electrons and becoming zinc ions. 2. The electrons flow from the zinc anodes to the iron hull. 3. In return, the iron hull does not corrode, preserving its structural integrity and preventing further damage. By attaching sacrificial anodes made of zinc, the USS Monitor's hull would be protected from further corrosion by diverting the corrosive process to the more reactive zinc anodes.

Key concepts

Corrosion Protection

Corrosion protection is essential to prolonging the lifespan of metal structures exposed to harsh environments. In the context of the USS Monitor, a Civil War ironclad, its iron hull faced severe rusting due to seawater exposure. Corrosion occurs when the metal reacts with an electrolyte, such as seawater, leading to rust formation.
To protect metal surfaces, methods like using sacrificial anodes are employed. This strategy diverts the corrosive action from the primary metal by introducing a more reactive metal that will corrode first. By doing so, the structural integrity of the original metal, such as the iron hull of the USS Monitor, is preserved. This is a common method used in boating, pipelines, and metal structures exposed to corrosive environments.
Understanding how to effectively prevent corrosion can save historic artifacts and modern structures from deterioration, and it's a critical aspect of preservation efforts.

Galvanic Series

The Galvanic Series is a chart used to predict and understand the likelihood of metals to corrode when they are in contact with each other in an electrolyte. It ranks metals according to their electrochemical reactivity.
- Metals at the top of the list, like zinc, are more reactive and prone to corrosion. - Metals further down, like iron, are less reactive in comparison.
In the case of the USS Monitor, zinc is higher on the Galvanic Series than iron. This means that when zinc is in contact with iron in an electrolyte (such as seawater), zinc will corrode in preference to iron. By exploiting this characteristic, zinc acts as a sacrificial barrier, protecting the less reactive iron hull from corrosion.
The Galvanic Series is a valuable tool for engineers and conservationists, allowing them to choose appropriate sacrificial anodes for different metals.

Electrochemical Cell

An electrochemical cell is created when two different metals are connected in a conductive solution, such as seawater. This results in the flow of electric current due to the difference in reactivity between the metals.
In terms of sacrificial anodes, the more reactive metal acts as the anode, while the less reactive acts as the cathode. For the USS Monitor:

• The zinc is the anode, corroding and losing electrons (oxidation).
• The iron hull is the cathode, protected as it receives electrons (reduction).

This set up allows the sacrificial anode to degrade, while preserving the integrity of the protected metal!
Understanding electrochemical cells is essential for explaining how the transfer of electrons from the anode to the cathode prevents the corrosion of less reactive metals.

Iron Hull Corrosion

Iron hull corrosion is a significant challenge for maritime vessels, particularly those made from older materials or exposed to saltwater environments. Iron reacts readily with oxygen in water, forming iron oxide or rust.
This chemical reaction not only weakens the metal but also leads to structural compromises, especially in historic artifacts like the USS Monitor.
To combat this, employing sacrificial anodes becomes crucial. A more reactive metal, such as zinc, is attached to the hull, thereby using its higher reactivity to draw the corrosive process away from the iron. By doing this, the physical condition of the hull is maintained.
Iron hull corrosion showcases the destructive force of nature and highlights the importance of corrosion protection methods in preserving maritime history and technology.

Zinc Anodes

Zinc anodes are commonly used as sacrificial anodes in corrosion protection systems. Their effectiveness lies in their placement on the Galvanic Series, being more reactive than metals like iron.
When zinc anodes are affixed to a structure like the USS Monitor's hull, they undergo corrosion first, sparing the more noble metal from oxidizing. This creates what's termed a "sacrificial" effect.
- As zinc loses electrons, it turns into zinc ions, which later disperse into the electrolyte. - This process provides electrons to the iron, reducing its tendency to form iron oxide.
Zinc anodes are thus indispensable in maritime maintenance, ensuring that submerged or exposed metallic surfaces remain intact against the harshness of seawater environments.