Question

Determine whether or not each metal, if coated onto iron, would prevent the corrosion of iron. $$\begin{array}{lll}{\text { a. } \mathrm{Mg}} & {\text { b. } \mathrm{Cr}} & {\text { c. Cu }}\end{array}$$

Short answer

Mg would prevent the corrosion of iron due to sacrificial protection, Cr does not provide sacrificial protection but may protect iron through other means like formation of a passive oxide layer, and Cu would not prevent the corrosion of iron through sacrificial protection.

Step-by-step solution

Understanding Galvanic Series

To determine whether a metal will prevent the corrosion of iron when used as a coating, we need to refer to the galvanic series. This series lists metals in order of their relative activity, also known as their electrochemical potential. Coating iron with a more active metal (anodic) would protect it through sacrificial protection, while a less active metal (cathodic) might cause it to corrode faster.

Checking the Position of Metals in the Galvanic Series

In the galvanic series, metals that are more active (or anodic) than iron will corrode and protect the iron from corrosion. Metals that are less active (cathodic) than iron will not offer protection and might even accelerate corrosion under certain conditions.

Assessing Magnesium (Mg)

Magnesium (Mg) is more anodic than iron in the galvanic series. Hence, when coated onto iron, Mg will corrode in place of the iron, thereby protecting it from corrosion. This is known as sacrificial anode protection.

Assessing Chromium (Cr)

Chromium (Cr) is less anodic and more noble than iron in the galvanic series. This means it will not corrode before iron, and it does not provide sacrificial protection. However, Cr is often used to form a passive oxide layer that protects iron from corrosion for reasons other than being anodic.

Assessing Copper (Cu)

Copper (Cu) is less anodic than iron, much like chromium, meaning it will not corrode before iron. Therefore, it will not provide sacrificial protection to iron.

Key concepts

Galvanic Series

When metals come into contact with each other in the presence of an electrolyte, such as salty water, they can form a galvanic cell, leading to corrosion. The Galvanic Series is a chart that ranks metals and alloys by their electrochemical potential. It is used to predict how metals will interact when in contact. Metals towards the top of the series, like magnesium (Mg), are more active and are known as anodic. They tend to corrode faster than those towards the bottom, which are referred to as cathodic and are more noble, like gold (Au).

For preventing iron corrosion, a coat of a more active metal from the galvanic series serves as a sacrificial layer. This layer corrodes preferentially, thus preserving the iron. As such, using knowledge of the galvanic series aids in selecting suitable metals for corrosion protection, much like in the exercise which looks into Mg, Cr, and Cu as potential coatings for iron.

Sacrificial Anode Protection

One of the practical applications of the galvanic series is the use of sacrificial anode protection. This approach involves attaching a more reactive metal, the sacrificial anode, to the iron structure we intend to protect. Over time, this anode corrodes instead of the iron, effectively sacrificing itself.

For example, magnesium, as mentioned in the exercise solution, corrodes before iron, offering a shield as a sacrificial layer. This technique is particularly useful for underground pipelines, ships' hulls, and water heaters. It's an economic and effective way to enhance the durability of iron-based structures against corrosion. The correct selection of a sacrificial anode is essential and is informed by the galvanic series, ensuring the anode is more anodic relative to the metal being protected.

Electrochemical Potential

The term electrochemical potential refers to the tendency of a metal to gain or lose electrons in an electrochemical reaction, a critical concept in understanding corrosion processes. It is the driving force behind galvanic corrosion and the operation of batteries. Metals with higher electrochemical potentials readily give up electrons and are oxidized more easily; these metals serve as anodes in galvanic cells.

In sacrificial anode protection, the metal with the higher electrochemical potential corrodes first, relinquishing electrons to the less reactive metal, or the cathode. Therefore, when selecting a coating for iron corrosion prevention, one would opt for a metal with a higher electrochemical potential than iron, ensuring it acts as a sacrificial anode and not the other way around. As was clear in the exercise, Mg has a higher electrochemical potential relative to iron, which is why it can serve as an effective protective coating, while Cr and Cu cannot.