Question

Which of the following metals are suitable for use as sacrificial anodes to protect against corrosion of underground iron pipes? If any are not suitable, explain why:

(a) Aluminium

(b) Magnesium

(c) Sodium

(d) Lead

(e) Nickel

(f) Zinc

(g) Chromium

Short answer

Aluminium, magnesium, sodium, zinc and chromium are used as sacrificial anodes to protect against corrosion of underground iron pipes.

Step-by-step solution

Sacrificial anodes:

Sacrificial anodes: metals that are used to prevent the corrosion of iron.

In the rusting process, iron acts as both cathode and anode, but it is lost only at the anode. So, in order to prevent corrosion, iron has to act as cathode (it needs to be reduced), and the other metal has to be easily oxidized (acts as anode, hence the name sacrificial anode).

Therefore, we will write down redox reaction for all the given metals. If ${\text{E}}^{\text{°}}$of a given metal is less than ${\text{E}}^{\text{°}}$of iron, oxidation of metal will occur, hence, it can be used as sacrificial anode. On the other hand, if ${\text{E}}^{\text{°}}$of a given metal is larger than ${\text{E}}^{\text{°}}$of iron, oxidation of iron will occur, hence, iron will corrode rapidly.

Redox reaction of iron

${\text{Fe}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{-}\rightleftharpoons \text{Fe}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^{°}=-0.44\text{V}$

To use as sacrificial anodes:

a. Aluminum

${\text{Al}}^{3+}\left(\text{aq}\right)+3{\text{\hspace{0.17em}e}}^{-}\rightleftharpoons \text{Al\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^{°}=-1.66\text{V}$

Since ${\text{E}}^{\text{o}}$is lesser than ${\text{E}}^{\text{o}}$of iron, it can be used as sacrificial anode, but, its efficiency is low because of the formation of oxide layer which passivate the anodes. Hence, it is not a good choice.

b. Magnesium

${\text{Mg}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{-}\rightleftharpoons \text{Mg\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^0=-2.37\text{\hspace{0.17em}V}$

Since ${\text{E}}^{\text{o}}$ is lesser than ${\text{E}}^{\text{o}}$of iron, it can be used as sacrificial anode.

c. Sodium

${\text{Na}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\rightleftharpoons \text{Na\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^0=-2.71\text{V}$

Since ${\text{E}}^0$ is lesser than ${\text{E}}^0$ of iron, it can be used as sacrificial anode, but, its efficiency is low because it reacts with water.

d. Lead

${\text{Pb}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{-}\rightleftharpoons \text{Pb\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^0=-0.13\text{V}$

${\text{E}}^0$larger than ${\text{E}}^0$of iron, hence, it can't be used as sacrificial anode.

e. Nickel

${\text{Ni}}^{2+}\left(\text{aq}\right)+2{\text{\hspace{0.17em}e}}^{-}\rightleftharpoons \text{Ni\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^0=-0.25\text{V}$

${\text{E}}^0$larger than ${\text{E}}^0$ of iron, hence, it can't be used as sacrificial anode.

f. Zinc

${\text{Zn}}^{2+}\left(\text{aq}\right)+2{\text{\hspace{0.17em}e}}^{-}\rightleftharpoons \text{Zn\hspace{0.17em}\hspace{0.17em}}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^0=-0.76\text{V}$

Since ${\text{E}}^0$is lesser than ${\text{E}}^0$of iron, it can be used as sacrificial anode.

g. Chromium

${\mathrm{Cr}}^{3+}\left(\text{aq}\right)+3{\text{e}}^{-}\rightleftharpoons \mathrm{Cr}\left(\text{s}\right){\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}E}}^{°}=-0.74\text{V}$

Since ${\text{E}}^0$is lesser than ${\text{E}}^0$of iron, it can be used as sacrificial anode.