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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 ${\text{E}}^{\text{掳}}$of a given metal is less than ${\text{E}}^{\text{掳}}$of iron, oxidation of metal will occur, hence, it can be used as sacrificial anode. On the other hand, if ${\text{E}}^{\text{掳}}$of a given metal is larger than ${\text{E}}^{\text{掳}}$of iron, oxidation of iron will occur, hence, iron will corrode rapidly.

Redox reaction of iron

${\text{Fe}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{鈭�}鈬�\text{贵别鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^{掳}=鈭�0.44\text{V}$

a. Aluminum

${\text{Al}}^{3+}\text{鈥�}\left(\text{aq}\right)+3{\text{鈥塭}}^{鈭�}鈬�\text{础濒鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^{掳}=鈭�1.66\text{V}$

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

${\text{Mg}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{鈭�}鈬�\text{惭驳鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^0=鈭�2.37\text{鈥塚}$

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

c. Sodium

${\text{Na}}^{+}\left(\text{aq}\right)+{\text{e}}^{鈭�}鈬�\text{狈补鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^0=\text{鈥�}鈭�2.71\text{V}$

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

${\text{Pb}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{鈭�}鈬�\text{笔产鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^0=鈭�\text{鈥�}0.13\text{V}$

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

e. Nickel

${\text{Ni}}^{2+}\left(\text{aq}\right)+2{\text{鈥塭}}^{鈭�}鈬�\text{狈颈鈥�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^0=\text{鈥�}鈭�0.25\text{鈥�}\text{V}$

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

f. Zinc

${\text{Zn}}^{2+}\left(\text{aq}\right)+2{\text{鈥塭}}^{鈭�}鈬�\text{窜苍鈥夆赌�}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^0=鈭�0.76\text{V}$

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

g. Chromium

${\mathrm{Cr}}^{3+}\left(\text{aq}\right)+3{\text{e}}^{鈭�}鈬�\mathrm{Cr}\left(\text{s}\right){\text{鈥夆赌夆赌塃}}^{掳}=鈭�0.74\text{V}$

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