Sacrificial protection takes place when a magnesium block is attached to the steel on oil rigs. Which metal is 'sacrificed' and
which is protected? Explain your answer.

Explained below.

Explanation:

In oil rigs, Sacrificial protection is the protection of the steel against corrosion by using magnesium block which is a more reactive metal. This means that steel which is to be protected from corrosion will be the cathode while magnesium block preventing it from corrosion will be the anode and as such the block will be the one to corrode instead.

The mechanism behind this sacrificial protection is that the steel pipe on the rig will be connected to the magnesium block through copper wires and then the magnesium block will donate its electrons to the steel thereby preventing it from rusting. Thus is a reversible reaction and as such, after the steel Iron undergoes oxidization by receiving electrons from the magnesium block, it will be immediately be reduced to iron as depicted in the equation below.

Mg⇌Mg^(2+) + 2e-

Fe^(2+) + 2e-⇌Fe

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catya your not at all which answer i've seen you do this on another question!