The Foundations of Chemistry

Figure 21-13a shows an iron pipe connected to a strip of magnesium, a more active

metal, to protect the iron from oxidation. The magnesium is preferentially oxidized. It is

called a “sacrificial anode.” Similar methods are used to protect bridges and the hulls of

ships from corrosion. Other active metals, such as zinc, are also used as sacrificial anodes.

Galvanizing (coating the iron with zinc), combines these two approaches. Even if the zinc

coating is broken so the iron is exposed, the iron is not oxidized as long as it is in contact

with the more reactive zinc (Figure 21-13c).

Aluminum, a very active metal, reacts rapidly with O 2 from the air to form a surface

layer of aluminum oxide, Al 2 O 3 , that is so thin that it is transparent. This very tough,

hard substance is inert to oxygen, water, and most other corrosive agents in the environ-

ment. In this way, objects made of aluminum form their own protective layers and need

not be treated further to inhibit corrosion.

876 CHAPTER 21: Electrochemistry

Magnesium bar (anode)

Iron pipe (cathode)

(a) (b)

O 2

OH–(aq)

O 2 + 2H 2 O + 4e– → 4OH–

(reduction) Fe (cathode)

Zn coating

(anode)

Zn(s) → Zn2+ + 2e–

(oxidation)

Zn2+(aq)

Water drop

Figure 21-13 (a) Cathodic protection of buried iron pipe. A magnesium or zinc bar

is oxidized instead of the iron. The “sacrificial” anode eventually must be replaced.

(b) Cathodic protection of a ship’s hull. The small yellow horizontal strips are blocks of

titanium (coated with platinum) that are attached to the ship’s hull. The hull is steel (mostly

iron). When the ship is in salt water, the titanium blocks become the anode, and the hull the

cathode, in a voltaic cell. Because oxidation always occurs at the anode, the ship’s hull (the

cathode) is protected from oxidation (corrosion). (c) Galvanizing is another method of

corrosion protection. Even if the zinc coating is broken, it is still oxidized in preference to

the less reactive iron as long as the two metals remain in contact.

Acid rain endangers structural
aluminum by dissolving this Al 2 O 3
coating.

(c)