John Marcum Wells, age l2, of Charleston, . Va., or his question:
How does water cause iron to rust?
It is disappointing to find that the wintery weather has caused the barbecue grill to crumble to flaky rust. A mighty bridge weakened by rust would be disastrous. Our sturdy iron and steel structures must be sealed from both water and oxygen, and this rust proofing cost is immense.
The rusting of iron is a chemical process called oxidation. It is related to the chemical activity of fire and to the slow form of combustion that goes on when our bodies change nourishment into energy. Some elements are more active and eager to form chemical compounds with others. One of the most active of these is oxygen. It is oxygen that fuels the oxidation of fast fires and the slower forms of combustion.
Oxygen is needed to carry on the rusting process of iron and steel. For this particular job, however, it must be dissolved in liquid moisture. When iron comes in direct contact with air and water, atoms of dissolved oxygen unite with atoms of iron and form molecules of iron oxide. Oxygen, of course, is a gas, and iron is a sturdy metal. The compound iron oxide is very different from the parent elements from which its molecules are made.
It is a rusty red substance, soft and crumbly. And both water and oxygen are needed to cause iron and steel to rust by oxidation. Metal surfaces tend to be smooth and cool, and there is moisture in even the driest air. The rusting process begins when this moisture condenses on our sturdy structures of iron and steel. Atoms of dissolved oxygen combine with atoms of iron, and the metal is covered with greenish film.
Soon the fuzzy, green film turns reddish brown and when the moist film evaporates, some of the hard iron has become soft rust. The rust soon flakes away, leaving the metal pitted and scarred. These holes entrap and hold more moisture, and, once started, the rusting goes on at a faster and faster rate.
Rust proofing should be done before the rusting begins. Surfaces of iron and steel are coated with paint or plastic to seal them from air and moisture,. And fresh coats are added before the old ones wear thin. The endless job of rust proofing san francisco's magnificent golden gate bridge keeps 24 painters busy throughout the year.
The war on rust is expensive, but the oxidation of iron is vital to life. For plants and animals need iron, and this vital iron must be served in the form of iron oxide. Plentiful supplies of iron are oxidized in the moist soil, and plants absorb the rusty iron oxide dissolved in the ground water. We get our vital iron by eating iron rich vegetables and leafy salads.