Here is an instructive story about a disastrous blunder an American millionaire once made.

    He wanted to have the best yacht in the world. He put in an order and thought up a romantic name "The Call of the Sea." He spared no money. The contractors did their utmost to please their client. It remained only to finish the interior decorations..

    But the yacht never went to sea; it never had a chance to. A short time before the day the yacht was to be launched its body and bottom were found to have completely corroded.

    Why? Because corrosion is an electrochemical process.

    The shipbuilders had decided to plate the bottom of the yacht with a nickel-copper alloy called German silver. It was a good idea because this alloy, though expensive, resists corrosion in sea water very well. It resists corrosion all right, but it is not very strong. And therefore many parts of the ship had to be made of other metals, special steels.

    And this was the undoing of the yacht. Powerful galvanic cells formed at the points of contact between the German silver and the steel and the bottom immediately began to disintegrate. The end was sad.

    The millionaire's grief was indescribable, and the yacht builders remembered ever after one of the laws of corrosion, that its rate increases sharply if to the principal metal are added other metals capable of forming a galvanic cell with it.

---