Why can zinc protect steel even beside a small scratch?

A galvanized steel fence has a coating of zinc. When that coating is intact, it forms a barrier between steel and its surroundings. A small scratch seems to defeat a barrier, yet nearby zinc can still help protect the exposed steel. Its second job depends on electrical and chemical connections.

In a damp environment, zinc and steel can form parts of an electrochemical circuit. Zinc atoms can lose electrons and enter reactions that consume the zinc. The electrons travel through the connected metal toward the steel. Reactions at the steel surface receive electrons, while dissolved ions move through the moisture to complete the charge-carrying routes.

Corrosion of iron involves iron atoms losing electrons as they leave the metal. Supplying electrons from the more readily reacting zinc helps suppress that iron-losing process at the nearby steel. The protection therefore reaches across a small break in the coating through a connected circuit; it is not a new layer instantly filling the scratch.

Zinc is gradually used up while providing this protection. That is why it is called sacrificial protection. Electrical contact between the metals and the surrounding conditions matter, and the effect has a limited reach. Once the nearby zinc is exhausted, that source of protection ends.

The useful coating combines two mechanisms: an intact physical barrier and a supply of metal that can react in preference to steel. A scratched surface can therefore behave differently from an equally scratched paint layer, even though both initially looked like continuous covers.