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Ask Dr. Knowledge

What exactly is meant by galvanizing steel, and how can it be measured?

August 22, 2011

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Galvanization is the protection of iron and steel against corrosion, or rusting, by coating them with zinc. This was traditionally done by electroplating in a galvanic cell, but today it is often done by dipping the steel or iron into molten zinc.

Galvanization can protect the underlying metal well, even parts that get exposed when the zinc coating is damaged. This process affords much more protection than using paint.

Corrosion is an electrical process, and zinc is more reactive than iron or steel, so the electrical current involved with the corrosion will go preferentially to the zinc. The zinc is then said to be “sacrificed’’ to protect the iron or steel.

The big problem with iron and steel is not so much that they oxidize, or rust, but that rust is fragile stuff. As it falls off, it exposes more and more metal, which is what makes it so damaging.

The amount of zinc coating on a piece of galvanized iron or steel can be measured by using a magnet, which pulls on iron and steel but not on zinc. The thicker the coating, the weaker the pull. A properly calibrated device can mechanically or electronically indicate the thickness of a zinc coating without damaging it.

Ask Dr. Knowledge is written by a Northeastern University physicist, John Swain. E-mail questions to him at drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.