Protecting Mild Steel from Corrosion using Zinc

The Principle of Galvanic Protection of Steel
Various methods are available to protect steel from corrosion. One of the most elegant of these is the use of zinc. Zinc lies below steel in the galvanic series and therefore will corrode preferentially to steel, offering “galvanic protection”.

When galvanic protection is in operation, gaps in the zinc layer do not result in loss of steel but instead accelerate the corrosion rate of zinc. The conditions that need to be met to achieve galvanic protection are:

the steel has to be electrically connected to the zinc
the materials are close enough for a protective electrical circuit to be made through the corrosive environment
The second of these conditions is by far the harder to judge since service conditions vary greatly. So, in atmospheric corrosion, the metals must be immediately adjacent to each other, but in seawater immersion much greater separation of steel and zinc may be tolerated.

Options for Zinc Protection
There are three main options for the protection of large structures by zinc. Each has key advantages so it is important to assess each application’s particular circumstances.

Galvanized Steel
Galvanizing is extremely reliable but needs to be built into a project in the design stage to ensure that the components can be prepared and coated prior to manufacture.

Galvanized steel can come as hot-dipped or electrogalvanized. Hot dipped galvanized steel is generally exceptionally reliable and predictable, and can be supplied at a number of different thicknesses.

The lifetime of the material is extended by the intermetallic layers (iron-zinc) that forms as the steel is dipped into molten zinc.

This layer ensures that the coating cannot fail by delamination and actually corrodes slower than the outer zinc layer, so visible rusting of a hot-dipped galvanized structure does not automatically mean that the structure itself is suffering physical damage.

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