Hot zinc plated: triple corrosion protection

Structural steel is widely used as an effective building material, which provides designers and architects with almost unlimited creative freedom.

Corrosion protection

For buildings and structures, which are exposed to weather conditions and other harsh natural influences, it is very important to protect the steel from corrosion. Often, the design life of buildings reaches 50-100 years, which requires strong and durable protection of steel elements of buildings from corrosion.

Hot zinc plated - just that, what is needed to protect steel from corrosion. It provides three levels of corrosion protection to steel: barrier protection, cathodic protection and zinc patina protection.

Zinc coating: barrier protection

The first line of defense for hot-dip zinc coating is barrier protection.. Like conventional paints, hot-dip zinc coating provides protection by simply insulating the steel against the effects of electrolytes from the environment.. As long as this barrier remains intact, steel protected, and no corrosion occurs. but, as soon as this barrier is damaged, corrosion begins.

To provide protection against corrosion, the barrier must remain intact. Therefore, it must have two important properties - the strength of adhesion to the base. (steel) and abrasion resistance. The dense and impermeable nature of zinc makes it a very good barrier coating material.. In contrast to zinc coating, paints are prone to the formation of puncture-like through defects such as pin pricks.. Elements penetrate through these defects, which cause corrosion of the steel under the protective coating.

Zinc coating: cathodic protection

In addition to barrier protection, hot zinc plating provides cathodic protection to the steel. It means, that zinc will corrode itself, to protect the underlying metal. This happens in accordance with the so-called "galvanic range of metals" (cm. below). Cathodic protection of steel with zinc is ensured even then, when the damage to the zinc coating on steel to "bare" steel reaches 5-6 mm: no corrosion of steel occurs, until she "eats" the surrounding zinc.

Electroplating range of metals

The galvanic series of metals is a sequence of metals in order of decreasing corrosion properties.
The higher the metal in the column, the higher its corrosive properties and the less "noble" it is.
The lower the metal in the column, the more resistant it is to corrosion and the more it is "noble".

Anode metals,
less noble

Magnesium
Zinc
Aluminum
Cadmium
Steel
Lead
Lead
Nickel
Brass
Bronze
copper
Nickel-copper alloys
Stainless steels (passivated)
Silver
Gold
Platinum

Cathode metals,
more noble

This sequence of metals determines, what metal will be the anode, and which one is the cathode, when they both form electrolytic cell. Metal, which is higher in this list, is anodic to metal, which is below it. It means, that in the connection the "upper" metal will protect the "lower" metal from corrosion, being exposed to it myself. Therefore, zinc also protects steel.

Zinc patina protection

The last line of defense for hot-dip zinc coating is the formation of a zinc patina.. Zinc patina is the formation of a coating of zinc corrosion products on the surface of steel. Zinc, like all metals, when interacting with the atmosphere begins to corrode. When the zinc coating is exposed to moisture at the same time, and air, zinc corrosion products will naturally form a coating on its surface. Formation of these corrosion products - zinc oxide, zinc hydroxide and zinc carbonate - occurs gradually during dry and wet periods of operation.

Fully formed zinc patina significantly reduces the corrosion rate of zinc and acts as an additional impermeable barrier for the hot zinc coating.

Source: http://www.galvanizeit.org