Steel is the most abundant and the most efficient and cost effective metal which is available in the market and it provides specifiers design freedom. However the projects which are most exposed to atmosphere & harsh conditions are subject to critical coating for corrosion protection.
The reason for the extensive use of hot-dip galvanizing (HDG) is the three-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically-bonded zinc coating that completely covers the steel surface and seals the steel from the corrosive action of the environment. Additionally, zinc’s sacrificial behavior protects the steel, even where damage or a minor discontinuity in the coating occurs. Finally, the natural weathering of the coating results in the development of an additional layer of protection on the surface.
Barrier Corrosion Protection
This is perhaps the oldest & the most widely used method of corrosion protection in which the base metal is isolated from the environment. Here hot dip galvanized coating offers barrier protection to the steel metal. As long as the barrier is intact, the steel is prevented from corrosion. But if the barrier is breached the corrosion is again set to begin.
Since the barrier must remain intact to provide corrosion resistance, the two important properties of barrier protection are adhesion to base metal & abrasion resistance. A tightly bonded layer of zinc makes this galvanizing method an excellent barrier protection. Coatings such as paint that have pin holes are more susceptible to underfilm corrosion which if spread rapidly can create havoc.
Cathodic Corrosion Protection
Cathodic protection is one of the most effective ways to resist corrosion on metallic surfaces. It basically follows a principle where an element of corrosion circuit is introduced by a new corrosion element which thus becomes the cathodic element of the circuit.
Hot dip galvanizing protects steel cathodically which is similar to the sacrificial anodic method. Basically, a metal (zinc) anodic to the base metal (steel) is placed in the circuit to corrode in place of the base metal. The Galvanic Series of Metals is a list of metals arranged in order of electrochemical activity in seawater (the electrolyte).
Zinc Patina Corrosion Protection
This method is different than the barrier & cathodic protection. In this the zinc patina protects the zinc coating from getting corroded. Zinc like all metals begins to corrode when exposed to the environment.
As galvanized coating is exposed to free air the zinc patina formation begins with a thin layer of zinc oxides. Then when these pieces are exposed to the environment & moisture like rainfall, dew & humidity, zinc oxide particles react with water to form a porous gelatinous zinc hydroxide. Then during dry cycles, carbon dioxide reacts with the zinc hydroxide and converts to a thin, compact, and tightly adherent layer of zinc carbonate.
A fully developed patina is a passive, stable film that adheres to a zinc surface and more importantly it is not water soluble which means zero chances of washing off in rains or snow. Hence because of this, zinc patina corrodes very slowly & protects the galvanized coating underneath and thus highly reducing the corrosion rate to about 1/30th of the rate of steel corrosion in the same environment. Zinc patina formation is critical to long lasting corrosion resistance of hot dip galvanizing.