Metallurgical aspects of steel galvanization - Steel Casting

Metallurgical aspects of steel galvanization

Exposure to environment can affect components or parts made of steel. Changes in temperature or exposure to moisture and air can cause corrosion of steel. It causes a chemical reaction, which results in the oxidation of iron present in the steel. As iron occupies a majority of the volume of steel, corrosion leads to increase in volume but decrease in the strength of the steel product. It is, therefore, important to prevent this chemical reaction to maintain the quality of the steel.

Galvanization

There are a few ways by which the steel surface can be protected from oxidation, and galvanization is one of the most commonly used and reliable ways to achieve that. Galvanization is the process by which steel is coated with zinc that prevents its exposure to the atmosphere and consequently prevents rusting. Galvanization can protect the steel surface for about 15 years or more. Molten zinc is applied on the surface of steel by one of the two methods: hot dipping or electrolytic deposition. Both these methods are successful in preventing the oxidation of iron present in steel and hence avoid corrosion.

Metallurgical bond

Galvanization of steel creates a barrier between the atmosphere and the steel surface. During the process of galvanization when the steel surface is coated with molten zinc, chemical reaction occurs. The zinc and the iron present in the steel form a series of zinc-iron alloy bonds. The addition of zinc coating provides impact resistance to the steel surface. The coating adds to hardness and ductility offering greater protection to the steel surface. They are also shield against rough handling of the zinc coated steel. However, the thickness of the zinc coating does not have any effect on the shelf life of the galvanization. This is because the durability or shelf life of galvanization depends on the total coating mass.

Impact of galvanization

Zinc is more electrochemically active than steel. In an electrolyte, it becomes the anode for steel. This prevents the formation of cathode and anode areas on the surface. This makes the electrons to flow from zinc anode to steel that creates positive charged zinc ions. Further, the negatively charged electrons are attracted to positive hydrogen ions and reacts with them creating hydrogen gas. Between the electrolyte and the steel cathode there occurs no chemical reaction. This is called as cathodic protection and it prevents the corrosion of the steel. On the anode surface, the positively charged zinc ions react with hydroxyl ions in the electrolyte that are negatively charged. Further, over years, when there is physical damage to the zinc coating, the cathodic protection acts as a shield against corrosion.

Presence of steel impurities

More often, steel contains impurities and these affect the process of galvanization. For example, phosphorus as impurity can create tougher coatings of zinc and enhance the resistance to corrosion. Silicon acts similar to zinc during the galvanization process. When zinc is coated on the surface of steel, phosphorous and silicon act as catalyst and enhance the bonding of zinc-iron producing more layers.