what is galvanisation and crystallisation ?

What is Galvanising?

The first use of hot dip galvanising on steel was reported by one P J Maloiun in 1742. A French patent was issued to Sorel in 1837 and an English patent to H V Craufurd in the same year.

Hot dip galvanising is the process of coating iron, steel or ferrous materials with a layer of zinc, by passing the metal through a molten batch of zinc at a temperature of around 860â—¦f (460â—¦c), which further reacts with carbon dioxide (C02) to form zinc carbonate (ZNC03), a usually dull grey, fairly strong material that stops further corrosion in many circumstances and protects the steel from the elements. Galvanised steel is widely used in applications where rust resistance is needed, and can be identified by the crystallisation patterning on the surface, often called a spangle.

Hot dip galvanising protects steel and ferrous material from corrosion by providing a tough metallic zinc envelope, which completely covers the steel surface and seals it from the corrosive action of its environment. The galvanised coating provides outstanding abrasion resistance. Where there is damage or minor discontinuity in the sealing coat of zinc, protection of the steel is maintained by the cathodic action of the surrounding galvanised coating.

Virtually any ferrous article may be coated. Articles ranging in size from small fasteners to structures hundreds of metres high may be protected. Large galvanising kettles, together with modular design techniques of construction and double-end dipping allow almost any sized structure to be galvanised.

Metallic zinc is strongly resistant to the corrosive action of normal environments and hot dip galvanised coatings therefore provide long-term protection for steel. By contrast, most organic paint coatings used on steel need frequent renewal and when coatings are breached, corrosion begins at the exposed area of steel, spreading rapidly beneath the coating film.

What is the Galvanising Process? Preparation of work for Galvanising

It is critical to ensure that any item to be hot dipped galvanised is completely clean in order for the molten zinc to form a sound and complete metallurgical bond with the item. To ensure the material surface is completely clean the material is passed through a pickling process prior to entering the galvanising kettle. In this process any scale, rust, oil, paint and other surface contaminants are carefully removed from the steel by suitable preliminary treatment and subsequent acid cleaning or pickling in sulphuric or hydrochloric acids, followed by rinsing. Rolled steel surfaces covered by heavy mill scale may require abrasive blast cleaning prior to acid cleaning.

Fluxing

The acid-cleaned steel article is immersed in a flux solution, usually 30% zinc ammonium chloride with wetting agents, maintained at about 65â—¦C. The flux solution removes the oxide film which forms on the highly reactive steel surface after acid cleaning, and prevents further oxidation before galvanising. The work is then dried ready for galvanising.

Galvanising

On immersion in the galvanising bath the steel surface is wetted by the molten zinc and reacts to form a series of zinc-iron alloy layers. To allow formation of the metallurgic bond the work remains in the bath until its temperature reaches that of the molten zinc, in the range 445â—¦C to 465â—¦C. The work is then withdrawn at a controlled rate and carries with it an outer layer of zinc which solidifies to form the relatively pure outer zinc coating.

The period of immersion in the galvanising bath varies from several minutes for relatively light articles, up to half an hour or longer for major structural members.

The resulting galvanised coating is tough and durable, comprising relatively pure zinc and zinc-iron alloy layers bonded metallurgically to the underlying steel, completely covering the article externally and internally, providing unmatched resistance to abrasion. The fact that hot dipped galvanising provides internal as well as external protection is a major advantage compered to normal paint only systems.

An important advantage of the galvanising process is that visual inspection shows that work is completely protected and gives an excellent guide to coating quality.

1. To cause to form crystals or assume a crystalline structure.2. To give a definite, precise, and usually permanent form to: The scientists finally crystallized their ideas about the role of the protein.3. To coat with crystals, as of sugar.