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Article updated on 02/03/20 by Priyom Bose
Zinc is a material that has gone through substantial changes in the way that it is used as well as in the way that it is perceived, both by specialists and by the general public.
Zinc Coatings on Steel
One of the biggest applications of zinc can be found in the protective coatings for steel. The development of the numerous types of zinc coatings arose from two happy accidents of chemistry, of which included the recognition of zinc’s relatively slow and predictable rate of atmospheric corrosion as compared to that of steel, as well as the relative positions of zinc and iron in the electrochemical series.
Zinc will preferentially corrode to provide cathodic protection to iron when both are in contact with an aqueous medium. This is used to protect immersed structures, such as ships’ hulls, drilling rigs and pipelines. This unique property also ensures the continuous protection of any bare areas in a zinc coating on steel, which may arise as a result of damage or operations such as cutting or drilling, due to the surrounding presence of zinc. Taken together, these two factors provide the basis of a unique corrosion protection system that uses approximately 4 million tons of zinc annually to protect about 100 million tons of steel. This represents almost half the total world consumption of zinc.
Other Zinc Coatings
Originally, zinc coatings were applied by a process known as hot-dip galvanizing, which involves the dipping of prepared steel in molten zinc. Typically, this process is done with fabricated steel or with sheets of steel, many of which are profiled to produce the ubiquitous ‘corrugated iron’.
The first development from hot-dip galvanizing was during the production of continuous strip steel with a galvanized coating. Since then, several refinements of this process have allowed for the greatest part of the total tonnage of steel carrying zinc coatings. Other processes are available and used when specific characteristics are required, such as electroplating, otherwise known as electrogalvanizing, flame sprayed coating and several others.
Recent Zinc Galvanizing Alloys
Process developments in continuous galvanizing have enabled numerous coatings to be produced with very closely controlled thickness and surface finish. These can be formed and joined without significant damage to the protective coating. Along with these developments, a series of alloys for coatings have been produced. The most important of these are Galvalume and Galfan.
Galvalume consists of about 55% aluminum and 45% zinc with a small amount of silicon. This material is being used extensively around the world, as it has better atmospheric corrosion resistance than pure zinc; however, it loses the ability to protect any exposed steel, such as that at cut edges. Galfan, on the other hand, is a zinc alloy that contains 5% aluminum and small amounts of rare earth metals. To this end, Galfan has a substantial and growing niche market, in which its properties are valuable. In addition to its ability to retain some cathodic protection capabilities, the corrosion resistance of Galfan is also better than that of zinc.
Other Zinc Alloys
Zinc (Zn) is an abundant metal found in the Earth’s crust with a myriad of industrial and biological uses. At room temperature, zinc is brittle and bluish-white in color but can be polished to a bright finish. Zinc is currently the fourth most widely used metal in the world after iron, aluminum, and copper. It has strong anticorrosive properties and bonds well with other metals.
Several zinc alloys have been found to offer varying benefits for their given applications, some of which include:
- Brass: 3-45% zinc by weight. Brass zinc alloys can be used in musical instruments, valves and hardware.
- Nickel silver: 20% zinc by weight. This is mainly used for the shiny silver appearance in jewelry, silverware, model train tracks and musical instruments.
- Zinc diecasting alloys: Less than 78% zinc by weight. These alloys usually contain minuscule amounts of lead (Pb), tin (Sn), copper (Cu), aluminum (Al) and magnesium (Mg) to improve diecasting characteristics and mechanical properties. These alloys are used to make small intricate shapes and are suitable for moving parts in machines. The cheapest of these alloys are referred to as pot metal and serve as inexpensive replacements for steel.
Galvanized steel with a very fine surface finish is used to produce the parts of car bodies that are vulnerable to corrosion. The surface finish of the coated steel is such that there is no visible difference in its appearance after painting between panels, both with and without the zinc protection.
Building and Construction Industries
Building and construction industries use at least two-thirds of all produced coated steel strip, mainly for roofing and cladding of commercial and industrial buildings. Much of the material used in buildings has a mill-applied organic coating that sits on top of the zinc. Buildings in this ‘color coated’ steel are a familiar sight, particularly in out of town shopping centers and industrial estates. Color coated steel can provide lifetime protection for such buildings. More recently, the use of galvanized steel in domestic buildings has grown considerably.
Hot Dip Galvanizing
Hot dip galvanizing in its original form, as applied to fabricated steelwork, is also a growing industry. This is unique, as many industries, like the hot-dip galvanizing industry, that are more than 150 years old are no longer viable. The reasons for the process’s continued success are in the intrinsic relationship that exists between iron and zinc, the improved organization of the handling of work in the plant and, most importantly, a better appreciation of the economics of corrosion protection. As these factors are better understood by design engineers, the tonnages of steel galvanized have risen steadily.
Zinc alloy castings are unique, particularly when produced by the pressure die casting process. These materials can be made to extremely close tolerances, with excellent surface finish, have a range of useful mechanical properties and can receive a wide range of applied finishes. As a result, zinc castings can be found in a number of applications ranging from automobiles to zip fasteners.
A further benefit associated with recent improvements that have been made on the pressure die casting process includes its ability to make castings that are much thinner, thereby reducing the amount of metal required for a given product. As a result, casting manufacturers achieve significant cost and weight savings, which is a highly valuable point within many industries, particularly in automotive applications, while also improving the quality and consistency of the material.
Zinc can be recycled indefinitely without degradation. This has been appreciated for many years and zinc scrap and residues are classified, traded and priced according to their zinc content and to the economics of turning the scrap into a useful product. Old scrap zinc sheets are easily returned into metal ingots.
Process residues from galvanizing are highly valued raw materials for the production of zinc pigments and other chemicals. When zinc-coated steel is recycled, the zinc volatilizes and is captured in the flue dust, which allows for its reuse alongside zinc ores during the production of primary zinc metal.
Pharmaceutical Uses & Other Commercial Use
Zinc oxide is also present in many common commercial products, some of which include batteries, paint, plastics, rubber products, floor coverings, inks, cosmetics, soap, and textiles. Zinc is also a natural insect repellent.
Zinc, both in its elemental and salt forms, has also been used as a therapeutic modality for centuries. In fact, zinc oxide is used in the preparation of calamine.
Similarly, zinc pyrithione has been in use as photo protecting, in soothing agents or as an active ingredient of antidandruff shampoos. Zinc is also used for the treatment of several dermatological conditions such as infections like leishmaniasis and warts, inflammatory dermatoses, such as acne vulgaris and rosacea, pigmentary disorders like melasma, and neoplasias, such as basal cell carcinoma. The role of oral zinc is well-established in human zinc deficiency syndromes including acrodermatitis enteropathica. This element is also an essential micronutrient for infant growth and development.
Primary author: Dr. Anthony Wall
Source: Materials World, Vol. 6 no. 4, April 1998.