Zinc Metal

Zinc is the first element of the twelfth column of the period table.

Under standard conditions zinc is a bluish white lustrous metal that is brittle at ordinary temperatures but malleable at 100°C to 150°C. It is harder than lead but softer than copper.

Zinc has a relatively low melting points for a metal. It is a fair electrical conductor.

When zinc comes into contact with the air it reacts with carbon dioxide to form a thin layer of zinc carbonate. This layer protects the element from further reaction.

Zinc is fairly active and will dissolve in most acids and some alkalis. However is does not readily react with oxygen.

Zinc Metal

Zinc is currently the fourth most widely consumed metal in the world after iron, aluminium, and copper.

It has strong anticorrosive properties and bonds well with other metals.

Consequently, about one-half of the zinc that is produced is used in zinc galvanizing, which is the process of adding thin layers of zinc to iron or steel to prevent rusting.

You do not have to look far to see an iron or steel structure that has been galvanized.

Why Galvanize Steel an Iron

As discussed, zinc has a relatively low melting point and can therefore safely melted in a zinc bath to become a liquid.

Steel and iron can be prepared and dipped into the zinc bath containing the zinc liquid. The zinc metal will bond to the steel, cool, go hard and protect the steel from corrosion.

If we look at the galvanic series you will see that zinc is more active (anodic) than iron and steel and will corrode in preference to the steel or iron surface.

The galvanized steel can even be overpainted to extend the life of the zinc.

Zinc Anodes

As zinc will protect steel against corrosion it is frequently used for steel that is immersed in water.

The water acts as the electrolyte and the steel the metallic path.

This process is called cathodic protection.

Zinc metal (anodes) are attached to the structure. No power is required.

Providing the correct calculation is made, there is no maintenance.

The zinc anodes can be used on bare or coated steel.

Aluminium is used for the same process.

Zinc

Metal ore is mined out of the ground.

The mining takes place globally with deposits in USA, Canada, Austria, Peru, China, Scandinavia and Ireland.

The process of mining to production has various stages commencing with the collection of the zinc sulphide from the ground through to the treatment plant.

Zinc-Galvanizing

Zinc coatings protect steel in two main ways:

• Zinc weathers at a slow rate which can give a predictable life.
• Zinc provides cathodic protection to the steel substrate. If there is damage to the zinc these areas may be protected from corrosion as the zinc will sacrifice itself rather than the steel.

Galvanizing Surfaces

You will see galvanizing used in everyday life from electrical pylons, architectural steelwork, handrails and stairwells to garden gates and garage doors.

In some environments galvanized steel has been known to last over 50 years.

The zinc coating itself may not always be easy to see as it is often topcoated for aesthetics and additional protection.

Galvanizing Surfaces

The galvanizing process is frequently used for corrosion protection where access for maintenance will be difficult when the structure is in operation.

An example of this is Railway overhead gantries.

Hot dip galvanizing is commonly used for overhead gantry steelwork as illustrated.

Other examples of widespread use includes motorway gantries, lamp posts, parapets and handrails.

It is possible to over paint these items at a later stage in their life providing proper preparation and cleaning is conducted prior to overcoating.

Cathodic Protection

Cathodic protection is based on the knowledge that anodic metals have a greater tendency to lose electrons than more noble metals.

Zinc is more anodic than steel.

When the two are connected in the presence of an electrolyte, the zinc becomes the anode on the corrosion cell and is slowly consumed, while the steel is the cathode and is protected.

The zinc in the galvanizing process provides the cathodic protection.

Zinc and Steel

Zinc is generally referred to as a 'sacrificial' Coating.

By design, the zinc coating corrodes preferentially over time, sacrificing itself to retard corrosion of the steel.

Its life is directly proportional to its thickness and the elements to which it is exposed.

This galvanic activity is a desirable characteristic with respect to the corrosion behaviour of steel, especially at surface scratches and cut edges, where the base steel will be exposed to the elements.

The galvanising will protect these surface scratches and cut edges.

Characteristics of Hot-Dip Galvanized Coatings

The hot-dip galvanizing process creates a dense coating of zinc that is metallurgically bonded to the steel surface. As the molten zinc diffuses into the steel surface, it creates four distinct layers that vary in their zinc to iron ratio.

The different layers of the three distinct iron-zinc alloy components are defined as a percentage of zinc/iron:

• Zeta (94% Zn 6% Fe)
• Delta (90% Zn 10% Fe)
• Gama (75% Zn 25% Fe)
• Also the layer of free zinc at the surface.
• Eta (100% Zn)

Hot-Dip Galvanizing Process

The hot-dip galvanizing process will be discussed in other training units, however, the process consists of pre-cleaning and fluxing the article to be galvanized and dipping the article in molten zinc.

There are restrictions depending upon the length of the zinc bath, although double-dipping is possible.

As the molten zinc metal dries (cools and solidifies to a hard coating) immediately on leaving the bath, the article can be put into service without any drying or curing that would be necessary with painted articles.

How does zinc deteriorate?

The corrosion of hot-dip galvanizing begins immediately after the molten zinc layer has cooled.

The appearance of hot-dip galvanizing can vary. Typically, the zinc surface is smooth with a pattern of random spangles resembling snowflakes in zinc.

When the steel contains trace elements of silicon and phosphorus outside of recommended ranges, it is considered a “reactive” steel.

The hot-dip galvanizing of reactive steel produces a matte finish after cooling. After exposure to atmospheric oxygen and moisture, the zinc surface begins to corrode, developing a thin layer of zinc oxide.

Continued atmospheric exposure results in the formation of zinc hydroxide.

Eventually, after about two years of exposure, the zinc surface layers have been converted largely to a metallic grey patina composed of insoluble zinc carbonate and zinc oxide.

At this point, the zinc surface has essentially stabilised and continued atmospheric corrosion is substantially reduced.

Design

The decision to protect steel by hot dip galvanizing should always be taken at the design stage.

Consultation between the designers, fabricator and galvanizer is essential to achieve the best results from the process.

A correct design will allow easy handling and the free flow of molten zinc will enhance the quality of the coating from an aesthetic and technical point of view.

Design Considerations

It is important that the design of the structure is considered before considering a galvanized process.

• Skip weld and crevices may trap acid which causes damage to the galvanizing as it could vaporize during the process or stain the coating at a later date
• Ensure there are no areas where air pockets or air bubbles may form preventing full contact with the molten zinc
• Ensure the structure is of a size that can fit into the zinc tank
• Warpage of the article is a possibility due to unequal thickness of the article or structure
• The article or structure should be made of the same alloy
• Faying surfaces should not be galvanized as this would have to be removed later

Environments

The designer must check the specific environment that the hot dip galvanizing can be exposed too.

Various ISO and other standards give advice on the use of hot dip galvanizing or zinc in specific environments.

The hot dip galvanizing can also be topcoated to suit specific environments, give additional life and look more pleasing to the eye.

Advantages and Disadvantages

Hot dip galvanizing has a number of advantages and disadvantages.

Advantages:

• Proven performance.
• Economical.
• Can be topcoated/painted.
• Can be maintained.

Disadvantages:

• Factory application required.
• Limited to size of galvanizer's bath.
• Potential adhesion problems with overcoating.
• Zinc oxide deposits (white salts) if exposed and not painted.

Standards and Guides

There are numerous standards and guides for hot-dip galvanizing.

Various countries have developed their own standards.

A number of galvanizing organizations also exists who supply information and advice on the hot-dip galvanizing process.

A detailed training unit on standards and guides is contained within the training program.

Summary

Within this training unit we briefly looked at “what is galvanizing”.

We discussed the properties of zinc metal and its corrosion properties on iron and steel surfaces.

We discussed the design implications with regards to the article and environment.

We also discussed standards along with the advantages and disadvantages of hot-dip galvanizing.