By the end of this lesson, students will be able to:
Explain the physical and chemical properties of zinc and its role in protecting steel through galvanizing and cathodic protection.
Describe the hot-dip galvanizing process, including coating layers, design considerations, and environmental factors influencing corrosion resistance.
Evaluate the advantages, limitations, and applications of galvanizing in real-world structures such as pylons, gantries, handrails and other structures.
Zinc Metal
Centuries before it was identified as an element, zinc was used to make brass (an alloy of zinc and copper) and for medicinal purposes.
Metallic zinc and zinc oxide were produced in India sometime between the 11th and 14th centuries and in China in the 17th century. However, the discovery of pure metallic zinc is credited to the German chemist Andreas Marggraf, who isolated the element in 1746.
Zinc corrodes more slowly than steel, so it is used as a means of protecting steel against corrosion.
This process is known as galvanizing and has gained a reputation for good corrosion control properties in specific environments.
Some zinc facts:
Symbol: Zn
Atomic Number: 30
Atomic Weight: 65.38
Classification: Transition metal
Phase at room temperature: Solid
Melting Point: 419°C 1665°F
Zinc Metal
Zinc is the first element of the twelfth column of the periodic 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 point 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 relatively active and will dissolve in most acids and some alkalis. However, it 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?
As discussed, zinc has a relatively low melting point and can therefore be 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, harden 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 as the metallic path.
This process is called cathodic protection.
Zinc metal (anodes) is 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.
Mining takes place globally, with deposits located in the 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, galvanised steel has been known to last over 50 years.
The zinc coating itself may not always be easy to see, as it is often top coated for aesthetics and additional protection.
Galvanizing Surfaces
The galvanising 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 include motorway gantries, lamp posts, parapets and handrails.
It is possible to overpaint these items at a later stage in their life, provided proper preparation and cleaning are conducted before 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 in 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)
Eta (100% Zn)
Also, there is a layer of free zinc at the surface.
Hot-Dip Galvanizing Process
The hot-dip galvanizing process will be discussed in other training lessons, 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 mainly converted 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 (within the bath) will enhance the quality of the coating from an aesthetic and technical point of view.
Design Considerations
The design of the structure must be considered before considering a galvanized process.
Skip welds and crevices may trap acid, which causes damage to the galvanizing as it could vaporise 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 complete 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 the 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 in which the hot-dip galvanising can be exposed.
Various ISO and other standards advise on the use of hot-dip galvanizing or zinc in specific environments.
The hot-dip galvanizing can also be top coated 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 the size of the 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 organisations also exist that 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 lesson, 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 regard to the article and environment.
We also discussed standards along with the advantages and disadvantages of hot-dip galvanizing.
