Galvanising – steel surface protection

Galvanising is one of the most popular ways of protecting structural steel, which in general oxidises easily. As rust weakens the bonds of the metal itself, it is important to protect the steel which will be exposed to oxidising.

Rust is caused by reaction among iron and oxygen together with the water or moisture. Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide which weakens bonds of the metal itself.

There are three popular ways of protecting steel from rust – paint, cathodic protection and galvanizing.

1. Paint/primer is used to exclude the water from the steel, therefore, preventing the oxidising reactions from happening. You can achieve different aesthetic effects by treating the steel with a primer. Paint is usually very rich in zinc phosphate. The compound in the paint disrupts the normal formation of anodes on the surface, hydrolyzing in water producing zinc ions (cathodic inhibitors) and phosphate ions (anodic inhibitors). The primer is applied directly on the steel simply by using a paint roller. This method is not suitable for outside conditions thought. At Steelo, we treat structural steel with red primer as a standard, unless specified otherwise.

2. Cathodic protection – used to minimise the corrosion by making the steelwork act as an electrical cathode. It takes reactive metal alloys such as magnesium alloy or zinc and connects it to the steel in the electrolyte. This method connects the metal to be protected to a more easily corroded metal to act as the anode. The sacrificial metal then corrodes instead of the protected metal. This method is often used in protecting e.g. fuel pipelines and steel storage tanks.

3. Galvanising – there are three types of galvanizing suitable for different types of steel.

• Zinc plating – The steel is placed in a zinc bath by using a current of electricity. This method is suitable for protecting the screws. As this method provides a very thin layer –  just 10 microns thick – it is a thinner coating than hot dip galvanizing making it unsuitable for outdoor applications.
• Continuous or in-line galvanizing – this method is also a hot-dip process but is only applied to a steel sheet, strip, and wire. It is used for example for brick ties.  Cannot be used if the material is to be cut after galvanising – the cut edges will be left without a protective zinc surface.
• Hot dip galvanizing – a method used commonly to protect the structural steel. If required, we use hot-dip galvanising to protect the steelwork we fabricate.

Hot dip galvanising

Hot dip galvanizing is one of the most effective methods of protecting the structural steel. It is a method of coating the steel with a protective layer of zinc. Zinc bonds with steel in the chemical process and forms a series of zinc-iron alloy layers. The final layer is 100% zinc. These layers are extremely abrasion resistant as are harder than steel. Zinc makes its own protective oxide carbonate film on the top. Steel is protected even in case of scratches.

Galvanised steel is mainly used on the outside, as the protective layer of zinc is relatively thick. This way, it will protect steel from corrosion even in extremal atmospheric conditions for 50 to 100 years.

Advantages of using zinc galvanizing

• protection against the corrosion – zinc has got a protective oxide carbonate film on its surface
• durability
• extremely abrasion resistant – the inner metallic layers are harder than base deal
• has a low carbon footprint
• versatility
• longevity
• complete coverage (both side, awkward corners and narrow gaps)
• maintenance-free (over 40 years)
• protect steel in case of abrasion or if scratches occur- even if there are some scratches in the zinc coating, zinc will not be undercut by rusting as steel is not corroding in the adjacent area of zinc
• most cost-effective and sustainable method
• environmentally friendly
• aesthetics – can be also painted or powder coated
• recyclable

Hot dip galvanising – 3 steps

1. Surface preparation (design stage, cleaning process: degreasing – removing dirt oil and organic residues; mild acid or pickling bath for removing mill scale iron oxide and etch the steel; fluxing for removing any remaining oxides and coats the steel with the protective layer to prevent any further oxide formation)
2. Galvanizing – once the steel is clean and dry it can be dipped into a bath of molten zinc. Zinc will cover the entire structure. Zinc-iron intermetallic layers are formed and an outer layer of pure zinc occurs.
3. Inspection – visual inspection of each structure, checking if the coverage is proper and checking the thickness of zinc cover

Galvanising – did you know?

• Galvanized steel is suitable for high-temperature applications of up to 200 °C. The use of galvanized steel at temperatures above this will result in the zinc peeling off at the intermetallic layer
• Structural steel that has been galvanised my arrive on site with some additional holes, that the client didn’t require. The holes are designed for transport, tubular venting and floating zinc reasons. Inadequately vented fabrications may explode when immersed in molten zinc.
• The temperature of a bath of molten zinc is around 840 °F (449 °C)
• Galvanized steel will maintain magnetic properties. Galvanized steel has a protective coating of zinc, which will not affect the magnetic properties of the steel, iron or other metal type that it is protecting.
• Steelwork structures for galvanizing have to be adjusted to the size of the bath. We are able to make constructions up to this size: length 7.6m, width 1.4 m and 3.2m depth.
• Galvanized steel can be painted
• Galvanized steel can be welded. Galvanized steels are welded easily and satisfactorily by most commonly practised welding techniques. Closer control of welding conditions than for uncoated steel is usually necessary but procedures are simple and well established. Galvanized steels are welded easily and satisfactorily by most commonly practised welding techniques. Closer control of welding conditions than for uncoated steel is usually necessary but procedures are simple and well established.

Watch the video below to learn more about steel coatings: