Galvanisation is the process of coating iron or steel with a protective layer of zinc (Zn) by immersing or ‘dipping’ the base metal into a bath of molten zinc.
The temperature of these baths reaches around 449 °C (840 °F) or 550-560 °C for small objects. The higher temperature is used for smaller objects to make the zinc more liquid, so it flows into smaller nooks and crannies better.
Steel and iron are galvanised with zinc because zinc acts as a protective layer to prevent rusting, corrosion and weathering of the base metal.
Every millimetre of the base metal that comes into contact with the zinc is galvanised, creating a permanent protective layer.
The zinc bonds with the iron or steel metallurgically. This metallurgical process creates an iron-zinc alloy on the steel surface. A layer of pure zinc is left on the outside when the part or product is withdrawn from the zinc bath.
Galvanisation baths
Galvanisation baths, also known as zinc kettles, are where the magic happens.
Most sizes and shapes of steel fabrications can be hot dip galvanised, with the galvanising company limited only by bath size.
Galvanising kettles can have a flat or round bottom. This choice is normally determined by what works best for the site space
Galvanising companies will often have baths and galvanising kettles of various sizes. Zinc baths are designed to hold a maximum weight for the molten metal and the product. It’s important for them to be properly sized and specified for this reason.
At PDFL, we build these baths for galvanisers and help to maintain them as a specialist fabricator. Baths can be built to any size and specification.
We have particular expertise in building internal linings for galvanising baths made from HDPE, PP, steel and GRP. Many existing zinc bath liners are prone to failure from misuse, harsh chemical cleaners and age.
HDPE is a particularly versatile material for liners. It looks very similar to butyl rubber but costs significantly less with better mechanical strength. Another common liner is PVC, which can be chosen instead of HDPE for smaller tanks.
We can also specify and design the correct liner/tank/bath to use for an individual process in the whole process. This includes pre-treatment, pickling, washing, fluxing, hot dipping and modern rinse treatments in the galvanisation process.
Other tanks and baths
It’s important to note that the galvanisation stage of galvanising is only one part of the whole process. There are several other stages involved in galvanising (see the section below under “process”) and all of them are as crucial as one another.
In addition to galvanisation baths, the following baths are used:
- Pickling tanks: These contain a bath of diluted hydrochloric acid
- Washing tanks: These contain filtered water
- Rinse tanks: These are empty tanks with a rinse function
- Fluxing tanks: These contain a bath of zinc ammonium chloride
We offer a wide range of tanks including GRP, polypropylene and mild steel lined tanks for storing fluids or solids. This takes care of pickling and fluxing.
Plant and equipment
Galvanising plants require a variety of equipment, including:
- Galvanising baths
- Chemical receiving stations
- Acid containment linings
- Ground protection sheets
- Air containment tanks
- Dosing stations
- Chemical storage tanks
- Fume scrubbing machines
- Degreasing and flux tanks
We are a leading designer, manufacturer, supplier and installer of tanks, vessels, ducting and related plant used in galvanising plants.
The galvanisation process
Galvanisation companies have a precise process to ensure a high standard of galvanisation no matter the steelwork in question.
The process looks like this:
Steel inspection
The steel to be coated is inspected to ensure it’s suitable for hot dipping. Things to look out for include condition, venting holes and contamination.
Pickling
The steel is dipped into a tank of diluted hydrochloric acid. These tanks are made from HDPE. The hydrochloric acid removes dust and mill scale.
Washing
The steel is washed in a tank of filtered water to wash off the hydrochloric acid. It may be resubmerged several times.
Fluxing
Fluxing precedes the steel’s molten zinc bath treatment. The steel is submerged in a bath of zinc ammonium chloride to remove remaining traces of oxide.
Drying
The steel is placed in a dryer so that there is no moisture on it.
Galvanising
The steelwork is submerged into a bath of molten zinc running at a temperature of around 449 °C (840 °F) for large objects and 550-560 °C for small objects. The steel is submerged for around 10 minutes, but this time can vary.
Cooling
The freshly galvanised steel is withdrawn from the tank and left to cool in the air for 24-hours. Alternatively, it can be cooled with water.
Inspection
In the United Kingdom, galvanised steel must pass the specifications set out by British Standard BS EN ISO 1461 (2009). The finished product is inspected to ensure it meets these standards, and then it is weighed for shipment.
Service and maintenance
Whether we are talking about chemical storage tanks, flux tanks or galvanisation baths, service and maintenance is essential for safety and performance reasons.
With regards to galvanisation tanks, you will want to conduct spark leak tests to detect if metal contaminants are getting accidentally mixed with the zinc. Containment tests should also be performed to ensure the liner is doing its job.
Mechanical and electrical equipment, including scrubbers, ducting and odour control systems (if applicable) should be tested and serviced in line with manufacturer guidelines. It will be worth your while taking out an S & M package for this.
At PDFL, we provide annual service and maintenance packages for the products we sell including fume scrubbers, piping systems, storage and chemical tanks and liners. If you are in need of galvanising plant and equipment and you want maintenance included, we offer a variety of solutions to suit your unique requirements.
For a chat about your needs, call us on +44 (0)1922 418005 or email sales@pdfl.uk.
