Galvanised Steel - Hot-dipped, PreGalv, DuraGal or AllGal?
Understanding Galvanising Types
Feb 16, 2021
A number of different forms of galvanising are used to protect steel from rusting (oxidation). Different forms of galvanising offer different levels of protection and, consequently, make the product protected suitable for differing applications.
We notice some confusion out there with regards to the different types of Galvanising. This is not made any easier with the use of trade names, such a DuraGal & AllGal, to describe similar products.
To clear up some of the confusion the main galvanising types are described here, and we also provide an overview of the level of protection provided by the different forms of galvanising.
In this article we discuss:
- How Galvanising Protects
- Types of Galvanising
- Galvanising Standards
- United Steel’s Galvanised Product
On reading the full article you should have an improved understanding of the different forms of galvanising, and have some links to useful guidelines & references.
How Galvanising Protects
Galvanising provides two different types of protection for steel. These include:
- Firstly, physical or barrier protection, by enveloping the steel in a coating of zinc, which oxidises at a slower rate than steel. The rate of oxidisation of zinc is 1/17th to 1/18th that of steel.
- Secondly, chemical protection, by using the principles of “Cathodic Protection”, where even when the physical envelop is compromised exposing the steel, the surrounding zinc will still oxidise in preference to the exposed steel.
The Zinc Oxide on the surface of the galvanised steel undergoes further reactions with compounds in the atmosphere (i.e. water and carbon dioxide) to form a stable film, which enhances the barrier protection properties. This stable film is known as a Zinc Patina, it takes 6-12 months to develop, has a matt grey colour, is made from Zinc Carbonate, and has a corrosion rate of 1/30th that of steel.
When Galvanising is applied with the batch Hot Dipped (HD) Galvanising process a mechanically hard coating is produced that has excellent resistance to abrasion and mechanical damage. Batch processed HD Galvanising creates a four layer metallurgically bounded barrier. The layers consist of a Zinc/Iron alloy, and the iron composition of the alloy is highest in the layers closest to the steel surface. Two of the middle layers have a surface hardness that is greater than the underlying steel.
If the physical barrier created by galvanising is damaged, Cathodic Protection has the ability to protect exposed areas of steel up to a width or diameter of 5mm. There is also the possibility of a degree of self-healing. Thus, Cathodic Protection can offer a limited amount of protection when galvanised steel is cut or drilled.
Types of Galvanising
Galvanising can be applied to the steel using a variety of processes including, for example, Hot-Dipping, Electroplating and In-Line Galvanising. A number of different forms of galvanising are discussed in this section.
The level of protection offered by galvanising is most influenced by the thickness of the zinc coating applied, rather than the method of application. The following graph, from the British Iron & Steel Research Association, demonstrates the linear relationship between the thickness of the zinc coating and the service life of the coated steel. A number of different galvanising processes are plotted, with the thickness of the zinc coating on the horizontal axis and service life on the vertical axis.
There are three primary processes used to Galvanise bar, hollows & plate, of which the greatest percentage is hot-dipped galvanised. There are two types of hot-dipped galvanising – batch & continuous. The three types of galvanising and their characteristics are:
- Hot-Dip Galvanising: This is the most familiar form of galvanising, where steel items are galvanised in batches, by dipping them in a bath of molten zinc or zinc alloy. When the steel surface comes into contact with the molten zinc a chemical reaction takes place to form the zinc-iron alloy layers described in the previous section. With this process the galvanising thickness is determined by the mass of the steel being galvanised, and as a consequence an operator independent standard minimum coating is applied. This process allows objects that are a variety of complex shapes to be galvanised inside and outside in one operation. The molten zinc is able to cover corners, seams & rivets, seal edges & penetrate recesses unlike some other forms of galvanising. The galvanising is also slightly thicker on corners and narrow edges, giving greater protection than organic anti-corrosion treatments.
- Continuous Galvanising Processes: Is also known as pre-gal, in-line or mill galvanising. In this galvanising process, continuous ribbons of steel & steel plate are run through a molten zinc bath at speeds of around 180m/sec, molten zinc on the surface is cleaned off using a gas knife to provide a tightly controlled coating thickness & surface finish. The length of time in the zinc bath is much shorter than in the batch process (2-4s vs 4-8mins) and as a consequence the metallurgy is different, with the surface coating being almost pure zinc. The softer surface coating means steel galvanised in this manner can be worked without cracking the coating, but it is less abrasion resistant. These thinner coatings may require supplementary protection for outdoor exposure.
- Electro-Galvanising: Is a form of galvanising where steel is placed in a zinc salt solution, and an electric current is passed through the solution to deposit a layer of zinc on the surface of the steel. The terminals used to introduce the electric current into the steel, are the steel itself (cathode) & an inert anode or sacrificial zinc anode. The positively charged zinc particles (ions) are attracted to the negatively charged steel. The zinc layer deposited using electro-galvanising is thinner, has a more aesthetic appearance (i.e. bright & uniform), is softer and is more ductile than with other methods. Electro-galvanised sheet is often used in the manufacture of car & truck bodies.
Continuously Galvanised products are often sold under a variety of trade-names, such as AllGal, DuraGal, GalForce, and Flo-coat to describe the coating. These products can have differences in terms of the galvanising process, galvanising thickness, interior treatment for hollows (e.g. paint or galvanising), and supplementary exterior coatings that are applied to further protect the product.
|Continuous Galvanising Line|
Other types of Galvanising that are used for protecting steel from corrosion include:
- Thermal Spray or Metallising, where semi-molten zinc is sprayed onto the surface of the steel. This process has the advantages that thick coatings can be applied and coatings can be applied in the field, but obtaining internal & sharp edge/corner coverage is difficult.
- Sheradising, where the part to be coated is heated while being placed in a rotating steel drum with zinc powder. The zinc evaporates and bonds to the steel surface. The coating thickness is related to the time in the drum. This method is often used to coat fasteners (e.g. screws) & has a hard dull grey finish.
- Mechanical Plating (Peen plating), where the part to be coated is rotated in a drum with zinc powder and glass-beads. The glass beads press the zinc powder into surface of the part to be coated. This method is also often used to coat fasteners, and in particular threaded components or those requiring close tolerances.
- Zinc Rich Paints, where zinc dust is suspended in a paint solution. These paints are used for repair of damaged galvanised coatings or on-site protection of large structures
There are a number of standards covering the different types of galvanising applied to “off-the-shelf” and fabricated product. These standards provide guidance on the types & thicknesses of galvanising to be applied in different environments, the galvanising manufacturing processes & tolerances, and the design of items to be galvanised. Specifically:
- AS/NZS2312 - “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings” provides guidelines on the design of steel parts to minimise corrosion and to ensure effective application of galvanised coatings. It also provides guidance on the influence of different environments on corrosion, corrosion maps for different locations in NZ, and the life to first maintenance of different galvanising processes, with specified minimum coating thicknesses in the different environments. This standard comes in three parts, Part 1 covers paints, Part 2 covers HD, and Part 3 covers Thermal Sprays
- NZS 3404.1 Steel Structures Standard, has an appendix covering the corrosion protection of steel structures.
- AS/NZS 4680: Hot-dip galvanized (zinc) coatings on ferrous fabricated sections. Describes the composition of the molten zinc in the galvanising bath, information to be supplied by the purchaser of the hot-dipping service, draining & venting requirements, testing and inspection of hot-dipped galvanised items, acceptable renovation of uncoated areas, & provision of a certificate of compliance when required. The testing & inspection includes the surface finish and the thickness of the coating. The purchaser needs to provide the standard to which the HD process is to conform (i.e. NZS 4680), and may include information such as the composition of the base metal, special pre-treatments, and required finish
- AS/NZS 4792: Hot-dip galvanized (zinc) coatings on ferrous hollow sections, applied by a continuous or a specialized process. This includes hollow sections galvanized on both surfaces in a specialized plant, made by welding pre-galvanized steel strip; and galvanized on external surfaces only, within a continuous (in-line) process.
- AS 4750 Electro-galvanised Coatings on Ferrous Hollow and Open Sections
- AS 1397 – Hot dipped zinc coated and aluminium/zinc coated sheet & strip covers the requirements for the composition & properties of the base steel, and coating mass requirements for different steel thicknesses.
- Other standards exist for galvanising tube & pipe, and fasteners. Examples include, AS/NZS 4534 for continuously galvanized wire, AS/NZS 1559 for hot-dip galvanized steel bolts and AS/NZS 1214 for hot-dip galvanized coatings on threaded fasteners
In test certificates you may see reference to coating designations that are of the form, for example, Z100. Coating designations of this form are from International Standards such as ISO 3575 and ASTM A653 (ASTM is the American Society of Testing & Materials). The letter relates to the composition of the coating (i.e. Z for Zinc & ZF for Zinc Iron Alloy) and the number relates to the thickness of the coating in g/m2. A Z100 coating is typically 7 micro m thick on both sides.
United Steel Galvanised Product Overview
United Steel stocks various galvanised steel products, which includes:
- Square & Rectangular Hollow Sections (SHS & RHS), which are predominately continuous or electro- galvanised product, with HD available in a few sizes. The continuous & electro- galvanised product has coatings described by a variety of tradenames and an overview of these is provided later in this section.
- Pipe, which is HD galvanised to NZS1074
- Sheet is available in both continuous & electro galvanised. Both types of sheet are available in a range of thicknesses from approximately 0.6mm to 3mm.
- Angles are HD galvanised, with Equal Angle available in a range of sizes from 20x3 up to 125x10, and Unequal Angle available from 100x75x6 up to 125x75x10.
- Flats are HD galvanised and available in a range of sizes from 25x3 to 100x10.
- Reinforcing Bar is HD galvanised
- Reid Bar is HD galvanised
United Steel is able to supply Brick & Block Lintels, by cutting and drilling Hot-Dipped Galvanised Angle to the customer's requirements.
|Hot-Dipped Brick and Block Lintels|
By way of overview, the different tradename coatings include:
- ALLGAL, which has an electro-galvanized high purity zinc coating applied to both sides of the sheet feedstock, before it is cold formed into hollow section. The weld is then restored with zinc alloy applied by thermal spray and the hollow is coated in a Clear-Tec clear polymer protective coating. Thus, all ALLGAL products are protected from corrosion on the inside and out. The minimum average zinc coating weight is 50 g/m2 on the inside and outside. The galvanising complies with NZS 4750
- DuraGal are cold formed hollow sections that are hot dip galvanized to AS/NZS 4792 and have a minimum average coating mass of 100 grams per square metre applied to the exterior of the section. DuraGal is made from hot rolled coil that is in-line galvanised.
- DuraGal Plus - 100 grams per square metre minimum average coating mass (internally and externally). DuraGal is made from pre-galvanised Hot Rolled Coal. The galvanising complies with NZS 4792
- GalForce™, which is manufactured from HD galvanised steel strip, and so is galvanised on both the inside and outside. The galvanising complies with NZS 4792
The different mills in Australasia and Asia tend to use feedstock with one of the above coatings, or are licenced to produce feedstock coated with one of the above coatings. Thus, the coatings will differ from steel producer to steel producer.
|Continuous Galvanised and HD Galvanised SHS|
As can be seen United Steel stocks a variety of galvanised products, coated using a variety of different processes, including the more durable Hot-Dipped galvanised product. United Steel is happy to discuss your galvanised product requirements with you, and advise the product options best aligned to your needs.