Galvanised Steel Angle: What It Is, How the Coating Works, Where It Excels, and When Aluminium Is the Better Choice
Steel Protected by Zinc: How Galvanising Changed Construction
Mild steel is one of the most useful materials in construction. It is strong, stiff, affordable, easy to cut and weld, and available in every standard profile. But it has one critical weakness: it rusts. Expose mild steel to moisture and air, and it begins to corrode. Leave it unprotected, and the corrosion is progressive, eating deeper into the metal year by year until the section weakens and fails.
Galvanising solves this problem by coating the steel with a layer of zinc. The zinc protects the steel in two ways: it forms a physical barrier that prevents moisture and air from reaching the steel surface, and it provides sacrificial protection, meaning that if the zinc coating is scratched or damaged, the zinc around the damage corrodes preferentially, protecting the exposed steel until the zinc is consumed.
Galvanised steel angle, the L-shaped profile coated with zinc, is one of the most widely used structural components in UK construction. It provides the strength of steel with the corrosion resistance of zinc, making it suitable for external structural applications, agricultural buildings, fencing, industrial frameworks, and any situation where the angle needs to survive outdoors without rusting.
But galvanising is not a permanent, unlimited solution. The zinc coating has a finite thickness, a finite lifespan, and specific limitations that affect its performance in different environments. Understanding these limitations is essential for choosing between galvanised steel and the increasingly popular alternative: aluminium.
How Hot-Dip Galvanising Works
The most common and effective form of galvanising for structural steel angle is hot-dip galvanising, a process governed by BS EN ISO 1461 in the UK. Here is what happens.
Cleaning
The steel angle is first cleaned to remove all oil, grease, dirt, paint, and mill scale from the surface. This is done in three stages: a caustic (alkaline) wash to remove organic contaminants, acid pickling (typically in hydrochloric or sulphuric acid) to dissolve the mill scale and surface oxides, and a rinse to remove the acid residue. The cleaning must be thorough because the zinc will not bond to a contaminated surface.
Fluxing
After cleaning, the steel is dipped into a flux solution (typically zinc ammonium chloride) that prevents the clean steel from oxidising before it enters the zinc bath. The flux also helps the molten zinc wet and bond to the steel surface. The fluxed steel is dried before galvanising.
Galvanising
The prepared steel angle is immersed in a bath of molten zinc at approximately 450 degrees Celsius. The zinc reacts with the steel surface to form a series of zinc-iron alloy layers that are metallurgically bonded to the steel. These alloy layers are harder than the underlying steel and provide the primary corrosion resistance. On top of the alloy layers, a coating of pure zinc solidifies as the steel is withdrawn from the bath.
Cooling and Inspection
The galvanised angle is allowed to cool in air or is quenched in water. Once cool, the coating is inspected for thickness (measured with a magnetic gauge), uniformity, and freedom from defects. The minimum coating thickness is determined by the steel section thickness: for steel thicker than 6mm, the minimum mean coating thickness is 85 microns; for steel between 3mm and 6mm, it is 70 microns.
The Result
The finished galvanised steel angle has a bright, spangled, silvery appearance that will gradually weather to a dull matte grey over time as the zinc surface oxidises. The coating is tough, abrasion-resistant, and provides corrosion protection that can last 50 years or more in typical UK atmospheric conditions, depending on the environment and the coating thickness.
Types of Zinc Coating: Not All Galvanising Is the Same
The term "galvanised" is used loosely in the building trade, but there are significant differences between the types of zinc coating that affect performance and suitability.
Hot-Dip Galvanising (Batch)
This is the process described above, where fabricated steel sections are immersed in a molten zinc bath. It produces the thickest and most durable coating (typically 70 to 85+ microns) with a metallurgical bond to the steel. Hot-dip galvanising is the standard specification for structural steel angle used in construction, infrastructure, and external applications. It is governed by BS EN ISO 1461.
Continuous Hot-Dip Galvanising (Pre-Galvanised)
In this process, steel strip or coil is passed continuously through a molten zinc bath before being formed into the finished profile. The resulting coating is thinner (typically 20 to 45 microns) than batch galvanising and does not have the same metallurgical bonding at cut edges and formed corners. Pre-galvanised steel angle is cheaper but provides less corrosion protection, particularly at the cut ends where the steel is exposed. It is suitable for internal or sheltered applications but is not a substitute for hot-dip galvanised steel in exposed external environments.
Electrogalvanising (Zinc Plating)
Electrogalvanising deposits a very thin layer of zinc (typically 5 to 25 microns) onto the steel surface using an electrochemical process. The coating is uniform and bright but significantly thinner and less durable than hot-dip galvanising. Zinc-plated angle is suitable for indoor use only (shelving, furniture, light framing) and will rust quickly if used outdoors.
Duplex Coating (Galvanising Plus Powder Coating)
A duplex system combines hot-dip galvanising with a powder-coated finish applied over the zinc. This provides the corrosion protection of galvanising plus the colour and appearance of powder coating, with a synergistic effect: the combined lifespan of the duplex coating is 1.5 to 2 times longer than either coating alone. Duplex-coated galvanised steel angle is specified on projects where both corrosion resistance and colour matching are required, though the additional cost narrows the gap with aluminium.
Where Galvanised Steel Angle Is the Right Choice
Galvanised steel angle is the standard specification for several categories of construction work where its combination of strength, corrosion resistance, and cost make it the most practical option.
Structural Steelwork
In buildings, bridges, and infrastructure, galvanised steel angle provides structural support, bracing, and connections. The high strength and stiffness of steel are essential for these load-bearing applications, and the galvanised coating provides the corrosion protection needed for external exposure. Hot-dip galvanised structural steel is specified to BS EN ISO 1461 and is the norm for exposed steelwork on commercial, industrial, and public buildings.
Agricultural Buildings
Barns, livestock buildings, storage sheds, and agricultural infrastructure use galvanised steel angle extensively for framing, bracing, and connections. The agricultural environment is particularly corrosive (moisture, ammonia from livestock, chemical fertilisers), and the thick zinc coating of hot-dip galvanising provides the robust protection needed to survive these conditions for decades.
Fencing and Barriers
Galvanised steel angle is used as fence posts, gate frames, horizontal rails, and barrier supports in industrial, commercial, and agricultural fencing. The strength of steel handles impact loading and the galvanised coating resists corrosion from ground contact and weather exposure.
Industrial Racking and Storage
In warehouses, factories, and distribution centres, galvanised steel angle forms the framework of shelving systems, pallet racking, and storage structures. The load-bearing capacity of steel is essential for heavy storage, and galvanising protects the steel from the damp, unheated conditions typical of industrial buildings.
General Fabrication
For welded fabrication projects such as frames, brackets, supports, and bespoke metalwork, galvanised steel angle is the default choice when the finished product will be used outdoors. The angle can be hot-dip galvanised after fabrication, ensuring that all welds, cuts, and joints are coated.
The Limitations of Galvanised Steel Angle
Galvanised steel angle is an excellent product for many applications, but it has limitations that need to be understood and managed.
The Zinc Coating Is Finite
The zinc coating on a galvanised steel angle has a measurable thickness, and it is consumed over time by atmospheric corrosion. In a typical UK urban or suburban environment, the zinc consumption rate is approximately 1 to 2 microns per year. A hot-dip galvanised coating of 85 microns will therefore provide approximately 40 to 85 years of protection before the zinc is consumed and the underlying steel begins to rust. In coastal or industrial environments, the consumption rate is higher (2 to 5 microns per year), and the coating life is correspondingly shorter.
Cut Edges Are Vulnerable
When galvanised steel angle is cut after galvanising (for example, on site during installation), the cut edge exposes bare steel. The surrounding zinc provides a degree of sacrificial protection to the cut edge, but this protection is limited in extent (typically 1 to 2mm from the zinc coating). On larger cuts or in aggressive environments, the exposed steel at the cut edge can begin to rust. Touch-up with zinc-rich paint is recommended at all cut edges.
Galvanic Corrosion With Other Metals
Galvanised steel can cause galvanic corrosion when in direct contact with certain other metals, particularly copper and brass. In the presence of moisture, the zinc preferentially corrodes, which can accelerate the consumption of the galvanised coating. When galvanised steel angle is used alongside aluminium components (fascia, coping, gutter), the galvanic couple between zinc and aluminium is relatively mild and generally acceptable, but best practice is to isolate the two metals with a non-conductive barrier where possible.
Appearance Changes Over Time
The bright, spangled finish of newly galvanised steel weathers to a dull matte grey within the first few years of exposure. This is a natural and harmless process (the surface zinc is forming a stable zinc carbonate patina), but it means the appearance of galvanised steel is not stable or predictable over time. For visible architectural applications where consistent colour and appearance matter, galvanised steel is not the best choice unless it is overcoated with powder coating (duplex system).
Limited Colour Options
Galvanised steel angle in its standard form is available in only one "colour": the natural silvery-grey spangled finish, which weathers to matte grey. If colour is required, the galvanised surface must be overcoated with powder coating or paint, adding cost and complexity. Aluminium, by contrast, can be powder coated in any RAL colour directly, without the intermediate galvanising step.
Weight
Galvanised steel angle is approximately three times heavier than aluminium angle of the same dimensions. For rooftop, balcony, and elevated applications where structural load limits apply, this weight penalty is significant. Steel is also harder to handle, transport, and install at height, increasing labour costs and health and safety considerations.
Galvanised Steel Angle vs Aluminium Angle: Making the Right Choice
The choice between galvanised steel and aluminium angle comes down to what the application demands.
Choose galvanised steel when: The application is structural and requires the maximum strength and stiffness of steel. The angle will be concealed or the natural grey appearance is acceptable. Heavy loads need to be supported. Budget is the primary constraint. The angle will be welded into a larger steel fabrication. The environment is general UK atmospheric conditions (not coastal or highly corrosive).
Choose aluminium when: The angle will be visible and appearance matters. Colour coordination with the building's fascia, coping, or cladding is required. The application is on a rooftop, balcony, or elevated structure where weight matters. The angle will be in contact with other aluminium components (eliminating galvanic corrosion risk). Non-combustible classification (A2-s1, d0) is required for fire safety. The application is in a coastal or highly corrosive environment where the zinc coating on galvanised steel would be consumed more quickly. Zero-maintenance performance over 40+ years is the goal.
For building envelope applications specifically, the fascia boards, soffit panels, copings, gutters, downpipes, and drip trims that form the visible roofline of a building, aluminium is now the standard specification for both commercial and higher-specification residential projects. Metal Profiles Ltd manufactures the full range of aluminium building envelope products in-house at their Chelmsford, Essex facility: fascia boards, copings, box gutters, downpipes, drip trims, and all associated accessories. Every product is polyester powder coated on-site in any RAL or BS colour and certified to A2-s1, d0 fire classification. For the visible building envelope, aluminium provides the colour flexibility, corrosion resistance, fire compliance, and maintenance-free longevity that galvanised steel simply cannot match.
Cost Comparison: Galvanised Steel vs Aluminium Over Time
At the point of purchase, galvanised steel angle is cheaper than aluminium. For standard sizes, galvanised steel angle costs approximately 30 to 40% less per metre than aluminium angle of the same dimensions. This upfront saving is the primary reason galvanised steel remains the default for structural and industrial applications where appearance and maintenance are not priorities.
However, when the total cost of ownership over the life of the building is considered, the picture changes. Galvanised steel may require touch-up painting at cut edges and damage points. In aggressive environments, the zinc coating may be consumed within 20 to 40 years, after which the steel begins to rust and the angle needs replacement or recoating. Aluminium requires no maintenance, no recoating, and no replacement for 40 to 50+ years.
For concealed structural applications (where the angle is hidden behind cladding, inside walls, or beneath the building), galvanised steel is usually the most cost-effective choice. For visible, exposed applications (where the angle is part of the building's appearance and accessible only with scaffolding), the cost of future maintenance and potential replacement makes aluminium the better long-term investment.
Standard Galvanised Steel Angle Sizes
Galvanised steel angle is available in a comprehensive range of standard sizes. Equal angles are the most commonly stocked, with sizes from 25mm x 25mm x 3mm up to 200mm x 200mm x 20mm. Unequal angles range from 40mm x 25mm x 4mm to 200mm x 100mm x 12mm. Standard lengths are typically 6 metres, but 12-metre lengths are available for larger sections.
The most commonly specified sizes for general construction applications are 40mm x 40mm x 4mm, 50mm x 50mm x 5mm, and 75mm x 75mm x 6mm in equal angle, and 65mm x 50mm x 5mm in unequal angle. For lighter-duty applications (brackets, trims, edging), smaller sizes from 25mm x 25mm x 3mm are used.
When ordering galvanised steel angle, specify whether you require hot-dip galvanised (to BS EN ISO 1461, for maximum corrosion protection) or pre-galvanised (for lighter-duty, sheltered applications). The difference in coating thickness and long-term performance is significant.
Galvanised Steel and Aluminium: Both Have Their Place
The construction industry needs both materials. Galvanised steel provides the heavy-duty structural backbone of buildings: the frames, the braces, the lintels, and the concealed supports that hold everything up. Aluminium provides the visible, colour-coordinated, maintenance-free building envelope: the fascia boards, the copings, the gutters, and the trims that define the building's finished appearance from the outside.
The best buildings use each material where its strengths are most relevant. Steel behind the scenes. Aluminium on display. Galvanised where cost and strength dominate the decision. Aluminium where colour, corrosion resistance, fire compliance, and aesthetic quality are priorities.
Understanding the strengths and limitations of galvanised steel angle, and knowing when aluminium is the better alternative, is a fundamental skill for anyone involved in specifying, procuring, or installing building materials in the UK. The right choice depends on the application, the environment, the visibility of the component, the maintenance budget, and the expected lifespan. Get that choice right, and the material will do its job without complaint for decades.
Frequently Asked Questions
How long does galvanised steel angle last outdoors?
In a typical UK suburban or urban environment, hot-dip galvanised steel angle (with a minimum 85-micron coating) will last approximately 40 to 85 years before the zinc is consumed and the underlying steel begins to rust. In coastal environments, the lifespan is shorter (20 to 40 years) because the salt spray accelerates zinc consumption. In rural environments with low atmospheric pollution, the lifespan can exceed 100 years. Pre-galvanised (thinner coating) and electrogalvanised (very thin coating) products have significantly shorter lifespans outdoors.
Can galvanised steel angle be painted or powder coated?
Yes. Galvanised steel can be overcoated with powder coating or paint to add colour and extend the life of the corrosion protection. This is known as a duplex system. The galvanised surface must be properly prepared (typically by light abrasion or specialist etch primer) to ensure the paint or powder coat adheres to the zinc. A duplex-coated galvanised steel angle provides the combined benefits of galvanising and paint, with a total lifespan that is 1.5 to 2 times longer than either system alone.
Is galvanised steel angle safe to weld?
Yes, but with important precautions. Welding galvanised steel produces zinc oxide fumes that are toxic if inhaled, causing an illness known as "metal fume fever". All welding of galvanised steel must be done with adequate ventilation or respiratory protection (a P3-rated mask as a minimum). The weld area should be cleaned of zinc before welding (by grinding back the galvanised coating around the weld zone) and the area should be touched up with zinc-rich paint after welding to restore the corrosion protection.
What is the difference between galvanised and zinc-plated angle?
Hot-dip galvanised angle has a thick zinc coating (70 to 85+ microns) that is metallurgically bonded to the steel and provides decades of outdoor corrosion protection. Zinc-plated (electrogalvanised) angle has a very thin coating (5 to 25 microns) that is electrochemically deposited and provides only short-term protection, suitable for indoor use only. The visual difference is also significant: galvanised steel has a spangled, crystalline appearance, while zinc-plated steel has a uniform, bright, chrome-like finish. For any outdoor or structural application, always specify hot-dip galvanised, not zinc-plated.
When should I use aluminium angle instead of galvanised steel?
Use aluminium when the angle will be visible and colour matters, when it will be in contact with other aluminium components, when weight is a constraint (rooftops, balconies), when non-combustible fire classification (A2-s1, d0) is required, when zero-maintenance performance over 40+ years is needed, or when the environment is coastal or highly corrosive. For concealed structural applications where maximum strength is needed and appearance is irrelevant, galvanised steel is usually the most cost-effective choice.
Further Reading
For more technical detail on galvanising processes, standards, and performance, the following resources are recommended:
Galvanizers Association - The UK industry body for hot-dip galvanising, providing technical guidance, standards information, design advice, and CPD material for specifiers and engineers: galvanizing.org.ukThe Metal Store - Practical comparison of aluminium angle vs mild steel angle iron, covering strength, weight, corrosion resistance, cost, and fabrication characteristics: themetalstore.co.uk
