Types of anticorrosive coatings for sheet metal: zinc, zinc-magnesium and aluminium-zinc

The durability of a metal roof or a metal facade largely depends on the type of anti-corrosion coating applied to the steel. Although many customers focus primarily on sheet thickness, color, or profile, one of the most important factors influencing the material’s long-term performance remains the layer that protects the steel against corrosion.

The best-known solution is galvanized zinc coating, which has been used for decades in the roofing and facade sheet metal industry. However, in recent years, other types of coatings have been developed to increase corrosion resistance, particularly in more aggressive environments or in applications where the material is exposed to additional stresses.

Among the most commonly used solutions are:

• conventional zinc coating – Z200, Z225, Z275 and other similar variants
• zinc-magnesium coatings
• aluminium-zinc coatings

Each of these technologies has different properties and may be better suited to certain types of projects. Understanding the differences between them is important for property owners as well as architects, installers, or designers seeking to choose a durable metal roofing or facade system.

CONVENTIONAL ZINC COATING – THE INDUSTRY STANDARD

Most roofing and facade sheet metal systems use galvanized steel. In this case, the protective layer consists of zinc applied through a continuous hot-dip galvanizing process.

The amount of zinc is expressed in grams per square meter and usually appears in the form of codes such as:

• Z200
• Z225
• Z275

These values indicate the total mass of the zinc coating applied to both sides of the sheet.

Zinc provides two important types of protection.

Barrier protection

First, the zinc layer isolates the steel from direct contact with the external environment. As a result, moisture, oxygen, and other corrosive agents have more difficulty reaching the steel core.

Galvanic protection

Second, zinc is more reactive than iron. This means that when a scratch or a small exposed area appears, the zinc corrodes first and protects the surrounding steel. For this reason, zinc is also known as a sacrificial metal.

This combination of barrier protection and galvanic protection is why conventional zinc coating remains the industry standard for many types of roofing sheet metal.

WHAT Z200, Z225 OR Z275 VALUES ACTUALLY MEAN

For many customers, codes such as Z225 or Z275 may seem like technical details that are difficult to interpret. In reality, they are very important when assessing the durability of a product.

The greater the amount of zinc, the greater the reserve of anti-corrosion protection. This reserve is particularly important in sensitive areas such as:

• cut edges
• perforations
• accidental scratches
• areas heavily exposed to weather conditions

For example, the difference between Z225 and Z275 means 50 g more zinc per square meter, which can influence the duration of the material’s protection over time.

Therefore, when comparing two types of galvanized sheet metal, the zinc coating level is one of the key indicators worth considering.

ZINC-MAGNESIUM COATINGS

In recent years, new types of metallic coatings have emerged in which zinc is combined with magnesium and, in some cases, with a small amount of aluminium.

These coatings were developed to improve the performance of conventional zinc and increase corrosion resistance in certain environments.

The typical composition of such a coating is generally:

• zinc – the main component
• 1–3% magnesium
• sometimes a small amount of aluminium

Although the percentages of magnesium are relatively small, their effect on the behaviour of the protective coating can be very significant.

THE ROLE OF MAGNESIUM IN STABILIZING ZINC

Adding magnesium changes the way the coating reacts with the environment.

Instead of forming only zinc oxide, the surface can develop more stable compounds such as:

• zinc hydroxycarbonate
• zinc-magnesium compounds

These compounds form a denser and more stable protective layer that can:

• slow down the corrosion process
• reduce zinc consumption
• stabilize the metal surface over a longer period

For this reason, in certain environments, zinc-magnesium coatings can provide corrosion resistance several times greater than conventional zinc coating.

This makes them attractive for projects where high durability is an important requirement.

SELF-HEALING PROPERTIES OF ZINC-MAGNESIUM

One of the advantages most commonly associated with zinc-magnesium coatings is the phenomenon sometimes referred to as self-healing.

In areas where the coating is damaged, cut, or scratched, local reactions can occur in which zinc and magnesium ions migrate toward the exposed area.

Through this process:

• protective compounds form around the defect
• the spread of corrosion can be reduced
• cut edges and perforations can remain better protected

This effect does not mean that the coating completely “repairs” itself, but it can help limit corrosion around:

• cut edges
• scratches
• perforations
• cut or profiled areas

For this reason, these coatings are valued in applications involving numerous cutting, forming, or perforating operations.

Aluminium-zinc coatings

Another important category is represented by aluminium-zinc coatings. In this case, the protective layer is an alloy in which zinc is combined with aluminium.

The best-known compositions are:

• 55% aluminium – 43–44% zinc – 1–2% silicon
• variants with 65% aluminium

This type of coating is frequently used when very good protection is required in aggressive atmospheric environments or at higher temperatures.

The role of aluminium in anti-corrosion protection

Aluminium behaves differently from zinc. When exposed to air, it quickly forms a stable layer of aluminium oxide, which protects the metal surface.

In aluminium-zinc coatings:

• aluminium provides highly effective barrier protection
• zinc continues to provide galvanic protection in certain localized areas

This combination gives aluminium-zinc very good performance in environments exposed to atmospheric corrosion and high temperatures.

For this reason, this type of coating is often used for metal panels, facades, or systems heavily exposed to environmental factors.

DIFFERENCES BETWEEN ZINC, ZINC-MAGNESIUM AND ALUMINIUM-ZINC

Although all these coatings are designed to protect steel, they work slightly differently and may offer specific advantages depending on the application.

Conventional zinc

Conventional zinc coating provides:

• very good galvanic protection
• proven long-term performance
• widespread use in roofing and facade systems

Zinc-magnesium

Zinc-magnesium coatings provide:

• zinc stabilization
• high corrosion resistance
• very good performance in cut or profiled areas
• partial self-healing effect

Aluminium-zinc

Aluminium-zinc coatings provide:

• very good resistance to atmospheric corrosion
• good performance at high temperatures
• effective barrier protection thanks to aluminium

The optimal choice depends on the conditions in which the product will be used and on the project requirements.

WHERE THESE COATINGS ARE USED

These technologies are used in a wide range of metal construction applications, including:

• metal roofs
• ventilated facades
• metal panels
• architectural systems
• profiled construction elements

The choice of coating type depends on several factors:

• the environment in which the building is located
• the degree of exposure to moisture and aggressive agents
• the type of roofing system
• durability requirements
• installation and operating conditions

In areas with normal conditions, conventional zinc coating may be sufficient. In more aggressive environments or in projects requiring enhanced performance, zinc-magnesium or aluminium-zinc may be more suitable options.

CONCLUSION

Anti-corrosion coatings for steel have evolved significantly in recent decades. In addition to conventional zinc coating, technologies such as zinc-magnesium and aluminium-zinc provide improved performance in certain applications and environmental conditions.

Each of these solutions offers specific advantages. Zinc remains the industry standard, zinc-magnesium provides a high level of stability and protection, while aluminium-zinc stands out for its very good performance in aggressive atmospheres and at high temperatures.

Understanding these differences helps in selecting a suitable solution for durable metal roofs and facades that can withstand the test of time and meet the actual requirements of the project.

If you are comparing several types of sheet metal for a roof or facade, do not compare only the profile or color. Also check the type of anti-corrosion coating, as it directly influences the long-term durability of the system.