Introducing the metallic coatings of today and of the future

Corrosion is a problem which costs our economy billions each year. While almost all materials are affected by corrosion, ArcelorMittal’s revolutionary new coating Magnelis^® will ensure that steel solutions resist year after year.

Defined as a change in the properties of a material, corrosion affects all materials including wood, plastics, concrete and even stainless steel. Most often the term is encountered in connection with steel which, in its raw state, is susceptible to oxidisation.

Corrosion is caused by the interaction of a material with its environment which leads to a change in its properties. It can have negative effects both in terms of security and the costs of repair or replacement.

Typically the costs of corrosion accrue because it: 

• Reduces the strength of a material (due to the reduction in its thickness) which can lead to failure
• Removes the application from service until repairs can be carried out
• Allows the contents of pipes and tanks to escape, causing contamination to the environment

While it is difficult to put a value on the cost of corrosion to the economy, direct damage (not including indirect costs of events such as environmental catastrophes) may be as high as 5% of gross domestic product (GDP). In the Benelux countries alone, this represents a sum of more than €50 billion.

The risk of corrosion depends on the environment in which a material is utilised. The corrosion risks of different ambient conditions have been classified in the ISO EN 12994 standard. The classifications start at C1 (non-aggressive environment) and go up to C5 (most aggressive). Distinctions are made between indoor and outdoor environments (see Table 1).

Table 1: Classification of ambient conditions (ISO EN 12994-2)

Improve security, reduce costs

The most common way to prevent corrosion in steel applications is by applying a coating to the steel. Today, sophisticated metallic coatings are available which can slow the progression of corrosion dramatically (see Figure 1).

Figure 1: Results of salt spray tests on different coatings (20 µm coating on each side)

All metallic coatings protect the steel in two ways:

• By providing a barrier between the steel and the corrosive elements in the environment.
• By exploiting the sacrificial properties of one or more elements in the coating (for example, zinc). 

The sacrificial properties of certain elements have been exploited for many years. Steel-hulled ships typically have zinc anodes attached to the hull. The zinc corrodes more easily than the steel, forming an electrochemical barrier against the corrosive water in a process known as cathodic protection.

Overview of metallic coated products

The corrosion resistance of a metallic coating is not only defined by the thickness of the coating. Its chemical composition is also critically important.

Today, the best known steel coatings are hot dipped galvanisation (HDG), electrolytic galvanisation and Aluzinc^® (see Table 2). Electrolytic galvanisation offers a homogeneous and very thin coating which is utilised in indoor applications such as appliances, electronics and furniture.

Table 2: Types of steel coatings and their applications

The most widely used metallic coating is standard hot dip galvanisation (HDG). This is a pure zinc coating which is used in applications such as roofing, domestic appliances, cabinets, and electrical machinery.

Galfan contains zinc and 5% aluminium. Compared to HDG, galfan exhibits improved corrosion resistance thanks to the addition of aluminium.

By contrast, Aluzinc^® is composed of 55% aluminium, 43.4% zinc and 1.6% silicon. This combination provides Aluzinc^® with its superior performance compared to coatings such as HDG. The attractive spangly finish of Aluzinc^® also makes it a popular choice for architectural projects.

Magnelis^®: best corrosion protection, even on cut edges

ArcelorMittal is continuously researching new metallic coatings. The latest is Magnelis^®. A metallic zinc coating, Magnelis^® includes 3.5% aluminium and 3% magnesium and offers the ultimate corrosion resistance in the most aggressive ambient conditions.

The magnesium in the coating’s composition plays the key role in ensuring a stable barrier against corrosion across the entire surface, particularly in chloride- and ammonia-rich environments. Its improved corrosion resistance also means the thickness of the coating can be reduced.

The application of Magnelis^® ensures the preservation of natural resources as it uses less zinc than pure-zinc coatings. Zinc runoff into the environment is also reduced significantly.

Unlike other metallic coatings, Magnelis^® also offers excellent protection for cut edges thanks to its self-healing effect. A film of zinc magnesium chloride hydroxide forms on exposed edges, providing a protective coating against corrosion. This makes Magnelis^® a very cost-effective alternative to batch galvanisation.

Figure 2: Magnelis^®: an alternative to post-galvanising and other metals

The effectiveness of Magnelis^® enables ArcelorMittal to offer significant guarantees on the performance of this coating. Magnelis^® is the first coating to offer 20-year guarantees against corrosion in marine applications. In less aggressive environments, guarantees of up to 25 years are offered.

While Magnelis^® provides the best corrosion protection of any metallic coating available today, ArcelorMittal continues to search for new solutions. Future developments will certainly see the cost of corrosion decrease and extend the life of steel applications even further.