1. Introduction

Today, we use many different materials for producing equipment and components designed for areas with a potentially explosive atmosphere. They can be basically classified in the table below. 

When choosing the best materials to be transformed into finished products, it is important to consider the limiting factors of nature. All materials, including the ones we use, have three enemies:

• environment;

• temperature;

• time.

Temperature and time are familiar factors, while the environment, where our products are used, is not easy to control.

We are not talking about familiar potential hazards caused by an explosive atmosphere (which can be controlled by laboratory tests and guaranteed by certification), but rather deterioration caused by highly aggressive environments like chemical and petrochemical plants.

Corrosion resistance is a relative factor, as it depends on the actual environmental conditions that significantly influence the nature of the attack.

This is why Cortem Group constantly tests its materials in accordance with ASTM standards (American Society for Testing and Materials) – for example B117 in salt fog, G31 in hydrogen sulphide and hydrochloric acid, etc. - and carries out in-depth research into their resistance in outdoor environments. It is therefore able to choose the right material based on objective experience and to guarantee long-term product safety.

2. The aluminium alloy

Aluminium alloy is one of the world’s most widely used materials in the production of explosion-proof enclosures.

It is highly corrosion-resistant and, therefore, universally known as the most effective and versatile material for most applications. It is much lighter than cast iron, so it makes the equipment much easier to install and maintain; moreover, it is highly corrosion-resistant and does not require surface protection, unlike cast iron which has to be galvanized or coated.

Nevertheless, Cortem Group protects its products with a surface coating, RAL 7035, that contains stainless steel particles, a protection against mechanical impact, and characterizes our production avoiding imitations and fake products. Furthermore, the Coralum surface protection treatment based on an electro-ceramic coating applied by electrolytic deposition directly on the aluminium alloy is now available on request.

Aluminium is also much cheaper than stainless steel. 

The mechanical properties of aluminium alloy castings are more than adequate for ensuring explosion-proof electrical protection.

In the past, either aluminium-copper alloys were used, but they were not at all corrosion-resistant, or aluminium-magnesium. Although being corrosion-resistant, magnesium, when mechanically hit, can generate sparks with energy values able to trigger an explosion (technical standards limit the magnesium presence at 7,5% in mass). Today we use the aluminium-silicon alloys, where copper is present as impurity. Its main features can be summarized as follows:

• fairly high mechanical resistance;

• adequate ductility;

• good denseness;

• corrosion resistance (oxidation only).

The aluminium- silicon alloys we use mostly are the AlSi10Mg(a) (EN AB 43000) and the AlSi12(b) (EN AB 44100) and their chemical compositions, in accordance with EN 1706 standard, are contained in the table below.

3. The corrosion resistance

The corrosion resistance is a relative factor, as it’s necessary to consider the environmental conditions that affect the nature of the attack.

Aluminium and its alloys have generally excellent corrosion resistance in various and different environments.

Despite of it’s a very chemically active metal, its behaviour is stabilized by the formation of a protective oxide film on its surface. This film, which in case of breakage is able to reproduce itself immediately, has a thickness, if formed in the air, ranging from 50 to 100 Å.

In case of use in more aggressive atmospheres, or when improved with artificial growth processes (anodization), the film becomes thicker.

This oxide film is transparent, hard, adherent to the surface and not leafed. Accidental abrasion of the surface of the film are automatically repaired. Therefore, the conditions that cause corrosion of aluminium and its alloys are those that mechanically abrade the protective film or that favour chemical conditions that degrade it and minimize the availability of oxygen for its reconstruction.

In general, the protective oxide film is stable in aqueous solutions with Ph between 4.5 and 8.5, and it’s not attacked by acids and alkaline solutions, such as, for example, nitric acid, acetic acid, silicate sodium, ammonium hydroxide.

As for other metals, the corrosion phenomena are related to the passage of current between the anodic and cathodic areas, then the potential difference of the different areas. In this regard, it should be noted that the size and the morphology of the corrosive phenomena are related to many factors, including the composition of micronutrients, their localization and their quantity.

The improved corrosion resistance is achieved by the pure aluminium, but silicon alloys are also highly resistant to corrosion in marine environments or in the presence of strongly acidic or strongly basic vapours which normally characterized hazardous areas, where explosion-proof enclosures are normally installed.