October, 2015

In chemical, petrochemical, oil plants and, generally, in all types of industrial facilities which present a danger of explosion due to the presence of gas in the form of cloud or flammable dust, there are also harsh chemical substances that can be harmful and source of deterioration of electric and non-electric equipment.

For these reasons, it’s necessary to pay great attention, during the plant design, to a correct selection of the types of materials, depending on the specific needs of the plant in which such equipment must operate.

This short article does not want to describe the characteristics of the materials that can be used in this type of location, but rather to give an overview of the possible applications and contraindications on their use, depending on the type of chemical, petrochemical or oil product treated, processed or stored.

Stated that there is a multiplicity of chemical substances and many types of materials that are usually used for the construction of the equipment, in this paper we would like to give a guideline of the possible applications recalling that all the information collected for the preparation of this paper come from reliable sources. Cortem Group didn’t carry out specific tests of validation and, therefore, it does not assume any responsibility for the correctness of this information.

Each application has a number of conditions such as exposure time, concentration, temperature etc. For these reasons, Cortem Group recommends to submit to practical tests any materials that might come into contact with chemicals, check the applicability depending on the nature of the chemical, its concentration and its possible emission in free air.

However, we’d give few examples of compatibility or incompatibility of the ferrous and non-ferrous metals that can be installed in all types of industrial facilities that present hazardous atmosphere due to the presence of gas, in the form of cloud or flammable dust, and, at the same time, the presence of aggressive chemical substances. In Table 1 are listed some chemical substances and the compatibility/incompatibility of some ferrous and non-ferrous metals, some plastics materials and the borosilicate glass to these substances. 

Table 1

Table 1 should be considered as a basic guideline, it will not and cannot be exhaustive for all the possible cases. It’s virtually impossible to state all of them, pointing out that the levels of compatibility/ incompatibility must always be validated by the designer, in collaboration with the responsible for process plant or the project designer. However, in order to a better understanding of the above, we give examples of "behavioural variable" with chemical substances not used in their natural state (100%).

1. The borosilicate glass

From the technical point of view, the most important chemical property of the borosilicate glass is the chemical inertness towards acidic or alkaline solutions. From the available literature, we can state that the only chemical compounds which may give rise to corrosion phenomena are hydrofluoric acid, concentrated solutions of sulphuric acid and combinations of caustic solutions with high pH and high temperatures.

2. The aluminium

Let’s see the behaviour of the technical aluminium and the aluminium alloy (Al-Cu alloys excluded) in the presence of hydrocarbons and halogenated compounds (source Alluminio Manuale degli impieghi). Halogenated hydrocarbons in the presence of water can decompose giving rise to the corresponding acids (e.g. hydrochloric acid), which attack the natural oxide film destroying it. It’s also possible the development of complex reactions starting from aluminium halides. The trend to reactivity is related to the stability of the halogen-bond organic radical. In each case, the corrosion problems occur at elevated temperatures, such as those of boiling of chemical compounds. Some phenomena may occur in the presence of moisture, also in phase of storage, if the pieces subjected to degreasing with halogenated hydrocarbons are not well dried.  

Let’s see the behaviour of the technical aluminium and the aluminium alloy (Al-Cu alloys excluded) in the presence of compounds with aromatic rings (source Alluminio Manuale degli impieghi). The non-chlorinated aromatic compounds do not present serious problems of susceptibility to corrosion towards aluminium and its alloys. The aromatic acids are an exception to this rule, in particular salicylic acid, in the presence of moisture.

Let’s see the behaviour of the technical aluminum and the aluminium alloy (Al-Cu alloys excluded) in the presence of compounds with oxygen functional group (source Aluminium Manual loans). The alcohols and organic acids can attack the light alloys and their aggressiveness depends on water content. Ethers, ketones, esters and anhydrides are classes of products virtually inert.

3. The stainless steel

Behaviour of stainless steels used in the chemical, petrochemical and oil plants, those non magnetic, resistant to most chemicals organic and inorganic, with Cr content> 11%.

Stainless steel AISI 304 and AISI 316L are those most frequently used for such purposes.

4. The surface treatment of ferrous and non-ferrous metals

The materials that are usually used in plants described above, except for the steels, which do not need further protective treatments, if properly selected for their installation, need of surface treatment suitable to the type of possible corrosion present at the place of installation.

Aluminium, for example, may require anodising or other surface treatments to be protected against aggressive agents or painting treatments, always for the same reasons, choosing such treatments according to the system design specifications.

Furthermore, the carbon steel, commonly indicated as "Iron", requires protective surface treatment such as: a system of electrolytic galvanizing, if the aggressiveness is slight and if the material will be installed in confined areas (Indoor); hot-dip galvanized if installed in highly aggressive and outside areas (Outdoor); coating systems like "wet on wet" or electrostatic (powder), with defined cycles of preparation and treatment for the types of aggressive agents.

Cortem Group, always focused on these issues and constantly looking for new technologies for the treatment of materials, for several years uses a system for surface treatment, depending on the specific system requirements. It is able to apply a protection to materials usually used to manufacture its equipment: lighting fixtures, sockets, distribution boxes (junction box), switching boxes (marshalling box) or panel boards for starter, control and distribution that, if left in their natural state and in the presence of harsh chemicals that are not compatible, they can be attacked even irreversibly.

5. Conclusions

As you can see, the variables that could occur in the various cycles of the production process are many and not always predictable, if not in the phase of the plant engineering. Therefore, the aim of this discussion was to highlight how many and which may be the factors of the degradation of materials that can be used in industrial plants, leaving to the designer the choice of the correct type of material.