Materials which are used in petrochemical plants, are continuously exposed to the corrosive effects produced by the petroleum processing units.

These corrosive effects are enhanced for processes operating at high temperatures, leading to a faster degradation of mechanical components. Therefore, material stability in corrosive high-temperature environments is of great importance with regard to industrial applications.

Mixed gas environments are very characteristic of metallurgical and chemical processes, where the presence of molecular species such as oxygen (O2), hydrogen (H2) and nitrogen (N2) sulfur (S) and carbon (C) often requires the use of coatings to avoid the accelerated degradation these mixtures are able to promote.

The performance of aluminum coated steel is beneficial for products that require the advantages of good corrosion resistance. For some products, the behavior of the aluminum-iron interfacial compound dictates the overall resistance of the coating system to oxidation, scale formation and abrasion.

Aluminum coated steel products are used successfully in corrosive (oxidizing, sulfidizing) environments in which the temperature ranges from that of outdoor exposure to 600°C. Among the atmospheric corrodents to which these coated products are exposed, one can list high-sulfur containing industrial atmospheres, marine environments, organic acids in food waste, etc.

The choice of aluminum coatings
Methods of applying aluminum coatings include hot dipping, pack cementation, cladding, and thermal spraying, amongst others. The choice of one method is determined by the coating performance required, intended use of the product, shape, size, production volume and cost.

Thermal-spray coatings have become an interesting alternative to the economical and efficient operation of many engineering components. Many industries have introduced this versatile technology into the manufacturing environment. The main advantages of the process are:

• Versatility with respect to feed materials (metals, ceramics and polymers in the form of wires, powders, rods, etc).
• The ability to create freestanding structures for near net-shape manufacturing.
• The capacity to form barrier and functional coatings on a wide range of substrates.
• The rapid solidification synthesis of specialized materials.

One drawback of the coating is the oxide content generated during spraying. A second drawback is the presence of open pores and crevices within the coating structure, through which the corrosive environment penetrates towards the substrate. And a third drawback is the degradation of coating materials during spraying. Some elements such as Cr and Al are partially oxidized and due to this phenomenon, chemical composition of the coating material changes.

However, during the last decades, many efforts and new technologies and techniques have been developed in order to minimize or prevent the adverse effects of these coatings. The use of sealers and specially pre-heating treatments (when the dimensions of the product are not too wide) applied to thermal spray products, have often shown to be quite effective, for avoiding corrosion species penetration via porosity.

Aluminum sprayed coatings can be expected to offer good protection for aggressive environments containing oxygen and sulfur, due to its ability to form a dense and homogeneous layer of aluminum oxide, avoiding the diffusion of external corrodents elements to the interior of the material.

The presence of S, C and O in the samples corroded in the fluid catalytic cracking unit (FCCU), are able to diffuse to the interior of the coatings, via short-circuit paths, reaching the steel substrate and initiating internal corrosion there.

It is believed that the porosity in the coating provided short-circuit paths for corrodants to penetrate through the coating and that this effect was enhanced by the fact that most of the pores present in thermal spray coatings happen to be linked in an interlamellar interconnected manner. After some time, cracks formed due to the corrosion process started also to show a high interconnection. Another point to be considered is the structure of sulfide scales. Sulfides are, by nature, porous and the presence of such structures within the material, can contribute even further for the short-path diffusion of the gaseous species into the metallic substrate.