The corrosion protection of large steel structures such as bridges, pipelines, oil tanks, towers, radio and television masts, overhead walkways and large manufacturing facilities in the metallurgical, chemical, energy, and other industries is a key issue. The protection of structures exposed to moist atmospheres and seawater such as ships, offshore platforms, seaports, is even more difficult.
In most cases the surface to be protected is thousands and even tens of thousands of square meters, requiring that coating costs are competitive with those of traditional painting methods. The coating rate must be at least 10 m2/h and coating must be, if possible, deposited in one unique pass; the equipment must be mobile and autonomous for operation in field conditions and can work under manual control, automation being generally difficult for large scale operations, and at last the spray gun can be up to 30 m away from other elements of the equipment.
Flame and wire-arc spraying meet such requirements. These equipments are widely used in industry because the investment is rather low and also coating adherence is generally good (over 20 MPa), with almost no heating of the substrate. However coatings obtained by these methods are relatively porous (up to 20 %). This porosity can be reduced by shot peening just after spraying.
Therefore, the main use of thermal sprayed coatings is as sacrificial coating with typical thicknesses between 50 and 500 µm. Referring to section 3, such coatings must have a cathodic behavior relatively to the ions of the metal to be protected, in almost all cases steels. As illustrated in Figure 5b, the cathodic protection can be porous without any corrosion of the underneath metal. Metals used are then zinc, aluminum and zinc-aluminum.
Zinc performs better than aluminum in alkaline conditions, while aluminum is better in acidic conditions. If resistance to wear must be improved, aluminum coatings can be sprayed with alumina particles, e.g. by using cored-wires. For the protection of steel reinforcement in concrete, zinc is generally used, but titanium has also been used.
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