ABSTRACT

The objective of this research is to study the long-term performance of thermal-sprayed Aluminum, Zinc and Zn/Al coatings in tropical marine environments. To that effect, carbon steel coupons (10 cm x 15 cm x 3 mm) were sprayed using two different processes (electric arc and flame spray), with and without the application of two additional layers of sealer. Three sealers were evaluated: two wash primers of different industries, and a phenol sealer. The sheets were exposed for two years in two atmospheric stations, La Voz (coastal marine atmosphere), and the Towers of the Lake Maracaibo Crossing (marine atmosphere), supplemented by laboratory-accelerated tests: Climatic Chamber and intermittent salt spray (ISO 11474). The sheets were characterized by means of initial weight, thickness, metallography, adhesion, roughness, with monthly evaluations by visual and photographic inspection for two years. The results showed that in these aggressive atmospheres, where the wind effect is high (> 11.3 km/h), the performance of the thermal sprayed Zn/Al coating is excellent, even without sealer, providing galvanic protection to the substrate. In the case of Zn and Al coatings, a phenol-based pore sealer should be used to provide a complete cover for the irregularities of this type of coating.

INTRODUCTION

On a worldwide level, many organizations carry out studies to determine the efficiency of new types of coatings on metallic substrates that would enable them to control corrosion problems in aggressive environments, such as marine environments, where maintenance operations are very frequent and costly and interrupt the normal operations of structures or equipment. In the United States and Europe and recently in Japan1 thermal-sprayed coatings are receiving considerable attention because of their promising resistance to corrosion, new application processes and many advantages with regard to other coating systems2. To mention but a few: they withstand mechanical damage, depending on the material applied, they can provide sacrificial protection, they can be applied with greater thickness than galvanizing, they can be applied in the field, there are no limitations with respect to the piece to be covered, thicknesses can be restored, their surface is rough enough to serve as anchor for any other coating used as finish, they have no problems of incompatibility with other coatings, etc. Their performance in the above-mentioned countries has been well documented. The "American Welding Society"3 reported a study on the durability of these coatings, here thermal sprayed zinc and aluminum, with and without sealer, applied by the flame spray process on steel panels exposed in seawater environments, both marine and industrial, provided protection against corrosion for 19 years. British Standard (BS5493, 1977)4 reports that: "sealed thermosprayed aluminum coatings provide 20 years or more protection against corrosion for steel with no maintenance in environments with seawater splash zones". In 1984, Morrow H.5 reports antecedents of economic studies in terms of cost/useful life of thermal sprayed Zn coatings and two painting systems recommended by the Federal Highway Administration.

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