Coatings continued protective action

Biocides are by their nature intrinsically toxic, in this respect any adventitious release to the environment requires an assessment of the relative risk posed. The 5th Environmental Action Plan of the EU is committed to a substantial reduction in the use of biocides. In particular, the Biocidal Products Directive (98/08/EC) is concerned with controlling biocidal products in the market place. Compliance with this directive is required from all member states by 14th May 2000. In this context, a strategy to control the release of biocides is timely, if continued protection is to be afforded to industry and consumer alike. One approach to controlling the release of biocide is to encapsulate in an inert inorganic framework, prior to incorporation in the coating.1... 

The reducers for iron and acid reductions are sometimes equipped with side and bottom cast-iron lining plates, which may be reversed or replaced, in order to protect the vessel against the continuous erosive action of the iron borings. Alternatively, the reducers may be lined partially or entirely with acid-resisting brick or tile. Such a protective coating lasts almost indefinitely and performs satisfactorily in service. 

These low-alloy steels have improved corrosion resistance in outdoor atmospheres in rural areas, or in areas having relatively low pollution levels. The protective action of copper and other alloying elements is due to a resistant form of oxide that forms a protective coating imder atmospheric conditions, but has little or no favorable effect when immersed continuously in water or when exposed to severe industrial corrosive conditions. 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

Since the barrier action of a conversion film is dependent on its solubility and continuity, it is evident that the phosphate films provide only limited protection. However, they do provide an excellent base for paint, plastic, and mbber coatings. 

Tidal—The tidal zone is an environment where metals are alternately submerged in seawater and exposed to the splash/spray zone as the tide fluctuates. In the submerged condition, metals are exposed to well-aerated seawater and biofouling does occur [11,121. A continuous cover of biofouling organisms protects some metal surfaces such as steel, while the presence of biofouling on stainless steel surfaces can accelerate localized corrosion. Steel is influenced by tidal flow, where increased movement due to tidal action causes an increased steel corrosion rate [121. Curve (b) in Fig. 1 shows that steel corrosion at exposed coating defect sites is as severe in the tidal zone as it is in the splash/spray zone. 

Many corrosion cells are formed on the surface, accelerating the corrosion rate as a sacrificial metal and its alloy-coated materials are exposed in the natural atmosphere. During this time, corrosion products are gradually formed and converted to a stable layer after a few months of exposure. After the stable layer has been formed, the corrosion rate becomes constant. This secondary barrier of corrosion protection continuously regenerates over a long period of time. In most cases, the service life of a sacrificial metal coating depends on the secondary barrier action of the corrosion product layer. 

Erosion-corrosion arises from the eombined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosion-corrosion. It is especially harmful to alloys that passivate by forming a protective surface film the abrasive action may erode away the film, leaving exposed a bare metal surface. If the coating is not capable of continuously and rapidly re-forming as a protective barrier, corrosion may be severe. Relatively soft metals such as copper and lead are also sensitive to this form of attack. Usually it can be identified by surface grooves and waves having contoms that are characteristic of the flow of the fluid. 

---