BOILER CORROSION 11.1 DEFINITION A boiler is a vessel in which water is continuously vaporized into steam by the application of heat. The steam that the boiler produces can be used for power generating, water heating and manufacturing processes. 1 1 . 3 REQUIREMENTS OF AN EFFICIENT BOILER To meet the minimum requirement of an efficient boiler, the boiler should: (a) generate maximum amount of steam at required pressure and temperature with minimum fuel consumption, (b) be light in weight and occupy minimum space, (c) confirm to safety regulations,

1 1.4 CLASSIFICATION OF BOILERS Boilers are classified according to use, pressure, materials of construction, tube content, firing mode and heat source. (a) Classification Based on Use They are either stationary or a mobile-type. Following are their uses: • Building heating • Plant process steam • Power generation • Waste heat removal (b) Classification Based on Pressure Following are the major types under the above category: • Low-pressure boilers: This category includes all steam boilers that do not exceed 150 psi, and all hot water boilers that do not exceed 160 psi. • Power boilers: All boilers exceeding the limits of low-pressure boilers, are called power boiler.

7.3 CLASSIFICATION OF COATINGS Coatings can be classified in the following categories according to corrosion resistance: (a) Barrier coatings (b) Conversion coatings (c) Anodic coatings (d) Cathodic coatings. 7.3.1 BARRIER COATINGS Barrier coatings are of four types - anodic oxides, inorganic coatings, inhibitive coatings and organic coatings.

(1) Anodic Oxides A layer of AI2O3 is produced on aluminum surface by electrolysis. As the oxides are porous, they are sealed by a solution of potassium dichromate. The object of sealing is to minimize porosity. However, chromates have health hazards and are not allowed in some countries. (2) Inorganic Coatings These include coatings like ceramics and glass. Glass coatings are virtually impervious to water. Cement coatings are impervious as long as they are not mechanically damaged. (3) Inhibitive Coatings In several instances, inhibitors are added to form surface layers which serve as barriers to the environment. Inhibitors, like cinnamic acid, are added to paint coatings to prevent the corrosion of steel in neutral or alkaline media. (4) Organic Coatings Epoxy, polyurethane, chlorinated rubber and polyvinyl chloride coatings are extensively used in industry. They serve as a barrier to water, oxygen, and prevent the occurrence of a cathodic reaction beneath the coating.

7.3.3 ANODIC COATINGS By anodic coating, it is meant that a coating which is anodic to the substrate, such as zinc aluminum or cadmium coatings. On steel such coatings are generally called sacrificial coatings. They protect the substrate at the expense of the metallic coating applied. The zinc coatings protect the substrate by acting as a sacrificial anode for the steel which is cathodic to zinc: Consider nickel coating on a steel substrate. A layer of bright nickel is laid on the dull layer of nickel. Over the bright nickel a layer of chromium is laid. The bright nickel (high sulfur content) is more noble than the steel substrate. Such a coating system is called duplex coating. 7.3.5 MISCELLANEOUS COATINGS These include glass coatings, porcelain coatings and high-temperature coatings. Any breaks in the coating cause the anodic oxidation of Zn to occur

7.3.4 CATHODIC COATINGS In this type of coating, the metals which are deposited are electropositive to the substrate. For instance, for copper coated steel, copper (£° = +0.337 V) is positive to steel ( F =-0.440 V). The coatings must be pore-free and thick. Electroplated coatings are generally pore-free and discontinuities are not observed.

Corrosion Engineer Work closely with the engineering staff in working out new designs or modifying existing ones to reduce the opportunity for corrosion. Close contact with the maintenance engineers. Work with the production department. Collaborate with accounting department in establishing the actual cost of corrosion. Work closely with the purchasing department. Assist the sales department by helping discover any deficiencies in products. Keep management informed of his need and his accomplishments so that he may be provided with the support he will require to be effective.