1. 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.
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,

2. 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.

33. 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.

34. (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.

35. 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

36. 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 (£° = V) is positive to steel ( F = V).
The coatings must be pore-free and thick. Electroplated
coatings are generally pore-free and
discontinuities are not observed.

43. 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.