Page 168 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

146 CORROSION CAUSES

(a)

(b)
Figure 3.14 (a) Pressurized storage tanks; (b) Unpressurized storage tank (7).

prevention. ASTs are prone to a variety of modes of corrosion mechanisms. Internal
and external modes of corrosion that may occur on an AST are shown in Fig. 3.16.

3.10.1.1 Internal Corrosion There are several different corrosion conditions in
the interior parts of an aboveground tank. Vapor-phase corrosion can occur in areas
exposed to vapor above the stored products and includes general, crevice, and pit-
ting corrosion, depending on the temperature and the characteristics of the material.
Product side corrosion can occur on the internal wall plate when corrosive materials
are stored. This type of corrosion includes general and pitting corrosion. At the inter-
face of liquid and gas in a tank, the corrosion rate is often accelerated because the
oxygen or moisture concentration gradient at the interface varies with depth in the
liquid. Aqueous phase corrosion can occur when water contamination and settling
in petroleum products result in a layer of water on the bottom of the tank. Although
the product may be noncorrosive, the presence of contaminants such as sludges and
deposits may result in internal bottom and wall general corrosion, crevice corrosion,