Corrosion control general operations

The precise protocols necessary to achieve effective corrosion control will vary dependent on individual boiler design and operation. For example, control of alkalinity is fundamental in controlling corrosion mechanisms. In small to midsize, general-purpose and industrial boilers, it is common practice to obtain adequate BW alkalinity as part of any water treatment program that operates under a free-caustic regimen. This approach generally is perfectly acceptable, and such programs normally can be relied on to ensure a clean, scale- and corrosion-free boiler. 

With regard to higher pressure industrial and power boilers, the more exacting waterside and operating conditions demanded generally dictate that an alternative approach to corrosion control be employed, which usually precludes the presence of free-caustic. 

In a modern business environment, successful enterprises cannot tolerate major corrosion failures, especially those involving personal injuries, fatalities, imscheduled shutdowns, and environmental contamination. For this reason considerable efforts are generally expended in corrosion control at the design stage and in the operational phase. This is particularly true for industries where harsh chemicals are handled routinely. 

Inhibitors The use of various substances or inhibitors as additives to corrosive environments to decrease corrosion of metals in the environment is an important means of combating corrosion. This is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, it has also proved to be economicaUv attrac tive for many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective as the result of their controlling influence on the cathode- or anode-area reactions. 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

It is generally agreed that the causes and effects of poor water chemistry, mechanical problems, boiler section corrosion, metal failure, and poor boiler plant operation are all closely interrelated. Thus, effective control over the various corrosion processes that may occur in a boiler and its auxiliary equipment is fundamental to the realization of the full life expectancy and safe operation of the plant. Corroded and wasted metal cannot be replaced easily, and the failure of a boiler in service is both potentially dangerous and expensive. 

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