Corrosion control chemical additives

Treatment programmes to deal with scale formation and crystallisation generally include mixtures of additives so that a wide spectrum of activity is accomplished and may additionally include corrosion control chemicals and biocides. Each system will have unique characteristics and will involve the need "to tailor" the programme to meet the specific needs of that system. 

Corrosion monitoring can be used to provide operational infonnation. If corrosion can be controlled by maintaining a single variable (e.g., temperature, pH, chemical treatment, etc.) within limits previously determined, then that variable can be used to predict changes in corrosion patterns as the limits are exceeded in both a positive and negative direction. An extension of this technique is to use a monitored variable to control chemical addition directly through automatic feed systems. 

Chemical suppHers include basic manufacturers of active ingredients, formulators, and distribution or service industries. The relative importance of each depends greatly upon the industry being suppHed. In many instances, the vendor may supply a number of performance chemicals (eg, corrosion control agents or stabilizers) in addition to the antimicrobial agent. 

Corrosion. Anticorrosion measures have become standard ia pipeline desiga, coastmctioa, and maintenance ia the oil and gas iadustries the principal measures are appHcation of corrosion-preventive coatings and cathodic protection for exterior protection and chemical additives for iaterior protectioa. Pipe for pipelines may be bought with a variety of coatiags, such as tar, fiber glass, felt and heavy paper, epoxy, polyethylene, etc, either pre-apphed or coated and wrapped on the job with special machines as the pipe is lowered iato the treach. An electric detector is used to determine if a coatiag gap (hoHday) exists bare spots are coated before the pipe is laid (see Corrosion and corrosion control). 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

Fruit and vegetable juices packed with 21-26 in. of vacuum and stored in uncoated aluminum cans caused severe corrosion as shown in Table III. The corrosion rate brought about by the juices depends more on the nature of the organic acid present and the buffering capacity of the juice than on the total titratable acidity (11). The use of coated aluminum containers considerably minimized corrosion problems. Product control under extended storage conditions may be achieved by using specific chemical additives. However, more work is needed in this area before final conclusions can be reached. 

Chemical treatment programs are often individually designed for particular boiler plant systems but usually contain oxygen scavengers, pH boosters, and corrosion inhibitors. In addition, the formulations employ materials specifically designed to limit the degree of deposition and control the mechanisms of deposition. 

Chemical additives are needed at some plants with recirculating cooling water systems in order to prevent corrosion and scaling. Chemical additives are also occasionally used at plants with once-through cooling water systems for corrosion controls. 

Some chemical additives such as corrosion inhibitors, wax crystal modifiers, detergents, and demulsifiers provide performance which is difficult to duplicate through refining without adversely affecting some other fuel property. Other additives such as metal chelators, fuel sweeteners, biocides, lubricity improvers, foam control agents and combustion enhancers can also be used to solve fuel performance problems. 

The brief survey of chemical additives for the control of corrosion illustrates the many ways in which control can be achieved. The choice of method will depend on the effectiveness required, coupled with the costs involved. 

Corrosion Control. Surfaces that become wetted by a lubricant and its additives are typically much less prone to corrosive damage from water, acids, bacteria, and other similar corrosion agents. Additives can neutralize acids as well as form a barrier film, which repels water and other chemically aggressive contaminants. 

This chapter identifies socioeconomic benefits in major electrochemical market sectors, both present and future. These sectors include energy, industry, national security, and health, among others. The domestic economic contribution, excluding costs of corrosion, approaches 30 billion per year, or about three-fourths of 1 percent of the gross national product (which amounted to 3800 billion in 1984). Within a decade, substantially greater sales are projected for batteries, fuel cells, semiconductors, sensors, corrosion control, and membranes. In addition, introduction of new technology could slow the loss of major markets in electrochemical production of metals and chemicals and in electroplating. 

In a water-cooled reactor the coolant is processed continuously for control and removal of chemical and radioactive contaminants. In a PWR the lithium formed by (n, a) reactions in dissolved boron will add to whatever natural lithium is present as a contaminant and for corrosion control, but the continued processing will hold it at some steady concentration. For the purpose of this estimate we shall assume a concentration of 1.0 ppm of lithium in the coolant and will neglect the additional Li produced by reaction (8.50), However, after the coolant lithium has been exposed to thermal neutrons for a few years it will become depleted in the Li, because of the high absorption cross section of Li. A typical isotopic composition of lithium in the coolant of a PWR is 99.9 percent Li [S2]. Applying Eq. (8.55) for tritium produced by Li(n, a) yields the yearly production of 34 Ci listed in Table 8.10. The yearly production of the tritium from Li reactions is estimated at 4 Ci [S2]. 

The terminology anodic and cathodic inhibitors is based on these functions. Anodic protection prevents or limits electron flow to the cathode area. Cathodic inhibitors generally reduce the corrosion rate by forming a barrier at the cathode thereby restricting the hydrogen ion or oxygen transport to the cathode surface. Tables 14.6 and 14.7 provide some information on common corrosion inhibitors. Specific corrosion control requirements are usually based on blends of two or more of the listed chemicals perhaps, in addition to chemicals to control scale formation and biological activity. 

There is no commercially available controller on the market, but development costs for this relatively simple system are estimated to be 2,000 (easily justified by chemical additive savings and/or reduced corrosion rates). 

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