Metal, oxygen scavenging

Oxygen Control. To meet industrial standards for both oxygen content and the allowable metal oxide levels in feed water, nearly complete oxygen removal is required. This can be accompHshed only by efficient mechanical deaeration supplemented by an effective and properly controlled chemical oxygen scavenger. 

Condensate systems can be chemically treated to reduce metal corrosion. Treatment chemicals include neutralising amines, filming amines, and oxygen scavenger-metal passivators. 

The use of neutralising amines in conjunction with an oxygen scavenger—metal passivator improves corrosion control in two ways. First, because any acidic species present is neutralized and pH is increased, the condensate becomes less corrosive. Second, most oxygen scavenger—passivators react more rapidly at the mildly alkaline conditions maintained by the amine than at lower pH levels. For these reasons, this combination treatment is gaining wide acceptance, particularly for the treatment of condensate systems that are contaminated by oxygen. 

Here, the use of inhibitor formulations having a less dramatic effect on TDS (such as certain tannins) may be extremely beneficial. Formulations are available that are based on tannin chemistry and contain blends that act as oxygen scavengers and metal passivators, with additional sludge dispersant and antifoam properties. 

Demineralized water is often employed for HTHW systems, and here it is preferable to employ an organic oxygen scavenger. Hydrazine, although a health hazard, is often used, although diethylhydroxylamine (DEHA) is preferred from an overall safety position or where there are high levels of nonferrous metals present. The pH typically is maintained at 8 to 10, depending on specific circumstances. 

Certain types of oxygen scavenger have additional functions, such as pH neutralizing and metal chelating effects. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

Overall, DEHA is a low toxicity product, performs extremely well as a rapid oxygen scavenger and metal passivator, and is an excellent replacement for hydrazine. In addition, it is competitively priced and consequently a very popular product. 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

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