Inhibitors chromate-phosphates

Typical examples of inhibitors used for minimizing corrosion of iron and steel in aqueous solutions are the chromates, phosphates, and silicates. Organic sulfide and amine materials are frequently effective in minimizing corrosion of iron and steel in acid solution. 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however, it has also been found to be attractive in some once-through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions, organic sulphides and amides are effective. 

Anodic inhibitors reduce corrosion by polarizing the anodes rapidly. If insufficient anodic inhibitors are used, severe localized pitting will occur in the unprotected areas. Examples of anodic inhibitors are phosphates, silicates, nitrates, and chromates. ... 

The mitigation of corrosion can be achieved economically by the use of corrosion inhibitors. Chromate has been extensively used in an aqueous environment for the protection of aluminium, zinc and steel. Although chromates are cheap and effective, they are not acceptable because of their toxicity. Alternate inhibitors such as molybdates, organic inhibitors such as phosphonates, mixtures of phosphates, borates and silicates and surfactants like sulfonates have been used in place of chromates. Chromates are anodic inhibitors and help to form passive oxide on the metal surface. 

In the late 1950 s, chromate-phosphate systems incorporating zinc as another cathodic inhibitor were introduced, followed by zinc-chromate inhibitors, without phosphate. Using chromate-zinc, or polyphosphate-chromate-zinc inhibitors it was possible to cut working concentrations still further. It was necessary to control pH to make this inhibitor system function effectively. At increased pH (above pH 7.5)tendency for zinc loss by precipitation increases. Furthermore, pH rise may cause heat exchangers to become fouled by zinc hydroxide slimes or zinc phosphate. 

A. Inorganic Inhibitors. The inorganic inhibitors may be classified on the basis of the role of oxygen. Inhibitors that can function without external oxygen are sometimes called passivators. These compounds include chromates and nitrates, which are readily reduced and able to oxidize the metal surface, usually iron, to form a passive oxide film. Other inorganic anions used in inhibitors include phosphates, silicates, and borates. 

Some ions are inhibitors and will reduce either the anodic or cathodic reactions that occur during the corrosion of aluminum. Examples are chromates (an anodic inhibitor) and phosphates (a cathodic inhibitor). Inhibition of circulating water systems is complex and professional consultation is recommended for the design of water treatment systems. 

Makeup fresh water in recirculating water systems must be added to replace losses from entrainment (drift, or windage), evaporation losses, and blowdown. Windage losses can be estimated as 0.1 to 0,3 percent of the recirculation rate for induced-draft towers. If makeup water introduces dissolved salts (hardness) which will otherwise accumulate, a small amount of water is deliberately discarded (blowdown) to keep the salt concentration at some predetermined level. A calculation is demonstrated in Illustration 7.11. Chlorine treatment of the water to control algae and slime and addition of chromate-phosphate mixtures to inhibit corrosion have been common in the past, but restrictions on discharges to the environment by the blowdown have led to the use of nonchromate inhibitors. Many other practical details are available [14, 15]. 

It is very rare that a single inhibitor is used in systems such as cooling water systems. More often, a combination of inhibitors (anodic and cathodic) is used to obtain better corrosion protection properties. The blends which are produced by mbdng of multi-inhibitors are called synergistic blends. Examples include chromate-phosphates, polyphosphate-silicate, zinc-tannins, zinc-phosphates. Phosphonates have been used to cathodically protect ferrous materials. Following are the major applications of synergistic blends of inhibitors. 

There is now a search for less toxic, greener alternative corrosion inhibitors. Chromates have combined effectiveness with relatively low cost. Sinko has reviewed the quality parameters needed for corrosion inhibitors in coatings, and generally concluded that inorganic non-chromate inhibitors such as phosphates, molybdates, borates and silicates are inferior to chromates. Rare earth salts have been proposed and show promise as alternative corrosion inhibitors, and we have been developing rare earth carboxylates as potential dual function inhibitors combining the roles of the rare earth and the aromatic carboxylates. If rare earth-based corrosion inhibitors are to be used, there is a need to understand the chemistry of the elements and of the inhibitors. 

Inorganic inhibitors. Usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the negative anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors. 

The most effective way to prevent SCC in both stainless steel and brass systems is to keep the system clean and free of deposits. An effective deposit control treatment is imperative. A good corrosion inhibitor is also beneficial. Chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions. 

Precipitation and Vapor-Phase Inhibitors. Precipitation inhibitors are film-forming compounds that produce barrier films over the entire surface. Phosphates and siUcates, which are the most common, do not provide the degree of protection afforded by chromate inhibitors, but are useful in situations where nontoxic additives are required. Two main drawbacks to the use of phosphates and siUcates are the dependence on the water composition and the control required to achieve maximum inhibition (37,38). 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

To reduce the effects of corrosion, inhibitors are added, typically sodium chromate in the salt brines and sodium phosphate in the glycols. These are alkaline salts and help to counteract the effects of oxidation, but periodic checks should be taken, and borax or similar alkali added if the pH value falls below 7.0 or 7.5 [1]. 

Adsorption inhibitors act by forming a film on the metal surface. The action of traditional oil-based red lead paint formulations presumably involves the formation of soaps and the precipitation of complex ferric salts that reinforce the oxide film. There has been substantial interest in recent years in development of replacements for lead-based and chromate-based inhibitor systems. Adsorption inhibitors based on pol3rmers have been of particular interest. In this volume, Johnson et al. and Eng and Ishida discuss inhibitors for copper 2-undecylimidazole is shown to be effective in acid media, where it suppresses the oxygen reduction reaction almost completely. Polyvlnyllmidazoles are shown to be effective oxidation inhibitors for copper at elevated temperatures. Also in this volume, Chen discusses the use of N-(hydroxyalkyl)acrylamide copolymers in conjunction with phosphate-orthophosphate inhibitor systems for cooling systems. 

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