Cathodic inhibitors polyphosphates

Although orthophosphates are themselves passivating, anodic inhibitors (and also cathodic inhibitors, forming a calcium phosphate barrier film), the film strength is weak, even in simple HW systems and they are not used for this purpose. Nevertheless, despite the thermal instability of sodium hexametaphosphate and other polyphosphates, phosphates in general have several important properties that make them useful in boiler plant operations. These properties include ... 

Polyphosphates are employed because they provide a cleaning action (hence their use in POC formulations), they act as cathodic inhibitors (by forming large colloidal cations with calcium), and they also revert to O-PO4, an anodic inhibitor. 

As described in the discussion of heat exchanger fouling elsewhere in this encyclopedia, anodic and cathodic reactions occur. Chemicals may be added to prevent these reactions they are termed anodic and cathodic inhibitors. Cathodic inhibitors form a barrier at the cathode reducing or eliminating H" " or O2 transport to the cathode. They include nitrites, silicates, tannins, and orthophosphates. Anodic inhibitors prevent or restrict electron transfer and include polyphosphates, polyphosphonates, and molybdates. Some of these chemicals represent nutrients for aquatic life and may encourage the growth of microorganisms. 

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. 

Synergistic Effects In practical formulations, it is common to use several different inhibitor species, frequently a combination of anodic and cathodic inhibitors. The simultaneous use of two or more different inhibitor species often results in a more efficient inhibition than the sum of the individual effects of the inhibitors. For example, the inhibition of mild steel in chlorideSynergistic effects often depend sensitively on the ratio of the different inhibitor species. 

Cathodic inhibitors in near-neutral solutions interfere with the oxygen reduction reaction by restricting the diffusion of dissolved oxygen to the electrode surface. These substances usually form thick surface layers with poor electronic conductivity (the latter is an important prerequisite to avoid oxygen reduction on the film surface). Examples are phosphates, polyphosphates, silicates, borates, and inorganic inhibitors, such as Zn +, which precipitate as Zn hydroxide, and Ca ", which forms calcium carbonate films in the presence of CO3 [3]. These inhibitors... 

Formulations usually contain a combination of different anodic and cathodic inhibitors. Commonly used are ortho- and polyphosphates, phosphonates, tannins, lignins, benzoates, silicates, chromates, molybdates, nitrites, nitrates, zinc salts, aromatic azoles, carboxylic acids, amides, amines, soluble oils, and oxygen scavengers, such as hydrazine and sulfites [3, 46]. Some of these substances (e.g. silicates) are employed predominantly in synergy with other inhibitors, whereas in other cases the combination of inhibitors may have adverse effects (e.g. nitrites and organic amines or amides may form carcinogenic nitrosamines at elevated temperatures). 

Polyphosphates are effective cathodic inhibitors for aerated cooling systems [5,6]. They are also known as condense or polymer phosphates. They bond with calcium ions to form positively charged colloidal particles that form a barrier film on the cathode (Fig. 14.7) [6], A disadvantage of this inhibitor is hydrolysis of phosphoms oxygen bond, resulting in orthophosphate, a less desirable inhibitor. 

As an example of these types of inhibitors, polyphosphates, phosphates, silicates, and benzotriazole can be mentioned. The action of these inhibitors is highly dependent on the environmental factors such as pH and redox potential. Therefore, they are anodic under certain conditions and cathodic otherwise. 

The polyphosphate molecule bonds with divalent calcium and other ions to form positively charged colloidal particles which are attracted to the cathode and form a protective film. As some metal ions, such as iron, may also be adsorbed on the film, polyphosphate also shows a partial anodic behavior although basically they are cathodic inhibitors. The mechanism of corrosion prevention by polyphosphates is shown in Fig. 6.12. 

Zinc salts are well-known cathodic inhibitors in cooling vrater systems. They are, however, not used alone as the films formed by them are unstable. They are, however, used very effectively with polyphosphate as a synergistic blend to maximize the effect of inhibition. These synergistic blends... 

Polyphosphates also act as corrosion inhibitors. They are cathodic polarizers and form a durable corrosion-inhibiting film that includes adsorbed calcium. 

Chemical Treatment. A wide variety of chemicals and water treatments are used for corrosion control. Corrosion inhibitors usually act by forming some type of impervious layer on the metallic surface of either the anode or cathode that impedes the reaction at the electrode and thereby slows or inhibits the corrosion reaction. For example, various alkali metal hydroxides, carbonates, silicates, borates, phosphates, chromates, and nitrites promote the formation of a stable surface oxide on metals. The presence of these chemicals in the electrolyte allows any faults in the metal surface or its oxide film to be repaired. If they are used in too small a quantity as anodic inhibitors, they may promote intense local attack because they can leave a small unprotected area on the anode where the current density will be very high. This is particularly true of chromates and polyphosphates. 

In the early 1950 s, combinations of alkali chromate (an anodic inhibitor) and polyphosphate (generally accepted as cathodic) came into prominence for cooling system corrosion inhibition. The combination of chromate with phosphates proved highly efficient in comparison with straight phosphate or straight chromate, and could be used at substantially lower concentrations. 

Examples of the latter are chromates, which are reduced to Cr(III) hydroxide or oxyhydroxide on the metal surface, or polyphosphates, in which decomposition and subsequent precipitation of Ca phosphate has been suggested [8]. The precipitation reactions will depend on the local solution composition (pH, metal ion concentration) in the near-surface region of the corroding metal, which may pronouncedly deviate from that in the bulk. For instance, the production of OH in the cathodic partial reaction will raise the surface pH and thus promote the precipitation of compounds, such as Zn hydroxides, even in noticeable acidic solution. In a similar way, the pore-plugging ability of anodic inhibitors may be enhanced by reactions with local metal ion accumulations in the vicinity of active pores in a passive film. 

Many inhibitors used in neutral environments form surface layers by precipitation or polymerization reactions. The polyphosphates, (NaP03) and the organophosphates belong to this category of inhibitor. Acting principally on the cathodic reaction (diffusion barrier), they lower the corrosion rate in the presence of oxygen. 

Inhibitors may be classified as shown in Fig. 6.1. There are two major classes inorganic and organic. The anodic type of inorganic inhibitors includes chromates, nitrites, molybdates and phosphates, and the cathodic type includes zinc and polyphosphate inhibitors. The film forming class is the major class of organic inhibitor as it includes amines, amine salts and imidazolines - sodium benzoate mercaptans, esters, amines and ammonia derivatives. 

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. 

Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel... 

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