Passivation polyphosphates

Figure 10.1 Types of phosphate structures, (a) Where x = 12 to 14, the structure represents sodium polyphosphate, a phosphate typically used in HW heating and industrial steam boiler formulations. The structure is ill defined and described as glassy rather than crystalline. Where x = 2, it represents sodium tripolyphosphate, (b) This is the structure where effectively, x = 0, and represents trisodium phosphate (sodium orthophosphate), which is commonly supplied in either crystalline or anhydrous powder form and used as an alkalinity booster, boiler boil-out cleaner, and metal surfaces passivator.

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 ... 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Offline passivation involves treatment of equipment currently out of service. Treatment levels are typically higher consequently, passivation is completed more quickly. Passivation of nonchromate treatment generally uses either a polyphosphate, zinc, molybdate or other nonchromate-based inhibitor in combination with various surface-active cleaning agents. The passivation solution should be disposed of after the pretreatment stage, rather than dumped back into the cooling system where the potential for fouling can exist due to the precipitation of pretreatment compounds such as zinc or phosphate. Table 8.1 outlines both online and offline pretreatment procedures. 

Most usually, a preoperational cleaning (POC) process/passivation program uses a chemical cleaner formulation based on a polyphosphate such as SHMP or STTP, together with various dispersants and surfactants. Where polyphosphate is not permitted to be discharged to sewer, silicates can often be used. Formulations may also include NaEDTA and sometimes specific corrosion inhibitors such as tolyltriazole (TTA). 

Zinc polyphosphate has also been used successfully as a passivating agent, often in combination with chromate. The zinc is a cathodic... 

The presence of polyphosphate detergent in the cleaning formulation (some of which will revert to orthophosphate) will, under most circumstances, lead to passivation of the clean metal surfaces. Additionally, as part of the water treatment program start-up procedure, it may prove useful to supplement this initial passivation with 2 to 3x the normal reserve of maintenance inhibitor chemical treatment for a period of 1 to 2 weeks. 

The primaiy role of sulfur species in the tribochemical processes is passivation by sulfur species on nascent surfaces caused by a severe wear process (extreme pressure conditions) Fe2+ + S2" - FeS, the heat of formation AHf (FeS) = -1.04 eV. The sulfide can prevent adhesion and also the attack of oxygen species, AHf (FeO) = - 2.82 eV (Mori, 1995). There is enough sulfur in engine oil to initiate acid-base reactions in the formation of zinc sulfide (Martin, 1999). Organic sulfur species other than in thiophosphate form can react with the ZnO produced by the phosphate reactions according to the following ZnO + S2 - ZnS + O2. The zinc sulfide can also be directly produced if the polyphosphate contains sulfur atoms in the polymer chain (thiophosphate), for example ... 

The first example of QDQWs described in the literature is the system CdS/HgS/CdS. The synthesis, which is outlined in detail in Ref. 56, starts with a standard polyphosphate-stabilized CdS colloid in aqueous solution. To these particles with an average diameter of 5.3 nm and a fairly narrow distribution of sizes, Hg(C104)2 is added at neutral pH. The mercury salt dissociates readily in this medium, allowing the mercury ions to react with the CdS colloids. Since the solubility product of HgS is more than 20 orders of magnitude smaller than that of CdS, Cd ions are readily replaced by the Hg ions. As is demonstrated by polarography and mass spectrometry, this substitution reaction proceeds until the surface Cd " is replaced by Hg ", thus forming a surface layer of HgS on the CdS core, which then passivates the particle toward further substitution. Addition of H2S to this... 

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. 

Sodium benzoate [6, 7] (CeHsCOONa), sodium cinnamate [8] (CeHs-CH-CH COONa), and sodium polyphosphate [9,10] (NaPOsjn (Fig. 17.2) are further examples of nonoxidizing compounds that effectively passivate iron in the nearneutral range, apparently through facilitating the adsorption of dissolved oxygen. As little as 5 X sodium benzoate (0.007%) effectively inhibits steel in... 

The mechanism of inhibition in the case of sodium polyphosphate solutions may depend in part on the ability of polyphosphates to interfere with oxygen reduction on iron surfaces, making it easier for dissolved oxygen to adsorb and, thereby, to induce passivity. Other factors enter as well there is, for example, evidence of protective film formation of the diffusion-barrier type on cathodic... 

The inhibition of metal corrosion in industrial water systems was first achieved by the use of inorganic salts or their blends, including chromates (Evans, 1936 Mayne and Pryor, 1949), nitrites (Hatch, 1952), phosphates (Patterson and Jones, 1952), borates (Mercer, 1990), silicates (Lehrman and Shuldenen, 1952), zinc salts (Hatch, 1965 a) and other cations (Hinton, 1989). Additionally, chromates and nitrites were mainly applied, and from the end of the 1950s the use of polyphosphates increased (May et al., 1981 Hwa, 1971). Treatments with anodic inhibitors such as nitrites or chromates require a high initial dose and a relatively high continuous dose in order to achieve an effective passive layer on the metal surface. The concentration of chromate and nitrite can be decreased in the presence of polyphosphates and zinc ions. 

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... 

In this case, minimal concentrations of inhibitors such as sodium chromate, sodium nitrite, polyphosphates, sodium benzoate, or borax are effective. Steel is easily passivated in demineralized or distilled water because the pH is neutral and there are no dissolved ions to interfere with formation of the passive layer. 

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