Corrosion polyphosphates

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Organophosphonates are similar to polyphosphates in chelation properties, but they are stable to hydrolysis and replace the phosphates where persistence in aqueous solution is necessary. They are used as scale and corrosion inhibitors (52) where they function via the threshold effect, a mechanism requiring far less than the stoichiometric amounts for chelation of the detrimental ions present. Threshold inhibition in cooling water treatment is the largest market for organophosphonates, but there is a wide variety of other uses (50). 

Figure 5.1 Effect of oxygen concentration on corrosion of mild steel in slowly moving water containing 165 ppm CaCl2 48-hour test, 25°C. [Courtesy of H. H. Uhlig, D. N. Triadis, and M. Stern, Effect of Oxygen, Chlorides, and Calcium Ion on Corrosion Inhibition of Iron by Polyphosphates, J. Electrochem. Soc. 102, p. 60 (1955). Reprinted with permission by The Electrochemical Society, Inc. ]...

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. 

In nitrogenous fertiliser solutions of the NH4NOJ—NHj —HjO type corrosion of steel can be prevented by 500 p.p.m. of sulphur-containing inhibitors, e.g. mercaptobenzothiazole, thiourea and ammonium thiocyanate. However, these inhibitors are not so effective where most of the NHj is replaced by urea. For these solutions phosphate inhibitors such as (NH4>2HP04 and polyphosphates were more effective... 

Orthosilicates are not suitable for corrosion inhibitor purposes, and metasilicates are not generally used because they tend to increase the caustic alkalinity too much, leading to the potential for caustic-induced problems. However, glassy polysilicates are widely used. They have inhibition properties similar to those of polyphosphates and also have an indefinite crystalline structure. 

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. 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

Utility systems for equipment and space heating and cooling frequently use heavy metal corrosion inhibitors in their heat transfer fluids. Chromate compounds are among the best corrosion inhibitors available. Nonchromate inhibitors that have proved to be feasible substitutes include polyphosphates, organophosphates, zinc, molybdates, and aromatic azoles. Some of these compounds have their own environmental impacts, however. Azoles, for instance, can be quite dangerous to human health. 

It is rare in actual applications that only one corrosion inhibitor is used. Synergistic blends of two or more inhibitors can take advantage of the strengths of each. Table 10-1 compares the performances of several of these blends to that of a chromate-zinc blend under different operating conditions. Note that while the chromate-zinc blend offers the best corrosion inhibition with no contaminants present and at high temperatures, other blends are close, and do not present the environmental problems that chromates do. With petroleum ether, hydrogen sulfide, or hexane contaminants present in the cooling water, the Polyphosphate-HEDP-Carboxylate blend performs the best, followed by Zinc-HEDP. 

Oxygen, carbon dioxide and various chemicals used to reduce scaling can cause corrosion. Corrosion control is provided largely by the use of inhibitors such as chromates, polyphosphates, silicates and alkalies. 

The first methods of cooling tower corrosion control involved adding several hundred parts per million of sodium chromate, as chromate is capable of excellent anodic corrosion control at these dosages. However, these early programs were both inefficient and expensive. The advent of synergized zinc chromate-polyphosphate treatments not only made corrosion control more... 

Polyphosphates are also used in cooling systems to attain sufficient corrosion control. Cooling towers are operated in a pH range of 6.0 to 7.5 to provide optimum stability for the polyphosphate. The feasibility of cooling tower operation at higher pH levels, in which the potential for corrosion is decreased, has increased the popularity of low-chromate programs. 

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

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). 

Fire-retardant chemicals used by the commercial wood-treating industry are limited almost exclusively to mono- and diammonium phosphate, ammonium sulfate, borax, boric acid, and zinc chloride (4,8). It is believed that some use is also made of the liquid ammonium polyphosphates (9). Some additives such as sodium dichromate as a corrosion inhibitor are also used. Aqueous fire-retardant treatment solutions are usually formulated from two or more of these chemicals to obtain the desired properties and cost advantages For leach-resistant type treatments, the literature shows that some or all of the following are used urea, melamine, dicyandiamide, phosphoric acid, and formaldehyde (10-12) ... 

Titanates are valuable in other paint applications. Corrosion-resistant coatings have been described for tinplate, steel, and aluminum (440—444). Incorporation of phosphoric acid or polyphosphates enhances the corrosion resistance. Because titanates promote hardening of epoxy resins, they are often used in epoxy-based paint (445). Silicones (polysiloxanes) are often cured by titanates. Pigments, eg, Ti02, Si02, Al O and Zr02, are frequently pretreated with titanates before incorporation into paints (441,446). In these applications, the Ti(OR)4 compounds are often mixed with Si(OR)4, Al(OR)3, Zr(OR)4, and other metal alkoxides (12). 

Corrosion inhibitors such as chromates, silicates, polyphosphates, nitrites, nitrates, borates and mercaptobenzothiazole have been used in corrosion inhibition of aluminum and its alloys.45... 

Organophosphorus and polyphosphate compounds also have been used as fire retardants. In one study, ammonium polyphosphate was used at loading levels of 96 kg/m to achieve a flame-spread index of 15 according to ASTM E 84 (J2). This treatment produced low smoke yields however, this treatment was corrosive to aluminum, slightly corrosive to mild steel, but not corrosive to brass (77). In a patent by Clermont (78), phosphorus pentoxide, dimethylformamide, and urea were used to produce fire-retardant paper or veneer. Other patents (79, 80) describe the reaction of ammonia with partial esters of polyphosphoric acid. All patents demonstrated some leach resistance of the phosphorus. 

Due to the high corrosivity of melts containing sodium polyphosphate, it is produced in vats lined with zirconium silicate bricks. When the reaction is complete, the melt is quenched on cooling rollers. In a process developed by Hoechst AG (FRG), the polymeric phosphates are obtained by phosphorus combustion with air in the presence of sodium hydroxide in a graphite-lined tower. The total energy for the process is provided by the combustion of the phosphorus. 

Phosphates Polyphosphates Orthophosphates Antisealants, corrosion control, sequestering agents... 

---