Synergistic corrosion protection

For larger, more complex LPHW systems and for LP steam heating systems, silicates are seldom used alone but are formulated with nitrite or molybdate inhibitors to provide synergistic corrosion protection. 

The synergistic corrosion protection reported by Ho, coupled with research indicating that larger size, increased molecular weight and asymmetry of organic inhibitors often improve their effectiveness, " led Markley et al. to investigate other rare earth organophosphate compounds, namely cerium diphenyl phospliate (Ce(dpp)j) and mischmetal diphenyl phosphate (Mm(dpp)3). 

Cooling water systems are one of the most important fields where synergistic corrosion inhibitor blends are applied for temporary corrosion protection. Several combinations have been developed which are capable of achieving increased protection under various plant operating conditions. Some of the most important synergistic mixtures are chromate-zinc, phosphonate-zinc, polyphosphate-zinc, polyphosphate-chromate, polyphosphate-phosphonate, and ni-trite-phosphonate. 

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. 

Ochoa, N., Moran, R, and Pebere, N., The synergistic effect between phosphonocarboxylic acid salts and fatty amines for the corrosion protection of a carbon steel. Journal of Applied Electrochemistry, 2004. 34(5) 487-493. 

Although molybdate compounds have been advocated for corrosion inhibition purposes they have not been used as inhibitors in concrete practice. Experiments to ascertain the synergistic effect of a calcium-nitrite- sodium-molybdate combination (4.5 parts to 1 part) on corrosion of steel in concrete [64] showed that at the inhibitor-chloride ratio of 1 11 the combined admixture protected steel from corrosion and that it was more effective than when calcium nitrite was used alone. 

Molybdate s low toxicity to fish and other aquatic life has helped to gain recognition in recent years as a corrosion inhibitor. Molybdate forms its protective film by adsorption on metal surfaces. When chloride and sulfate anions are present in the cooling water environment, they compete for adsorption, and high concentrations of molybdate are needed for effective passivation of the metal surfaces. In order to be able to reduce the molybdate concentration for cost-effective levels, synergistic blends are made up that include other inhibitors such as phosphorates and zinc. 

Molybdate is never used on its own in open cooling systems, as the dose rate for protection of steel is on the order of 75 to 150 ppm Mo. It tends to be formulated primarily with various organic corrosion inhibitors in alkaline solution, and certain synergistic ratios permit the molybdenum content to be reduced so that the reserve of Mo in the cooling water is only 3 to 10 ppm. 

Nitrite is usually formulated with TTA (yellow metal protection) and borax (to provide a buffered pH of 8.0 or greater). Sometimes nitrite is formulated with benzoate, an anodic inhibitor effective at preventing waterline attack, or silicates or molybdates, to provide synergistic inhibition and to reduce corrosion risks with softer waters and multimetal systems. 

In the case of codeposition of Co and Mo on Au and Fe electrodes, very strong synergistic effects of coplating of these two elements were found (168), with Tafel slopes of around 60 mV in the low current-density range being observed. With Ni and Mo coplating on Ni or Fe, slopes of 23-28 mV are observed (75). The Co plus Mo electrodeposits remain cathodically protected during the Hj evolution but on open circuit evolve H2 rapidly-from active Mo centers, as in decomposition of Raney nickels. However, this may be due as much to desorption of codeposited H as to evolution of H2 by corrosion of the base metal. Mo. 

Chem. Descrip. Basic calcium zinc molybdate-based Uses Corrosion inhibitor for protective coatings Eeatures Economical stable synergistic with zinc phosphate Properties Wh. fine powd. < 0.1% 325 mesh residue 3.2 p mean particle size nodular shape sp.gr. 3.0 oil absorp. 17 g/100 g < 0.5% moisture... 

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