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Identifier

000461915

Title

The chemistry of silicate and phosphate species with applications in automotive coolant technologies

Alternative Title

Χημεία των πυριτικών και φωσφορικών ειδών με εφαρμογές σε τεχνολογίες ψυκτικών υγρών

Author

Σκορδαλού, Γεωργία Ι

Thesis advisor

Δημάδης, Κωνσταντίνος

Reviewer

Μήλιος, Κωνσταντίνος
Σπύρος, Απόστολος
Στούμπος, Κωνσταντίνος
Κουλουμπής, Αντώνιος
Άγγλος, Δημήτριος
Κατσαράκης, Νικόλαος

Abstract

The use of cooling fluid is imperative for heat transfer applications and has multiple functions, such as protection against overheating or freezing. However, due to the heat stress conditions that occur inside such systems, these fluids can also contribute to problematic issues in the cooling system, such as scaling/deposition on the heat transfer surfaces and corrosion of the metallic components. Therefore, a variety of chemical additives are added in the coolant or their concentrates aiming to protect the metal surfaces from corrosion, prevent precipitate scaling and deposition, and maintain the pH at an acceptable levels. Silicate- and phosphate-based additives are among the most common inorganic corrosion inhibitors, frequently used in combination with other organic or inorganic inhibitors. Unfortunately, if the processes are not carefully controlled, both silicate and phosphate can participate in scaling events, depleting the system from active corrosion inhibitor, and harming the heat transfer efficiency. Silicon dioxide plays a vital role in both the industrial and biological world. Thus, silicic acid polymerization and silica scale inhibition are intensively studied processes in aqueous media. The silica chemistry and behavior in non-aqueous or mixed aqueous/non-aqueous media was thoroughly investigated in this work, within the scope of engine coolant technology. Unwanted silica precipitation of silicate containing coolants, which are usually mixtures of water and glycols (most commonly monoethylene glycol, MEG), may obstruct coolant flow in car engines, leading to engine overheating and breakdown. It is therefore imperative to understand the formation mechanisms of these silicate precipitates in the operating conditions of a car engine. As only limited knowledge is available in the relevant literature on the fate of silicates in non-aqueous or mixed aqueous/non-aqueous media, compared to the aqueous systems, this presents an intriguing subject both from an academic and industrial point of view. The aim of our approach is the systematic study of the polycondensation chemistry of silicate and the variables (physical or chemical) that exert either positive or negative influence on this process. Next to silicate precipitation, uncontrolled phosphate deposition in the car engine may also hamper the heat transfer from engine to coolant, as a thermally insulating layer can be formed on hot surfaces. The precipitation and deposition of insoluble mineral precipitates on critical surfaces like heat exchangers due to the presence of dissolved ions is a common problem in the use of water for industrial processes such as cooling, boiler, desalination, and geothermal systems. The presence of metal ions, such as Al³⁺, Fe³⁺, Cu²⁺ and Ca²⁺ even at low concentrations can cause the precipitation of phosphate. The inverse solubility features of these precipitates allow immediate and irreversible precipitation under the heat stress conditions occurring inside a car engine. The metal ion sequestration approach was used in this work to control the formation of each metal phosphate scale separately. Finally, the performance of selected sequestrants was evaluated in the presence of different blends of metal ions.

Language

English, Greek

Subject

Corrosion inhibitor

Dissolution

Engine coolant

MEG

Metal ions

Polycondensation

Sequestration

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