[bigben2k]

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Coolant Chemistry
(a discussion from the Triumph Car list)
....The major component of coolant is water which is a great heat transfer fluid.
....The next major component is the base of the concentrated coolant, as purchased at the store.
....There are two major base chemicals that commonly are used. Ethylene glycol (EG, generally green) is the most common base. Less common is propylene glycol (PG, generally orange or reddish), which has been used for years in Switzerland owing to poison laws and is a recent entry in the U.S. market.
....One function of the coolant is to lower the freezing point of the mixture in the system. Another function of the coolant is to raise the viscosity (thickness) of the coolant mixture. Higher viscosity mixtures will reduce cavitation at the water pump. PG (orange stuff) and EG (green stuff) will both raise the coolant viscosity, methanol will not.
....A variety of different chemicals are added to coolants to inhibit corrosion. Called inhibitors, the function of these additives may be to form a stable, protective film on the metal surface or to alter the solution properties of the coolant. Really, the precise mechanism of protection of some additives is not known by anyone who is willing to publish their results.
....Common corrosion inhibitors include: sodium phosphate, sodium nitrate, sodium tolytriazole, sodium molybdate, sodium borate, sodium benzoate and sodium silicate. These are all sodium salts. Actually, only the right hand group of these salts is chemically the inhibitor, i.e., benzoate or silicate. These salts dissociates in water, in other words, they separate into sodium, with a positive charge, and the inhibitor, with a negative charge. The sodium salts are used because of the high solubility (readily splits off and stays in solution) of sodium. There would never be sodium deposits in your engine.
....Different inhibitors protect different metals.
....Aluminum heat-transfer corrosion is best inhibited by silicate and most poorly by phosphate and borate.
....Copper is best inhibited by molybdate and most poorly by benzoate; high-lead solder best by molybdate and phosphate and most poorly by nitrate, silicate and benzoate; low-lead solder best by tolytriazole and molybdate and most poorly by nitrate and silicate;
....Mild steel best by molybdate, phosphate and nitrite and most poorly by tolytriazole and benzoate;
....Gray cast iron best by nitrate and most poorly by benzoate, tolyriazole, and borate; and cast aluminum best by silicate and most poorly by phosphate and molybdate.
....Phosphate is the most ubiquitous and most controversial inhibitor. It is a well known inhibitor of ferrous metal corrosion, hence trisodium phosphate is used to clean off sheet metal. American car manufacturers have specified phosphate in coolants because it is highly effective at preventing cavitation. Europeans specify non-phosphate coolants because phosphates have a propensity to precipitate in hard water. Also, phosphates have a negative effect on the corrosion rate of aluminum. This beneficial effects peak at concentrations of about 3 gm/liter and decreases at both lower and higher concentrations. Typical concentrations in coolants range from 0 to 8 g/l.
....Nitrate is included in virtually all coolant formulations because of its efficacy in preventing aluminum radiator pitting, with presumably no negative side effects for other metals. A typical concentration is 2 g/l.
....Tolytriazole is similarly included in virtually all formulations owing to its effectiveness in preventing cupreous metal corrosion. A typical concentration is 1 g/l.
....Molybdate is a broadly beneficial additive. It prevents corrosion in many metals and acts synergistically with phosphates and silicates to prevent corrosion in others. Molybdate also seems to prevent cavitation damage; it is usually selected to perform this function in non-phosphate coolants. Typical molybdate concentrations are 2 to 3 g/l.
....Borate is the most commonly used buffer for coolant systems. Off the shelf, American coolants tend to have a pH of 10 or higher (this is an alkaline pH), while European coolants tend to have a pH of 7 to 8.5 (which is near neutral to very slightly alkaline). In service, the pH of American coolants often drops to 8. Unfortunately, borate tends to have a direct and negative effect on aluminum corrosion. In spite of this, the importance of keeping coolants well buffered is great enough to keep borate in coolant formulations. A typical concentration is 4 g/l.
....Benzoate (and Nitrite, which is not mentioned here) are part of the British Standards Institute's [BSI] Corrosion Inhibited Ethanediol Anti-freeze formulation. Benzoate is more common in European coolants than American coolants and is described as a ferrous metals corrosion inhibitor.
....Finally there are silicates, which appear to be ultra necessary in the protection for aluminum. The problem is that silicates are not indefinitely stable in solution. While other additives can be used to stabilize silicates somewhat. The lifespan of coolants could be considered by the presence of an adequate silicate concentration. 2 g/l is an effective concentration of silicate.
....Bringing all of these inhibitors together, a combination of benzoate, molybdate, borate, nitrate, tolytriazole and silicate is a good additive package that doesn't use phosphates. The non-silicate part of the package is fairly effective in preventing aluminum corrosion, and makes a good back-up system in for an aluminum block engine, should the silicates become depleted.
....Other additives appear in coolants as well. These agents are typically used to stabilize the inhibitors or the metal salts which are corrosion by-products. This type of additive is called a sequestrants. Another required additive is the colorant.