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Date:         10 Oct 1994 05:33:07 GMT
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         scott@psy.uwa.edu.au (Scott Fisher)
Subject:      

Here's my article (written for Australian Jaguar owners, where anti-freeze is not required)...

ANTIFREEZE, CORROSION and ETHYLENE GLYCOL

OK Here is an article I wrote (for my local Jaguar Club magazine), it has been edited for international readin...

ETHYLENE GLYCOL: Ethylene glycol is a clear colourless, odourless liquid which has the property of both lowering the freezing point and raising the boiling point of water. It's ease of handling makes it a convenient and popular anti-freeze. In 1989 approximately 6,700,000 tons of ethylene glycol was manufactured, about 50% of this was used as anti-freeze. At about $2-8 per litre (tax-free, direct from a chemical company) you can see that glycol production for antifreeze purposes is big business. Another 40% is used in the fibre industry to manufacture polyester. This turns up mainly as clothing and recyclable plastic bottles. The final 10% of uses are as hydraulic fluid, solvent and a plasticizer. In other words ethylene glycol is used in cooling systems, paints, plastics, clothing, hydraulic systems, cosmetics and in some unfortunate instances, french wine.

In the context of the automotive cooling system ethylene glycol is not an anti-corrosive agent, it is in fact corrosive. To offset this fact manufacturers add anti-corrosives (inhibitors) to the glycol mix/preparation (usually 30% ethylene glycol + inhibitor formulation and 70% water). These preparations while in good condition perform well in their intended role in both minimising corrosion and preventing freezing of the coolant, however, over the life of the coolant the anti-corrosion properties of the inhibitors are depleted. Once they fall too low you may have a coolant that is acidic (containing among other things oxalic acid) which is corrosive to metals.

I should mention that plain water is also a corrosive if used alone in your cooling system, however, water is corrosive for different reasons than glycol, as will be discussed later. Let's consider the two properties of ethylene glycol that are of concern, toxicity and corrosiveness.

1: CORROSIVITY: The terms "glycol", "inhibitor" and "coolant" are often used interchangeably and hence confused, in the context of automotive cooling systems. There are important differences, a coolant has the task of transferring heat from the engine through some type of heat exchanger (radiator) to the air. Almost any liquid can serve as a coolant. However a good coolant should also minimise corrosion damage to the cooling system. Water and Glycol alone or in combination may both serve as coolants as they are liquids. An inhibitor has the role of minimising or preventing corrosion that is often caused by coolants. Both water and glycol alone or in combination are corrosive and need inhibitors added to them to prevent corrosion.

Water aids corrosion in three main ways; 1) bringing free oxygen in close contact with the metals so that corrosion (oxidation) can occur. 2) Water is conductive, once water has been flowing in your cooling system for some time it's conductivity will rise as it picks up metal ions and the water may serve to promote electrical activity which may erode metals by galvanic action. 3) Some of the metal ions in the water may also react directly with the metal surfaces.

Why is glycol corrosive? Apart from supporting the above three processes ethylene glycol has the added unfortunate property that it oxidises through several stages to oxalic acid. Most of the series of oxidation products to and including oxalic acid are directly corrosive to metals. Added to this, oxalic acid is highly toxic. Conditions conducive to breakdown of ethylene glycol into corrosive acids are present in an engine cooling system. These include heat (specifically engine hot-spots), galvanic or electrolytic couples of the several metals that are in the system, occasionally the presence of combustion gasses, water impurities and elastomer additives.

If you overheat (boil) glycol based coolants they must be replaced immediately as this accelerates the oxidization process of the glycol to acids. For the thrill-seekers among you the products of ethylene glycol oxidation by oxygen and subsequent reactions include: aldehydes, carboxylic acid, nitric acid , glycolic acid, glyoxylic acid, oxalic acid, formaldehyde and formic acid.

To combat the above acids and other corrosion activity, antioxidants and alkaline formulations are added to the glycol mix. These include many compounds which are used in cooling systems where antifreeze properties are not required and include primary, secondary and tertiary amines; organic and inorganic phosphates, silicates cresols and other phenolic substances; a wide variety of sulphur compounds; soaps; alkali metal salts; and borates.

These inhibitors slow down the corrosion process caused by the glycol and the water. They may coat the metal surfaces and prevent corrosion by passivation. Passivation is the process where the a protective film forms on the metal which prevents further contact with the solution. Unfortunately in all coolant preparations (with or without glycol) the inhibitor system (during engine operation) is being continuously depleted in the performance of these actions. For this reason, proper cooling system maintenance is critical.

One aspect of cooling system maintenance that we can all easily follow is to minimise "aeration" of your coolant. It is essential to keep your coolant from aerating as this accelerates the uptake of free oxygen from the atmosphere. As free oxygen is one of the essential ingredients for corrosion (by oxidation) the importance of minimising it's uptake is clear. To this end you should make sure all your hoses are in good condition and clamped tightly (especially the inlet side to the water pump - to help prevent cavitation) with high quality clamps. "Closed systems", where an expansion tank and recovery system closed to the atmosphere is used, also help in this regard.

With respect to the corrosion inhibition properties of glycol based coolants. In 1972 the US army set up a study to determine the rate at which the inhibitors in a glycol mix were being depleted. A prime function of these inhibitors is to provide alkalinity to neutralize the organic acids formed by the oxidization of the glycol. The test was conducted over a period of several years in all types of vehicles and showed that in a glycol mix 40% of vehicles needed attention. Of this 40%, 3% had antifreeze in their systems that was highly corrosive. In light of this it would seem that a "ideal" inhibition system would allow you to replenish these inhibitors without having to completely change the coolant.

2: TOXICITY: We all know not to drink coolant when we are stuck in the desert dying of thirst, here's why; The projected LD50 (The dose at which 50% of a experimental population die) in humans for ethylene glycol is around 1.4g/kg. In other words the minimum fatal dose for a 30% ethylene glycol 70% water mix is around 200 ml in an adult male and about 40 ml (one large mouth-full) for a 4 year old child.

Unfortunately some of the corrosion inhibitors added to glycol mixes such as sodium nitrite or sodium benzoate are even more toxic than the glycol. So, the truth of the matter is that upon drinking a glycol based coolant you are likely to die from nitrite poisoning well before the glycol has a chance to kill you. It has been pointed out that this is unfortunate because there is an antidote for glycol poisoning, alcohol (I kid you not), alcohol keeps enzymes busy that would otherwise break down the ethylene glycol into toxic oxalic acid...now we know why your average radiator mechanic has lasted so long.

Effects of ethylene glycol poisoning: If ingested or inhaled (avoid the vapour from a hot glycol-infested radiators) in sufficient quantities its immediate effect is on the central nervous system (CNS) where it can result in headaches, tremors, drowsiness and convulsions. The short term effects of high doses includes kidney damage leading to a reduced ability to urinate and accumulation of liquid on the lungs.

WHY USE ETHYLENE GLYCOL?: Given the above, the question must be raised why ethylene glycol is used in places where anti-freeze properties are not required? It has been suggested that one reason may lie in the internationalising of the vehicle industry.

The majority of vehicle manufacturers are international organizations with headquarters in the Northern hemisphere. As most vehicles are sold to markets where antifreeze is essential (North America, Japan, Europe), there is little economic incentive for manufacturers to re-design for non-freezing climates. Vehicle manufacturers often treat these places identically to their frozen cousins and supply their own antifreeze/coolant preparation and specify that only this product is the only product to be used if you want your warranty to stay intact.

Many "corrosion inhibitors" not only have glycol but they contain it in bizzare quantities...

SQ36 for example in Australia comes in a 500 ml bottle containing 25.7% ethylene glycol. As we know that ethylene glycol is not a corrosion inhibitor one may ask why SQ36 contains glycol at all? It certainly would not provide real anti-freeze protection (not that the SQ36 claims to have any anti-freeze properties) as 128 ml of ethylene glycol (25.7% of 500 ml) when it has been diluted into approximately 20 litres of cooling system, comes out at a concentration of about 1/2 a percent (0.64%). To enjoy any real anti-freeze benefits of ethylene glycol you would need at least 50 times that amount making a coolant concentration of at least 30% ethylene glycol. As for what is providing the corrosion inhibition in SQ36? well, that's what the other "mysterious" 74.3% of the bottle is for (no one can tell me what it actually contains).

Another reason many local manufacturers may recommend glycol based coolants could be that they want to avoid damage claims when Mr & Miss Sahara-Desert take their glycol-free car on a skiing holiday and find next morning that their engine resembles some kind of metal-ice sculpture. It is easier for companies to make sure all cars have anti-freeze protection and save the worry. However, given the case with SQ36 this supposition does not appear well supported.

In conclusion there is no reason why Australian vehicles must use glycol based coolants. In fact, when you have glycol in your coolant you add to your headaches (no pun intended) in that you then have to combat the acids that are produced when glycol breaks-down. Why not simply forget about glycol altogether? This is in fact what may people (including myself) have done with their cars. Fortunately, the alternative coolant preparations are often much cheaper than glycol and have an excellent performance record.

ALTERNATIVES:

The Paranoid Jaguar (Corrosion Inhibition: Part 2)

Welcome back, in the previous article we considered the few of the benefits and many evils of ethylene glycol (you can see I am being objective about this :-). I concluded that ethylene glycol is an anti-freeze agent not an anti-corrosive agent. Ethylene glycol is in fact corrosive as it breaks down into (among other things) oxalic acid. The anti-corrosion properties that come with glycol based mixes are due to added inhibitors not the glycol. Since we don't need anti-freeze protection in WA (and most of Australia for that matter) why not simply chuck the glycol and just use the inhibitors. This article discusses doing just that, focusing on one group of non-toxic, environmentally friendly inhibitors, the tannins.i

TANNIN & TANNIN PHOSPHATE:

First it should be pointed out that corrosion inhibition is somewhat of a "black-art". If something works, it works! and often little definite is known as to why this is so. I discovered this when I trundled over to the Department of Chemistry at UWA and located a group of friendly chemists huddling around a test-tube. I coughed politely, as to gain their attention, and announced "I'd like to know how tannin based corrosion inhibitors work?". I smiled inwardly knowing I had stated my question in a succinct and direct manner; a fitting question for the scientific mind to ponder.

There was a silence while they shuffled uncomfortably and then one of the chemis wrinkles in a stained laboratory coat he paused in deep thought and ran his right hand through the few remaining hairs on his head, that unlike the rest had found nowhere better to be. Finally he said "Tannin based corrosion inhibitors, how do they work? Hmmmmm...that's a tough one, an automobile cooling system is a complex environment with many dissimilar metals, some in electrical contact some not, there are cyclic thermal changes and localized temperature differences, varying thermal gradients and unpredictable pH variations. Galvanic and other electrochemical interactions are likewise unpredictable. Then we have the problem of impurities in the metals. Combustion products may or may not be present in the coolant, while the coolant itself well that's another problem...here read this PhD that will explain it!" He handed me a voluminous work titled "Some Electrochemical Studies of Corrosion Inhibition" and turned back to study the test-tube that was now foaming over onto the bench and eating a hole through the laminate

The following information is my understanding of some fairly complicated chemical and electrochemical interactions. First let us consider the nature of the inhibitor in question, the tannins.

The tannins used in inhibitors are usually extracted from tree barks and wood materials. Yes, we are talking cup-of-tea tannin here. As I have heard it the history of tannin based corrosion inhibitors extends back to a time when people would collect the unused tea from the pot and decant it directly into the radiator of their SS100. It was long noted how metal tea-pots could survive a life-time protected by a thin dark-brown coating of tannin, while the kettles used to heat the water for the tea would develop scale and corrode within a year or two.

As for their chemistry, tannins are predominantly large molecular structures. Hydrolysis (mixing with water) of these compounds results in the formation of polyhydroxy-phenolic compounds such as phragllol and gallic acid. Tannins are really a diverse collection of water soluble, high molecular weight poly-phenolic compounds [this has been included for the thrill-seekers among you].

It appears that although tannin based inhibitors do prevent corrosion (I have seen this with my own eyes under both experimental conditions and in my own and other peoples vehicles) the mechanisms by which they actually perform this task are still not fully understood. There are six somewhat related ways in which tannins are proposed to aid in corrosion inhibition, by providing:

1) pH Buffering 2) Scale reduction 3) Pit capping/ion scavenging (localised protective cap at corrosion sites) 4) General passivation (general formation of a protective layer) 5) Oxygen scavenging 6) Low conductivity

1. Tannin phosphates have been found to act as a buffer, ensuring that potentially destructive pH changes do not occur through the oxidation and reduction other products in the coolant mix.

A buffer solution is a mixture of compounds that will collectively maintain a near constant pH even when acid or base is added. So, if there is another component in the solution which decomposes to an acidic or basic product, the buffering action will tend to combat and minimise the pH change. This is important as the rate of corrosion and the passivity of the metal will depend on pH.

2. Tannin phosphates have been shown to react scale to form soluble complexes. Scale (like the stuff that builds up on kettle elements) is removed by the hydrolysis products of the tannin, yielding a di-chelate, which diffuses into the solution bulk (in other words, the scale is broken down and washes away). Scale is unwanted because it can reduce the ability for waste heat to make it from the engine to the coolant as it may act as an insulating layer. If scale becomes extremely severe it may even block the cooling passageways in the engine. Both conditions can result in both localised or general overheating and may promote rapid localised corrosion.

3. Tannin phosphates promote pit capping and ion sequestering.

Free metal ions are a product of corrosion processes, they cause a problem because these ions both increase conductivity of the coolant and can sometimes react further with the metals in your cooling system resulting in yet another form of corrosion. Tannin phosphate readily reacts with ions in the coolant and prevents these ions interacting with the metal surfaces. Pitting of the metal surfaces is minimised because the metal ions generated by a corrosion process will react with the phosphate to form an insoluble layer. This layer will tend to seal the corrosion pits (where ion concentrations are greatest) and prevent further attack. Pit capping is essential for if a pit (the usual mechanism by which corrosion causes component failure) is allowed to form they can rapidly grow in size.

4. Tannins promote metal preservation by passivation.

The tannins generally react with oxygen at the metal surface, and with metal ions formed by anodic reaction (described above), to form a hard protective film (similar to the inside of a tea pot). The reduction in anodic sites continues rapidly until the metal surface is fully protected. Once formed, only small concentrations of tannin and oxygen are required to maintain the integrity of this film. It should be noted that this film is very thin and does not impede heat transfer from the engine to the coolant like scale can.

5. Tannins may act as oxygen scavengers, reducing the bulk oxygen concentration

Tannins react with free oxygen in the coolant. In an engine cooling system, closed to the atmosphere, this reduces the concentration of free oxygen. Remembering that free oxygen is required for oxidation corrosion it is clear why you want as little oxygen in your coolant as possible.

6. The low conductivity of tannin and tannin-phosphates helps minimise galvanic corrosion.

Many commercial inhibitors contain large quantities of inorganic salts and as a result have a high conductivity. When inhibitors fail (ie when they are exhausted) the high conductivity can assist current flows between dissimilar metals that are often found in cooling systems (your engine starts to behave like a big battery), this can result in rapid galvanic corrosion. Tannin and tannin phosphate compounds in a solution of pure water (reverse osmosis, distiled or de-ionised) have a very low conductivity and even if they do become depleted their low conductivity helps minimise the rate of galvanic corrosion attack.

PRACTICALITIES:

In practice using tannin based inhibitors is easy and cheap. The cheapest way to buy them is in tablet form and you simply add a fixed dose per litre of water (soft, de-ionised, reverse-osmosis or distiled, avoiding tap water) initially and then an even smaller dose every 5000 km to maintain the tannin levels in the system. This ability to maintain the inhibitor levels makes the tannin based systems appealing as you can always be sure your inhibitor is working.

Problem: Joe average won't check the air in his tyres...how do we get him to add some tannin to his radiator occasionaly?

The economic aspects of tannin based inhibitors have not been lost on Transperth (local bus service for the city of Perth). They are presently in the throws of converting our bus fleet from a glycol based cooling system mix to a tannin based system. It is expected that cooling system maintenance costs will fall from approximately $48,000 to about $8000 a year. Throwing a calculator at my own situation, tannin works out at about 16 cents per litre to inhibit the system initially (I flush my cooling system once every 12 months) then 4 cents per litre every 5000 km (to maintain the correct inhibitor levels). Over a 12 month period for an XJ6 (19 litre cooling system) that travels 20,000 km/yr this would work out at $6/yr and that's about the only time you will ever see the word Jaguar and a single digit maintenance cost in the same sentence.

The final but not least important benefit that we should consider is the environmental one. Tannins are already present in the environment in large quantities as they occur naturally in plants, indeed this is where they are extracted from in the first place. The tannins when used in a cooling system break down into harmless compounds that stay suspended in your coolant and are safely disposed of. Glycol on the other hand is toxic and the less of it that is poured down the drains, onto driveways and into back yards the better.

Scott Fisher _--_|\ N Department of Psychology / \ W + E University of Western Australia. Perth [32S, 116E]--> *_.--._/ S Nedlands, 6009. PERTH, W.A. v Joy is a Jaguar XJ6 with a flat battery, a blown oil seal and an unsympathetic wife, 9km outside of a small remote town, 3:15am on a cold wet winters morning. -------------------------------------------------------------------------------


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