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