The carboxylate corrosion inhibitors form a thin coating on metal (or
at least the parts that are beginning to corrode) and restrict access
to the metal access by ions that promote corrosion. The sites I saw
were vague on how the carboxylate attached to the metal. Possibly the
carboxylate ends attach to (partly oxidized ?) metal and the non-polar
/ hydrophobic / "fatty" ends face out toward the antifreeze solution
and keep ions and water(?) away from the metal; however, I located no
evidence for the exact nature of the attachment to the metal.
* Statements regarding forming a thin layer:
"The orange-colored Dex-Cool coolant that GM started using in some
1995 cars is a non-silicate formula. After 3,000 miles, Dex-Cool has
coated the internal passages with an organic acid. This passivating
coating will last for five years or 100,000 miles. If you replace
parts such as a water pump or radiator, the coolant must recoat the
new part. That's why, in normal maintenance or repair procedures, it
is not a good idea to replace Dex-Cool with ordinary silicate
coolants. In emergency situations, Dex-Cool can be mixed with standard
green coolants. However, mixing with green coolants will reduce the
life of Dex-Cool to that of ordinary coolant and you must inform the
customer of the shortened life. Why do it when you can sell the
customer on the benefits of Dex-Cool?"
http://www.asashop.org/autoinc/october/techtips.htm
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"By measuring coupons? mid-section temperatures in the Heat Transfer
test, the ?single layer protection? mechanism of Shell ELC Corrosion
Inhibitor technology is demonstrated: unlike conventional inhibitors,
the carboxylic acids form only a very thin protective layer. This
results in improved heat transfer. "
The above statement is for a Shell corrosion inhibitor that is
carboxylate-based; but they don't say what carboxylate is used.
"Shell ELC Corrosion Inhibitor is a heavy duty inhibitor concentrate
based on a unique extended life carboxylate inhibitor system
formulated to provide corrosion protection in aqueous solutions.
Mixed with the appropriate amount of water, this product provides
maximum protection of all cooling system and heat transfer systems
metals, including brass, copper, solder, steel, cast iron and
aluminum."
http://www.shell-lubricants.com/products/pdf/ELCCorrosionInhibitor.pdf
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"As you may have already discovered, antifreeze technology has become
significantly more complex since General Motors/Texaco introduced
DexcoolŽ to the market in the mid 1990?s. Dexcool is a
carboxylate-salt-based, extended-life antifreeze; Dexcool gave this
technology, popularly called organic acid technology (OAT), a position
of prominence in the U.S. market. Prior to the mid 1990?s, the U.S.
market had been dominated by the so-called conventional antifreeze
formulations, which are based on inorganic salts (sodium borate,
sodium nitrate, sodium phosphate, sodium molybdate, sodium nitrite,
etc.). Currently, conventional antifreeze formulations still have the
largest part of the market, but OAT-type extended-life formulations
have a major and growing share.
These two broad categories of antifreeze each have advantages and
disadvantages relative to each other, and they have many similarities
as well as differences. OAT, carboxylate-based antifreezes are touted
as having longer potential service lives than conventional, inorganic
salt inhibited antifreezes. This is based on the fact that carboxylate
salts are not chemically consumed as they perform their function of
inhibiting corrosion. Carboxylates protect metals from corrosion by
forming a thin, molecular coating on them and thereby insulating them
from any corrosive impurities that may form in the glycol/water base
of the antifreeze. As long as the antifreeze volume in the cooling
system is kept sufficiently high to coat all the metallic surfaces,
and coolant is not lost due to leakage nor diluted by top-off with
water or conventional antifreeze, it continues to be present in
sufficient concentrations to function properly."
http://www.webacorp.com/technicalresources-artic3.html
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The following quote presents a slightly different view of how carboxylates act:
"Carboxylates provide corrosion protection by interacting chemically
at the metallic corrosion sites, rather than forming a layer of
inhibitors that covers the total surface."
A bit later, this site seems to say the carboxylates act only at
certain sites on the metal:
"A third type of coolant is the extended-life ?full? carboxylate-based
variety, which was developed to be superior in performance as well as
meeting global compositional requirements (no silicates to meet the
Japanese specifications and no phosphates to meet the European
requirements). This technology is also known as Organic Additive
Technology (OAT). Since carboxylates provide corrosion protection by
chemically interacting with the metal surfaces only where they are
needed, the improved protection mechanism provides a host of
advantages including extended life protection, unsurpassed high
temperature aluminum protection, and heat transfer advantages."
The statements by this site seem to be a minority opinion. The site is
http://64.233.187.104/search?q=cache:a4pktCHb4woJ:noln.net/columns/12-04/chevrontexacodec04.html+dexcool+corrosion+inhibitor&hl=en&client=safari
-------
Somewhat supporting the above view is the following statement, where
"passivity" generally indicates a thin protective layer of oxidized
material:
"Recent results showed that oxygen is essential to develop the
inhibitive characteristics
of several compounds. For example, sebacate and octanoate require
significant oxygen levels, whereas
2-ethyl hexanoate and decanoate maintain passivity even at very low
oxygen levels. It has been also
reported that some compounds require much lower critical current
densities for passivation than others, such as in the case of
decanoate compared with octanoate. "
http://64.233.187.104/search?q=cache:DG2WsFDuatsJ:www.nace.org/nacestore/assets/paperabstracts/2004/04412.pdf+%22ethyl+hexanoate%22+corrosion&hl=en&client=safari
--------------------------------------------------------
* Composition of Dex-Cool:
"Equilon (owned by TexacoŽ and ShellŽ) markets a European coolant
technology (OAT) that consists of ethylene glycol inhibited with a
combination of sebacic acid and 2-ethylhexanoic acid supplemented with
tolyltriazole. It was originally called "Long Life", but a lawsuit
brought by Warren Oil, who markets a fully formulated coolant under
the brand name "LongLifeŽ) forced the retraction of that term form the
DEXCOOL, Texaco and CaterpillarŽ packaging. The combination of a mono
and dibasic carboxylic acids permitted Texaco to obtain a patent on
the specific combination. Other companies have obtained similar
patents, by varying the mixture somewhat and by using similar, but not
exactly the same, chemistry."
http://www.penray.com/bulletins/dexcool.htm
------
"What's In DexCool?
You should know that the term "orange coolant" has come to mean a
DexCool-approved brand but that doesn't mean it's really true. If you
make this assumption, you'd be wrong. It's what Fords warning labels
could be interpreted to refer to, so that label doesn't serve to
clarify things (certainly not when a Ford system contains a very
different-non-DexCool, but orange-dyed-coolant). The discontinued
Mercury Cougar was an exception; it did contain an orange coolant
similar to DexCool.
The "DexCool" designation means the coolant passes General Motors
performance testing. Although DexCool is not a specific formula, all
three brands that have the label (Texaco Havoline, Prestone Extended
Life and Zerex Extended Life) are somewhat similar. In particular,
they're OAT coolants, but the similarities go beyond that basic
description.
All DexCool-approved coolants to date use two organic acid
rust/corrosion inhibitors, one called sebacate, the other called 2-EHA
(which stands for 2-ethylhexanoic acid). These organic acids are very
stable and last a long time, although they take thousands of miles to
become fully effective in protecting coolant passages."
http://www.findarticles.com/p/articles/mi_qa3828/is_200408/ai_n9453107#continue |
