Val:

What can I say? Mechanical Emngineers are our lifeblood (no pun intended) in my business, and I use them all the time. But I have never had such a succinct and thoughtful response, on any subject, such as yours.

I am just glad that we have had the opportunity to validate many years of experience - use distilled water, but if you cannot, then make sure that your 50-50 coolant is specific for cooling systems of fine autos like MB.

Finally, if you use MB Coolant, you will have that little tell-tale checmical which will pinpoints any leaks. Cheap insurance, in my calculated judgment - and yes, I have done the calculations!!!!.......Andras

Well, Val hasn't posted very often on MercedesShop but when he does, LOOK OUT!
Being an engineer myself (electrical), I must admit, I had to drop everything and read his post.
I don't know what it is about engineers but, when we are in the presence of someone who really knows their stuff, it's a spiritual experience.
Thanks for the post!

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Bill Wood - Webmaster
MercedesShop.com, LLC

I'll probably be banned from this site but

Excellent analysis of pure deionized water. Unfortunately the question regarded a 50-50 solution of DI water and antifreeze. I mixed the same today and the pH was 10.78 which is alkaline compared to the acidic pure DI. In water treatment at my workplace the elevated pH is proportional to the non corrosive water characteristics.

Case in point is a water pump seal failure after 210,000 miles on a vehicle I purchased new.

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1991 350 SDL
114,000 miles

The alkalinity rule is generally a good one. Most ferrous metals are indeed passive in an alkaline anvironment. Aluminum is as well, to a degree. Aluminum is a metallurgical oddball. It's amphoteric. It will react in either an acidic or alkaline environment. The aluminum oxide is generally considered stable in a narrow pH band of 5-8. Ultrapure water (and usually distilled and Ordinary DI water) has a pH of 5-7. Your tap water probably does,too. The "acid-like" qualities of the ultrapure water come not from the [H]+ concentration, but from the polarity of the water molecule itself. By far the biggest disqualifier for tap water is the presence of halogen ions. Chlorine, Bromine, Iodine, any of the 'ines. After water, chloride is the most common element on earth. If you have city water, it likely treated with chlorine. Forgot to hose off the road salt after that last snow storm? Rut-roh Raggy! The most prevalent form of corrosion related failures for ferrous metals is chloride related. Add into the equation that corrosion mechanisms are approximately doubled for every 10 degrees C increase in temperature and tap water could spell disaster. Unlike ferrous metals, Aluminum is very resistant to most acids. Unfortunately, HCl isn't on of them. Chlorine in solution forms hypochlorous acid. In equilibrium state, some HCl exists as well. This basically means that there are a ton of free Cl- ions floating about. What is the most reactive metal in your engine? Aluminum. Only Zinc and Magnesium are more reactive. So...you have two elements which represent the opposite ends of the potential chart..and viola! say hello to Mr. Galvanic Cell, say goodbye to Mr. Anode, in this case your aluminum engine components. The lovely passive oxide layers that protect the aluminum and iron substrates also condemn them. The oxides are ceramics, ie insulators. And an insulator between two conductive materials is a? Anyone? Anyone? Capacitor! Oh, Baby can you feel it! The potential across the film layer serves as an electromotive force to help the Cl- ions out. Ouch. Oh, yeah, I almost forgot the combustion process introduces sulfate into the crankcase. It condenses as?...you guessed it... Sulfuric Acid! This is why your oil and coolant become more acidic over time. The additives that maintain the alkalinity of the coolant are there to do just that, maintain. The extent of the alkalinity is less important than its existence and for aluminum a high pH sucks. The goal therefore is to maintain a low to moderately alkaline solution. Then throw in some stuff to prevent metal ions in solution from forming metal salts which will precipitate out and gum up the cooling passages. Add some silicates for the Aluminum...and we've got the start of something that just might prevent your engine from becoming a big pile of red-brown rust with a few white blobs of aluminum oxide here and there. Most reputable brands of anti-freeze (aluminum compatible) are more than adequate to protect your engine for a long, long time. Assuming you don't muff it and forget to change it or top up with other than 50/50. Next time....the importance of good oral hygiene!


val

If the water is good enough to drink then just mix it to 50/50 and more important, change it at least every three years like the manual says.

I haven't read this much on this site since the great debate on oil change intervals.

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metricman, that would be too easy, hah hah hah

Seriously though it is nice to hear the theory on the subject. Greater understanding makes for better decision making in the future.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)

Job 20-010 of the MB M104 Engine Service Manual states:

"Use water which is clean and not too hard. Drinking water fequently, but not always, satisifes requirements. The content of dissovled substances in the water may lead to corrosion. If in doubt, use distilled water."

Unless this has advice has been superceeded, it is MB's official word.

Also, "de-ionized" water is "hungry" for something, so I wouldn't use it, when distilled water is cheap and plentiful.


:-) neil
1988 360TE AMG
1993 500E

BTW- here's a link of the actual text, scroll down to bottom, and read the last sentence: