Acidic corrosion happens when the coolant drops the pH level below 7.0, which happens when there are more positively charged hydrogen ions than negatively charged hydroxide (OH-) ions. Technically, only H30+ (called hydronium) ions exist, because free hydrogen ions quickly combine with H20 molecules to form H30+ ions. The result of this excess is that metal surfaces are corroded with pits as the positive ions rip up and then combine with atoms from metallic and other surfaces.



The pH level is actually defined in an interesting manner. It is defined as the negative log10 of the moles of H30+ ions per liter of water. Water naturally ionizes like this: H20 -> H30+ + OH- about once for every 107 molecules. So, in one mole of pure water, there is about 10-7 moles of H30+. The more H30+ ions you have, the lower the pH level becomes. The pH scale of 0...14 relies on the fact that the product of H30+ and OH- concentrations is always 10-14. If you are interested in a bit more detail, try this page on pH Scale. There are some details here that a doctorate in chemistry (such as our staff member pHaestus) could share, but, thankfully, those details are mostly irrelevant to our discussion.

Moles of Hydronium Ions in One Mole of Water
pH Level

100 Moles = 1.0
0 - Perfect Acid

10-1 Moles = 0.1
1 - Very Acidic

10-2 Moles = 0.01
2 - Lemon juice

10-4 Moles = 0.0001
4 - Wine

10-7 Moles = 0.0000001
7 - Neutral (neither acidic or basic)

10-10 Moles = 0.0000000001
10 - Detergent for Washing Clothes

10-12.5 Moles = 0.00000000000032
12.5 - Household Bleach

10-14 Moles = 0.00000000000001
14 - Perfect Base


In theory, you will see corrosion start when the pH level drops below 7.0. However, copper and aluminum will typically form a thin hydroxide coating that is stable until pH levels drop below 4 or 5. To complicate matters, however, you can go too far in the other direction too. Making the coolant too basic (called an alkaline pH level) can cause corrosion too. A strong base can strip the protective aluminum hydroxide layer that forms on an aluminum surface nearly as easily as a strong acid. And in situations where metal oxide coatings do not form, a strong base can actually be even more corrosive to elemental solid than an acid.

Coolants typically perform best when the pH level is from 9.0 to 11.0. As the coolant ages, it becomes more acidic and starts attacking the metal surfaces, so manufacturers add buffering chemicals. These buffering chemicals (usually borates and phosphates) absorb the acidity as it forms, preserving the safety of the coolant. The buffering capacity of a solution is known as alkalinity. You do not need buffering chemicals if you change your coolant before it becomes too acidic, and many cheaper brands of antifreeze-style coolant skip the buffering chemicals to save money.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".