The Chemicals of OC'n and Super Cooling - Coolants

By: Brian 9/16/02

Prologue by Joe-
Brian has come up with an article to complement the first Chems of OC'n and Super cooling - Insulation article with a Coolants article. I am impressed with this article and I think it will help the water coolers out there get a better understanding of the chemistry that actually happens in their cooling systems.  Now with that said lets get on with it!

With a water cooled computer, there is a tendency to fill your system with the cheapest coolant and then leave it alone.  After all, most of us are inherently lazy (ed. change that to be effort conscious or work sensitive please), and maintenance is not nearly as fun as carving out a new window or even soldering a motherboard to up the voltage. Beware!  Your computer needs love and attention just as much as any car! One caveat, however. This article covers the important details for sealed and nearly sealed water cooling loops. For "bong" cooling systems (evaporation), there are a different set of goals and requirements.

Coolant Types

Before we go into why you need maintenance, we should probably go into the different types of coolant. By definition, water cooling uses water as the coolant, but that is really a simplification.  Most of us add things to the water.  The main additives I have seen are methanol, isopropyl alcohol, ethylene glycol, soap, bleach, humidifier additives, and Water Wetter™ (or one of the Water Wetter™ clones).  More importantly, the reasons we add these chemicals are to:

1. Reduce acidic corrosion,
2. Reduce galvanic corrosion
3. Reduce biological growths
4. Increase heat movement

Acidic Corrosion

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.

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.

Galvanic Corrosion

Galvanic corrosion results from having more than one type of metal in your system. Technically, galvanic corrosion occurs when dissimilar metals are in physical contact, but even when the metals are separated in a cooling system there is a potential for problems.  Some water cooling systems stick with just one metal, but most of us use a combination of copper, aluminum, brass, and lead-based solder because of the heater cores we use.  When different metals exist, the water allows metal ions to be stripped from one side (with the higher potential energy) to the other. So, one side becomes pitted and the other side to get covered by the deposited metal ions.  Several methods exist to prevent galvanic corrosion: chelating agents, protective coatings, maintaining a zero voltage differentials and sacrificial anodes.

Using de-ionized water will not solve galvanic corrosion.  In fact, it may initially speed up the corrosion! De-ionized water may be non-conductive, but it is corrosive to metals. When it dissolves those metals, it immediately becomes conductive, as it now has an ample supply of ions. De-ionized water doesn't stay de-ionized.  Even CO₂ from air will dissolve into de-ionized water and (through a chain of reactions) result in a supply of negative and positive ions.

Antifreeze (either ethylene glycol or propylene glycol) is a chelating agent.  This means that they actually bind with metal ions in solution (before they deposit on the opposite surface), form complexes that are stable in water, and thus prevent an electric circuit from forming. With galvanic corrosion, the voltage that exists between dissimilar metals rips metal ions from one side and transports them to the other.  With a chelating agent like ethylene glycol, those ions are intercepted and trapped before they reach the other side and start to deposit.  However, because there is a set amount of ethylene glycol molecules in the coolant, they will all eventually chelate metals so you must change and refresh the coolant mixture.  Otherwise, when the

None of the other additives will affect galvanic corrosion.  However, you can add a sacrificial anode (zinc) to help. The zinc will dissolve instead of your cooling system. Some large-scale cooling systems actually use voltage monitoring and control to zero out the voltage differential. When there is no voltage differential, there is no galvanic corrosion.  This requires more sophisticated equipment than is practical to use in a PC cooling situation though.

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