For the truly geeky...

There are quite a few different grades of Deionized or DI water. The one that is like an acid is ultrapure or electronics grade DI water. With a resistivity of 18Megohms and a ton of specifications as to total organic and dissolved solids, this stuff is indeed, very aggressive. This is due to its polar nature. Ultrapure water acts like an acid in quite a few environments. The piping for ultrapure has to be made from a special grade of Teflon Resin-PFA HP, either 340,350,440,or 450. Other materials generally considered to be corrosion resistant like PVDF (kynar) or ECTFE, are attacked aggressively by ultrapure water. Analysis of these materials after exposure yields results similar to exposure to highly concentrated sulfuric acid. 316 Stainless Steel suffers as well. It is generally considered unacceptable for use with ultrapure water due to the ability of ultrapure water to leach impurities out of the bulk matrix, thereby contaminating the water. Not good for semiconductor applications where limits for these ions is in the ppb. Under certain conditions, however, ultrapure water will reduce the passive oxidation layer and begin to corrode the bulk material. Localized boiling is one of these conditions. The mechanism here is the same for aluminum and mild steel as well. The boiling caused water hanmmer at the vapor nucleation sites. This breaks up the oxidation layer exposing the raw 316 stainless steel underneath. Normal corrosion proceeds as usual. Ultrapure water cannot exist outside of very carefully controlled conditions. Exposure to air quiclky drops its resistivity from 18 megohms to around 8 due to the saturation of CO2 in solution. I don't think anybody drinks this stuff or has it lying around. Normal Deionized or DI water has much lower organic and dissolved solids specs and measures out at around 1Megohm max. There are enough ions in solution here to tie up a lot of the polar water molecules, greatly reducing its aggressiveness. Aluminum has a wonderfully passive oxide layer that is impervious to just about everything. Under static conditions, that is. Moving fluids change everything. So does boiling and cavitation. Thus, susceptible components for corrosion would be water pumps, heads, radiators. Radiators are susceptible due to the highly tortured path that the coolant takes through it. This flow can strip the passive layer and expose the base aluminum, which corrodes quickly. There are a ton of corrosion mechanisms at work in any engine, (only the truly super-geeky need read the list at the bottom of the page...) The key is to minimize the effects by tying up those ions which cause the most damage. Chlorides kill. Phospates precipitate out and foul heat transfer surfaces. I'll save the full list for another episode of the dull-and-truly-geeky. The bottom line is that distilled or low grade deionized water will very likely reduce corrosion in your engines. Could've just said that first, I guess....

Val

Proceed only if thou art desperately in need of a life...
Forms of Corrosion
General/Uniform Corrosion:
Corrosive attack dominated by uniform thinning due to even regular loss of metal from the corrosion surface Atmospheric corrosion or degradation of material exposed to the air and its pollutants rather than immersed in a liquid
Galvanic corrosion that occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte
Stray-current caused by an externally induced electrical current
General biological corrosion of metals generally over the entire exposed surface in aqueous environments
High-temperature Oxidation corrosion by direct reaction of exposed metals to oxidizing agents at elevated temperatures
Other forms
Localized Corrosion:
all or most of the metal loss occurs at discrete areas
Filiform occurs on metallic surfaces coated with thin organic film, typically .1 mm thick, characterized by the appearance of fine filaments in semi-random directions from one or more sources
Crevice corrosion in narrow openings or spaces in metal to metal or non-metal to metal component sites
Pitting extremely localized corrosion marked by the development of pits
Localized microbiological cases where biological organisms are the sole cause or an accelerating factor in the localized corrosion
Metallurgically influenced corrosion: form of attack where metallury plays a significant role
Intergranular occurs when the corrosion rate of the grain boundary areas of an alloy exceeds that of the grain interiors
Dealloying a form of corrosion characterized by the preferential removal of one constituent of an alloy leaving behind an altered residual structure
Mechanically assisted degradation:
form of attack where velocity, abrasion, hydrodynamics etc. play a major role
Erosion removal of surface material by the action of numerous individual impacts of solid or liquid particles
Fretting combined wear and corrosion between contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations
Cavitation & Water drop impingement occurs on a metal surface in contact with a liquid, pressure differentials generate gas or vapor bubbles which upon encountering high-pressure zones, collapse and cause explosive shocks to the surface
Fatigue occurs in metals as a result of the combined action of a cyclic stress and a corrosive environment
Environmentally induced cracking:
forms of cracking that are produced in the presence of stress
Stress cracking service failures in engineering materials that occur by slow environmentally induced crack propagation
Hydrogen damage results from the combined action of hydrogen and residual or tensile stress