Here are a few things I found online (so it must be factual, right?). Sorry for the long post:
"According to the U.S. Motors's Table of Corrosive Chemicals, hydrochloric and sulfuric acids are known to damage aluminum parts in motors, drives and gears. You can reduce hydrochloric acid's effects through dilution. Very weak solutions of sulfuric acid will not damage aluminum parts if you keep them at room temperature. Boric, carbonic, lactic and nitric acids usually do not cause significant damage to aluminum. Chromic acids cause moderate damage, depending on both the concentration of the acidic solution and the temperature."
Another source from the aircraft industry stated, "Weak acids such as citric acid, formic acid and sulphamic acid are suitable for use with metals such as aluminium, zinc, copper and nickel. Although these metals will be affected by these acids, this will be much less severe (than with) strong acids such as hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid."
The strength of a acid is a good indicator of how corrosive it will be on the head. A lower pH number means a stronger acid. A acidic pH of around 2.5 should be the lowest (strongest) to use for aluminum. Although that might sound like a strong acid, especially when hydrochloric acid has a pH of 2, but it really isn't. The pH scale is logarithmic, so a pH of 2 is ten times stronger than a pH of 3. Examples of acids around 2.5 pH include the ones mentioned in the aircraft reference; citric, formic, and sulphamic. Those were listed as causing low damage to aluminum. By comparison, the pH for the weak acids listed in the other reference (boric, carbonic, lactic, nitric) as causing "no damage" to aluminum, are between 3 and 5. It might be easy to say, let's play it safe and use one with a pH of 5. However recall that 'scale' is harder than aluminum, so it will take a stronger acid to eliminate it. Kind of a trade off; the stronger the acid the better it gets rid of the scale, but the more it eats the head also. And keep in mind, all of these are for pure forms of the acids. Dilution lowers the pH, so the more water you add the higher the pH number, and the weaker the acid. Therefore you could adjust the pH by the amount of (pure or distilled) water you mix the acid with (always add the acid to the water, and not the other way around). Temperature and length of time of exposure also have an effect, as discussed earlier. And consider the availability and cost of these acid choices. Some can be difficult to buy locally in the quantities needed, and very expensive.
What are some options for our application:
We would have to do some shopping to see what is readily available and affordable.
Oddly distilled vinegar is listed as having a pH in the upper 2's, so it might actually work...especially if not diluted and kept heated and left soaking long enough. But check the label, the stuff you buy at the grocery store might be diluted and a weaker pH? A temp of around 150 degrees F will help.
A solution of 16 oz oxalic acid per 2 gals water was discussed for cooling system flushes. I don't know what pH that would yield...a simple litmus paper test strip will find out. It could be mixed in a stronger concentration if needed. In my area it is not commonly found. But in areas with wet climates it is often sold as "wood bleach", used to clean the discoloration off wood decks. I bought some in powder form online for a moderate price, but hopefully it is less expensive if found locally.
Phosphoric acid works great for rust. That does not necessarily mean it is good for 'scale' though. Although it is stronger than the others, it is considered a mild acid so it might be a good option for a head (depending on dilution). If I recall, the stuff you can get easily at local hardware stores is around 20% solution, which should be about the right strength (need to test the pH). I have a few gals of it left over from cleaning a couple X1/9 fuel tanks. It was highly diluted in order to fill the tanks, so it will be very weak (it was really too weak for the fuel tank cleaning). Unfortunately it also got badly contaminated with that ugly tar crud we discussed with those tanks. But I might try filtering it and check its pH. I know it makes a great aluminum cleaner/polisher when wiped over the surface, so it will be interesting to see what happens when scaled aluminum is soaked in it.
According to the MSDS sheet, "CLR" is a mix of water with lactic acid with a little gluconic acid and lauramine oxide. The first two are very mild acids used in lots of industries, including medicine, food, cosmetics (makes a great facial peel). Apparently their role in CLR is similar to that of the acids discussed earlier, to break down the minerals in scale. The last one is a very common surfactant, so it's role is likely to clean things up as the acid goes to work. I think it would be too mild to be really effective for our use. The concentrated version (CLR Pro) isn't exactly cheap (I think close to $30 a gal), and it should definitely be used at full strength. So I'm not keen to even try it, I think there are better options.
I might still have some muriatic acid left over from other uses. It could be diluted a lot to get the pH up to the 2.5 range. However it gets into an area of extreme caution for use on aluminum, so I'm not certain it is appropriate.
After my holiday guests have left and I can get things back to normal, I'll experiment with some of this. Especially the ones I have easy access to (oxalic acid, vinegar, and phosphoric acid).