It's still not clear to me how/why an inhibitor added to acid protects metal without also protecting scale. For example:

http://www.chemtexlimited.com/sulphamic ... caler.html

But I'm not a chemist. My simplistic conclusion - if I use an acid, I'll use one with inhibitors in it.

RO water keeps my boiler clean and even cleans it some.

Avoid sodium hydroxide if it might touch aluminum.

Using sodium hydroxide to remove calcium sulphate deposits is a metathesis reaction. There are problems with it. I always have regarded concentrated NaOH as extremely hazardous. We try to avoid using concentrated NaOH in laboratory courses. I think it a greater hazard than concentrated sulphuric acid or concentrated HCl. I put it up there with nitric acid. One little appreciated chemical hazards from it is the rapid reaction of aluminium with it to produce hydrogen and sodium aluminate. The reaction has been used to produce hydrogen!

Strong carbonate solutions have been used too, this produces a calcium carbonate precipitate that may be loose so it can be washed out, and then residues removed with dilute acids.

Inhibitors tend to produce an ultra thin layer on metallic surfaces that is ultra thin but strongly protective.

So in theory descaling should occur anytime water is added to the boiler that is softer than the scale? If so this is good news for me because we have very soft water here. I've always assumed that a thin layer of scale would prevent metal leaching from the boiler to the brew water, so I never descale.

My old Sama came with a lot of scale but I just rinsed it well rather than do an aggressive descaling. If I want to descale it a little more just filling it with soft water and leaving it to descale should do the trick?

Scale will dissolve if the water be pure enough. (Solubility product rules.)

The thing that protects copper and many other metals is very thin film of metal oxide rather than calcium carbonate or sulphate scale. Acidic descaling materials tend to remove this, and it has to reform again. For copper the metal oxide coat is most stable at pH around 7.5. Copper in boilers without scale tends to look dark from the colour of copper oxide.

Why doesn't this thread come to the top of the list when we post in it?

So how pure does the water need to be? Is soft tap water ok, or should I use distilled for a while?

Doesn't scale also protect against metal leaching? The Sama boilers are brass.

PC

The carbonate and sulphate scales tend to be porous. The copper oxide and zinc compounds are not. Carbonate scales are also alkaline, especially if they be heated to the point of decomposition (like when they are on heating elements.) Acidic descaling agents can remove some of the protecting copper oxide. It is the oxygen, not the water that reacts with copper. (Including the copper in brass.) Again, the copper oxides are most stable at a pH of about 7.5.