I recently acquired a 1974 Cremina (there's a thread from a few weeks back detailing my heating element woes with this machine; Used Olympia Cremina trips GFCI outlet Happily those have been addressed and it's up and running).

I noticed while I was working on it that the boiler has some pinholes, which are currently blocked with scale. The boiler also has some crud that possibly leached out of the compromised element, and I would like to get rid of that stuff. Using vinegar or citric acid worries me as I don't want to descale the pinholes. I'm guessing my only option is a soft metal brush, some mild soap and a bit of elbow grease?

Wait, are you saying the boiler actually has tiny holes (that do not belong there) and they are being blocked by scale?

Hi Greg:

There's a well-known solution to this, Loctite 290 wicks into the pinholes and seals them. You don't need to keep them sealed with scale. I've done this myself and it works.

Using Loctite 290 to seal small cracks and pin holes in boilers

I concur with the Loctite/Bondloc 290 trick.

The stuff is intended as a thread sealer, but can be used in many crevice leak situations. I recently used it to seal all 10 thread bushes in my boilers lid flange and a pipe-to-boiler weld leak. With 10 stud holes and a crack all addressed with 290 the boiler will hold 1.3 bar dropping to 1.0 over 24hrs. The pressure of an espresso machine is relatively low and there's next to no risk of a 290 crevice fix being compromised.

On occasion in the past where:
- a pin hole leak has developed into something larger after descaling (like a brass boiler lid that's suffered dezincification) then I have drilled the hole through to say 4-5mm and put a stainless bolt-nut through with copper washers. Cover the bolt/nut/washer assembly with 290, wipe off the excess and give it 24hrs. Works well.
- a crack has developed in the flange of a stainless boiler then I have brazed them over with silver-solder. Stainless doesn't conduct heat very well so its possible with even just a small DIY propane torch to get the area red-hot. To be honest this trick requires a certain amount of experience with silver-soldering and also you need a silver-solder with a little nickel in it to prevent crevice corrosion in the weld (specific only to brazing stainless). If your boilers copper then you'll need a bigger propane torch and then there'll be other things to consider as well.

On the whole 290 covers a lot of situations. If it were me, I'd see the it through with the citric-acid descale. You need to uncover and address these things rather than rely on scale to hold it.

What's the boiler made of?

I often wondered if boilers that have been subjected to the kind of abuse that results in pin holes could be rescued by electroplating a very heavy layer of copper on them from the inside. I once was on Sabbatical leave working with a copper company, and I know from that that there is a very specific mixture that is utilised to make a smooth even coating of copper.

It also makes sense to avoid conditions that result in boiler corrosion to keep boilers healthy. Aluminium boilers really have an intrinsic problem. It is a poor material for boilers, but cheap.

Brass is really zinc bronze. Especially under acidic conditions the zinc has a tendency to dissolve out, often called dezincification. Brass seems to be most stable in water at pH 7.5 to 8 that contains some bicarbonate ion. (Zinc carbonate is insoluble and the zinc carbonate forms on the surface of the Zn.)

Copper is copper! It does NOT react with water, the electrode potentials are wrong. However, it does react with Oxygen, and we live on a planet with 21% oxygen. Normally copper gets a thin oxide coating that is fairly resistant to oxidation. However, during descaling operations this tends to get dissolved away, so more forms. When water contains chloride there is a vicious reaction involving both Cu1+ and Cu2+ that can create a situation similar to iron rusing, except involving copper chlorides. This happens badly with copper alloys as well. This is basically what "bronze disease" is.

Stainless steel is more resistant, but it does not tolerate being in contact with non stainless steel items for long periods when wet, and it also is in trouble with chloride.

Espresso (except for very dark roast) really seems to be best tasting when made with water that is near ideal from the standpoint of minimising corrosion!!!! To me the ideal is near 1 mMolar potassium bicarbonate, as I have often mentioned before. The brewed coffee itself contains a lot of calcium, magnesium, and (especially) potassium. It also contains chloride, and other anions. That is why we need to clean up immediately after making espresso!!!!

The ultimate bad thing is water with sulphate and massive carbonates and high calcium and chloride!!!

I've recently torn into an Oscar and found a boiler that looks like this after stripping away the insulation. I'm concerned about the indentation around some of the pinholes and wonder whether that's an indication of bronze disease. It might be an example where Loctite 290 isn't a sufficient fix.

Silver soldering or brass/bronze brazing is how you patch holes in non-code compliant pressure vessels.

Loctite is a Mil Spec regular strength thread locker, basically the Mil Spec extra cost version of 242.

The boiler I showed you for illustration may not be worth fixing unless you have good knowledge and equipment for brazing. I'm pricing a replacement boiler for the one above, or it may be better to part out the machine.

There are very sound technical reasons that ANSI/ASME frown on patching pressure vessels but to cut down on the workload of boiler inspectors there is a low pressure cutoff and espresso machines are not included due to low pressure and boiler capacity. If you calculate the volume of vapor released in a total boiler failure at full pressure and normal level you will boil lots of lobsters almost instantly. That boiler would probably fail an internal inspection as those holes start from the inside.

Perhaps we should be more concerned than we tend to be when boiler thickness from wall dissolution has decreased enough to produce pinholes. The possibility of catastrophic boiler failure under pressure could cause severe injury. One of the reasons I did not buy a plastic replacement for the cap of my La Pavoni when the threads were slightly damaged was also the possibility of catastrophic failure. Instead I made one from brass bar stock. (There is a steeper learning curve to making the things than I originally had thought especially when using equipment designed for small work.)

The advantage of using KHCO₃ in deionised or distilled water is that the stabilising copper oxide coat stays on the brass or copper and provides excellent corrosion protection. With brass, ZnCO₃ also forms on the surface and provides further protection. A second (flavour) advantage is that one can vary the concentration to match the characteristics of the coffee and the roast. (Dark roast has had most of the acidic compounds in it destroyed, and decomposition that occurs at higher temperatures produces alkaline compounds. Light roast tastes better with more KHCO₃ to reduce acidity.) Also using this results in an exact water composition so there are no surprises!