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Home > Espresso Machines > Boiler and Thermal System Materials: A Mania Column

Boiler and Thermal System Materials


Boiler types, sizes, and constitutions are hot topics according to our customer support reps. And there is good reason for this. A boiler, or other heating system like a thermal block, in conjunction with the pump are the most important active organs of your machine.  So what are the differences? First let me explain what the various types of boiler materials are.
  • Aluminum Boilers (like those found in most Gaggia and in some Espressione), are usually less expensive and normally have good corrosion resistance.

  • Stainless Steel Boilers (like those found in Saeco, Nespresso, Capresso, and Briel) have good heat retention properties and are resistant to corrosion.

  • Copper / Brass (brass is an alloy made up of copper and zinc) boilers (like those found in Rancilio, Pasquini, and FrancisFrancis!), being the most noble of the three metals, are the best of all at resisting corrosion. That is why most potable water runs through copper lines.  Their heat retention properties are good.

So which is the best type of metal for a boiler? The answer depends on many factors, not just the material per se. It seems that many folks are scared off of aluminum boiler machines, in particular. The thought is that this type of boiler is very susceptible to water corrosion. Is this thought well-founded, or are other metals just as susceptible to corrosion? In order to understand corrosion susceptibility, you need to recognize the necessary conditions for water corrosion: oxygen, water, metal, and some sort of catalyst.
The catalyst is the important bit here. It breaks the shield that is the oxide layer between the metal and the ambient water. Under normal conditions, pure water sitting in aluminum, brass, or stainless steel reservoirs may not corrode the reservoir for a very long time. However, if you introduce certain particles or chemicals, the protective layer that exists between the water and the surface of the boiler could become abraded in some area. This local abrasion will eventually give way to a general abrasion, and you wind up with a pitted out boiler.
So, just filter out the large particles? Unfortunately, it is not that easy. Some of the most mischievous bodies in your water are microscopic. Chloride, the second most common chemical found in nature (outside of water), is often used to treat water so that it is potable. Chloride can wreak havoc on an espresso maker boiler by plaguing it with local corrosion, as described above. So either filter your water or clean and descale your machine regularly! And preferably both.
Aside from direct local abrasion by a foreign body, temperature can also affect corrosive activity. Obviously, we want the boiler to get very hot, so there is not much we can do about this one. But if your machine was not designed to remain on for long periods of time, TURN IT OFF! You may be adding more life to your boiler.
The final cause of corrosion I will discuss is galvanic corrosion. In this type of corrosion, your espresso machine becomes a battery. This can happen when there are two distinct metals which contact one and the same body of water at the same time. But not just any water, water with electrolytes. One of the metals acts as the positive end of the battery, the other as the negative end. And that positive end begins corroding rapidly, much faster than it normally would, while the negative end’s corrosion rate slows considerably. So no worries so long as your water lacks electrolytes, right? Right. But it doesn’t, or at least most tap water isn’t lacking electrolytes. Two common Electrolytes are chloride, as stated above the most common chemical used to treat drinking water, and calcium, America’s number one contributor to water hardness.
Not all dissimilar metals will be susceptible to galvanic corrosion. So long as the metals have similar nobility, or atomic stability, there will be no problem. Since most of us don’t have an ordered list of noble metals, here are some basic combinations to avoid: Aluminum/Chrome, Aluminum/Brass, Aluminum/Copper, Stainless Steels/Copper, Stainless Steels/Brass, and Aluminum/Stainless Steel.
In conclusion, there are three common metals used in espresso machine boiler construction. Some folks are worried about aluminum boilers, but with proper maintenance and preventative measures (like using filtered water and/or frequently descaling the machine), galvanic and abrasive corrosion can be prevented or significantly slowed.