Recently there have been posts about problems with the MCAL groups. The problems seem to stem from corrosion and pitting of the cylinder wall. It seems to me that the design of the group makes it very susceptable to corrosion from water contaminated with chloride and other ions.

Machines like all three designs of the La Pavoni levers are not open to the atmosphere above the pistons. When contaminated water leaks past pistons in these machines, it remains above the piston, and is forced back into the boiler. The MCAL is different.

When traces of water leak past the MCAL pistons, it evaporates into the air through the small hole on the back of the group, leaving the salts dissolved in it on top of the piston and between the upper seal and the top of the piston. Over time more and more salts accumulate and the water there becomes laced with salts. The real villain amongst the salt ions is probably chloride. CuCl is a stable +1 valence copper compound. It serves as a catalyst for oxidation of more and more copper until deep pits form. With copper alloys (bronzes) the other metal can dissolve away leaving a porous surface where the CuCl can accumulate and where more and more oxidation can occur. Brass is really zinc bronze, and is highly subject to this process. This chloride induced corrosion is often called "bronze disease".

Almost all surface and well water contains chloride. When surface water flows into depressions in extreme desert areas, the water evaporates leaving behind salt. The image below of el salar de Atacama in northern Chile that I took a few weeks ago shows what happens over time in this situation. Everything on the image in front of the mountains in a distance is salt.



Salt can obviously accumulate above an MCAL piston just like this! Water analyses are available for most municipal water systems. It seems apparent that high chloride water is an invitation to corrosion.

Unfortunately the salts in the water that gets past the piston are trapped--Just like the salts in El Salar de Atacama!

It is apparent that one either needs to use very low chloride water or else frequently remove accumulated materials from above the piston in order to avoid this problem.

God I love having a Phd Chemist on this forum.

+1

+2...that knows his machines!

For reference, this is the first pitted cylinder:



It would be great to get images of the others...trouble shooting the MCaL pictured above has not been easy and it would be valuable to all MCaL rebuilders to know what to look for to avoid this problem.

Assuming that the pitting is the cause of the jerky lever movement, would it be possible to use silver solder to coat the pitted surface of the group and then refinish it back to the original shape? This would be a relatively inexpensive fix for a capable welder (based on past experience I would guess that the shop I take this kind of work to could probably get it done for less than $60).

It is interesting to note that the pitting is at the top of the cylinder--the part where salts can collect. I suspect that once a pit begins to form more water gets squeezed into the pit when the next shot is made, and then the water evaporates leaving more salt, and we have a vicious circle. Once damage is done more leakage occurs compounding the problem.

Does the pitted area feel a bit rough to the touch? I suspect there is general corrosion in addition to the pitting.

(I did not realise it, but there is a Wikipedia reference to this problem:

http://en.wikipedia.org/wiki/Bronze_disease

It shows the equations that I did not write out that demonstrate how the cuprous salts act as catalysts to bring about oxidation of the metallic copper until it can be totally destroyed.)

The Wikipedia article is correct that the chlorides must be completely removed from the part to stop the deterioration. If one could then coat the inside of the group with more metal by one means or the other one could then bore it smooth and solve the problem.

I suspect another solution would be to bore out the group and then machine an oversized piston. This should not take too much shop time. I suspect the stock piston seals would stretch to fit a slightly oversized one? This would result in a slightly larger shot volume, but that would not be bad.

Another possible solution is to bore out the group and insert some sort of metal sleeve. Both brass and stainless tubing is available with inside diameters of 38 and 41 millimeters.

All of us with these machines should clearly be vigilent. We should carefully inspect the walls of the group cylinder frequently, and take preventative measures should the slightest evidence of damage appear. We should always clean out the bore of the group and rinse both the walls and the piston with clean pure water at each servicing.

I wonder if heavily used machines might retain smooth walls, but have the bore of the cylinder wear unevenly from this type of corrosion?

Does a Brita waterfilter remove the chloride ??

I been told that leaving water standing for some time ,will lett some of the chloride evapurate ,
those into plants do this

about silversolder , its done with a flame , i dont know how the chroming would stand up to this
and it would be difficult . once molten the silver behaves like water ,

hard chroming the inside , and then hone it down again , only leaving the chrome in the pits (bore now oversize)
and as a finish yet another chroming ,againg followed by a honing ,repeat until all pits are filled up with chrome , and the inner surface is back to the original diameter ,,but hard chromed

but i think the price would be very high

a sleve ,,well the bore that enters the shower screen ,is very thin allready
making a sleve that would fit into that ,would have to be very very thin

one could make the sleve soo big than the original part that goes into the shower screen is completely machined away ,and the sleve would be the new end of the bore

Only organic chloro compounds will evaporate. The problem here is metal chlorides. They have essentially zero vapour pressure, NaCl boils at over 1400 C! Calcium chloride has an even higher boiling point, around two thousand degrees. Thus solutions containing salts will evaporate leaving more and more concentrated salt solutions. That is the problem not just here, but in many other systems. Espresso machines are heated, and that makes the problem much worse because chemical reactions occur much faster at higher temperature.

Reverse Osmosis systems can remove chloride ions, activated carbon filtration will not. There are also ion exchange systems to remove it.

The water analysis in my town shows 160 ppm chloride! It also shows hardness of 320 ppm! It is an example of water that should not be used for espresso! I even have to descale pots and pans after boiling vegetables in it!

***

So, It sounds like the answer is a reverse osmosis water filter? Does anyone have a line on one of these things for the espresso machines?

Sonny,

It probably makes more sense to install a system like one of these at the kitchen sink. That way you'll have the water for cooking and other needs.

I've used one of these combined with a whole-house activated carbon sediment filter and a whole-house water conditioner for 15 years. I've never had a scaling problem with my coffee machines, my All-Clad cookware or my copper pots. I have "city" water and change the filters annually.