The M6 (and later M4-P) top-plate made from zinc was an economy-move by Midland and later by Wetzlar/Solms. By using centrifugal casting processes, they could make a lightweight and strong top-plate that needed very little "post" casting machining and polishing. Zinc also takes fairly well (note the "fairly" wording) to black chrome and chrome plating. I saw some of the first zinc top-plates and also some zinc cast baseplates in 1983 at Midland. The engineer that showed them to me exclaimed, "Ah, Zinc iz wunderbar!" - There were problems with the early tops for the R-series, the zinc would react with the chrome and start bubbling and they changed the composition of the zinc and also changed the process for plating.
The old process of making brass top-plates required a very complex set of dies, 6 of them, and each costing more than a top of the line Mercedes (their price-guide!). The process they used, and still use, is a vacuum draw die stamping. The brass is heated to a high temperature and thus made more 'malleable" and then sucked into the appropriate die and shaped, then moved down the line to the next die and the process repeated until the top-plate was finished. Time consuming and costly.
The zinc casting is an automated process, molten zinc alloy is poured into the die, spun at high speed and the centrifugal force exerted removes bubbles and uneven spots. It is not a bad system and I looked into it when I started making the Rapidwinder housing. The cost of the die is very expensive and the process is not cheap either. To give you an ides of the cost involved, I wanted to make the plastic roller on the winder from cast alloy, the cost for the die was quoted at $22,000 and the cost per unit at $5 (and on top of that comes cleaning off casting edges and anodising).
Today Leica could make this top-plate by CNC and in more suitable alloys, even in alloyed titanium or a stainless steel compound. The cost per unit would be reasonable; probably less than $100/piece in the volumes they produce cameras.
The Zinc is not bad; it absorbs and dissipates impact fairly well. If you hit it really hard it can crack, but a brass housing hit with the same force, would have collapsed and crushed the finder assembly and metering circuitry. The biggest risk with using zinc is if you manage to scratch through the coating (chrome/black chrome. nickel and acid copper sealer) into the zinc itself. If you expose the "wound" to very salty air (sea spray, East Coast salted road slush) you can create a galvanic effect and the zinc will turn into a white powder and the plating will lift off. This being said, I have never had that happen to any of my M6's or late M4-P's, and they have not been babied over the years either and I do live less than 150ft/50meters from the Pacific Ocean.
Evidently the M6 TTL will have a brass top-plate again and in spite of the zinc alloy's improved impact features I like that. Somehow having a white/grey surface show up after a couple of 1000 rolls in your camera doesn't look right. Well-worn Leicas show brass on the edges!
Oh, they abandoned the zinc baseplate idea. It was too difficult to make the "hinge" where the plate attaches to the camera strong enough and after use it started to crack. Brass is more "flexible" and could take the on/off and twisting motion. Another reason why I abandoned zinc for the Rapidwinder housing. The alloy I use is much more flexible and also much stronger than the zinc alloy (and considerably more expensive too!) and can be CNC-machined to very tight tolerances.
Of course when Leica goes back to the brass top-plate, we can then strip the chrome plating off and paint them black and we will then have our brassy M's again, only this time with meters in them!
Brass takes to plating very well, is easily machinable and has a give to it when bumped (albeit small), and thus will bend instead of cracking. It is one of the best metals to use when you have metal sliding against metal because it takes minimal lubrication to keep from wearing. It is also relatively stable through a wide range of temperatures.
[…] Put simply, Leica (and other manufacturers) used brass for cameras and other optical instruments from the beginning. It is corrosion-resistant (hence its use in boat fittings) and engineers know its coefficient of expansion and other details inside-out. It is malleable and therefore more easily machined/stamped/bent into complex shapes (but therefore it also dents!). Leica's brass gears contribute to the very smooth 'feel' of the M2/3/4. But are too soft for the pounding of a motor, so were changed to steel in the M4-2 and later cameras with winder connections.
All the M4-P's with flush windows have zinc top plates. The reason why the cameras with brass top plates in black chrome get "silvery" on the edges instead of braising is because of the nature of the black chrome plating process. It is not a single black chrome plate, but rather a series of platings applied to the metal part. On the zinc top plates, first a flash coat of copper is applied, then nickel plating, chrome plating and then the finish black chrome plating. I believe the copper flash coat was not used on the brass top plates. Since the bright chrome plating is very durable, it is rare to wear past this on the black chrome cameras. Black paint cameras on the other hand do not have these coatings and wear through to the brass rather easily.
Zinc does have some interesting properties. It resists dents like nobody's business, but will crack under extreme stress. It is also somewhat porous, which makes the copper flash coat necessary. If the zinc is scratched down to the bare metal, it will begin to oxidize and tun into a powder around the scratch. Zinc is not as ductile as brass. That's the reason the zinc top plates lack the fine detail of the brass versions, hence the flush windows. Zinc is also cheaper to make, as they are cast and not formed under high pressure in a gigantic press like the brass top plates were. BTW, I've heard that Leica will be going back to brass top plates on the M cameras in the near future.
Although it rarely happens, some early-model M6's or R4's have been known to corrode. Typically the body top plate or back-door starts to develop bubbles, giving the camera a "smallpox" appearance.
So corrosion can happen, for zinc is after all a highly reactive metal (hence its use galvanised steel). Thankfully there have been no reports of problems with later model M6's or M6TTLs.
Aside from the Titanium M6 and one-off special editions like the black-paint LHSA M6, until q1 2002 the "back to brass" rumours proved false, with all general production M6 TTL variants being made the standard way using zinc alloy top plates.
In June 2002 however, Dr Joseph Yao reported that the final batch of M6 TTLs did come with a brass cover.
There are regular issue M6TTL bodies with brass tops as I have one of them! I know one dealer here in HK very well and told me about seeing a few M6TTL bodies with paper inner boxes. He said he noticed the finish of these bodies looked exactly the same as the M7 finish.
One day I got a call from him so I ran down to look at the latest batch with these paper inner boxes. He had 3 chrome 0.58x M6TTLs with paper inner boxes and I compared them to other M6TTL bodies with plastic inner boxes as well as chrome M7 bodies. I was convinced they were the same finished as the M7 which is brass of course. The chrome is much duller more like a M6 Titanium without the yellow cast.
I bought one on the spot. I asked him to keep an eye on other brass bodies especially a black one but apparently they were pretty rare as he never came across one again. I don't know how else I can positively confirm thing other than take off the top and look inside.