galileo42 Posted January 1, 2009 Share Posted January 1, 2009 <p>I am looking to buy this nice black M6, with an okay price, but the seller says it has «chicken pox» on some places. In fact, on the photos, I can see some faint little grey spots on the lower back and on the bottom plate. Nothing dramatic, just like tiny discolorations. Any idea what this is? Is »chicken pox» a known Leica disease? Is it bad? Can it spread and/or get worse? Otherwise the camera looks in pretty good shape. Thank you.</p> Link to comment Share on other sites More sharing options...
ray_dicecca Posted January 1, 2009 Share Posted January 1, 2009 <p>Michel, mine has a bit of it as well. I noticed it in the 80's but it has not spread and am not worried about. Here is a bit of info on the problem (see "M6 Zinc Corrosion?") in the following:</p> <h1><a href="http://leica.nemeng.com/042b.shtml">Brass vs. Zinc top plates</a></h1> Link to comment Share on other sites More sharing options...
galileo42 Posted January 1, 2009 Author Share Posted January 1, 2009 <p>Thanks, Ray, very informative. Zinc frame? This could also explain why the wear on the edge the top plate, underneath the advance lever at rest, where the thumb could rub the top while activating the advance lever, shows as white-ish not brass-ish. And, come to think of it, I've never seen so much surface wear at that precise place. Unless it's caused by the lever itself, in which case it does not look good to me.</p> Link to comment Share on other sites More sharing options...
ronald_moravec1 Posted January 1, 2009 Share Posted January 1, 2009 <p>Zinc corrosion only gets worse. An alloy mix with impurtities and Leica knew this at the time, but the said they were taking precautions to be sure it did not happen. So much for their experiment. They went back to machined brass.</p> <p>I have two very early ones and they look like I removed them from the box yesterday. Things maybe got sloppy at the end.</p> <p>Essentially this is internal corrosion and it can not be stopped. I have seen cameras totally covered with it.</p> Link to comment Share on other sites More sharing options...
galileo42 Posted January 1, 2009 Author Share Posted January 1, 2009 <p>Thanks, Ronald. I think I'll pass on this one.</p> Link to comment Share on other sites More sharing options...
aplumpton Posted January 1, 2009 Share Posted January 1, 2009 <p>Zinc is normally used not alone as a metal, but as an alloy of more than one metal, which has superior physical and corrosion qualities to the metal. Zinc gravity casting alloys for thin section components may have 27% aluminium alloyed with the zinc.</p> <p>Like all alloys (brass is one), the choice of appropriate composition and control of casting and subsequent machining is important to the quality of the final product.</p> <p>Zinc gravity casting alloys possess interesting and attractive attractive foundry properties. Owing to their low melting temperatures (below 540°C) and casting temperatures, energy required is minimised. They are easily cast into thin sections - less than 2.5 mm, in sand molds. The melting and the casting of these alloys is effectively pollution free. Fluxing and degassing of the mold is not required, and due to the low casting temperatures, minimal pollution from the sand mold occurs.</p> <p>The mechanical and physical properties of the zinc alloy castings, and to a lesser degree their corrosion behaviour, are dependent upon the specific alloy type, the casting process used and the quality of the castings produced, the amount of ageing or service life of the casting and the amount of of impurities in the alloy casting, amongst other considerations.</p> <p>Both copper and magnesium increase the alloy strength properties, reduce ductility and inhibit <strong>intergranular corrosion</strong>. Iron is present as small FeAl3 particles and does not influence mechanical properties unless it exceeds 0,1%. When limited to the specified amounts shown in the relevant ASTM (American society for Testing Materials) specifications, lead, tin and cadmium do not cause intergranular corrosion, or the lowering of physical and mechanical properties. Other impurity elements such as chromium, nickel, manganese, indium, antimony, arsenic, bismuth and mercury do not normally occur in sufficient quantities in zinc casting alloys to be of concern regarding corrosion or effect on mechanical properties of the casting.</p> Link to comment Share on other sites More sharing options...
kens Posted January 1, 2009 Share Posted January 1, 2009 <p>Ahh...what was the question?</p> Link to comment Share on other sites More sharing options...
todd_angood Posted January 1, 2009 Share Posted January 1, 2009 <p>Get an M6 titanium. I have the ttl version and 3 titanium lenses. No chicken pox.</p> Link to comment Share on other sites More sharing options...
aplumpton Posted January 1, 2009 Share Posted January 1, 2009 <p>If you aren't in a hurry and are not being offered a sensationally low price (say under $900), probably better to wait and pick up an uncorroded black or chrome one at a few hundred dollars more. M6s I have seen, purchased and used have not had this problem.</p> Link to comment Share on other sites More sharing options...
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