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Harryrag
Tinkerer
Username: Harryrag
Post Number: 5
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 29, 2008 - 12:28 pm: | 
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has anyone ever heard of tin pest on camera or binocular bodies? any info is greatly appreciated |
Glenn
Tinkerer
Username: Glenn
Post Number: 363
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 29, 2008 - 04:24 pm: |
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As everybody knows, this is the allotropic transformation of tin at temperatures below 13deg C, into an unstable allotrope. I believe it can be shown that in pure tin, the onset of this problem takes approximately 18 months.
Obviously the phenomenon should only occur in tin, or high tin content alloys. However there is a serious problem with tin alloys, as even small amounts of alloying metals greatly reduce the autocatalytic effect.
I know of no camera or binocular bodies made of pure tin. I have not seen any degradation, in the alloy body castings of the many old and not so old cameras, I have worked on. As for binoculars; not 3 feet away from me as I write this, are my late father's naval issue Barr & Strouds which he used on the wartime Arctic convoys. Despite plenty of low temperatures, they are still sound in both body and optics.
If you found this specific instance of tin pest on the internet, I would suggest that it belongs to that ever increasing area of the internet labeled 'rubbish/untrue'. If by the slightest chance it is true; the answer is simple, do not keep your cameras in the freezer and only holiday near the equator!
The crumbling of early die cast toys made from Mazak (Zn/Al) alloy is well known, but I have never come across a camera body made from this alloy. Most seem to be manufactured from Al or Mg alloys that do not exhibit zinc content properties. So no crumbling camera bodies from this cause either. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 123
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 30, 2008 - 02:00 am: |
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I must admit I hadn't heard of tin pest! Self-destructing zinc alloy definitely can be a problem with cameras and lenses, though, Glenn. I have one of the original type Braun Paxettes which is disintegrating from this cause.
A few weeks ago I stripped an Enna Sockel auto mount. I was surprised to find that the helical guides were zinc alloy diecastings and both of them were badly cracked. Fortunately one of them still had just enough strength to hold things together. The other one fell to bits as I was using it as a pattern to make a steel replacement.
Not all zinc alloy die-castings seem to suffer from the problem, but I don't know whether this is due to different types of alloy, different casting technique, or different environment. The material used to be known as "pot metal" in the US, but it is also known by several much less polite terms on both sides of the Atlantic! |
Harryrag
Tinkerer
Username: Harryrag
Post Number: 6
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 31, 2008 - 03:38 am: |
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Maybe my initial question was too blunt or incorrect from a more sophisticated point of view of those who are on a par with metallurgists, but the reason why I am still a bit puzzled about all those pot or diecast alloys is that some of my gadgets. A Braun Paxette and most notably several binoculars suffer from disintegrating metal parts, corrosion, rust, whatever the right term may be.
The metal where the protective layer of paintwork is gone gets dark grey and thicker, it feels like being wet and can be scraped off easily. Can anything be done to stop the process? |
Glenn
Tinkerer
Username: Glenn
Post Number: 365
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 31, 2008 - 06:07 pm: |
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Steve,
My memory is going, I should have remembered the only Paxette I ever tried to repair - but I did not connect with Zinc as it also showed heavy damage from a drop. From conversations with the works metallurgist many years ago, regarding the condition of early die cast toys. i seem to recall that the problem arose from poor quality control, at the alloy preparation stage. It seems that the Mazak could be blended from scrap aluminium, thus the exact composition was very variable. I have read on the web casting temperature is an important factor, whilst another source states environmental issues. However no definitive answer unfortunately!
I certainly think moisture can be a problem. Anybody that has restored a classic car, will have had problems with pitted/damaged chromed die castings. This even happens in countries that do not use salt in the winter months, so that only leaves moisture as the obvious culprit.
Harry,
The only way you will slow the degradation of the exposed base metal, is to apply a sealant to the exposed surface. You may find that frequent applications of a good wax polish will do the job. I have used a product called Renaissance Wax on early silver finish Russian lenses. These lenses have barrels of polished aluminium, that were originally lacquered. In use the lacquer is softened by skin oils, thus wearing off in places. I remove the damaged lacquer and re-polish the alloy barrels. The wax polish has stood the test of time and use, the barrels have remained polished and un-corroded. I re-apply the wax, according to how frequently the lenses are used.
You could refinish the exposed metal with paint, but this will need more care in preparation and application. Also if patch painting, the results can look worse than the worn look. |
Gez
Tinkerer
Username: Gez
Post Number: 29
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, June 01, 2008 - 04:39 pm: |
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While on the subject of metal corrosion I recently had to deal with the alkaline products of mercury battery leakage attacking a brass assembly. This is not unusual, and vinegar does a good job of cleaning and freeing the parts, however one part that was stuck fast appeared to have been factory treated with a spray-on moly anti friction coat. Has anyone encountered such coatings reacting badly with battery leakage? |
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