Oct
24
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MC20151024.mp3 [90.4MB 01:33:55 64kbps]
A podcast about all things Macintosh. For Mac geeks, by Mac geeks. Episode 545. New Apple hardware. iMacs, Magic stuff, etc. Recent Apple software updates. Tim Cook opens up at WSJDLive. Apple TV orders start Monday. Apple Retina Macbook Pro replacement program. Apple kills electric motorcycle company. Follow-up on Apple Watch lugs. My transition to OS X El Capitan. Less time to buy AppleCare with iOS devices. Thing of the Moment: Sphero SPRK
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Adam,
I left this info in voice jail but I was a little long winded so maybe this comment is a better option.
I’m not an engineer, chemist, or metallurgist but I did stay at a Holiday Inn Express last night and… Seriously I’m a pilot by trade and I am also an accident investigator (think NTSB but I was trained by one of the military services). As part of our training we are taught about the type of corrosion you have been discussing on your last two shows. It’s called galvanic corrosion and this wikipedia article will explain everything you need to know, https://en.wikipedia.org/wiki/Galvanic_corrosion. In short, compatibility of metals is completely predictable based on anodic index, a measure of a metal’s electrochemical voltage relative to gold. The article has an anodic index table and explains that large anodic index differences combined with harsh environments e.g. proximity to sweaty skin will result in rapid galvanic corrosion for the metal with the most negative index.
Therefore you only want to mix metals with relatively similar indexes dependent on the environment. In arid desert metals can tolerate a higher index delta, moist salty environments require a small anodic index delta. A quick study of the anodic index table reveals stainless steel has an index of .50. The table doesn’t specifically call out 6000 or 7000 series aluminum but I have to assume it is in the .90 to .95 range. That’s a pretty high difference even for the best environments. Galvanic corrosion is a big problem in the aviation industry and has to be watched carefully in the design, maintenance, and repair of any aircraft; one wrong fastener used on a critical stress area can be disastrous! Apple reps are correct in pointing this out to customers when mixing and matching bands/lugs and watch cases. Consumers will have to be vigilant when purchasing after market bands. It’s really cool that Apple provides such divers materials in the manufacture of their watches but it does mean that the consumer must educate themselves a bit.
Thank you for the work you do, it is extremely useful and greatly appreciated!
Andy Campbell
Hi
I am an amateur astronomer and we have to deal with aluminium things such as mounts for telescopes, eyepieces and cameras out in all types of weather. With aluminium mounts, the only screws and bolts recommended are stainless steel. They do not react with aluminium and are the most durable for outdoors use. Aluminium can oxidise over years of outdoor exposure but this isn’t due to the contact with the stainless steel. It is just the natural oxidation that occurs when bare aluminium is exposed to oxygen.
Obviously it needs to be explained clearly to those who don’t truly understand the Galvanic corrosion charts. Basically aluminium is at the end of one side of that spectrum. So it is liable to react my intensely with the metals at the other end. These metals are most notably copper. Steel is near Aluminium on the spectrum so the rusting of iron process isn’t sped up by the contact. Having a non-reactive coating over reactive metals also halts the corrosion process as it requires bare metals to touch. Tin is often used in the electrical industry as it is an inert metal (middle of spectrum).
From this there is absolutely no problem using the stainless bands on the aluminium sports watch. There maybe for third party products if it isn’t stainless steel or coated adequately to prevent corrosion problems. Cooper is a very cheap metal.
I checked these facts with fellow astronomer and an electrical engineer who works primally with aluminium.
I hope this clears up the confusion about the Apple Watch band compadibity.
Love your show
earthlight
Australia