Note to self: Buy aluminum grease

[Akimbo]

A little more explanation might be in order. Two major factors are at play, Galling and galvanic corrosion. Stainless to stainless fasteners are very susceptible to galling. This is a problem on camera housings when stainless inserts are installed on high-use threads. Aluminum is very soft so threads that get used a lot often have inserts. The primary purpose of Anti-seizing compounds is to help prevent galling.

http://www.fastenal.com/content/feds/pdf/Article - Galling.pdf

As Wookie explained, galvanic corrosion happens anytime there are dissimilar metals and an electrolyte… like salt water. Thread compounds designed for the marine environment are optimized to minimize both.

 

[Storker]

Two major factors are at play, Galling and galvanic corrosion. Stainless to stainless fasteners are very susceptible to galling. This is a problem on camera housings when stainless inserts are installed on high-use threads. Aluminum is very soft so threads that get used a lot often have inserts.

It doesn't seem as if the NA-EM5 has steel inserts in the threads for the tray. The threads seem to have been cut directly into the housing body. Also, the way the screws loosened - slowly and syrupy, not the more abrupt loosening I'm used to for screws set in holes of similar metal - and the greyish-white powder that drizzled out of the holes after I was able to get the screws loose and looked like Al oxide makes me believe that this is a case of galvanic corrosion.

I'll try to check with the yacht center in Stavanger, but it doesn't seem as if they have a web store, and that town is a couple of hours' plane ride from where I am. So I'll probably check with the local boating store as well. Worst case, I'll have to change my rinsing routine to include taking off the tray and applying silicone grease or something like that on the screws after every dive (sigh.).

Anyway, thanks for the tips and info.

 

[Akimbo]

It doesn't seem as if the NA-EM5 has steel inserts in the threads for the tray… .

There is an insert at the top of each handle on mine. Check the bolt that holds the 1" ball for the strobes. I just re-lubed all the fasteners on mine. Man I hate those tiny (M2?/1.5 mm hex) cap screws. Those threads are especially vulnerable to stripping.

… Also, the way the screws loosened - slowly and syrupy, not the more abrupt loosening I'm used to for screws set in holes of similar metal - and the greyish-white powder that drizzled out of the holes after I was able to get the screws loose and looked like Al oxide makes me believe that this is a case of galvanic corrosion...

Galling happens no matter how carefully and slowly you rotate the wrench. It happens more on die-cut rather than rolled threads, like most bolts, but still happens. It is pretty amazing really. One second you are turning a stiff nut on a bolt when suddenly it locks. I have had nuts suddenly gall so badly that the bolt twisted off trying to turn it.

It doesn’t take long for the harder and relative rough surface of a stainless fastener to abrade away hard anodizing on the Aluminum. You will get oxidation (Aluminum Oxide white powder) at that point even without a fastener installed.

The Aluminum Oxide helps to protect the metal from further oxidation, but is easily scrapped the next time a fastener is installed. The Aluminum threads just keep getting weaker as metal oxidizes and erodes. A common “fix” is to tap out the thread larger and install a stainless reducing insert or Helicoil insert… often with a sealant or adhesive.