Corrosion on D500 Hotshoe

I took my Nikon D500 on a dive trip about 3 weeks ago. I don't dive with this camera, just use it for top side shots. As it happened, I barely used it taking it out of its bag only once. Maybe it got a tiny bit of salt water on it. I did not use the hotshoe at all. After the trip I put the camera away in a cabinet. It was turned off, but had a battery installed. I'm sure I would have noticed if this material was on the hotshoe at that time.

I just took out the camera this morning to take on a nature walk and instantly noticed this buildup of white corrosion or something on the hot shoe. There was no salt build up or any other indications of something weird anywhere else on the camera or lens. I cleaned off the material with some isopropyl alcohol and a tooth brush. Everything seems to work fine, and the battery still showed it was fully charged.

Does anyone have an idea of what might have happened here? I've used this and other cameras on the exact same trip and conditions many times and have never experienced this issue.

Ouch! That does not look good. Salt or corrosion? I'm guessing you didn't taste it to try and find out!

Very strange. Presumably no leaking battery nearby. I'd guess salt but how?

Good you were able to clean it up and that everything works ok. Phew.

This looks like electrolysis. Dissimilar metals in hotshoe. A bit of salt and some humidity. A close call....

I did not taste it... 

I am guessing there is some sort to residual current on the hotshoe, but I don't know what mechanism created the buildup.

1 minute ago, Tom_Kline said:

This looks like electrolysis. Dissimilar metals in hotshoe. A bit of salt and some humidity. A close call....

I need to read up on that. I think I'll put a multimeter on the hotshoe with the camera turned off and see what gives.

Maybe entropy. Looks like a car battery! The hotshoe became the battery in your case.

Most of the mineral (probably salt) deposit is on the metal hot shoe frame, which is the electrical ground for the camera's circuits.
Fortunately, the 4 contacts are free of deposit.  It may be that the hot shoe frame was a tad cooler then the ocean air and seawater vapor condensed on it.
You have done us all a favor by posting this. The lesson is: when out on the water keep the camera in a bag or case except when shooting, and before putting it away, wipe it off with a dry cloth. (Later on, also remember to remove all equipment batteries before storing.) 

It looks just like metal that has been exposed to saltspray, salt dries off but it's hygroscopic and pulls in moisture when the humidity is high enough and facilitates corrosion by completing the circuit between two dis-similar metals.  It can also produce deposits like this on a single susceptible metal component. I don't believe it has anything to do with the components being a ground or connected to any electrical circuits.  When the metal corrodes it forms metals salts which are bigger in volume than the parent metal so look like a build up.  

It's also nothing to do with condensation, salt spray travels large distances we get a little at home and we are about 3km from the ocean and 250m above sea level.  We see evidence on salt corrosion on our aluminium  screen frames which produces a white powder.  

It can happen with a salt film, which draws in moisture -there is only a little moisture so the chloride ion concentration is very high - this can cause rapid corrosion of many metals in the right circumstances.  Underneath these deposits it remains damp and corrosion accelerates.

The solution is simple - wipe down equipment with a damp cloth and dry it off after a day on the water.  Your camera may be "weatherproof" and have seals to prevent water getting inside, but it certainly is not salt spray resistant. 

If it has happened isopropyl alcohol is not the right solution - the corrosion products and the particularly the salt itself are generally soluble in water but not necessarily soluble in alcohol.  It is best to dry scrub to remove all traces of deposits then  wipe and scrub repeatedly with a damp cloth rinsing it it out each time to get rid of the salt.  Only when you have done that use alcohol to displace any residual water.  The concern is that unless you ran large quantities of alcohol to physically displace the salt there may still be a thin film of it left after the alcohol evaporates.