SCUBA-SI,
It is difficult to impossible to do a root cause failure analysis without looking at and evaluating the failed component(s) directly. Having said that I will attempt to provide some comments that may help with this problem and others of a similar nature involving marine corrosion. I apologize in advanced for the long post but corrosion is a very difficult and misunderstood subject:
1. Localized corrosion in the form of pitting is often the result of "crevice corrosion". This corrosion mechanism often occurs when a passive film former (e.g. aluminum alloys, stainless steels), are "covered" by a crevice former (e.g. barnacle attached to metal, gasket at a metal joint, adapter over a metal seat). Typically, tight, deep crevices are much more of a problem than loose, shallow crevices. For example, if you take an aluminum tube and stretch a tygon tube over its end (inner diameter of the Tygon the same or less than the outer diameter of the aluminum tube) and fully immerse it in natural seawater for a few weeks. Then remove the assembly from the water and take the Tygon from the end of the aluminum. You will notice significant pitting and etching on the outer diameter of the aluminum outer diameter surface. The tighter the crevice (Tygon fit over aluminum) and the deeper the crevice (distance the Tygon is placed over the end) and the longer the assembly is in seawater the greater the depth of attack.
2. Dissimilar Metal Corrosion (a.k.a. Galvanic Corrosion, etc.) occurs when dissimilar metals are in direct contact with another (although a metal wire can connect two metal components that are fairly remote from other). The active metal (the anode) will corrode preferentially to the noble metal (the cathode). This is the basis for cathodic protection which is purposefully placing an active metal in contact with another more noble component in order to preclude corrosion of the more noble component.
Dissimilar Metal Corrosion is often erroneously cited as the reason for numerous corrosion related failures mainly because it is easy to observe that two different metals are in contact with one another and draw the conclusion. Take the same assembly cited in paragraph 1 but slide a stainless steel sleeve tightly over the aluminum tube (specific alloys are important as is anodization but for simplicity sake let's not worry about it). All else being the same, pitting on the Tygon/aluminum assembly will be as significant if not worse than the Stainless/aluminum assembly.
Dissimilar Metal Corrosion becomes much more significant with unfavorable anode/cathode ratios. For example, if you place an aluminum rivet in a copper plate you will get much more attack than if you place a copper rivet in an aluminum plate (all else equal of coarse: size of plate, size of rivet, method of fastening, etc.).
Dissimilar Metal Corrosion is most problematic in full immersion conditions (assembly immersed in seawater for "long" periods of time). In alternate immersion conditions typically encountered with underwater camera systems, the dissimilar metals only see an electrolyte (seawater) for short periods of time. In the absence of seawater electrolyte, corrosion is nil/negligible. So if you rinse your system you will see little/no attack while in atmospheric conditions. If your system is not rinsed, a consistent salt mist is present, etc. then you can see some degradation above water but damage is limited to ~6mm (1/4") from the dissimilar metal juncture (interface). This limitation is due to the thin film electrolyte on the metal surfaces which results in high ohmic resistance and thus lower current density (corrosion damage is typically directly proportional to current density).
3. Electrolysis is a misnomer relative to corrosion. Corrosion is degradation by electrochemical reaction(s); Electrolysis is the dissociation of water.
4. Stray Current corrosion can be a problem when DC currents (and/or rarely with AC currents) enter and leave a metal component. Damage is seen where the current leaves the metal component. These types of failure tend to be very difficult to evaluate and mitigate. I am not qualified to comment much on stray current corrosion.
Now, on to your specific problem… I would guess that the primary mechanism for pitting is crevice corrosion. There may be some aggravation by dissimilar metal contact particularly if the aluminum adapter has a small nick(s) in its surface that damaged the hard anodized protective "film" (I assume that your hot-shoe is stainless steel?). Again, very small areas of exposed, bare aluminum are much more of a problem than if the entire surface was bare. This is the result of large current density (current densities on the order of pico amps per square centimeter are significant).
Since you experienced significant pitting in a very short period of time, it is also possible that stray-currents were contributing. I have little/no expertise in this area and it is difficult to imagine a source. Certainly the hot-shoe directly on the camera sees currents when the strobe is fired; however, most if not all housing hot-shoes are not "live" and are simply used to attach components such as spotting lights. Any one else think of a way that stray-currents may have entered and discharged through the adapter??? Keep in mind that a salt bridge can carry significant currents from one metal to another.
Camera housings are generally cast aluminum with small stainless steel “inserts/attachmentsâ€. Generally, this is an acceptable materials combination; however, design, workmanship, specific alloys, etc. are important to preclude corrosion issues.
Lastly, I believe the folks at ULCS use hard-anodized 6061 aluminum alloy. This material is successfully used in many components subject to marine “atmospheric†conditions (seawater mist, above waterline). A good example is window frames in oceanfront homes. The material generally does pretty well in alternate-immersion service but is marginal in full-immersion service (not regarded as marine grade where high-reliability components are concerned).
So, I would recommend that (a) you avoid a very tightly fitting adapter (this may not be possible with your specific parts), (B) make sure the housing hot-shoe/adapter juncture is rinsed topside, © somehow verify that there is not a stray current problem and (d) minimize relative motion between the hot-shoe and the adapter. Relative motion can be the result of (a) differential thermal expansion of the hot-shoe and the adapter, (B) galling type wear when sliding the adapter onto the hot-shoe, © abrasive wear if salt and or corrosion products develop in the crevice between the hot-shoe and the adapter, (d) handling the system by anything attached to the hot-shoe, etc.
Again, sorry for the long post. Hope this helps in some way.
Oh and bye the way, sacrificial anodes (typically zinc alloy) on the housing are of ZERO use at the surface. Furthermore, the system spends so little time in immersion conditions (perhaps 5 hours a day for 100 days a year even for the most prolific underwater photographers) that, in my opinion, they are of little value.
