Aqua_Soul,
It is difficult to determine the root cause(s) of degraded metals without actually inspecting, testing and evaluating the components; however, as you can tell, that doesn't stop many of us from trying. Metal failure occurs predominately through corrosion, fracture and wear. In my opinion, corrosion is misunderstood more than other forms of materials degradation. Assuming that your problem is pitting due to exposure in marine environment(s), I am fairly comfortable with and confident in the following comments/questions/suggestions:
1. I agree with Bob's comment above. Have you asked the manufacturer-of-the-housing if they know of similar problems with their product(s) and if they are will to evaluate... with hopes that they will repair or even replace?
2. You might consider visiting corrosion forums for further input...MarineCorrosion.org out of the UK is a good source but often very technical.
3. Generally, I would recommend you take action very carefully after weighing your options. It would be relatively easy to damage the housing well beyond its current state.
4. Based upon the limited information gleaned from your posts and observation of your photos, I would recommend the following:
a. Do not try to "locally" anodize the housing unless you are comfortable doing so and/or you solicit help from someone who is. It's not simply a matter of cleaning out dirt, debris and corrosion products followed by local application of an acid. Anodization is an electrochemical process. There are different types of anodized coatings and they are produced primarily by two methods: chromic acid and sulfuric acid. They produce a relatively thick, tenatious oxide layer that protects the underlaying base metal, aluminum alloy in this case. In my opinion, if re-anodizing is required, you need to match the process that was used originally and blend in a manner that you don't disrupt regions adjacent to the pits. Areas that presumably have retained integrity of the anodized layer.
b. Initially, I would not add sacrificial anode(s). They are relatively easy to install but care must be exercised to prevent other problems (e.g coating disbondment at the point of attachment, etc.). Furthermore, they only preclude corrosion during immersion. Most of us expose our housings to "alternate immersion" service. Consequently, even if the housing is protected via sacrificial anode, pitting can/will proceed while the housing is in spray-and-splash or even marine atmospheric conditions (like one often experiences on the deck of a boat even if you fresh-water rinse upon surfacing). Above the water's surface, a sacrificial anode can only protect the basemetal for a distance of approximately 1/4" (6mm)--radial distance from the perimeter of the anode. If you do pursue the sacrificial anode route, I would recommend zinc over magnesium (advantages include less probability of coating damage and lower consumption rate of anode itself).
c. Initially, I would focus on locally cleaning the pitted areas followed by an organic coating. Ideally, you'll want to use a system similar to your original coating whether it be enamel, epoxy, etc. Unfortunately I can't provide a specific cleaning procedure but I suspect there are many reliable solutions available online. I would suggest you only clean to the degree necessary (remove debris and any white powdery corrosion product). Should not rely solely on the unaided eye but rather use a magnifying glass or microscope at low power.
d. If "c" above does not mitigate the pitting then you may need to proceed to anodizing and/or installing a sacrificial anode(s)
e. Again, whatever actions you take you obviously want to make sure you do not do "more harm than good". If repairs are not done properly, you can aggrevate the surface and accelerate attack at/near the existing pits or elsewhere.
As for the cause of your pitting problem, there are several possibilities. I can't say with a high degree of confidence; however, I will provide the following comments:
--it is likely that you had defect(s) in your original coating system. Organic coatings (paint) often have "holidays" even if applied correctly. For aluminum housings, the below laying anodized layer is the next line of protection from pitting and other forms of corrosion. Without a painted overcoat, the relatively brittle, anodized layer can be damaged, mechanically in a variety of ways which provides a path for pitting of the basemetal (typically due to crevice corrosion via a "differential oxygen cell" mechanism).
--although less likely, you may have had a housing with some form of surface metallurgical defects (e.g. shrinkage porosity, gas porosity, non-metallic inclusions, etc.) In this case, the defects may have precluded the expected protection from anodizing and coating.
--also possible but not probable (and you are in the best position to know this) is that the housing experienced some very fine mechanical damage (hitting edges, sharp pointy object striking the housing, etc.
-- Pitting (particularly when crevice corrosion is an issue) can occur very quickly with passive film former (e.g. aluminum alloys, stainless steel alloys, etc.); however, depending upon the residual thickness of your base-metal, specific location of the pits, etc. your damage may not be urgently significant. Again, unless you go about repairs methodically and with care you could cause more harm than good.
--The role of dissimilar metals in your problem is easy to overstate. Aluminum and Stainless Steel are often successfully used in combination for Marine service regardless of the specific stainless steel alloy. The potential difference (driving force for dissimilar metal corrosion, a.k.a. "galvanic corrosion") of aluminum and stainless steel is typically acceptable particularly when the stainless steel component is small relative to the aluminum component (as is the case with most UW housings). The reasons include (a) a favorable anode/cathode ratio and (b) a phenomenon known as polarization...I will not get into this topics unless you are interested:)
--The role of stray-currents (the often used term, Electrolysis, is a bit of misnomer and erroneously used even by chemists, metallurgists, etc.--rarely does the corrosion engineer talk in terms of "electrolysis"), is unlikely for your problem.
Sorry for the long-winded response...please let me know if you would like further input...Good Luck!
Scott
