The axle is all the same 4140 steel covered by an aluminum wheel... no dissimilar metals... so as
long as the steel stays dry there's no corrosion and the axle looks like this...
But if you hit the assembly with a garden hose then don't bother removing the moisture... the
aluminum wheel traps the moisture and the steel corrodes... so the axle ended up looking like
this...
AAARGH.... Larry... no! The steel and aluminum corroded because they were in electrical contact with each other. Aluminum and steel are VERY dissimilar metals, spaced pretty far away from each other on the galvanic table. (Look it up, along with doing some homework on how galvanic corrosion works as a degradation mechanism).
The other reality is that while aluminum is very active on the galvanic table and easily corroded, aluminum oxide is not, and forms a very nice protective coating usually about 0.005 mils thick very quickly. However, this layer is easily removed by agitation, such as vibrations and mechanical motion, thus creating fresh aluminum oxide very quickly. The aluminum, thus being in electrical contact with the steel corrodes galvanically, and even the relative humidity in the air will enable the corrosion to occur (though much slower than spraying it with a hose or riding in the rain).
There are only 4 things you can do to mitigate galvanic corrosion:
1) don't use dissimilar metals on the galvanic table, or choose metals that are very close on the table and thus will have a very slow corrosion rate (not practical for probably 90% of mechanical design).
2) Use an impressed current system to counter the galvanic potential voltage and thus stop any corrosion. (Requires a power source, often not practical without some type of 'shore power' available, and interstingly enough, the presense of shore power in dense marinas has actually been shown to speed corrosion in boats due to the electromagetic fields created.)
3) Use a sacrificial anode. Often made of zinc. The zinc is in the galvanic electrical circuit with the dissimilar metals. Since zinc is so galvanically active, it corrodes readily. After it corrodes to a specified limit, you replace them. (Any of you with boats: Check your zincs every time you pull the boat out of the water! They've been known to weaken and fall off due to the expected corrosion they go through, and carefully follow the manufacturer's replacement criteria, and NEVER paint them.)
4) Electrically insulate the dissimilar metals from each other and thus eliminate the galvanic potential altogether.
Had I been the one to design it, I would have anodized the aluminum wheel and hub. A proper aluminum anodization (Al2O3) is about 1 mil thick, is harder than everything except diamond, is highly corrosion resistant, even in salt water, and most importantly, is highly electrically resistant and would require about 800 volts to pass a current through a 1 mil thick anodized layer. In other words, would have prevented any galvanic corrosion between the steel and the aluminum because the two
dissimilar metals would have been insulated from each other.
Makes me wonder if this was another case of a bean counter ruining a perfectly good design. Any mechanical or materials engineer worth his salt would know this intuitively. However, anodizing aluminum parts is not cheap and takes a lot of electricity in the process.
Thus endeth the engineering lesson for the day.
