There is a difference in electrical potential between steel and aluminum or steel and zinc. The electrical potential of Iron (steel, aka, carbon steel = carbon & iron) is -0.44V, zinc is -0.76V, and Aluminum is -1.67V (
https://www.engineeringtoolbox.com/electrode-potential-d_482.html). This is precisely why many transportation structures, such as light poles, sign structures, CMP, etc. are hot-dip galvanized...it is a sacrificial layer that will corrode before the steel can corrode. Furthermore, corrosion initiation does not necessarily occur at the interface between two metals, which is why you might not see corrosion at the connection of the spider arm to drum.
As far as how quickly the corrosion occurs, that is a function of the quantity of anode material, connection design, and certainly exposure conditions. I wasn't saying that an aluminum or zinc spider arm is guaranteed to fail in XX years, it may last for decades. I was simply saying it will begin corroding immediately from the second it's connected to a steel drum and will continue to do so unless a voltage exceeding the voltage potential of both metals is passed through the materials.
On that note, a grounding wire is not the same as active galvanic corrosion protection. A ground wire simply takes out any stray currents to a safe place (ground); whereas active galvanic protection systems pass a low-voltage current through the system to inhibit corrosion. Its entirely possible that higher-end and/or commercial washers use this type of active galvanic corrosion protection.
I know next to nothing about washers, so I'm not trying to make any claims about real-world experiences. I'm sure those of you who have torn apart hundreds or thousands of these things know how the design holds up in real usage conditions better than anyone else. I just know a lot about corrosion of dissimilar metals, and I didn't see anyone mention that as an explanation of the observed corrosion so I thought I'd bring it up. So I was only saying that it's a shock that they would design these spider arm using dissimilar metals. My guess is that the spider arms are purposely designed as a sacrificial anode to prevent corrosion of the drum and that they're designed (in theory) to provide enough sacrificial material to provide "full service life" (whatever that may be), but the designers failed to accurately account for environmental and usage conditions so many of these systems fail prematurely. There's also the argument that they're designed this way because of planned obsolescence, but that is basically just a conspiracy theory that would be very difficult to prove.