I seem to remember hearing that there could be a galvanic (?) reaction between aluminum and stainless? I'm not too good at scientific explanations.
Sounds like a question for....Battenkiller!
Say what? Putting me to work, are ye?
OK, I found this link that provides a good explanation of galvanic corrosion
Galvanic corrosion is primarily a problem in marine environments, but give the large number of trippers who spend at least some of their time in saltwater, I think it is best to address it here.
Bottom line is that dissimilar metals create a galvanic cell when immersed in an electrolyte (like seawater). One becomes a cathode and the other an anode. The one that is highest on the cathodic (more noble) end of the galvanic series becomes the cathode and will corrode the least. Therefore, stainless rivets will do better in aluminum gunwales than aluminum rivets will do in stainless gunwales. The link I provided actually illustrates this exact situation:
The approximate voltage difference for any two metals can be taken directly from Table 1. It is worth noting that marine slime films composed primarily of microscopic bacteria and diatoms can change the potentials of many of the alloys near the noble end of the Galvanic Series as indicated. The potentials of these alloys become more positive in the presence of slime films, thus increasing the voltage difference when these metals are placed in contact with more anodic alloys. This has been found to increase the corrosion rate of copper, steel, and aluminum anodes by a factor of 2 to 5, but to have no effect on the corrosion rate of zinc anodes. It may also change which metal in the couple becomes the anode.
When the two metals in a galvanic couple are close together on the series, such as manganese bronze and silicon bronze, their voltage ranges overlap, and either one can be the anode, depending on the exact exposure conditions. In this case, more detailed information than is given in this MAS Notes will be needed to predict the behavior of the couple. Several sources of additional information are listed in the Recommended Reading section.
The effect of the second factor above, the cathode-to-anode area ratio, C/A, is illustrated in Figure 1 for a rivet in a plate. In both couples A and B, aluminum is the anode, and stainless steel is the cathode. In couple A, the aluminum rivet is comparatively small, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small, and the C/A ratio is also small. Corrosion of the aluminum rivet in couple A will be severe. However, corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case.
Since no one I know is manufacturing stainless gunwales, I think that leaves out that disastrous combo. Stainless rivets in aluminum gunwales should be fine, although there might be some corrosion of the aluminum gunwale that may or may not enlarge the hole slightly over time.
Even so, this shouldn't be a problem in gunwales as they are rarely submersed in seawater. Still, if you are forced to use stainless rivets and
are paddling in a marine environment, it would be a good idea to rinse the gunwales with freshwater after a trip or before putting the boat into storage.