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The three main things to be concerned about are corrosion, galvanic corrosion[1] and galling[2]. Corrosion is more commonly known as rust, but that's technically reserved for steel (or, more accurately, iron) oxidation. Aluminum certainly corrodes, but it corrodes into aluminum oxide, or alumina, which is actually a ceramic that is stronger than aluminum. In fact, good luck using aluminum without a coating of alumina... it naturally occurs on the surface of atmospherically exposed aluminum. As a result, when a chunk of aluminum is broken off of a larger piece, the material almost immediately forms a protective layer of alumina. It's almost like aluminum self-heals in the presence of oxygen. Galvanic corrosion occurs when dissimilar metals come into contact amidst an electrolyte solution. In the case of vehicles, that's usually seawater/sea-mist or salted winter roads. While saltwater is also responsible for accelerating normal corrosion, it also plays a part in galvanic corrosion. So yes, having aluminum and steel touching is a recipe for galvanic corrosion, but galvanic corrosion is not black-and-white. Each metal has an anodic index; a high or low anodic index doesn't matter all that much. What matters is the difference between the anodic indices of the touching metals. For aluminum and steel, that difference is quite small, so galvanic corrosion isn't a huge issue, but it's not negligible either. Galling occurs when the sliding friction of two parts, say a screw and its threaded mate, results in plastic deformation, usually in the more ductile of the two pieces. This generally doesn't come into play in the construction of a vehicle because many parts are attached either by welding or by a bolt-nut combo. Choosing the correct materials for the bolts and nuts (basically, just use hardened, but not stainless, steel) gets rid of this concern in most cases. In cases where one must screw a steel fastener into a threaded aluminum piece, a helical insert is usually prescribed. While many manufacturers make these, we typically used Helicoil for spaceflight ops because they're tested and rated for spaceflight (batch traceability, proper composition docs, etc). [1] https://en.wikipedia.org/wiki/Galvanic_corrosion [2] https://en.wikipedia.org/wiki/Galling |