That's not entirely correct, I think. Acceleration is constant across the entire black box, but gravitational force depends inversely on the squared distance to the attracting mass, i.e. the force should be slightly different at the top and bottom of the box.

Both aluminum and steel are corroded by water. In fact, aluminum ions are more soluble in water than iron ions. The difference, however, is that iron oxides do not stick to the parent iron substrate and flake off, ever exposing new iron surface for corrosion. Aluminum on the other hand forms alumina (aluminum oxide, corundum), which is insoluble in water, has a very high hardness, and sticks strongly to the parent substrate, thus forming an inert layer all over the aluminum and preventing further corrosion. This is why aluminum roofs and siding works, without the aluminum dissolving in the rain water despite the very high solubility.

Garh... I should have previewed that :-( That should read "a body will not stop moving, unless something stops it"

Perpetual motion is actually a fundamental property of the universe, though we usually call it inertia: a body will not stop moving, unless somebody moves it. Therefore, linear perpetual motion is the norm, with change of velocity depending on an outside force.
To make it even more interesting: non-linear perpetual motion is actually also present in all molecules, at any given temperature, even at 0 Kelvin. Quantum chemistry shows that vibrational motion in a molecule changes by energetic quanta, where at least half a quantum is always present in a vibrational degree of motion (so-called zero-point energy). Hence, the atoms in a molecule are always in motion, even at 0 K, and the motion is non-linear. In first approximation, especially for diatomic molecules, it can be described as an oscillation with a parabolic energy profile, for multi-dimentional molecules one usually gets ergodic movement that is a bit more complex to describe, and is usually considered chaotic where only the statistical properties are relevant.
But no free energy, of course.