What effect does salt have on the corrosion of steel?

Corrosion does not always involve oxygen or salt. Corrosion is electro-chemical. It always involves electricity, an anode, a cathode, and an electric current. Salt provides an ionic solution with water which conducts electricity. The chemical reactions at the anode and cathode will have a strong influence corrosion. If you can protect the anode or the cathode from reacting, it prevents the current from flowing. Not sure about the chemistry of salt with iron but Fe readily forms a number of oxides, FeO, Fe2O3, and Fe3O4, different oxides, different colors. If you can get the steel to form an oxide layer that covers the surface and does not flake off, it allows it to rust but, once it is completely covered, it stops rust. Research Corten (I think?) steel used in architecture, the new building rusts, turns red (stains the ground and concrete around it) but then it protects the steel from then on.

I though that corrosion was due to oxygen, and the fact that other molecules are constantly colliding with the atoms of steel. Since salt is in a stable form, it'd wouldn't collide with the steel as much water. And since there are some free oxygen molecule in water, it'd corrode the steel faster. That's what I think anyway. I think the black layer is the product of steel and sodium/chlorine reacting together.

Just one point to add: An interesting effect of corrosion; and it is used by industry, is that of a sacrificial layer. A metal which has a small layer of rust on it's surface, carefully monitored and controlled, will save the lower structure metal from being eaten through. That's what the black layer you mention reminds me of.

Corrosion occurs most strongly when steel is momentarily dipped in salt water and allowed to dry and then dipped again over and over. Leaving it submerged greatly slows down the process.

I though that corrosion was due to oxygen, and the fact that other molecules are constantly colliding with the atoms of steel. Since salt is in a stable form, it'd wouldn't collide with the steel as much water. And since there are some free oxygen molecule in water, it'd corrode the steel faster. That's what I think anyway. I think the black layer is the product of steel and sodium/chlorine reacting together.

Corrosion occurs most strongly when steel is momentarily dipped in salt water and allowed to dry and then dipped again over and over. Leaving it submerged greatly slows down the process.

Just one point to add: An interesting effect of corrosion; and it is used by industry, is that of a sacrificial layer. A metal which has a small layer of rust on it's surface, carefully monitored and controlled, will save the lower structure metal from being eaten through. That's what the black layer you mention reminds me of.