Rust in Transit Is a Moisture Problem, Not a Steel Problem.
A container is a sealed steel box that swings through a large day-night temperature range every day at sea. As it cools at night, the warm, moist air trapped inside reaches its dew point and water condenses — on the ceiling, the walls and the coldest surfaces, which are usually the machined faces of the cargo. This is “container rain,” and it is why a machine loaded perfectly dry in a temperate factory can arrive streaked with rust after three weeks at sea. The steel did not change; the air around it did.
Because the cause is moisture, the cure is moisture control in layers, not thicker steel or more grease. You reduce the water in the air with desiccant, you stop water vapour reaching the metal with a barrier, and you arm the metal itself against any moisture that does reach it with a vapour-phase corrosion inhibitor. Used together, these three defeat container rain for the whole voyage; used alone, each has a gap the others cover.