My understanding is that shipping containers are usually made of COR-TEN steel, which forms a protective layer when it does rust (similar to the protective oxide layers that form on aluminum and stainless steel). But considering the containers are exposed to sea salt spray pretty much continuously, it makes sense that the additional protection of a coat of paint is necessary.

No, that's not going to work.

CLIMATES NOT TO USE COR-TEN®:

COR-TEN® steel requires alternating wet and dry cycles to form a properly adhered protective layer. Areas that have salt laden air, high rainfall, humidity, or persistent fog are typically not the proper environment for COR-TEN®.

http://www.corten.com/frequently-asked-questions.html

Aware of that one. The trail bridge at Pilarcitos Creek on the California Coastal Trail in Half Moon Bay was made of unpainted Cor-Ten steel. That will not last in the coastal zone, where it never dries out. The bridge collapsed when park staff drove a tractor across it.[1] No injuries, but the heavily used trail was out for most of two years.

The new bridge is made of marine-grade aluminum.

[1] http://www.hmbreview.com/news/groundbreaking-for-coastal-tra...

even if the containers are made with corten, it's not good for sea. The protective rust keeps getting washed off by the rain and the salt is too much for it to handle. Ends up rusting just as bad..

Especially in the ocean. A galvanizing zinc paint perhaps to stop rust? I think this is why cars now don't rust as much as they used too.

https://en.wikipedia.org/wiki/Galvanization

That's not why cars don't rust as much. Coatings have just gotten better and simulations have gotten better so engineers can know in advance that all the salty water that runs down the vehicle will be channeled through a few spots that will rust out in short order then do it anyway.

Galvanization was popular for a brief time while the industry was still trying to find replacements for lead in paint that didn't have crap performance.

Cold galvanization is expensive and involves nasty chemicals. Hot galvanization is cheaper but not as cheap as paint and involves equally nasty chemicals and molten metals.

Basically paint is cheap and good enough.

Unibody construction helps some as well. Less drilled holes, more plastic panels, better sealed seams, fewer dissimilar metals touching.