"Plastic" is lots of different things. I'd think polyethylene will be fine but not polylactide.

I was surprised to find that carbon fiber can sit in 50% H2SO4 indefinitely, but degrades above 65%, at normal temperatures.

https://www.gab-neumann.com/Corrosion-resistance-of-impervio...

At low concentration a sulfuric acid solution corrodes only by being an acid.

Neither carbon fiber is affected by acids, nor some types of plastic with carbon-carbon bonds, like polyethylene (but polyesters and other polymers made by polycondensation may be hydrolyzed by acids).

At high concentrations, a sulfuric acid solution begins to have an oxidizing behavior, even if not so strong as nitric acid, so it can convert the carbon from carbon fibers or some plastics into carbon dioxide, damaging them.

The difference in behavior is because the sulfate ions have a very high affinity for water. At low concentrations, they are strongly hydrated, so the attached water shields them from making direct contact with an immersed material. At high concentrations, there is much less water available for hydration and the sulfate ions can make direct contact with an immersed material. Then the oxidized sulfur atoms from the sulfate ions can oxidize any less electronegative elements.

Thank you, that was deeply enlightening.

That was fascinating. As a non- chemist it's easy to think the water is inert and 50% concentration just means 50% as much corrosion, but the water obviously plays an active role.

I think those may have fluoropolymer coatings on the inside. I recently got a ball-bearing cleaning container made of fluorinated HDPE, and they're surprisingly cheap. It's not bulk fluoropolymer; just ordinary HDPE treated with F₂ (g) as a post-processing step (says Google).

Neat. Wouldn’t a near 100% cloud be basically a liquid? What would it be like for a spacecraft to fly through that? A big puddle floating in the sky?

A near 100% H2SO4 cloud would be a cloud whose droplets have vanishingly little water in them.

As your balloon drifts up or down through it, the sheen of moisture it collects on its surface is maximum strength sulfuric acid. I hope your balloon is not of a material subject to oxidation, because if it is, you will soon discover a place even worse than in that cloud.

He meant that each droplet would be nearly 100% H2SO4, not the entire volume of the cloud.