Saltwater will eat metal over time, but it shouldn't just be eating important fixtures of the tank if things are set up and maintained properly. Tanks with metal rims can rust/corrode unless the metal is protected/coated, but usually those aren't good to use with saltwater for that reason. If the tanks are just glass and plastic, you shouldn't see any decay of the tank itself, and most other equipment these days should be pretty resistant to that kind of thing too. Generally, I would be pretty worried if I saw actual corrosion on some piece of equipment in my tank other than dip-in things like tongs and scrapers. If I saw visible evidence of corrosion on a heater, pump, or filter, that any other electrical thing in the water it would be removed immediately since it's a potential safety issue. However, I've never seen anything like that on my equipment except for one old, badly designed light fixture a long time ago (which I discarded as soon as I realized there was no way to stop salt deposits from getting into part of it).
If what you're seeing is salt crust rather than corrosion, that's just a fact of life with saltwater tanks and isn't harmful unless you let it creep towards or into electrical things. Salt deposits are controlled by leaving a gap between the rim and waterline and having less splashing of the water's surface. It's also easy to control just by giving the tank a wipe-down periodically (what I do since I have a lot of surface agitation). Metal tank stands need regular wipe downs with marine tanks since those can rust if salt crust gets on them and a rusting tank stand is bad for obvious reasons.
Actual live barnacles are not common long-term in marine tanks. If a store had a population of those long term I'd be pretty impressed. However, there are many other filter feeding organisms that are very common, so probably you're seeing those. Tanks with live rock will typically have more encrusting filter feeders and it's usually a good thing to have at least some. Biodiversity is a good thing in most marine tanks. I have lots of encrusting animals in my tanks and just leave them be along unless they're starting to block flow in a filter. The encrusting filter feeders eat particulates out of the water and my hermit crabs sometimes nibble on the encrusting filter feeders. If you want a "cleaner" tank you can accomplish that to a degree, but healthy reef tanks don't usually look sterile.
