In response to your question, Why steel and not aluminum. probably cost of aluminum versus steel and durability, steel harder, aluminum softer. These propane tanks can see alot of banging around, more so in industrial use.
As far as your compressor tank , They rust from the inside. Thats why it is very improtant to drain the water out often. That is one of the hidden dangers of compressor tanks especialy those in peoples garages, Where they may not be maintained properly, I use pressure vessels daily in my work They must be inspected and certified yearly by CAL OSHA and MSHA. ( Mining saftey and health administration ). And if not maintained properly can be lethal. I've seen the results of a vessel failure under pressure it aint pretty. Hence why they(people that fill your tanks) are required to check the exp. date on the tank But again most dont. kinda scary if you ask me.
Propane tanks only rust form the outside, Never the inside and thats only if you dont keep them in good
shape and painted. The paint is not there for looks it is the barrier between the air and the steel, We've all seen how fast unpainted steel rusts. After maybe years of service and if left to rust, steel gets thinner and if you sand and scrape what is it you think your removing, The Steel thickness. They only last so long people, bite the bullet and replace old tanks or get them properly inspected and recertified
Sorry about the hijack of the thread, but when I read what that propane person told you I was floored to say the least. People really dont know all the dangers associated with pressure vessels and i just had to speak up. They really can be deadly !!!!!
Best Regards, Roy