All acids and salts will corrode iron metals, but they take moisture, temperature and time. Some vapours will work pretty even rust on tools overnight, found that with printed circuit etching solution, when I left it overnight in one cabinet with some tools....
Ferric chlorid used to etch printed circuit boards leaves fairly nasty looking stains on most metals. PC-board solutions are designed to eat copper and will etch pretty effectively most metals, some will need some heat.
Mr. Muscle (liquid plummer) leaves fairly matte and metallic finish.
One etch will work on one effect, you may need to use several in succession to get the rich texture you are after.
You may consider hapharzad sand blasting and/or wire mesh before etching. Least good wash to remove oil and all that of the new steel plate.
Pekka