How can I force metal sheet to rust rapidly?
The corrugated certainly looks like galvanized steel where some galvanized has corroded. You need to remove the remaining zinc . Dissolving it with hydrochloridic / muriatic acid would be fast , 10% should be strong enough.Salt water, vinegar, citric acid etc, would do it but slower. Once the zinc is gone it should rust pretty fast , a little dilute salt or acidic water spray will speed it up.