10% w/v citric acid solution heated to 160°F is used as a safe alternative to nitric acid. 10 minutes contact is indicated in the ASTM Metals Handbook, but I like to leave it to cool overnight. You can also save the solution for reuse, just heat it back up. I've also been recommended to use 5% w/v citric acid and 5% w/v ascorbic acid, but ascorbic acid is more expensive.