Citric acid is an organic acid unlike nitric acid which is a mineral acid. It reacts with the iron much better than nitric acid and does not attack the elements in stainless steel alloy which nitric acid does. And citric acid, overall, is a less harmful chemical than nitric acid. Nitric acid raises problems with environmental disposal, health and safety. Citric acid on the other hand is derived from oranges, utilized in food and beverage products, is acceptable in some instances to go right to drain, depending upon contaminants and local environmental regulations, and much more friendlier when it comes to worker health and safety.
Even the process aspect of citric acid passivation is easier. Stainless steel must be cleaned prior to any passivation procedure. All surfaces contaminated with oil, grease, chips, grinding dust, lapping compound, buffing compound, coolant, and/or swarf must be removed.
