Green Chemistry for Stainless Steel Corrosion Resistance: Life Cycle Assessment (LCA) of Citric Acid versus Nitric Acid Passivation

Corrosion is a significant problem in many industries, and when using stainless steel, passivation is undertaken to improve corrosion resistance.
Traditionally, nitric acid is used within the passivation step, however, this has some detrimental environmental and human health impacts during its production and use. Reducing this impact is critical, and because of its toxicity, associated occupational risk and special disposal requirements, end-users of passivated stainless steels are exploring alternative passivation methods. However, it is also critical to understand the impact of any alternatives. Sustainable processing and manufacture embodies many elements, including; waste reduction, resource efficiency measures, energy reduction and the application of ‘green’ or renewable chemicals. In order to ensure the most effective system is utilised the impact, or potential impact of the system must be measured and options compared. The comparative environmental credentials of bio-based chemicals can be assessed using tools such as Life Cycle Assessment (LCA).

This paper is the first paper to evaluate the environmental impact of passivation using nitric and citric acid. It uses attributional Life Cycle Assessment (ALCA) to assess the environmental benefits and dis-benefits of using citric acid - produced biologically via fermentation, to replace nitric acid, whilst keeping the same level of corrosion resistance. The work is anticipatory in nature as the process is not yet undertaken on a commercial basis. The results therefore feed into future manufacturing and design.

Citric and nitric acids were compared using three different solutions: 4% and 10% citric acid solutions, and a 10% nitric acid solution (the conventional case). The results show that a scenario using a 4% citric acid solution is environmentally preferable to nitric acid across all impact categories assessed. However, a 10% citric acid solution used on low chromium and nickel steel was only environmentally preferable for 50% of the environmental impact categories assessed due to increased electrical energy demand for that scenario.