Passivation is a chemical process that enhances the natural corrosion resistance of stainless steel by removing free iron and contaminants from its surface. This treatment forms a uniform and protective chromium oxide layer, acting as a barrier against environmental factors that can cause corrosion.
It is crucial process in stainless steel production. During manufacturing processes like rolling, forming, or welding, contaminants such as grease, oil, and free iron particles can accumulate on stainless steel surfaces. These disrupt the protective oxide layer, making the steel susceptible to corrosion. Passivation restores and enhances this layer, guaranteeing the material's longevity and performance.
Nitric acid (HNO₃) is the ideal choice for passivation thanks to its powerful oxidizing properties. When stainless steel is treated with nitric acid, two key reactions occur:
First, nitric acid reacts with, for example, free iron particles on the surface, converting them into soluble iron ions (Fe²⁺ and Fe³⁺) that are removed from the surface.
Fe + 4HNO3 → Fe(NO3)3 + NO + 2H2O
The oxidizing nature of nitric acid ensures the formation of a thickened chromium oxide layer (Cr₂O₃), enhancing corrosion resistance.
Environmental Impact of Nitric Acid Disposal and Wastewater Contamination
However, the environmental impact of disposing of nitric acid must be considered. After passivation, the used nitric acid solution must be neutralized before disposal, which raises significant environmental concerns.
The acidic waste is neutralized using alkaline substances like sodium hydroxide or lime. This reaction produces nitrate salts (e.g. sodium nitrate), which are highly soluble in water. The nitrates that result in wastewater cause environmental problems such as eutrophication. Excess nitrates act as nutrients for algae and aquatic plants, leading to overgrowth, algal blooms and subsequent depletion of dissolved oxygen in water bodies. This process has a detrimental impact on aquatic life and disrupts ecosystems.
Furthermore, the neutralization process can precipitate heavy metals like iron, chromium and nickel from the stainless-steel surface into a sludge. This toxic sludge must be handled and disposed of carefully in accordance with environmental regulations to prevent soil and groundwater contamination. Improper disposal will inevitably result in the accumulation of hazardous substances in the environment.
SUSTEC's Breakthrough in Sustainable Acid Recovery for Stainless Steel Passivation Bathes
At SUSTEC, we recognized the need for a sustainable approach and undertook intensive research to address the problem.
Our research led to the development of a leading solution on the market to recover and recycle the used passivation bath acid completely. This process effectively regenerates the nitric acid, allowing it to be reused in passivation treatments and creating a closed-loop system.
Recovering the acid significantly reduces the generation of nitrates in wastewater and minimizes the production of toxic sludge. This approach not only mitigates environmental pollution but also promotes resource efficiency.
Recycling the acid is a cost-effective solution that reduces operational costs associated with purchasing fresh nitric acid and disposing of hazardous waste. It is a win-win for both the environment and the economy. This solution enables stainless steel producers to use the most efficient passivation chemicals and processes while eliminating the environmental impact typically associated with these conventional methods.
