Nitric acid

Additional physico-chemical information

Description of key information

Additional physico-chemical information is added to the dossier related to the corrosiveness to metals of nitric acid. A test for corrosiveness to metals has been waived. Corrosive properties from nitric acid to metals are well known and two supporting studies are added to demonstrate those properties. In addition, information on the production of concentrated nitric acid had been added to the dossier. The latter information is used in a weight of evidence approach.

Additional information

Corrosiveness to metals:

Data waiver:

Based on the experience with the substance and the extreme low pH of nitric acid, testing for corrosivity to metals is not considered to be necessary. Generally, extreme pH-values point to a higher likelihood that the substance or mixture is corrosive. However, it cannot lead to immediate classification in the hazard class corrosive to metals. For nitric acid, an example of corrosiveness to metals is added in the Guidance on the Application of the CLP criteria, page 255. Highly concentrated nitric acid (97%) (UN2031) is considered not corrosive to steel but corrosive to aluminium. In addition, it is classified as corrosive to metals. In contrary, red fuming nitric acid (UN2032) is not corrosive to steel neither to aluminium. It is not classified for corrosiveness to metals.

In a summary of Dechema (2013), it was concluded that while carbon steel or low alloy steel is strongly attacked by diluted nitric acid, it is resistant to corrosion in concentrated nitric acid (> 40%) due to the formation of a protective layer on the steel surface.

Crooks (2004) described that there are major discrepancies in published corrosion rates for various metals and alloys in nitric acid. This can be due to different nitric acid concentrations and test temperatures, but also due to the general variability in corrosion testing caused by differences in sample composition or finish, temperature control, test techniques, changes of oxidizing potential due to build-up of metal ions in solution etc. In general, stainless steels show the highest resistance against corrosion.

Aluminium alloys:

Pure aluminium has the highest corrosion rate in about 30% nitric acid with the rate being reduced at higher and lower strengths. Reported corrosion rates for aluminium 3003 in high nitric acid concentrations (93 -99%) are 0.05 -0.006 mm/y (at 32.3°C).

Iron and steel:

Ferrous alloys consisting of iron plus carbon or other elements in small amounts are passivated upon immersion in concentrated nitric acid, but the protective film is so easily damaged that they are not useful in nitric acid applications. In practice, both wrought and cast ferrous alloys corrode very rapidly. Reported corrosion rates for iron in various strengths of nitric acid (at 50°C) are 3,500 mm/y, 105,000 mm/y and 121,000 mm/y at 10%, 30% and 65% HNO3, respectively.

Stainless steels:

Stainless steels are the primary materials used in nitric acid service. Reported corrosion rate for 304 stainless steel in 65% HNO3 (at 50°C) is 10 mm/y and at higher nitric acid concentrations (93 -99%) 0.02 -0.58 mm/y (at 32.3°C).

Based on the available information on the corrosive properties of nitric acid to metals, the substance is classified as corrosive to metals category 1, H290.

Production of concentrated nitric acid

Keleti (1985) described indirect and direct methods for the production of concentrated nitric acid.

The overall reaction that yields nitric acid in industrial processes is the following:

NH3 + 2O2 -> HNO3 + H2O

It allows 77.8% wt % HNO3 as maximum. But process water must be added at the top of the absorption tower; therefore the produced industrial acid has a concentration between 50 and 70 wt%. This is well suited for fertilizer production, but, for example, for organic nitration concentrated (98 - 100%) nitric acid is needed. The simplest method, concentration of the diluted solution by distillation, is not possible: Nitric acid and water form an azeotropic mixture (68.4% at atmospheric pressure).

Concentrated nitric acid can be produced following either of two main routes: the indirect or the direct method. Indirect methods start with the weak nitric acid coming from a normal combustion-absorption plant and break down the azotropic mixture which would form in the distillation by adding a third component, a dehydrating agent, like concentrated sulfuric acid or magnesium nitrate to the weak acid, a technique called extractive distillation.

Direct methods do not start with weak acid, but elminate the excess water formed during the combustion of ammonia in the rapid cooler and process the nitrous gas mixture to arrive directly to a concentrated nitric acid. Normal absorption in water always gives diluted acid. There are nevertheless two possibilities. The first separates the fully oxidized nitrogen dioxide in liquid form from the gas mixture by absorption in concentrated acid and desorption and then lets it react under pressure with pure oxygen and water (or weak acid); in the second, the nitrogen oxides react with azeotropic acid to form a superazeotropic acid, which by simple distillation yields concentrated and azeotropic acids. The latter can be totally recirculated or used to produce diluted commercial-grade acid.

Kirk-othmer (2005) described that nitric acid concentration processes use extractive distillation to concentrate weak acid up to 99 wt%. A dehydrating agent, such as sulfuric acid or magnesium nitrate, is used to enhance the volatility of HNO3 so that distillation methods can surpass the azeotropic concentration of nitric acid. Weak acid and dehydrating agent are fed to a distillation column. Water removed from the acid dilutes the dehydrating agent, which is removed as a bottom stream and later concentrated for reuse in the process. Superazeotropic vapors pass to the bottom of a rectification section in which the acid is concentrated up to 99wt%. The strong nitric vapors are condensed overhead and a portion of the acid is returned to the column as reflux.