Red fuming nitric acid

Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of 84% nitric acid, 13% dinitrogen tetroxide and 1–2% water. The color of red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form nitrogen dioxide. The nitrogen dioxide dissolves until the liquid is saturated, and evaporates off into fumes with a suffocating odor. RFNA increases the flammability of combustible materials and is highly exothermic when reacting with water.

It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride; any such combination is called "inhibited RFNA" IRFNA) because nitric acid attacks most container materials.

It can also be a component of a monopropellant; with substances like amine nitrates dissolved in it, it can be used as the sole fuel in a rocket. It is not normally used this way however.

During World War II, the German military used RFNA in some rockets. The mixtures used were called S-Stoff (96% nitric acid with 4% ferric chloride) and SV-Stoff (94% nitric acid with 6% dinitrogen tetroxide) and nicknamed Salbei (sage).

Inhibited RFNA was the oxidizer of the world's most-launched light orbital rocket, the Kosmos-3M. IRFNA used in modified SS-1 Scud missiles as an oxidizer by the Iraqi military during the 1991 Persian Gulf War, was suggested as a possible factor causing Gulf War Syndrome. However, this theory was later refuted.

Other uses for RFNA include fertilizers, dye intermediates, explosives, and pharmaceutic aid as acidifier. It can also be used as a laboratory reagent in photoengraving and metal etching.

Compositions

 * IRFNA IIIa: 83.4% HNO3, 14% NO2, 2% H2O, 0.6% HF
 * IRFNA IV HDA: 54.3% HNO3, 44% NO2, 1% H2O, 0.7% HF
 * S-Stoff: 96% HNO3, 4% FeCl3
 * SV-Stoff: 94% HNO3, 6% N2O4
 * AK20: 80% HNO3, 20% N2O4
 * AK20F: 80% HNO3, 20% N2O4, fluorine-based inhibitor
 * AK20I: 80% HNO3, 20% N2O4, iodine-based inhibitor
 * AK20K: 80% HNO3, 20% N2O4, fluorine-based inhibitor
 * AK27I: 73% HNO3, 27% N2O4, iodine-based inhibitor
 * AK27P: 73% HNO3, 27% N2O4, fluorine-based inhibitor

Experiments

 * Hydrofluoric Acid Content of RFNA
 * When RFNA is used as an oxidizer for rocket fuels, it usually has a HF content of about .6%. The purpose of the HF is to act as a corrosion inhibitor. RFNA was tested for HF with a standard solution containing 12% of NO2 and a density of 1.57. These experiments were performed using an electrometric method. It was determined that the hydrofluoric acid content was about .5% by weight. This is very close to the usually .6% in rocket fuels.


 * Water content of RFNA
 * To test the water content, a sample of 80% HNO3, 8–20% NO2, and the rest H2O depending on the varied amount of NO2 in the sample. When the RFNA contained HF, there was an average H2O% between 2.4% and 4.2%. When the RFNA did not contain HF, there was an average H2O% between .1% and 5.0%. When the metal impurities from corrosion were taken into account, the H2O% increased, and the H2O% was between 2.2% and 8.8%


 * Corrosion of metals in RFNA
 * Stainless steel, aluminum alloys, iron alloys, chrome plates, tin, gold and tantalum were tested to see how RFNA affected the corrosion rates of each. This is important to understand because the containers that RFNA is kept in will affect the RFNA. Experiments were performed using 16% and 6.5% RFNA samples and the different substances listed above. Many different stainless steels showed resistance to corrosion. Aluminum alloys did not hold up as well as stainless steels especially in high temperatures but the corrosion rates were not high enough to prohibit the use of this with RFNA. Tin, gold and tantalum showed high corrosion resistance similar to that of stainless steel. These materials are better though because at high temperatures the corrosion rates did not increase much. It is interesting to note that corrosion rates at elevated temperatures increase in the presence of phosphoric acid. Conversely, the presence of sulfuric acid decreased corrosion rates.

Related

 * White fuming nitric acid
 * Nitric acid