Due to its strongly oxidising nature, HNO3 (Nitric Acid) attacks most metals, causing localised corrosion issues.
Exposure to concentrated Nitric acid or acid at a high temperature can cause intense attack in particular areas on the surface of a metal, also known as pitting.
Nitric Acid: Stainless Steel Exception
For nitric acid, stainless steel 304 is often used in a replacement for 316, which is the opposite solution for most corrosion resistant tactics (for nitric.)
The British Stainless Steel Association recommends for over 95% concentration that aluminium alloys can be considered.
Please see the table below for an overview of nitric acid resistant materials and their effectiveness against attacks.
According to the “Materials Selector for Hazardous Chemicals, Vol. 5: Nitric Acid” published by MTI,
304L material is preferable over standard 304to avoid sensitisation.
Nitric Acid Resistant Materials Charts
1 – Available for use
2 – Use with caution to concentration, temperature and pressure
3 – Do not use
302 and 304 stainless steels – 1
316 stainless steel – 2
Monel – 3
6MO stainless steel – 2 (Can provide greater protection in certain circumstances due to its higher chromium content.)
Hastelloy C276 – 3
Titanium – 1 (Advantage based on its strong oxide film when exposed to any oxidising substance.
)Inconel 625 – 2 (Good resistance but no initial benefit to 825 alloyunless other acids are present.)
Incoloy 825 – 1
Why Incoloy 825 Is A Powerhouse Against Nitric Acid
UNS N08825, also known as Incoloy or alloy 825, is one of the best metals to use in areas of Nitric acid corrosion.
Its chromium content allows a resistance to a variety of oxidising substances, with broad usefulness in higher temperatures.
Alloy 825 is used within industries that require all-round hardiness when exposed to strong substances.
These industries include the nuclear sector and acid production. The chromium content of 825 is 19.5-23.5, with other elements being 38.0-46.0 Nickel, 0.6-1.2 titanium and 2.5-3.5 molybdenum.
The types of corrosion that 825 can withstand include pitting, crevice corrosion, intergranular corrosion and SSC.
Due to its natural machinability at the annealed temper, Waverley Brownall is able to manufacture incoloy 825 fittings and valves to supply the chemical processing and food industries.
Remember that metals will react variably towards nitric acid according to concentration levels, temperatures, pressures and nature of metals.
Caution must always be taken when applying fittings, tube or valves into areas where nitric acid is present. Professional advice on the ideal material choices to work with nitric acid should be sought if there are no indicators within a company’s handbook.
