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Nickel based alloys- Performance in Nitric acid

Nitric acid is a strong oxidizing agent hence, alloys offering the excellent resistance are those which produce passive oxide layers. The alloys containing crucial magnitude of chromium provide superior resistance as compare to those having lesser Cr content. While nickel based alloys provide high resistance to pure nitric acid,these are specifically effective in the combined acid media such as nitric acid with reducing acids for example sulfuric or phosphoric acid). When small extents of chlorides or fluorides are available, nickel-chromium-molybdenum alloys offer outstanding resistance.

Nickel alloy 200 & Monel alloy 400

Nitric acid of concentration more than 0.5% quickly corrodes the nickel alloy 200 as well as Monel alloy 400.

Incoloy alloy 800

Due to presence of chromium, Incoloy all 800 provides supreme resistance to nitric acid. It also provides stress corrosion resistance better than austenitic stainless steels. Although, similar to Inconel alloy 600 and unstabilized stainless steels, it can be sensitized and subjected to intergranular corrosion through  prolong heating in the range 1000oF to1400oF or 538oC to 760oC temperature limit.

Incoloy alloy 825

Due to its high magnitude of chromium, Incoloy alloy 825 offers outstanding resistance to nitric acid solutions at whole content percentages, and temperature limits up to and including boiling 65%. It is stabilized against sensitization by inclusion of titanium. Hence, exposure to moderate temperatures while welding or stress relieving are not risky. Post weld heat processing prior to use in the oxidizing acid conditions is not usually needed.

Inconel alloy 600 and C-276

These alloys conssit of almost 16% chromium, are resistant to nitric acid in contents above 20% at room temperatures. In boiling acid 10%, Inconel alloy 600 offers superior functionality.

Although, it can be sensitized and made to attain high intergranular attack rates in warm acid, subjecting to moderate temperature can make these alloys susceptible, causing sensitivity to intergranular corrosion.

Inconel alloy 625

Inconel alloy 625 offers high resistance to nitric acid. A corrosion rate of about 30 mpy or 0.76 mm/a in boiling 65% acid is normal. It is stable against corrosion and doesn’t require post weld heat processing procedures prior to service in oxidizing acid conditions. In a boiling 15% nitric/ 3% hydrofluoric acid solution, where AISI 316 stainless steel corroded at 236 mpy or 5.99 mm/a, Inconel alloy 625 is attacked by merely 34 mpy or 0.86 mm per annum. 

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