Information

INCONEL alloy 690 (UNS N06690/W. Nr. 2.4642) is a high-chromium nickel alloy having excellent resistance to many corrosive aqueous media and high temperature atmospheres. In addition to its corrosion resistance, alloy 690 has high strength, good metallurgical stability, and favorable fabrication characteristics.

The high level of nickel imparts resistance to stress corrosion cracking in chloride-containing environments as well as to sodium hydroxide solutions.

The properties of INCONEL alloy 690 are useful for various applications involving nitric or nitric/hydrofluoric acid solutions. Examples are tail-gas reheaters used in nitric acid production and heating coils and tanks for nitric/hydrofluoric solutions used in pickling of stainless steels and reprocessing of nuclear fuels.

The alloy��s resistance to sulfur-containing gases makes it an attractive material for such applications as coal-gasification units, burners and ducts for processing sulfuric acid, furnaces for petrochemical processing, recuperates, incinerators, and glass vitrification equipment for radioactive waste disposal.

In various types of high-temperature water, alloy 690 displays low corrosion rates and excellent resistance to stress-corrosion cracking. Thus, alloy 690 is widely used for steam generator tubes, baffles, tube sheets, and hardware in nuclear power generation.

 

Stress-Corrosion Cracking

INCONEL alloy 690 has good resistance to stress-corrosion cracking in many environments including chloride containing solutions, high-temperature water,  polythionic acid, and moderate concentrations of sodium hydroxide.

In boiling 45% magnesium chloride, stressed U-bend specimens of alloy 690 did not crack in 30 days of exposure.

Alloy 690 has been tested extensively for resistance to stress-corrosion cracking in high-temperature water such as encountered in nuclear steam generators. The results show the alloy to be highly resistant to cracking in chloride containing water, oxygen-containing water under crevice conditions, and deaerated (oxygen of 0.02 ppm or less) water.

In polythionic acid, stressed U-bends of alloy 690 showed no cracking after exposure for 720 h to a solution that cracked sensitized Type 304 stainless steel within 1 h.

The specimens of alloy 690 were tested both in the annealed condition and after being exposed to 600��F (315��C) for up to 1000 h. The results of stress-corrosion-cracking tests in sodium hydroxide are given in Table 8. The tests were performed on U-bend specimens in nondeaerated sodium hydroxide.

Testing at 500��F (260��C) and 610��F (320��C) was done in autoclaves having air in the head space. The results indicate that alloy 690 resists cracking in boiling sodium hydroxide at concentrations up to about 50%. Severe general corrosion occurred at 500��F (260��C) and 610��F (320��C) in concentrations of 30% and greater. In deaerated 1% sodium hydroxide at 600��F (316��C), U-bend specimens did not crack in 9400 h. Additional tests in sodium hydroxide were performed on bent-beam specimens stressed at 50%, 90%, and 100% of yield strength (0.2% offset). No cracking occurred in 100 h of exposure to boiling 80% sodium hydroxide.

 

Heating and Pickling

Like other nickel alloys, alloy 690 should be clean before it is heated and should be heated in a low-sulfur atmosphere. Furnace atmospheres for open heating should also be slightly reducing to prevent excessive oxidation of the material.

INCONEL alloy 690 is a solid-solution alloy and is not hard enable by heat treatment. The alloy is normally used in the annealed condition.

 

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