Nickel-base alloys pitting corrosion

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

Nickel-based alloys withstand chlorides in the feed better than does stainless steel but under extreme conditions, pitting, SCC and crevice corrosion is present [19, 20, 21]. The corrosion takes place in non-annealed structures in the as welded condition. Performance of these alloys may be enhanced by post-weld solution annealing [22]. 

Steel phases have an influence on the rate of corrosion. Ferrite has a weak resistance to pitting. The presence of martensite can increase the hydrogen fragilization of steel. Intermetallic phases as Fe2Mo in high Ni content alloys can influence the corrosion resistance. The precipitate CuA12 in aluminum alloys the series 2000 is more noble than the matrix, with corrosion around the precipitate. The majority of case histories reported in the literature have involved austenitic stainless steels, aluminum alloys, and to a lesser degree, some ferritic stainless steels and nickel-based alloys.31... 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

Sulfite pulping chemical recovery consists of greater amounts of sulfite, hydrogen sulfide, and hydrochloric acid than those used in the Kraft process. Because of these corrosive species, the internal portions of the recovery boilers and the evaporators are generally constructed of reinforced plastics, type 316L stainless steel, type 317L stainless steel, or nickel-based alloys. To prevent pitting and crevice corrosion, scale build-up should be prevented, wet-dry zones should be avoided, and chloride concentration should be kept to a minimum. 

Nickel-base alloys (Inconel) Generally has good corrosion-resistant qualities. Sometimes susceptible to pitting Green powdery deposit... 

Oldfield, J. W., Test Techniques for Pitting and Crevice Corrosion Resistance of Stainless Steels and Nickel Base Alloys in Chloride Containing Environments, International Materials Reviews, Vol. 32, No. 3, 1987, pp. 153-170. 

Molybdenum, and sometimes tungsten, are also added to Ni-Cr-Fe allo3rs for improved localized corrosion (pitting and crevice corrosion) resistance. An example is alloy C-276 (UNS N10276), which contains about 57Ni-15.5Cr-15.5Mo-4W-5Fe and is one of the most pit-resistant nickel-base alloys available. 

Cast equivalents are available for many nickel-base alloys. Some test procedures, such as ASTM G 28, do not apply to cast materials. In most cases, castings should be expected to have lower corrosion resistance than wrought materials of similar composition. Also, selective phase attack may be confused with other corrosion mechanisms such as pitting. 

While general corrosion resistance is important, one of the major reasons that nickel-based alloys are specified for many applications is their excellent resistance to localized corrosion, such as pitting, crevice corrosion, and stress corrosion cracking. In many environments, austenitic stainless steels do not exhibit general attack but suffer from significant localized attack, resulting in excessive downtime and/or expensive repair and replacement. 

HASTELLOY alloy G—A columbium-stabilized. nickel-base alloy with excellent resistance to hot sulfuric and phosphoric acids Alloy G will withstand the corrosive affects of both oxidizing and reducing agents and can handle both acid and alkaline solutions. It resists pitting and has exceptional resistance to stress-corrosion cracking... 

Nickel-based alloys. Nickel-based alloys such as Inconel 625, Hastelloys C-276 and C-22, and titanium are not subject to pitting or crevice corrosion in low-velocity seawater, nor do they suffer impingement attack at high velocity. However, price limits their use to special applications in seawater systems. 

Most studies have shown that there should be few corrosion concerns for stainless steels or nickel base alloys in water aside from stress corrosion cracking, crevice corrosion and pitting however, the use of stainless steels and nickel alloys with NaK can present significant problems. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

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