Introduction

Precipitation
Strengthened Alloy

Nickel

Chromium

Molybdenum

Niobium

Corrosion
Resistance

88

Mechanical
Behavior

92

Purity

95

Overview

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INCONEL® alloy 725 (UNS N07725) is a nickelchromium-molybdenum-niobium alloy that is highly resistant to corrosion and is age hardenable for extremely high strength. It has essentially the same corrosion resistance as INCONEL alloy 625, which is widely used in a broad range of severely corrosive environments. The strength of age-hardened INCONEL alloy 725 is of the order of twice that of annealed alloy 625. Because the strength of alloy 725 is developed by heat treatment, not by cold work, ductility and toughness remain high. Also, strength can be imparted to large or non-uniform sections that cannot be strengthened by cold work.

High levels of nickel and chromium provide corrosion resistance in reducing and oxidizing environments. The substantial molybdenum content enhances resistance to reducing media and provides a high degree of resistance to pitting and crevice corrosion. Additionally, the combination of elements makes the alloy resistant to hydrogen embrittlement and stresscorrosion cracking.

The properties of INCONEL alloy 725 are useful for a range of applications that require outstanding corrosion resistance along with high strength. The alloy is used for hangers, landing nipples, side pocket mandrels and polished bore receptacles in sour gas service, where it resists the effects of hydrogen sulfide, chlorides and carbon dioxide. The alloy is also attractive for high strength fasteners in marine applications, where it resists corrosion, pitting and crevice attack in sea water.

Corrosion Resistance

High nickel, chromium and molybdenum contents enable INCONEL alloy 725 to resist a broad range of corrosive environments. The alloy is especially resistant to media containing carbon dioxide, chlorides and hydrogen sulfide, such as those encountered in deep sour gas wells. In such environments, INCONEL alloy 725 resists corrosion, pitting, hydrogen embrittlement and stress-corrosion cracking.

INCONEL alloy 725 is approved under NACE MR0175 for use in sour gas wells. INCONEL alloy 725 displays excellent resistance to general and localized corrosion in brines and sea water. Table 11 shows the results of crevice corrosion tests in sea water. These data show alloy 725 to be superior to alloy 625 in resistance to crevice corrosion initiation.

Physical Properties

Mechanical Properties