Corrosion Resistance of Alloy 716 in Chloride, Acidic, and Sulfur-Containing Environments

Element	Typical Content (%)
Nickel (Ni)	50–55
Chromium (Cr)	17–21
Iron (Fe)	Balance
Molybdenum (Mo)	2.5–3.0
Niobium (Cb) + Tantalum (Ta)	4.5–5.5
Titanium (Ti)	0.6–1.0
Aluminum (Al)	0.2–0.8
Carbon (C)	≤0.08
Manganese (Mn)	≤0.35
Silicon (Si)	≤0.35

This complex alloying design forms a stable, protective passive film on the surface, making it highly resistant to localized and general corrosion in chemical and marine conditions.

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2. Corrosion Mechanisms in Harsh Environments

Before discussing Alloy 716’s performance, it’s important to understand the types of corrosion it must resist:

• Pitting and Crevice Corrosion: Common in chloride-containing environments such as seawater or salt-laden atmospheres.

• Stress Corrosion Cracking (SCC): Occurs when tensile stress and corrosive media (like chlorides or H₂S) combine.

• Sulfidation: Caused by sulfur compounds in oil, gas, and refinery environments at elevated temperatures.

• Acidic Corrosion: Arises from exposure to mineral acids like sulfuric, nitric, and hydrochloric acid.

Alloy 716’s microstructure and chemical balance make it exceptionally resistant to these forms of degradation.

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3. Alloy 716 in Chloride Environments

Chloride ions are among the most aggressive species in corrosion science. They penetrate passive films and initiate pitting and crevice corrosion, especially in stainless steels and lower-grade nickel alloys.

A. Pitting Resistance

The combination of chromium and molybdenum in Alloy 716 enhances the formation of a dense passive oxide layer, significantly improving resistance to chloride-induced pitting.

In standardized ASTM G48 testing, Alloy 716 exhibits superior pitting resistance equivalent to or better than Alloy 718, making it suitable for marine, subsea, and offshore applications.

B. Crevice Corrosion Resistance

Alloy 716 performs well under stagnant or low-flow conditions — environments where crevice corrosion typically initiates. Its passive film remains stable even under differential aeration.

C. Chloride Stress Corrosion Cracking

Thanks to its nickel-rich matrix, Alloy 716 resists chloride-induced SCC up to temperatures exceeding 350°C (660°F). This makes it far more reliable than austenitic stainless steels such as 304 or 316 in brine and seawater exposure.

D. Typical Applications

• Subsea wellhead components

• Heat exchangers in seawater-cooled systems

• Offshore riser tubing and manifolds

• Pump shafts and impellers

The alloy’s excellent resistance to chloride attack ensures long service life and minimal maintenance in marine environments.

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4. Alloy 716 in Acidic Environments

Acidic media — including sulfuric (H₂SO₄), hydrochloric (HCl), and nitric (HNO₃) acids — are notorious for attacking metals, particularly at elevated temperatures or high concentrations.

A. Sulfuric and Hydrochloric Acids

Chromium provides a protective oxide layer, while molybdenum enhances resistance to localized acid attack. Alloy 716 performs well in dilute to moderate concentrations of sulfuric and hydrochloric acids, outperforming conventional stainless steels.

B. Nitric Acid and Mixed Acids

Due to its high nickel and chromium content, Alloy 716 shows excellent resistance to oxidizing acids such as nitric acid, where it remains stable and does not suffer intergranular corrosion.

C. Resistance to Acid Rain and Industrial Vapors

In chemical plants or power stations where acid vapors and condensates are present, Alloy 716’s passive film remains protective, preventing corrosion and material degradation over time.

D. Applications

• Chemical processing reactors and acid scrubbers

• Piping systems in refineries

• Heat exchangers in acid recovery systems

• Exhaust and flue gas components

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5. Alloy 716 in Sulfur-Containing (H₂S) Environments

One of the most demanding conditions for materials is exposure to hydrogen sulfide (H₂S), common in oil and gas wells. H₂S can cause sulfide stress cracking (SSC) and hydrogen embrittlement in many steels and alloys.

A. Compliance with NACE MR0175 / ISO 15156

Alloy 716 fully complies with the requirements of NACE MR0175 / ISO 15156, making it suitable for sour gas service. This certification ensures resistance to H₂S-induced cracking and pitting under high pressure and temperature.

B. Sulfidation Resistance at High Temperature

At temperatures up to 700°C (1300°F), Alloy 716 resists sulfur-induced oxidation and scaling due to the presence of nickel and chromium oxides that act as a protective barrier.

C. Resistance to Hydrogen-Induced Cracking

The alloy’s dense microstructure and controlled carbon content prevent hydrogen penetration, maintaining ductility and toughness in hydrogen-rich or sour environments.

D. Applications

• Downhole tools and packers in sour gas wells

• High-pressure valve bodies and manifolds

• Gas turbine exhaust components exposed to sulfur combustion

• Petrochemical reactors and reformer tubes

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6. Microstructural Stability and Heat Resistance

Alloy 716’s microstructure remains stable after prolonged exposure to corrosive and thermal environments. The precipitation of strengthening phases (γ’ and γ’’) ensures both mechanical stability and corrosion resistance, even after thousands of hours at elevated temperature.

This stability is particularly important in cyclic temperature operations such as turbines, reactors, and heat exchangers, where thermal fatigue and oxidation can otherwise lead to failure.

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7. Surface Protection and Maintenance

Although inherently resistant to corrosion, Alloy 716’s performance can be further enhanced by:

• Polished or passivated surfaces, reducing initiation sites for pitting.

• Proper heat treatment, restoring passive film integrity after welding.

• Controlled fabrication environments, avoiding contamination from carbon steel or chlorides.

With proper handling and installation, the service life of Alloy 716 components can exceed decades in aggressive conditions.

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8. Comparative Performance