Nickel-Based Alloy "Intergranular Corrosion"

In today’s highly industrialized world, stringent performance requirements are imposed on metallic materials across various fields, bringing intergranular corrosion (IGC) into sharper focus. From chemical engineering and marine projects to aerospace applications, once metal components suffer from IGC, their structural integrity and mechanical properties are significantly compromised. This may lead to catastrophic consequences, such as chemical pipeline rupture or aircraft component failure, which pose serious threats to production safety and human lives. Therefore, in-depth exploration of the essence, mechanisms, and preventive strategies of IGC is of paramount importance for ensuring the stable operation of industrial facilities and driving technological advancements across industries. 

Concept 

Intergranular corrosion is a type of localized corrosion that propagates along the grain boundaries of a metal. It primarily results from differences in chemical composition between grain surfaces and interiors, as well as the presence of impurities or internal stresses at grain boundaries. IGC disrupts the cohesion between grains, significantly reducing the mechanical strength of metals. After corrosion occurs, the metal or alloy surface may retain a metallic luster, showing no obvious signs of damage, yet the bond between grains weakens dramatically, degrading mechanical properties to the extent that the material cannot withstand impact, making it a particularly dangerous form of corrosion. IGC commonly occurs in brass, hard aluminum alloys, certain stainless steels, and nickel-based alloys. 

Differences Between Stainless Steel and Nickel-Based Alloy IGC 

1. Corrosion Mechanism 

• Stainless Steel: The primary cause is chromium depletion at grain boundaries. Within a specific temperature range (e.g., 450–850°C), carbon reacts with chromium at grain boundaries to form carbides. Due to the slow diffusion of chromium, chromium content near the grain boundaries decreases. When it falls below the level required for corrosion resistance, grain boundaries become preferentially corroded in corrosive environments. 

• Nickel-Based Alloys: In addition to elemental depletion, IGC involves the precipitation of secondary phases at grain boundaries. For example, in nickel-molybdenum alloys, elements like molybdenum segregate at grain boundaries or form intermetallic compounds under sensitization temperatures. This creates a potential difference between the grain boundary and the grain interior, triggering IGC. 

2. Sensitization Temperature 

• Stainless Steel: The sensitization temperature range is typically around 450–850°C, where grain boundary carbide precipitation leads to chromium depletion. 

• Nickel-Based Alloys: The sensitization temperature range is broader, generally between 600–1000°C. Variations in alloy composition result in differing sensitization processes and precipitate types compared to stainless steel. 

3. Corrosion Resistance 

• Stainless Steel: Corrosion resistance mainly depends on the passive film formed by chromium. Once chromium depletion occurs at grain boundaries, the passive film is destroyed, accelerating IGC, especially in acidic media (e.g., hydrochloric or sulfuric acid). 

• Nickel-Based Alloys: Nickel itself imparts good stability in many corrosive environments. Additionally, other alloying elements (e.g., molybdenum, chromium) enhance its broad-spectrum corrosion resistance, offering protection in both reducing and oxidizing acid environments. The impact of IGC on overall corrosion resistance in nickel-based alloys is more complex than in stainless steel. 

4. Corrosion Morphology 

• Stainless Steel: IGC often manifests as fine cracks along grain boundaries. In severe cases, materials lose strength and toughness, and macroscopic phenomena like surface spalling or powdering may occur. 

• Nickel-Based Alloys: Corrosion morphology varies with alloy composition and the corrosive environment. It may present as grooves deepened by grain boundary corrosion or as accumulated corrosion products at grain boundaries. 

ASTM G28 Intergranular Corrosion Testing 

The ASTM G28 intergranular corrosion test is a standard method for evaluating the susceptibility of forged nickel-rich chromium alloys to IGC. It includes Method A and Method B: 

• Method A: Uses a boiling solution of ferric sulfate and 50% sulfuric acid. 

• Method B: Employs a boiling solution containing 23% sulfuric acid, 1.2% hydrochloric acid, 1% ferric chloride, and 1% cupric chloride. 

Regardless of the method, samples must be properly prepared before testing. This typically involves cutting the sample to a suitable size and polishing the surface to ensure accurate results. In some cases, sensitization treatment may be required to simulate conditions where IGC is likely. 

After testing, the extent of IGC is assessed by measuring weight loss, observing surface changes (e.g., cracks or corrosion pits), and other indicators. Method A considers a corrosion rate of 12 mm/year as a reference, with typical test results for standard products ranging from 5–9 mm/year. Method B, however, does not specify precise numerical requirements but evaluates sensitivity to IGC by observing whether the alloy exhibits step-function increases in corrosion rate due to grain boundary precipitation issues. 

In industries such as chemical engineering, petroleum, and aerospace, nickel-rich chromium alloy equipment and components require excellent corrosion resistance. The ASTM G28 standard provides a unified and comparable testing method, enabling engineers and researchers to assess IGC risks under specific conditions, select appropriate materials, optimize processing techniques, and ensure product quality and safety.