Intergranular Corrosion

Intergranular corrosion in aluminum is primarily caused by the presence of impurities and alloying elements that precipitate at grain boundaries. Here are the main factors:

1. Heat Treatment: Improper heat treatment can cause the precipitation of alloying elements, such as magnesium and silicon, at grain boundaries, making them susceptible to corrosion.
2. Alloy Composition: Certain alloying elements, such as copper, can increase the susceptibility of aluminum alloys to intergranular corrosion.
3. Environmental Factors: Exposure to aggressive environments, such as those containing chlorides or high humidity, can accelerate intergranular corrosion.
4. Electrochemical Potential: Differences in electrochemical potential between grain boundaries and the grain interiors create a galvanic cell, leading to localized corrosion at the grain boundaries.

Prevention:

• Proper Heat Treatment: Ensure correct heat treatment procedures to avoid harmful precipitation at grain boundaries.
• Alloy Selection: Choose aluminum alloys with a composition that minimizes susceptibility to intergranular corrosion.
• Protective Coatings: Apply coatings or anodizing to protect the surface from corrosive environments.

By understanding and controlling these factors, the risk of intergranular corrosion in aluminum can be significantly reduced.

Intergranular oxidation is caused by the diffusion of oxygen along the grain boundaries of a material, leading to localized oxidation. Here are the primary causes:

1. High Temperatures: Elevated temperatures accelerate the diffusion of oxygen into the grain boundaries, promoting oxidation.
2. Alloy Composition: Certain alloying elements, such as chromium and aluminum, can form oxides more readily, contributing to intergranular oxidation.
3. Oxidizing Environment: Exposure to environments rich in oxygen, such as air or combustion gases, increases the likelihood of oxidation.
4. Stress Concentrations: Mechanical stresses can enhance the diffusion of oxygen along grain boundaries, exacerbating oxidation.

Prevention:

• Temperature Control: Minimize exposure to high temperatures or employ protective atmospheres during high-temperature processes.
• Material Selection: Use alloys with elements that form protective oxide layers, such as chromium in stainless steels.
• Protective Coatings: Apply coatings or surface treatments to prevent oxygen penetration.

By understanding the causes and implementing preventive measures, intergranular oxidation can be mitigated to protect material integrity and performance.

Stress corrosion cracking (SCC) is a complex phenomenon that requires three key factors to occur simultaneously:

1. Tensile Stress: The presence of tensile stress, either applied or residual, is essential for SCC. This stress can result from mechanical loads, thermal cycles, or welding processes.
2. Corrosive Environment: A specific corrosive environment is necessary to initiate and propagate SCC. Common environments include chlorides for stainless steels, caustic solutions for carbon steels, and ammonia for brass.
3. Susceptible Material: The material must be susceptible to SCC in the given environment. Factors such as alloy composition, microstructure, and heat treatment can influence susceptibility.

Preventive Measures:

• Stress Relief: Reduce residual stresses through heat treatment, annealing, or stress-relief processes.
• Environmental Control: Minimize exposure to specific corrosive agents known to cause SCC for the material in use.
• Material Selection: Use materials with higher resistance to SCC in the intended service environment.

By addressing these three factors, the risk of stress corrosion cracking can be significantly reduced.

The temperature at which stress corrosion cracking (SCC) occurs depends on the specific material and the corrosive environment. However, SCC typically occurs within a specific temperature range where both the corrosive environment and tensile stress are effective. Here are some general guidelines:

1. Stainless Steels in Chloride Environments: SCC is most likely to occur between 50°C and 150°C (122°F to 302°F). This temperature range provides optimal conditions for chloride-induced cracking.
2. Carbon Steels in Caustic Environments: SCC can occur at temperatures above 60°C (140°F), especially in concentrated caustic solutions.
3. Brass in Ammonia Environments: SCC is known to occur at room temperature to slightly elevated temperatures when exposed to ammonia or ammonium compounds.
4. High-Strength Alloys: SCC in high-strength alloys, such as certain aluminum or titanium alloys, can occur over a wide range of temperatures, depending on the environment and stress levels.