Material Degradation Processes

This entry discusses the destructive attack on a material by reaction with its environment. This phenomenon involves electrochemical or chemical reactions, leading to the deterioration of physical properties, structural integrity, and overall functionality.

Mechanisms of Material Degradation

• Uniform Attack: Degradation progresses evenly over the entire exposed surface, leading to a relatively predictable thinning of the material.
• Localized Attack: Degradation is concentrated in specific areas, resulting in more severe damage and potentially leading to catastrophic failure. Examples include pitting, crevice, and galvanic types.
• Galvanic Action: Occurs when dissimilar metals are in electrical contact in the presence of an electrolyte, causing one metal to corrode preferentially.
• Crevice Activity: Intense localized degradation within crevices and shielded areas on metal surfaces exposed to corrosive environments.
• Pitting: Localized attack resulting in small holes in the material. This is often difficult to detect and can rapidly lead to structural weakening.
• Intergranular: Preferential attack along grain boundaries of a metal, resulting in loss of strength and ductility.
• Erosion: Deterioration due to the combined action of mechanical wear and a corrosive environment.
• Stress: Degradation accelerated by tensile stress in a corrosive environment, leading to cracking and failure.

Affected Materials

A wide range of materials are susceptible, including:

• Metals: Ferrous (e.g., steel, cast iron) and non-ferrous (e.g., aluminum, copper, zinc, nickel, titanium).
• Polymers: Degradation due to UV radiation, oxidation, hydrolysis, and chemical attack.
• Ceramics: Though generally resistant, they can be attacked by specific chemicals, particularly acids and bases at high temperatures.
• Composites: Degradation can occur in the matrix material or at the interface between the matrix and reinforcement.

Environmental Factors

• Atmospheric Conditions: Humidity, temperature, pollutants (e.g., sulfur dioxide, nitrogen oxides, chlorides).
• Aqueous Environments: pH, salinity, dissolved oxygen, flow rate.
• Chemical Exposure: Acids, bases, solvents, oxidizing agents.
• Biological Activity: Microorganisms can accelerate degradation through biocorrosion.

Consequences

• Structural Weakening: Reduced load-bearing capacity and increased risk of failure.
• Equipment Failure: Malfunction and downtime, leading to economic losses.
• Safety Hazards: Potential for accidents and injuries.
• Environmental Damage: Release of harmful substances into the environment.
• Increased Maintenance Costs: Repair and replacement of degraded components.

Prevention and Mitigation

• Material Selection: Choosing materials that are resistant to the specific environment.
• Protective Coatings: Applying barriers to isolate the material from the environment (e.g., paints, galvanizing, plating, powder coatings).
• Cathodic Protection: Using an external electrical current or a sacrificial anode to prevent oxidation of the metal.
• Inhibitors: Adding chemicals to the environment to reduce the rate of degradation.
• Design Considerations: Avoiding crevices, sharp corners, and other features that can promote localized attack.
• Environmental Control: Reducing exposure to corrosive environments through ventilation, dehumidification, and other measures.