Corrosion and Degradation of Materials

• Corrosion and degradation affect various materials and causes significant global economic losses

Defining Corrosion and Degradation

• Corrosion: A material's reaction with its environment that leads to a decline in its properties
• Degradation: Deterioration of polymeric or ceramic materials, similar to corrosion but for non-metals
• Both corrosion and degradation involve environmental interactions and property deterioration
• Corrosion is a major problem causing significant economic losses worldwide

Corrosion vs. Degradation

• Corrosion applies to metallic materials and involves electrochemical reactions
• Degradation applies to ceramics and polymers and is caused by environmental factors
• Both terms describe material property deterioration from environmental exposure

Avoiding Material Deterioration

• Protective Coatings: Special paints shield metal from corrosion and protect the underlying material
• Material Selection: Choosing resistant materials reduces degradation, with some metals resisting corrosion better
• Maintenance: Regular checks prevent extensive damage, and early detection is important for avoiding major problems
• These methods extend material lifespan by preventing property deterioration

Classifying Corrosion

• Corrosion is classified by mechanism, morphology of the attack, and the medium that produces it
  • The classifications based on mechanism are Chemical (dry) or Electrochemical (wet)
  • The classifications based on morphology are Homogeneous uniform, selective, crevice, pitting, intergranular, cracking stress
  • The classifications based on medium are Acid, Atmospheric, Molten salt, Buried structures

Classifying Corrosion by Mechanism

• Chemical Corrosion (Dry): Occurs without moisture, typically at high temperatures, where the material behaves as a reactant with the environment
• Electrochemical Corrosion (Wet): Needs an electrolytic medium, forming galvanic cells on the metal. Anodic zones dissolve, creating a battery effect

Chemical Corrosion

• Occurs in high-temperature environments where gases and vapors react with metals
• Metals react with oxygen (or another corrosive gas) to form an oxide layer
• The properties of the oxide layer determine the level of protection, varies if its protective, or non-protective

Electrochemical Corrosion

• Requires a conductive medium for oxidation, with oxidation at the anode and reduction at the cathode
• Process leads to the formation of a corrosion cell, also know as a galvanic cell
• Anode oxidation and cathode reduction process results in battery formation in metallic materials
• Metal dissolution occurs in the anodic zone

Factors Affecting Corrosion

• Temperature: Affects the speed of chemical reactions
• pH: Influences the stability of corrosion products
• Contaminants: Create anodic and cathodic zones in the material

Statue of Liberty Corrosion Case

• The Statue of Liberty experienced electrochemical corrosion during a periodic inspection
• Shellac deteriorated between the statue's copper layers and wrought iron frame, causing the issue
• The problem was solved by applying a PTFE (Polytetrafluoroethylene) coating, this preventing both metals from contacting

Understanding Pourbaix Diagrams

• Pourbaix diagrams predict metal corrosion behavior in specific conditions by showing metal behavior (potential) at different pH levels
  • Corrosion: Metal will undergo corrosion
  • Immunity: Metal will not be altered
  • Passivation: Metal will suffer passivation
• Said conditions are predicted based on pH and potential, which aids in the proper material selection

Passivation

• A protective layer in the form of an oxide layer isolates metal from the environment, preventing further corrosion
• Aluminum, copper, and stainless steel passivate and form protective films
• Process involves creating a protective later on metal to prevent corrosions

Sacrificial Anode

• In 1824, engineer Davy introduced the use of zinc blocks to protect British warships, known as cathodic protection
• Zinc is used as a sacrificial anode, oxidizing instead of the ship's hull
• Elements like magnesium, aluminum, and their alloys can also be used as sacrificial anodes

Galvanization

• Steel is coated with zinc for protection against corrosion
• Zinc protects steel from corrosion, ensuring durability in harsh environments

Corrosion Protection Methods

• Coatings using special paints and galvanization
• Environmental control through pH adjustment and oxygen removal
• Cathodic protection via sacrificial anodes
• Material selection using corrosion-resistant alloys

Factors Influencing Electrochemical Corrosion

• Microstructure: Compact structures resist better than porous ones
• Composition: Heterogeneities promote corrosion
• Anisotropy: Crystal anisotropy creates potential differences
• Environment: Moisture, soil type, and aqueous media affect battery formation
• Microstructure, composition, and crystal structure affect affect corrosion
• Cold working generates dislocations, creating anodic zones

Environmental Factors

• Humidity is essential for corrosion, with carbon steel needing 80% humidity
• Soil type also affects the corrosion rate
• Aqueous media such as sea, waste, and fresh water contribute to corrosion

Corrosion Specific Types

• Bacteriological/Biological: Microorganisms create corrosive acids or sulfur as byproducts
• Erosion: Fluid flow causes corrosion over metallic surfaces
• Stress: Cracks form in metals under stress in corrosive environments, and requires high tension and connected corrosive media

More Corrosion Types

• Corrosion fatigue: Metal with fatigue stress (tension-compression) presents cracks, corrosive medium is not required
• Friction: When two materials subject each other to rubbing and wear stresses
• Crevice: Appears in crevices, cracks near welding seams, threaded areas. Occurs in gaps with varying pH and temperature

Material Selection for Corrosion Resistance

• Fe-Cr-Ni Alloys like stainless steel commonly used in cutlery
• Cu Alloys
• Light Alloys
• Metallic glasses with high resistance to corrosion
• Superalloys for high-temperature applications, common in the aerospace industry
• Material selection is vital to avoid corrosion and consider alloys that form low conductivity oxides

Polymers Degradation

• Polymer degradation differs mostly as physical and chemical instead of metals that are electrochemical
  • Polymers degradation involves breaking covalent bonds and it is irreversible
• Termo-oxidative / Self oxidation: Due to O2. Slow at low T and faster at high T (25 – 150 °C).
• Biodegradation: Caused by biological agents with enzymes
• Thermal: Breaking of covalent bonds at high T and inert environment.
• Mechanical: Occurs during the processing of polymeric parts, not during their use.
• Photodegradation / Photoxidation: UV radiation exposure in the presence of oxygen.

Ceramics Degradation

• Ceramics unusual degradation is due to chemical attacks and heterogeneities
  • Chemical attack: HF attacks silicates, and should not be stored in glass bottles
  • Concrete degradation: Acid rain can dissolve calcium carbonate present in concrete
  • Reaction with CO2: Calcium carbonate forms bicarbonates in the presence of moisture and CO2 making it easy to oxidize

Wood Degradation

• Termites: Insects degrade wood structure, also woodworms. Prevention: fumigation
• Fungi: Causes rotting and loss of mechanical properties
• Prevention: polishing and varnishing protects against biological degradation

Corrosion Testing

• Standardized Guidelines: Its important to use specific protocols for each type of material being tested
• Salt Spray Chamber: Standardized machines accelerating controlled corrosion, using a 5% NaCl atmosphere beam
• Visual Evaluation: Comparison based on standardized photographs of corrosion grades

Final Considerations

• Complexity: Corrosion and degradation are complex processes differing with the material and environment
• Prevention: Selecting the proper material and treatments helps minimize these phenomena
• Monitoring: Periodic inspections and testing of materials determine the effect of corrosion
• Evolution: Development continues in new materials and protection methods

Conclusions & Future Perspectives

• Corrosion remains a significant economic challenge
• New materials and techniques continue to improve corrosion resistance
• More studies are required to fully understand and prevent corrosion in new materials