Metal Surface Treatments and Corrosion Protection
Metal Surface Treatments
Nitriding
This surface hardening treatment increases corrosion resistance using ammonia gas in a high-temperature oven. Rapid cooling and subsequent annealing are performed.
Sulfinization
This treatment increases sulfur and nitrogen percentages in the steel’s surface layer.
Oxidation
Metals and alloys release electrons, increasing their oxidation state. A redox reaction occurs with another substance. All metals oxidize in dry, warm conditions, forming a compact, homogeneous, protective oxide layer (passivation). High temperatures or mechanical stress can crack this layer, leading to further oxidation. Moisture, salts, and acids accelerate this slow process. A material’s oxidation potential indicates its susceptibility to oxidation. Electropositive metals have a positive, high absolute potential, acting as anodes (reducers) in electrochemical processes.
Electrochemical Processes
Electrolysis
This process uses an electric current passed through electrodes in an electrolyte solution, generating a redox reaction. It’s applied in metal extraction, purification, and electrolytic processes.
Purification of Metals
Copper refining uses impure copper as the anode and pure copper as the cathode, immersed in a copper salt solution. Adjusting the potential prevents active metals from depositing on the cathode, while noble metals form anode sludge.
Electroplating
Used to protect metals and alloys by creating an impermeable layer. The metal to be protected acts as the cathode, the protective metal as the anode, and a fused metal salt solution as the electrolyte.
Galvanized Steel
An iron-zinc alloy. Electro-deposition uses an iron cathode, a zinc anode, and a zinc salt electrolyte. Zinc protects the iron from atmospheric corrosion, acting as the anode while iron acts as the cathode.
Corrosion and Protection
Corrosion
Material etching caused by destructive gases or acids, forming a porous oxide, salt, or other compound, leading to material destruction. Influenced by impurities, pressure, temperature, and mechanical stress.
Types of Corrosion
- Dry or Chemical Corrosion: Metal interacts with atmospheric gases, forming surface products that promote further gas absorption.
- Wet or Electrochemical Corrosion: Metal reacts with an oxidizing agent in the presence of electrolytes, creating a galvanic cell (anode and cathode), often favored by heterogeneities.
Corrosion Propagation
- Uniform: General attack across the metal surface.
- Localized: Attack occurs in specific areas.
- Pitting: Occurs at defects, impurities, or compositional differences, progressing inwards.
- Intergranular: Located at metal grain boundaries.
- Selective: One component of an alloy dissolves.
- Erosion: Occurs inside pipes due to chemical action and mechanical abrasion from fluids.
Corrosion Protection
- Heat Treatments: Homogeneous structures (normalized and annealed).
- Increased Material Purity
- Corrosion-Resistant Alloys
- Cathodic Protection: Making the protected metal the cathode (reduction site). Sacrificial anodes (more electropositive metals) are connected to the protected metal, oxidizing instead. These anodes need periodic replacement.
- Protective Coatings: Metallic (self-protective metal layers, often with a primary deposit for better adhesion) or non-metallic (paint, varnish, plastics, ceramics, bluing).
- Passivation: Forming an adherent, impermeable oxide layer to prevent oxidation and galvanic cell formation.