Corrosion in Dental Materials

Definition

Significance in Dentistry

• Weakens the restoration, reducing longevity
• Releases metal ions that may cause local tissue toxicity or allergy
• Causes discoloration (tarnish) and surface roughness
• Compromises the marginal integrity of restorations

Types of Corrosion

1. Chemical Corrosion (Direct Corrosion / Dry Corrosion)

• Occurs by direct chemical reaction between the metal surface and environmental agents (oxygen, sulfur compounds, acids)
• Does not require the presence of an electrolyte
• Example: Formation of silver sulfide (Ag₂S) tarnish on silver-containing alloys
• Results in a thin surface film (oxide/sulfide layer)

2. Electrochemical Corrosion (Wet Corrosion)

• Requires an electrolyte (saliva acts as the electrolyte in the oral cavity)
• Two half-reactions occur simultaneously:
  • Anode (oxidation): Metal loses electrons → M → M²⁺ + 2e⁻
  • Cathode (reduction): Electrons are accepted by the electrolyte
• This is the most clinically relevant form of corrosion

Subtypes of Electrochemical Corrosion:

• Occurs when two dissimilar metals are in contact in the presence of an electrolyte (saliva)
• Forms a galvanic cell: the more active metal (lower in the galvanic series) becomes the anode and corrodes; the nobler metal is the cathode
• Clinical example: Gold crown opposing amalgam restoration - amalgam corrodes preferentially
• The potential difference drives an electric current (galvanic current), which may also cause a galvanic shock felt by the patient
• The greater the separation of the two metals in the electrochemical/galvanic series, the greater the corrosion rate

• Occurs due to differences in ion concentration or oxygen concentration at different areas of the same metal surface
• Example: Crevice corrosion under plaque deposits or at crown margins where oxygen concentration is lower (that area becomes the anode)

• Occurs when a metal under mechanical stress is exposed to a corrosive environment
• Stressed areas have higher free energy and act as anodes relative to unstressed areas
• Can lead to premature fracture of a restoration

• Attack along the grain boundaries of an alloy
• Grain boundaries are more reactive due to compositional differences or impurities
• Can severely weaken the alloy

Tarnish vs. Corrosion

Passivation

• Some metals form a thin, dense, adherent oxide layer on their surface that protects against further corrosion
• Examples: Titanium (TiO₂), stainless steel (Cr₂O₃), chromium-cobalt alloys
• This phenomenon is called passivation
• The oxide layer acts as a barrier, preventing further attack
• If this layer is disrupted mechanically (scratches), corrosion resumes - this is called depassivation

Galvanic / Electrochemical Series

• In dental alloys, the noble metals (Au, Pt, Pd) are most corrosion-resistant
• Base metals (Ni, Cr, Co) rely on passivation for corrosion resistance

Factors Affecting Corrosion of Dental Alloys

1. Composition of alloy - More base metal content = greater susceptibility
2. Microstructure - Homogeneous single-phase alloys corrode less than multiphase alloys
3. Surface finish - Smooth, polished surfaces corrode less (less surface area, fewer grain boundary exposures)
4. Heat treatment - Proper annealing produces more homogeneous structure, reducing corrosion
5. Oral environment - pH, saliva flow, plaque, fluoride (accelerates corrosion of base metals), temperature fluctuations
6. Presence of dissimilar metals - Galvanic coupling

Corrosion of Specific Dental Materials

Amalgam

• Among the most susceptible dental alloys to corrosion
• Main corrosion products: tin oxides (SnO, SnO₂), tin chloride (SnCl₂), and mercury compounds
• Corrosion products may actually seal marginal gaps initially (self-sealing effect), but long-term corrosion weakens the restoration
• High-copper amalgams (with Cu₆Sn₅ / eta phase) are more corrosion-resistant than low-copper amalgams because the gamma-2 phase (Sn₇-₈Hg) - the weakest and most corrosive phase - is eliminated

Base Metal Alloys (Ni-Cr, Co-Cr)

• Rely on passive chromium oxide (Cr₂O₃) layer
• Fluoride-containing environments can disrupt this layer, increasing corrosion
• Used in metal-ceramic restorations, RPD frameworks

Gold Alloys

• Highly corrosion-resistant due to noble metal content
• Minimum gold + platinum group metal content ≥ 60% for Type III/IV alloys ensures good corrosion resistance

Stainless Steel (Orthodontic wires, bands)

• Contains at least 12-18% chromium for passivation
• Susceptible to crevice corrosion and pitting in high-chloride environments

Prevention of Corrosion

1. Selection of noble or high-noble alloys for restorations
2. Proper polishing of restorations to eliminate surface irregularities
3. Avoid mixing dissimilar metals in the same oral environment (minimize galvanic cells)
4. Fluoride exposure should be monitored in patients with base-metal restorations
5. Regular recall and maintenance - plaque removal reduces concentration cell corrosion
6. Proper heat treatment of alloys during fabrication

Clinical Relevance (Quick Points)

• Galvanic shock is felt when a new gold restoration contacts an old amalgam - usually transient as the galvanic current diminishes over time
• Corrosion products (metal ions) from Ni-Cr alloys can cause nickel hypersensitivity (most common metal allergy)
• Titanium is the most biocompatible dental metal due to its stable and self-repairing passive TiO₂ layer
• High-copper amalgam is preferred clinically because elimination of the gamma-2 phase significantly reduces corrosion and creep

Corrosion

Definition

Types of Corrosion

1. Chemical (Dry) Corrosion

• Direct reaction of metal with environmental agents (O₂, H₂S, SO₂) without an electrolyte
• Results in surface tarnish - e.g., silver sulfide (Ag₂S) film on silver alloys

2. Electrochemical (Wet) Corrosion

• Anode (active metal): M → M²⁺ + 2e⁻ (oxidation - metal is lost)
• Cathode (noble end): electrons are accepted by the solution (reduction)

Tarnish vs. Corrosion

• Tarnish - Superficial surface discoloration only; no structural damage (e.g., Ag₂S film)
• Corrosion - Deeper material degradation; compromises structural integrity
• Tarnish is a precursor to corrosion

Passivation

• Certain metals form a thin, protective oxide layer on their surface, resisting further corrosion
• Examples: Ti (TiO₂), Cr-Co and Ni-Cr alloys (Cr₂O₃)
• If this layer is scratched or disrupted, corrosion resumes (depassivation)

Galvanic Series (Most Active → Most Noble)

• The greater the separation between two metals on this series, the higher the corrosion rate of the active one
• Noble metals (Au, Pt, Pd) are most corrosion-resistant

Factors Affecting Corrosion

1. Alloy composition - higher base metal content = greater corrosion
2. Microstructure - multiphase alloys corrode more than single-phase
3. Surface finish - rough surfaces corrode faster; polishing reduces corrosion
4. Heat treatment - proper annealing produces homogeneous structure
5. Oral environment - pH, fluoride, plaque, temperature
6. Dissimilar metals - galvanic coupling accelerates corrosion

Corrosion of Amalgam

• Most susceptible dental alloy
• Corrosion products: SnO, SnO₂, SnCl₂
• Low-copper amalgam: gamma-2 phase (Sn₇₋₈Hg) is most susceptible - corrodes and creeps readily
• High-copper amalgam: gamma-2 phase is eliminated → significantly less corrosion and creep → clinically preferred
• Corrosion products may initially seal marginal gaps but long-term weaken the restoration

Prevention of Corrosion

• Use noble or high-noble alloys for restorations
• Polish restorations to a smooth finish
• Avoid dissimilar metals in close proximity in the same oral environment
• Minimize fluoride exposure in patients with base-metal restorations
• Regular recall and plaque removal

Clinical Significance

• Galvanic shock - Brief sharp pain when new gold contacts old amalgam; subsides as galvanic current diminishes
• Nickel hypersensitivity - Metal ions from corroding Ni-Cr alloys cause the most common metal allergy
• Titanium - Most biocompatible dental metal due to its stable, self-repairing TiO₂ passive layer
• Structural weakening, margin breakdown, and tissue toxicity are the main clinical consequences of unchecked corrosion