Potential Scale
Anodic Index Table
E vs. Reference
Electrochemical potential of metals relative to a reference electrode. More negative (anodic) metals corrode preferentially when coupled with more positive (cathodic) metals in the presence of an electrolyte.
Reference
SCE (Saturated Calomel Electrode)
Unit
V (Volts)
Metal Potentials (Sorted Anodic to Cathodic)
SCE (Saturated Calomel Electrode)| Metal | Potential (V) | Risk | Notes |
|---|---|---|---|
| Magnesium | -1.60 | Very High | Most anodic common metal |
| Zinc | -1.03 | High | Sacrificial anode material |
| Aluminum (Pure) | -0.76 | High | Forms protective oxide |
| Cadmium | -0.70 | Moderate-High | Plating material |
| Steel (Mild) | -0.61 | Moderate | Carbon steel |
| Cast Iron | -0.59 | Moderate | Gray iron |
| Stainless 304 (Active) | -0.53 | Moderate | Without passivation |
| Lead | -0.47 | Moderate | Chemical resistant |
| Tin | -0.44 | Low-Moderate | Solder component |
| Nickel (Active) | -0.35 | Low-Moderate | Plating layer |
| Brass | -0.30 | Low | Cu-Zn alloy |
| Copper | -0.24 | Low | Common cathode |
| Bronze | -0.23 | Low | Cu-Sn alloy |
| Stainless 316 (Passive) | -0.08 | Very Low | Highly cathodic |
| Titanium | -0.02 | Very Low | Noble behavior |
| Graphite/Carbon | +0.25 | Extreme cathode | Avoid coupling |
| Gold | +0.15 | Noble | Does not corrode |
| Platinum | +0.20 | Noble | Most noble |
Key Principles
- • Rule: More anodic metal corrodes; more cathodic metal is protected
- • Current flow: Electrons flow from anode to cathode through metal; ions through electrolyte
- • Rate factors: Potential difference, electrolyte conductivity, temperature, area ratio
- • Critical: Small anode + large cathode = worst case (high current density on anode)
Risk Spectrum
Corrosion Risk by Potential Difference
ΔE = E_cathode - E_anode
Risk assessment based on voltage difference between coupled metals. Greater potential difference drives higher corrosion current. Environmental factors (humidity, salinity) dramatically affect severity.
Reference
Potential Difference
Unit
V (Volts)
Risk Assessment by Voltage Difference
Potential Difference| ΔE Range | Risk Level | Guidance | Notes |
|---|---|---|---|
| 0.00 - 0.15 V | Negligible | Safe for most environments | Direct coupling acceptable |
| 0.15 - 0.25 V | Low | Acceptable in dry/indoor | Use caution outdoors |
| 0.25 - 0.50 V | Moderate | Problematic in humid/salty | Insulate or coat |
| 0.50 - 0.75 V | High | Severe in marine/industrial | Avoid direct contact |
| > 0.75 V | Very High | Rapid corrosion expected | Must isolate electrically |
Key Principles
- • Environment matters: Same ΔE more severe in seawater vs. dry air
- • Area ratio: Cathode area / Anode area ratio amplifies corrosion rate
- • Distance: Corrosion most severe near junction; decreases with distance
- • Time: Corrosion products may passivate surface and slow progression
Isolation Concept
Prevention Strategies
Mitigation Methods
Design approaches to minimize galvanic corrosion risk. Selection depends on application constraints, environment, and cost. Multiple strategies often combined for critical applications.
Reference
Best Practices
Unit
-
Mitigation Strategies
Best Practices| Strategy | Type | Method | Notes |
|---|---|---|---|
| Material Selection | Primary | Choose similar potentials | ΔE < 0.15 V ideal |
| Electrical Isolation | Primary | Gaskets, washers, sleeves | Break electrical path |
| Protective Coatings | Secondary | Paint, plating, anodizing | Coat BOTH metals |
| Sacrificial Anodes | Active | Zinc, magnesium blocks | Cathodic protection |
| Environmental Control | Supporting | Reduce humidity, salts | Dehumidifiers, sealants |
| Increase Anode Mass | Supporting | Oversize anodic part | Extend service life |
Key Principles
- • Best practice: Select metals within 0.15 V in expected environment
- • Isolation: Use non-conductive gaskets, bushings, washers (nylon, PTFE, rubber)
- • Coatings: Coat BOTH metals; coating only cathode risks pinhole corrosion
- • Sealants: Exclude electrolyte with caulking, potting compounds, conformal coatings
Galvanic Cell
Common Galvanic Couples
Real-World Cases
Typical metal combinations encountered in engineering. Some are intentionally designed (batteries, sacrificial anodes); others are failure modes to avoid (aluminum rivets in copper plate).
Reference
ΔE Values
Unit
V (Volts)
Common Galvanic Couples
ΔE Values| Couple | ΔE | Risk | Notes |
|---|---|---|---|
| Aluminum + Copper | 0.52 V | High | Avoid: Al corrodes rapidly |
| Steel + Copper | 0.37 V | Moderate-High | Common failure in plumbing |
| Zinc + Steel | 0.42 V | Moderate | Intentional: galvanizing |
| Aluminum + Stainless | 0.50-0.70 V | High | Use isolation bushings |
| Brass + Steel | 0.31 V | Moderate | Acceptable with coating |
| Titanium + Aluminum | 0.74 V | Very High | Must isolate completely |
| Carbon Fiber + Aluminum | >1.0 V | Extreme | Severe corrosion risk |
Key Principles
- • Aluminum+Copper: Classic failure - use transition fittings or isolate
- • Galvanized steel: Zinc sacrificially protects steel (intentional galvanic cell)
- • Carbon fiber: Extremely cathodic - never contact aluminum directly
- • Marine hardware: Bronze propellers require zinc anodes on aluminum hulls