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.
| 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 |
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.
| 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 |
Mitigation Methods
Design approaches to minimize galvanic corrosion risk. Selection depends on application constraints, environment, and cost. Multiple strategies often combined for critical applications.
| 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 |
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).
| 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 |