What is Galvanic Corrosion?
What is Galvanic Corrosion?
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte (moisture, salt water, etc.). The more "active" metal corrodes preferentially, protecting the more "noble" metal — but sacrificing itself in the process.
Also Called: Bimetallic corrosion, dissimilar metal corrosion
How It Works
Three conditions required:
1. Two different metals — With different electrochemical potentials
2. Electrical contact — Direct contact or conductive path
3. Electrolyte — Moisture, salt water, acids, etc.
The more active metal (anode) corrodes to protect the more noble metal (cathode).
The Galvanic Series
Metals ranked from most noble (least reactive) to most active (most reactive):
Most Noble (Cathodic)
| Rank | Metal | Behavior |
|---|---|---|
| 1 | Platinum | Most noble |
| 2 | Gold | Noble |
| 3 | Silver | Noble |
| 4 | Stainless Steel (passive) | Noble |
| 5 | Titanium | Noble |
| 6 | Monel | Noble |
| 7 | Nickel | Noble |
| 8 | Copper | Noble |
| 9 | Brass | Noble |
| 10 | Bronze | Noble |
| 11 | Copper-Nickel | Noble |
| 12 | Tin | Near-neutral |
| 13 | Lead | Near-neutral |
| 14 | Stainless Steel (active) | Variable |
| 15 | Cast Iron | Active |
| 16 | Carbon Steel | Active |
| 17 | Aluminum | Active |
| 18 | Cadmium | Active |
| 19 | Zinc | Active |
| 20 | Magnesium | Most active |
Most Active (Anodic)
Compatibility Guidelines
Safe Combinations (Similar Galvanic Position)
- Stainless steel + Stainless steel ✓
- Carbon steel + Carbon steel ✓
- Brass + Bronze + Copper ✓
- Aluminum + Zinc (galvanized) ✓
- Zinc + Cadmium ✓
Risky Combinations (Large Galvanic Gap)
- Stainless steel + Carbon steel ⚠️
- Stainless steel + Aluminum ⚠️
- Copper + Aluminum ⚠️
- Brass + Zinc ⚠️
- Steel + Magnesium ❌
Compatibility Matrix
| Fastener ↓ Base → | Steel | Aluminum | Stainless | Copper |
|---|---|---|---|---|
| Carbon Steel | ✓ | ⚠️ | ⚠️ | ❌ |
| Zinc Plated | ✓ | ✓ | ⚠️ | ⚠️ |
| Stainless A2/A4 | ⚠️ | ⚠️ | ✓ | ✓ |
| Brass | ⚠️ | ❌ | ✓ | ✓ |
| Aluminum | ⚠️ | ✓ | ⚠️ | ❌ |
Galvanic Corrosion Rate Factors
1. Voltage Difference
Larger gap in galvanic series = faster corrosion
| Gap | Corrosion Rate |
|---|---|
| Small (0-0.15V) | Negligible |
| Moderate (0.15-0.25V) | Slow |
| Large (0.25-0.5V) | Moderate |
| Very Large (>0.5V) | Rapid |
2. Area Ratio (Critical!)
Large cathode + Small anode = Very fast corrosion
| Scenario | Result |
|---|---|
| Large steel plate + Small SS bolt | Steel corrodes slowly |
| Large SS plate + Small steel bolt | Steel bolt corrodes FAST |
| Equal areas | Moderate corrosion |
The danger: A small steel fastener in a large stainless assembly will corrode rapidly.
3. Electrolyte Conductivity
| Environment | Conductivity | Corrosion Rate |
|---|---|---|
| Dry air | None | Zero |
| High humidity | Low | Slow |
| Fresh water | Moderate | Moderate |
| Salt water | High | Fast |
| Industrial chemicals | Variable | Often fast |
Fastener-Specific Guidance
Stainless Steel Fasteners
In carbon steel assemblies:
- SS fastener + Steel base = Steel base corrodes
- Usually acceptable (base material is sacrificial)
- Add isolation washer for critical applications
In aluminum assemblies:
- SS fastener + Aluminum base = Aluminum corrodes
- Use nylon washer for isolation
- Or use aluminum or zinc-plated fasteners
Zinc-Plated Fasteners
Why zinc plating works:
- Zinc is more active than steel
- Zinc sacrifices itself to protect steel
- Even scratched plating continues to protect
In various materials:
- Zinc + Steel = Zinc protects steel ✓
- Zinc + Aluminum = Compatible ✓
- Zinc + Stainless = Zinc corrodes (but limited contact)
- Zinc + Copper/Brass = Zinc corrodes rapidly ❌
Hot-Dip Galvanized Fasteners
Similar to zinc plating but thicker, longer-lasting protection.
Best for:
- Structural steel connections
- Outdoor construction
- Marine (with limitations)
Brass and Bronze Fasteners
Use with:
- Copper, brass, bronze (same family) ✓
- Stainless steel (similar nobility) ✓
Avoid with:
- Steel (brass accelerates steel corrosion) ⚠️
- Aluminum (aluminum corrodes rapidly) ❌
- Zinc (zinc corrodes rapidly) ❌
Prevention Strategies
1. Select Compatible Materials
Match fastener material to base material or select from similar galvanic positions.
2. Use Isolation
| Method | Application |
|---|---|
| Nylon washers | Electrical isolation |
| Plastic bushings | Isolate bolt from hole |
| Rubber gaskets | Seal and isolate |
| Paint/coating | Barrier between metals |
3. Apply Barrier Coatings
- Paint both surfaces
- Use epoxy coating
- Apply zinc-rich primer to steel
4. Control Environment
- Seal joints from moisture
- Use drainage to prevent water pooling
- Reduce salt/chemical exposure
5. Area Ratio Management
- Avoid small anodes with large cathodes
- If unavoidable, protect the anode (small part)
6. Use Sacrificial Anodes
In marine applications, intentional zinc anodes protect the assembly.
Common Applications & Recommendations
Aluminum Structures
| Fastener Choice | Notes |
|---|---|
| Best: Aluminum alloy | No galvanic action |
| Good: Zinc plated | Similar potential |
| Caution: Stainless | Isolate with nylon washer |
| Avoid: Plain steel | Aluminum corrodes |
| Avoid: Brass | Aluminum corrodes rapidly |
Steel Structures (Painted)
| Fastener Choice | Notes |
|---|---|
| Best: Same steel, same coating | Match everything |
| Good: Zinc plated | Zinc protects scratches |
| Good: Hot-dip galvanized | Best for outdoor |
| OK: Stainless | Steel may corrode at contact |
| Avoid: Copper/Brass | Accelerates steel corrosion |
Stainless Steel Structures
| Fastener Choice | Notes |
|---|---|
| Best: Same SS grade | No galvanic issues |
| OK: Brass/Bronze | Similar potential |
| Caution: Carbon steel | Steel corrodes at interface |
| Caution: Zinc plated | Zinc may corrode |
Marine/Offshore
| Fastener Choice | Notes |
|---|---|
| Best: 316 SS | Superior corrosion resistance |
| Good: Monel | Excellent marine performance |
| Good: Silicon bronze | Marine grade |
| Caution: 304 SS | Pitting in chlorides |
| Avoid: Carbon steel | Rapid corrosion |
| Avoid: Aluminum | In contact with copper-based |
Testing for Galvanic Risk
Quick Assessment
1. Identify both metals
2. Find their positions on galvanic series
3. Calculate potential difference
4. Consider area ratio
5. Evaluate electrolyte exposure
Potential Difference Guidelines
| Difference | In Industrial Atmosphere | In Marine Environment |
|---|---|---|
| <0.05V | Safe | Safe |
| 0.05-0.15V | Usually OK | Use caution |
| 0.15-0.25V | Caution | High risk |
| >0.25V | High risk | Severe risk |
FAQ
Q: Can I use stainless fasteners in aluminum?
A: Yes, with precautions. Use nylon washers to isolate, or accept that aluminum may show some corrosion at the interface. In dry environments, risk is low.
Q: Why did my zinc-plated bolt corrode when attached to copper pipe?
A: Copper is much more noble than zinc. The zinc plating sacrificed itself rapidly. Use brass or stainless fasteners for copper.
Q: Does galvanic corrosion occur without water?
A: No. An electrolyte (water, humidity, salt spray) is required. In perfectly dry conditions, dissimilar metals can be in contact without corrosion.
Q: Which corrodes — the bolt or the base material?
A: The more active (anodic) metal corrodes. Check the galvanic series to determine which.
Q: Can coatings prevent galvanic corrosion?
A: Yes, if the coating isolates the metals from each other and/or from the electrolyte. Any break in the coating can allow corrosion to start.
Q: Is galvanic corrosion the same as rust?
A: No. Rust is oxidation of iron/steel. Galvanic corrosion is accelerated corrosion due to dissimilar metal contact. Steel can rust without galvanic action.
Proper material selection prevents galvanic corrosion failures. When in doubt, consult a corrosion engineer or use isolation techniques.