How it is read
Each metal or finish has one number. The more active the metal, the higher its index; gold sits at zero and magnesium near the top of the scale. To screen a joint, take the indices of the two metals in contact, subtract, and compare the difference with the limit for the environment the assembly will see.
A common design rule uses three environments:
| Environment | Typical examples | Maximum index difference usually allowed |
|---|---|---|
| Harsh | Outdoors, high humidity, salt exposure | 0.15 V |
| Normal | Warehouses and indoor areas without temperature or humidity control | 0.25 V |
| Controlled | Temperature- and humidity-controlled spaces | 0.50 V |
These limits are a widely used design convention found in engineering handbooks and company standards, not a universal requirement. The individual metal values also vary between published tables, so use the table and limits that your own specification or customer invokes.
Where it fits
The anodic index is built for quick decisions on small hardware: fasteners, platings, connectors, electrical enclosures, brackets and aerospace or electronics assemblies. It answers practical questions fast — which plating brings a steel fastener within the limit for an aluminum panel, or whether a mixed-metal bracket can go outdoors without isolation.
What it does not tell you
The index compresses behaviour into one number and a pass or fail threshold. It ignores the area ratio between anode and cathode, says nothing about process chemistry, immersion, temperature or flow, and does not account for polarization or protective films. A pair that passes the index can still fail badly as a small anode wired to a large cathode.
For plant equipment in water, soil or process service, the right tool is the galvanic series measured in the actual electrolyte, read together with the area ratio and electrolyte conductivity. ASTM G82 is the guide for developing and using such a series. The galvanic series table sets out the seawater series next to the anodic index chart, and the galvanic corrosion entry covers the three conditions a couple needs and how to break them.