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Corrosion, galvanic action and service life · Lesson 1 of 4

Corrosion, galvanic action and service life: principles and terminology

The four parts of a corrosion cell and the three prerequisites of galvanic attack; what counts as an electrolyte; why a galvanic series is not a verdict; area ratio; deposition corrosion; and how zinc protects steel.

You will be able to: Name the parts of a corrosion cell, state the conditions galvanic corrosion needs, read a galvanic series with its limits, explain area ratio and deposition corrosion, and keep the safety boundary of inspecting at height.

13 min · 12 slides · 9 questions

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Slide 1 of 12

The four parts of a corrosion cell

GalvInfo says a corrosion reaction needs four components: an anode, a cathode, an electrical circuit and an electrolyte.

The anode is where metal is lost. The electrical circuit is the metal path between the parts; the electrolyte is the moisture path.

A corrosion cell on a galvanized part touching copperA galvanized piece in AFS red touches a copper piece under a moisture film. Ions travel through the water and electrons through the metal from the active galvanized part to the copper, and the galvanized part loses metal. Three ways to stop the attack are listed.Ions in the waterMoisture filmGalvanizedCopperElectrons in the metalDABCRemove any one and the attack stopsKeep the metals apartBreak the metal pathKeep the moisture off

A galvanized piece in AFS red touches a copper piece under a moisture film. Ions travel through the water and electrons through the metal from the galvanized piece to the copper, and the galvanized piece loses metal. A panel lists three ways to stop the attack: keep the metals apart, break the metal path, keep the moisture off.

A
Anode: the galvanized part (AFS metal in red), the active metal that loses metal
B
Cathode: copper (hatched), the more noble metal, which is protected
C
Electrolyte: the moisture film touching both metals
D
The metal path: where the two parts touch
The four parts a corrosion cell needs, drawn for this lesson, not to scale. The galvanized and copper pairing is one example.

Sources

Videos for this lesson

What this video shows: A worldstainless webinar on corrosion; its own chapter list includes galvanic corrosion at 17:05 (from the video's description). The webinar comes from a stainless steel industry body.

Video: Understanding Corrosion: Chapter 2 by worldstainless, on YouTube. Chosen from the video's published description.

Glossary

Corrosion cell
The set of four parts a corrosion reaction needs: an anode, a cathode, an electrical circuit (the metal path) and an electrolyte (the moisture path).
Anode and cathode
In a galvanic couple, the anode is the less resistant, active metal that corrodes, and the cathode is the more noble metal that is protected.
Electrolyte
Any moisture touching both metals that can carry current between them, such as condensate, dew, rain, damp debris or moisture in soil.
Galvanic corrosion
Corrosion that results when two dissimilar metals with different potentials are in electrical contact in an electrolyte, damaging only the active one.
Galvanic series
A published ranking of metals from the corroded, anodic end to the protected, noble end; a ranking is not a corrosion rate, and series depend on the electrolyte they were measured in.
Area ratio
The relative exposed area of the cathode and the anode; a small anode beside a large cathode is the case designs should generally avoid.
Deposition corrosion
Corrosion in which copper carried in water deposits as metal on aluminum while the aluminum dissolves, with no direct metal-to-metal contact needed.