• Uniform • Galvanic • Dealloying • Crevice • Pitting • Intergranular • MIC • SCC • HB-HE-HIC • Fatigue • Erosion • Fretting • Index

Different Types of Corrosion
- Recognition, Mechanisms & Prevention

Galvanic  or Bimetallic Corrosion

What is galvanic corrosion? Galvanic corrosion or Bimetallic Corrosion or Dissimilar Metal Corrosion, as sometimes called, is defined as the accelerated corrosion of a metal because of an electrical contact (including physical contact) with a more noble metal  or nonmetallic conductor (the cathode) in a corrosive electrolyte.

The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect.

The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced.

In this photo, a 5-mm thick aluminum alloy plate is physically (and hence, electrically) connected to a 10-mm thick mild steel structural support. Galvanic corrosion occurred on the aluminium plate along the joint with the mild steel. Perforation of aluminum plate occurred within 2 years due to the huge acceleration factor in galvanic corrosion.

What causes galvanic corrosion? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. When the corrosion potentials of various metals and alloys are measured in a common electrolyte (e.g. natural seawater) and are listed in an orderly manner (descending or ascending) in a tabular form,  a Galvanic Series is created. It should be emphasized that the corrosion potentials must be measured for all metals and alloys in the same electrolyte under the same environmental conditions (temperature, pH, flow rate etc.), otherwise, the potentials are not comparable.

The potential difference (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active  metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte will also affect the degree of attack. The cathode to anode area ratio is directly proportional to the acceleration factor.

• Select metals/alloys as close together as possible in the galvanic series.
• Avoid unfavorable area effect of a small anode and large cathode.
• Insulate dissimilar metals wherever practical
• Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good repair on the anode.
• Avoid threaded joints for materials far apart in the galvanic series.

Corrosion and Its Prevention (5-day module)
Corrosion and Its Prevention (2-day module)
Corrosion, Metallurgy, Failure Analysis and Prevention (3 days)
Marine Corrosion, Causes and Prevention (2 days)
Materials Selection and Corrosion (2 days)
Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)

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