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When Life Gives you Lemons, Make a Lemon Battery!

Inside most kitchens, a lemon leads a normal life, being integrated into a delicious dish or drink.  But many science students have found in their classroom experiments that a lemon is no mere fruit.

When connected to two dissimilar metals (such as copper and zinc), the lemon transforms into a battery, producing a small voltage potential.  A new found use of lemons is discovered, much to the delight of kitchen scientists everywhere.  For us here at Key Tech, it illustrates the positive use of a usually avoided event: Galvanic corrosion.

The process: break it down

Galvanic corrosion occurs when dissimilar metals are in the presence of an electrolyte, such as salt water (or citric acid in the case of lemons).  One metal forms the anode and the other metal forms the cathode. The anode is more active (less corrosion resistant) and the cathode is less active (more corrosion resistant). When they are electrically connected, a current is produced which causes a change in the corrosion properties of the metals:

  • The more active anode corrodes faster than it would by itself
  • The less active cathode corrodes slower than it would by itself

These metals must be in electrical contact and they must be exposed to an electrolyte. This type of corrosion will not occur if the metals are completely dry or electrically insulated from each other. Galvanic corrosion is a common enemy of ship builders, but this destructive process is of interest to any designer where metals and alloys interface, such as at fastener locations.  Material choice as well as surfacing options become critical at these junctions for the life and structural integrity of the system.

Welcome to the Galvanic Series (in seawater)

For those of us who love tables, galvanic series were developed to list metals and alloys based on their corrosion potential in a specific electrolyte.  In seawater, these materials are listed from most cathodic (inactive/resistant to corrosion) to most anodic (active/ease of corrosion).  The further apart the metals/alloys are in a particular series (thus the most dissimilar they are), the higher the risk of galvanic corrosion.

Least Active (Cathodic)

  • Platinum, Gold, Graphite, Titanium, Silver
  • Stainless Steel (passive)
  • Brass/Bronze
  • Stainless Steel (active)
  • Chrome
  • Nickel
  • Steel
  • Aluminum
  • Zinc
  • Magnesium

Most Active (Anodic)

How to prevent or minimize galvanic corrosion

  • Choose metals close together on the galvanic series for mating parts and fasteners.
  • Never place a small area of active metal on a large area of inactive metal (e.g. a zinc-plated fastener on stainless steel part).
  • Use coatings or other means to prevent electrical contact between parts:
    • Anodize
    • Chromate conversion (iridite)
    • Phosphate
    • Electroless nickel
    • Paint
    • Lubricants
    • Insulating tape
    • Non-absorbent washers
  • Use a sacrificial anode (e.g. galvanized steel)

Rules of Thumb: Know Your Environment

Where the metallic components will reside should be considered when designing your overall system.  The Anodic Index can be found here.  These anodic index values provide the comparison needed when deciding what type of fastener material to use in your designs to prevent galvanic corrosion.  The basic rules of thumb are (taken from corrosion-doctors.org):

  • Harsh Environments (outdoors, high temp, high humidity): the difference in Anodic Index should be less than 0.15V
  • Normal Environments (storage warehouses or non-temperature and humidity controlled environments): the difference in Anodic Index should be less than 0.25V
  • Controlled Environments (temp and humidity controlled): up to 0.50V difference in Anodic Index can be tolerated

A couple great resources for more detailed information on this subject are the ASM Metals Handbook (Vol 5 and 13) and “Handbook of Corrosion Engineering” by Pierre R. Roberge (2000).



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