Read the illustrated guide above or browse its accessible text below. For diagrams and tables, refer to the corresponding page image.
Galvanic corrosion. Why it happens and how to avoid it
We touched briefly on galvanic corrosion in Chapter 4 however this chapter gives you a deeper look into what it is, why it happens, and what you can do to avoid it.
3 things are required for galvanic corrosion to occur: two electrochemically dissimilar metals, an electrolyte solution, and electrical contact between the metals. A common scenario is marine equipment in a saltwater environment. When salt dissolves in water, it releases electrolytes which are attracted to the electrons of the nearby metals. An electrical contact or flow occurs when the electrolytes attach to both metals, creating the conditions for galvanic corrosion.
Damage from galvanic corrosion The damage from galvanic corrosion can be extensive and even result in failure of a structure or a joint. A classical example is the Statue of Liberty which has a copper surface on a cast iron frame. The two metals were originally separated by an insulating material which eventually failed resulting in significant galvanic corrosion. Repairs and improvements done in the 1980’s took two years and while originally estimated at $103 million, was actually much more.
The most severe corrosion occurs where the metals are in contact – like a joint or around fasteners. This site gives an example of a mild steel and aluminum joint with significant corrosion and perforations in the aluminum after 2 years. You can also see an example of brass and steel in contact.
Galvanic Corrosion Electrolyte (Saltwater)
Anode Cathode
Aluminum Copper (or less noble metal than cathode) (found in ACQ lumber)
Corrosion
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PROTECTING FASTENERS Fasteners are a common source of galvanic corrosion. The fastener typically has a smaller surface area than the material it is fastening, so if the fastener is the anode, corrosion can be very rapid. To avoid issues, consider the following when planning a project or assembly: • Whenever possible, use the same material for the fastener and the piece it is fastening • If using different materials, choose a fastener material that will be the cathode • Only use zinc coated fasteners on steel coated with aluminum, zinc, or galvalume • Do not use zinc or aluminum coated fasteners for copper or stainless steel
This table can help with the selection of fasteners for a sheet metal fabrication project.
Base Metal Fastener Metal
Zinc, Aluminum Steel and Brasses, Stainless Stainless steel galvanized and aluminum cast iron copper, bronzes, steel type type 302/304, steel alloys monel 410 303, 305
Zinc, galvanized A B B C C C steel
Aluminum and A A B C Not B aluminum alloys recommended
Steel and cast AD A A C C B iron
Teme (lead tin) ADE AE AE C C B plated steel
Brasses, copper, ADE AE AE A A B bronzes, monel
Stainless steel ADE AE AE A A A type 430
Stainless steel ADE AE AE AE A A type 302/304
Key: A: Corrosion of the base metal is not increased by the fastener B: Corrosion of the base metal is marginally increased by the fastener C: Corrosion of the base metal may be marginally increased by the fastener material D: Plating on the fasteners is rapidly consumed, leaving the bare fastener metal E: Corrosion of the fastener is increased by the base metal Source:
Sacrificial anodes Electrolytes are preferentially attracted to highly chemically active metals – called anodes. When a current flow is created between a highly active and a less active metal, the electrolytes move the electrons from the anode towards the less active metal, causing the anode to corrode more quickly and the cathode corrode more slowly. This process is used to slow galvanic corrosion by intentionally introducing a highly active metal to act as a sacrificial anode.
• Less chemically active metals are called cathodes and include platinum, titanium, stainless steel, and silver. • The more chemically active metals (anodes) include zinc, magnesium, and aluminum alloys.
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Preventing galvanic corrosion Galvanic corrosion can often be prevented through good design and planning. Start by choosing compatible metals to potentially avoid the problem altogether. If that’s not possible, coatings and non-conductive barriers are an option.
CHOOSE COMPATIBLE METALS If possible, choose metals that are compatible and less likely to create conditions favorable to galvanic corrosion. There are two things to consider: their distance on a galvanic table, and their difference in anodic index. • Choose similar, compatible metals whenever possible • Use metals close to each other on the anodic index (conditions dependant) • If you have to use dissimilar metals, try to make the surface area of the anode larger than the cathode to slow corrosion
A galvanic table ranks metals based on their tendency to interact galvanically, with galvanic corrosion more likely when the metals are further apart on this table. The galvanic table is meant as a guide as it doesn’t take into consideration other factors that might impact the corrosive potential of the metals.
The table lists the most active anode metals first and the least active cathodes last and different versions of the table are available for different environments or electrolyte solutions. Included below is the table for low oxygen seawater.
Galvanic series table for stagnant (low oxygen) seawater 1. Graphite 21. Niobium 1% Zr 2. Palladium 22. Tungsten 3. Platinum 23. Tin 4. Gold 24. Lead 5. Silver 25. Stainless steel 304 (active) 6. Titanium 26. Tantalum 7. Stainless steel 316 (passive) 27. Chromium plating 8. Stainless steel 304 (passive) 28. Nickel (passive) 9. Silicon bronze 29. Copper 10. Stainless steel 316 (active) 30. Nickel (active) 11. Monel 400 31. Cast iron 12. Phosphor bronze 32. Steel 13. Admiralty brass 33. Indium 14. Cupronickel 34. Aluminum 15. Molybdenum 35. Uranium (pure) 16. Red brass 36. Cadmium 17. Brass plating 37. Beryllium 18. Yellow brass 38. Zinc plating 19. Naval brass 464 39. Magnesium 20. Uranium 8% Mo
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The anodic index shows the electro-chemical voltage between a metal and gold. • In harsh environments with high humidity or saltwater there should be no more than 0.15V different between the metals • In less harsh environments without temperature or humidity controls there should be no more than 0.25V different between the metals • In temperature and humidity-controlled environments, a difference of 0.50V is possible
Metal Index (V)
Gold, solid and plated; gold-platinum alloy −0.00
Rhodium-plated on silver-plated copper −0.05
Silver, solid or plated; monel metal; high nickel-copper alloys −0.15
Nickel, solid or plated; titanium and its alloys; monel −0.30
Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nick- −0.35 el alloys; nickel-chromium alloys
Brass and bronzes −0.40
High brasses and bronzes −0.45
18%-chromium-type corrosion-resistant steels −0.50
Chromium plated; tin plated; 12%-chromium-type corrosion-resistant steels −0.60
Tin-plate; tin-lead solder −0.65
Lead, solid or plated; high lead alloys −0.70
2000 series wrought aluminum −0.75
Iron, wrought, gray, or malleable; low alloy and plain carbon steels −0.85
Aluminum, wrought alloys other than 2000 series aluminum, cast alloys of the silicon type −0.90
Aluminum, cast alloys (other than silicon type); cadmium, plated and chromate −0.95
Hot-dip-zinc plate; galvanized steel −1.20
Zinc, wrought; zinc-base die-casting alloys; zinc plated −1.25
Magnesium and magnesium-base alloys, cast or wrought −1.75
Beryllium −1.85
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COATINGS AND NON-CONDUCTIVE BARRIERS Coating either or both metals can protect it from electrolytes and therefore galvanic corrosion. This can work in 2 different ways:
1. A metallic coating becomes the sacrificial anode to protect the metal it is coating – which is a common use for zinc coatings. 2. A non-conductive material separates the two metals, removing the electric connection between them.
• Common coatings to prevent galvanic corrosion include: - Zinc plating - Galvanized dipped metals - Ecoating - Powder coating - Dacromet - Anodizing • If only one metal can be coated it should be the cathode • The thickness of a coating is important if it is intended to act as a sacrificial anode • Common non-conductive insulators include polymer or elastomer-based bushings, washers, gaskets, or coatings. • A non-conductive barrier must remain intact. Damage or degradation will lead to corrosion. • A non-conductive barrier must break all contact between the metals in order to be effective.
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