Concept · Coatings and corrosion
Galvanic corrosion
Why two different metals in contact corrode, why the area ratio matters, and how design and coating prevent it.
When two different metals touch in an electrolyte, the less noble one becomes the anode and corrodes faster. The more noble one is protected.
The why
Metals differ in how readily they give up electrons. The galvanic series ranks them in sea water. Magnesium, zinc and aluminium alloys are more active than steel. Steel is more active than copper. Aluminium is more active than stainless steel. The further apart two metals sit in the series, the stronger the drive of the cell.
Three things must be present: two different metals, an electrical contact between them, and an electrolyte that touches both. Take one away and the cell stops. In a dry, heated building a stainless bolt in carbon steel shows little attack, because no moisture film joins the metals.
In the cell, the active metal is the anode and loses metal. Electrons flow to the noble metal. There they reduce oxygen. The noble metal is the cathode and does not corrode. A higher conductivity of the electrolyte makes the attack faster.
The area rule
A small anode joined to a large cathode corrodes fast, because all the current concentrates on the small area. A big cathode supplies a large current, and the small anode has to deliver all of it.
Example: a scratch exposes 1 cm² of steel on a coated part. The part is bolted to 500 cm² of bare, more noble metal in damp air. The large cathode sets the current, and all of it leaves the steel through the scratch.
Check: if 500 cm² of steel were bare, the same current would spread over 500 cm². On the scratch it flows through 1 cm². The current density is 500 ÷ 1 = 500 times higher, and the attack is 500 times more concentrated.
The rule also tells you which way round a fastener pair is safe. A carbon steel bolt in a large stainless plate is the bad case, because the small steel bolt is the anode. A stainless bolt in a large carbon steel plate is much safer. The big steel plate carries the attack over its whole area, so the loss per square centimetre stays small.
Where it shows
The attack is worst near the contact. On an offshore platform, aluminium fittings held by stainless bolts in the splash zone corrode next to each bolt. A copper earthing strap bolted to a steel pile in sea water attacks the steel next to the joint. Both cases have a wet, salty electrolyte and a metal path, so the cell runs.
How it is avoided
- Use the same metal, or metals close together in the galvanic series.
- Insulate the metals from each other (gaskets, sleeves, washers) and break the electrolyte path.
- If only one metal can be coated, coat the more noble one (the cathode). A damaged spot then does not create a small anode.
The last rule catches people out. If the coating is on the anode only, every holiday becomes a small anode beside a large bare cathode. ISO 12944-3 says to paint the more noble metal when only one can be painted. It also bans spring washers, because they form crevices.
The same cell is used on purpose elsewhere. A zinc-rich primer makes the zinc the anode and the steel the cathode. Cathodic protection does the same with a sacrificial anode.
In the field
- Find the joints between different metals on the drawings and the structure. Typical pairs are stainless fasteners in carbon steel or aluminium, and copper against steel.
- Check that insulating washers, sleeves and gaskets are present before the joint is closed.
- Check that the coating covers the more noble metal, or both metals.
- Check that no spring washer or open crevice holds water at the joint.
- Check the coating at the joint for holidays and damage. Repair it before the joint sees an electrolyte.
What the inspector records
Record the joint location, both metals, the insulating parts fitted and which metal carries the coating. Record any joint where an insulator was missing or damaged and how it was fixed. Add the photographs and the date of the check to the report.
Exam traps
- Trap: "A corrosion cell needs two metals." One plate can have an anode and a cathode because of different oxygen supply.
- Trap: In a copper and steel couple, the steel corrodes, not the copper.
- Trap: Coating only the anode is the classic mistake. Coat the noble metal or both.
- Trap: "Any two different metals in contact corrode." A joint that stays dry has no electrolyte, so the cell does not run.
- Trap: Dezincification of brass is selective leaching. It is not a galvanic couple between two metals.
How the exams treat it
Corrosion is a domain of its own: 5 % of the CIP 1 blueprint and 9 % of CIP 2. The FROSIO book covers corrosion, cathodic protection and design.
51 practice items in the members area mention this topic (CIP 1: 14 · CIP 2: 23 · FROSIO written: 14).
Related
Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.
Take the free readiness checkSee the tracksAbout this page
Sources: ISO 12944-3:2017 · ISO 12944-2:2017.
Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.