Solve common problems

Electrolysis, Galvanic Corrosion

What is it and how do I solve this?

Electrolysis or galvanic corrosion, is the reaction caused by two dissimilar metals coming into contact with each other in a corrosive atmosphere or in sea water. Electrolytic corrosion is slightly different and is due to electrical current from other sources leading to problems.

NOTE: These subjects are highly complex and any continual blistering, corrosion or erosion on the underwater areas of steel or aluminium boats should be diagnosed by an expert on the subject. This short article can only give a very brief and incomplete view on the subjects.

How to recognise it

  • Paint blisters on the underwater areas of steel or aluminium boats.
  • Red areas of corrosion on the underwater areas of steel boats.
  • Areas of metal erosion and pitting on the underwater areas.
  • Blisters that reveal a bright clean metal surface and the liquid in the blister feels sticky or soapy.
  • White calcium type deposits in or around the protective zinc blocks or around blisters.    
  • Blistering that follows the lines of welds or along edges of chines, keels or any other sharp demarcation line or shape on a boat.
  • Protective blocks disappearing very fast.
  • Protective blocks still look like new.
  • Such issues are usually more prominent at the rear of hulls where the electrical activity is often the highest.

What causes it

  • Too many or not enough protective zinc/aluminium blocks.
  • Stray currents from shore based facilities.
  • Stray currents from onboard equipment such as high power use anchor winches, radio transmitting equipment, starter motor units and numerous others.
  • Incorrectly earthed shore lines.
  • Incorrect earthed or wired onboard equipment.
  • Nearby boats with electrical issues.
  • Welding carried out onboard without due consideration given to correct earthing procedures.
  • Faulty marina earth wiring (very unlikely these days).
  • Protective blocks painted over.

How to prevent it

  • Ensure all underwater appendages such as heat exchangers, propellers, stern drives, struts and outlet pipes are all correctly earthed.
  • Ensure any sacrificial blocks are the correct type, applied in the correct quantity and in the correct position on the hull.
  • Ensure all onboard wiring, switches, equipment and grounding points are installed correctly.
  • Ensure correct level of cathodic protection is used.
  • Have the boat’s ‘in water hull potential’ measured to ensure it’s in the safe zone.

How to treat it

  • Once the physical causes of the problem have been found and eliminated then it’s a relatively simple process to make good the paint systems as required.
  • You might be able to patch repair small areas.
  • If the areas are really large, you might have to remove and repaint the whole paint system.

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