Spark Erosion Due to Dissimilar Metals & Cathodic Protection Onboard Ships

Spark erosion can occur unintentionally in a vessel due to electrical discharge between dissimilar metals in the presence of stray currents—especially from cathodic protection systems.

Spark erosion - cathodic protection

When two dissimilar metals are in electrical contact and carry current, and there's a potential difference between them, a localized electric arc or spark can occur at the contact point, resulting in

erosion of material from the more anodic (less noble) metal, creation of small cavities or pits, and wear or structural weakening can occur over time.

A ship is built with various metals, such as Bronze or nickel-aluminum-bronze propellers, Steel hulls, and shafts, White metal or tin-based bearings, and cast-iron bedplates or crankcases. These dissimilar metals are all connected structurally and electrically, forming galvanic couples.

Now, add in the cathodic protection system (usually sacrificial anodes or impressed current systems), which intentionally introduces DC current to protect the hull from corrosion. This system can create stray currents through propulsion or hull-mounted equipment, and when improperly bonded or grounded, these currents can flow through engine components, bearings, or shafts.

Spark Erosion can happen in propeller shaft bearings (especially stern tube bearings), crankshaft–bearing interfaces, thrust bearings, and any rotating or sliding contact between dissimilar metals where stray current flows. Possible damages are pitting and cavities in bearing surfaces, overheating due to poor lubrication from uneven surfaces, loss of alignment or vibration from worn-down surfaces, and long-term damage to critical propulsion parts like shafts and crankpins.

Proper electrical bonding of all components, use of insulated bearings in critical locations, regular monitoring for shaft potential (shaft earthing brushes), maintenance of cathodic protection systems to avoid overcurrent and use of shaft grounding systems or electrostatic dischargers are recommended for damage prevention.

Source:

Turner, M. (2012). An Investigation of Galvanic Corrosion of Metals in a Seawater Environment. https://www.ewp.rpi.edu/hartford/~turnem4/EP/Other/Past%20Deliverables/7%202nd%20Progress%20Report.pdf