What Causes Rusty Electrodes on Spark Plugs?

Rusty spark plugs in your personal watercraft are less the product of wild maneuvers than they are simple condensation inside your craft's hull. When you see rust on the spark plugs, it means the watercraft's performance will soon drop off and rust will begin to spread to the rest of the engine. A straightforward, proactive effort recommended by all personal watercraft manufacturers will protect your vessel.

  1. How Rust Works

    • Oxygen in the air normally bonds to iron atoms over time to create iron oxide. As the molecules of water break down over time and in the presence of metal containing iron atoms, the oxygen atoms from the water bonds to the iron atoms to form molecules of iron oxide. The hydrogen atoms that were part of the water are also bonding to iron atoms molecules to form an acidic hydrogen oxide molecule that then bonds with the iron oxide molecules over time. The result is rust, a molecule of a ferrous hydrogen oxide.

    Spark Plugs and Rust

    • Spark plugs contain a ferrous metal – an alloy of iron, meaning spark plugs are susceptible to rust.

      Electricity is the movement of electrons through a conductor, such as a wire, or across an air gap, such as the narrow gap between the electrodes of a spark plug. When a spark plug is screwed into its bore, the “L” shaped electrode, the cathode, on the bottom of the spark plug is connected, through the engine, to the personal watercraft's common ground.

      The electrode that runs through the center of the spark plug, the anode, to which the spark plug wire is connected, is insulated from the cathode. When the charge from the engine's ignition coil passes through the cathode, a spark is produced as the charge jumps across the gap to the anode.

      Some of the electrons that move across the spark plug’s gap may be captured by the ferrous hydrogen oxide, increasing both the quantity of rust and the speed with which rust appears on the watercraft's spark plugs, both on the plugs’ electrodes and on the plugs’ sleeves that screw into the spark plug bores.

      As the rust increases, the efficiency of the spark plugs is reduced and the performance of the watercraft is degraded.

    Humidity in the Hull

    • When the watercraft’s engine is running, it produces heat, causing the water inside the sealed environment inside the watercraft’s hull to condense from the air around it. Most of this condensation will condense on the inside of the body of the watercraft. Gravity then pulls the condensation into the water trap at the bottom of the hull, the bilge.

      The condensation that doesn’t reach the inside walls of the body of the watercraft may condense on engine parts, including the spark plugs, where it contributes to the rusting process.

    Condensation in the Bilge

    • Most of the water that condenses from the air inside the watercraft’s hull is trapped in the bilge, the lowest part of the vessel. Most of the water that enters the bilge will remain there. Nonetheless, the surface of the water in the bilge is exposed to the heat from the engine. This water will evaporate, adding to the condensation inside the watercraft’s hull and increasing the potential for rust on the spark plugs.

    Mitigating the Problem

    • To help prevent rust on your personal watercraft’s spark plugs, you must reduce both the quantity of water and the length of time the water is present inside the craft’s hull.

      By removing the watercraft's seats and opening all the compartments – including both the storage and battery compartments – inside the hull, and by draining the bilge after every use, you can slow the rate at which rust will appear on your spark plugs and improve the performance of your watercraft.