Boat Rust And Corrosion

By Patricia Kearns

Keeping a sharp eye out for corrosion and taking action early are a critical part of preventive maintenance.

We may not be able to stop corrosion, but if we can identify what kind it is, we can probably control it or at least mitigate its effects. Below are some of the most common types of corrosion that occur on boats:

Atmospheric Corrosion

This is what happens to virtually every metal exposed to the air in my homeport in southwest Florida. The air is laden with moisture and salt, and it's a contest of wills to keep rust from forming overnight.

  • Photo of crevice corrison of chain plate assembly Custom-made sailboat chain-plate assembly is being attacked by water leaking through the deck at the chain plates. Stainless-steel chain plates are subject to crevice corrosion, when water soaks the deck core where it bears against the chain plate.
    Erosion corrosion of pipe elbow Both the threaded and hose-barb sections of the pipe elbow have wasted from erosion corrosion. Only two threads were engaged at the thru-hull fitting and the leak was a mystery to the boat's owner until a technician disassembled the fitting and removed the hose.

Immersion Corrosion

Put metal in water, especially saltwater, and it will corrode. This is a no-brainer. Regional variations influence this process.

Galvanic Corrosion

Occurs when you marry dissimilar metals and they get wet. The more noble metal will try to protect itself by "stealing" mass from the other (less noble). The transfer of mass occurs in the electrolyte (water). My description here is an oversimplification but the concept should not be unfamiliar. This is not "electrolysis."

  • Photo of a corroded engine mounting hardware Engine mounting hardware compromised by corrosion from leaks from engine exhaust fittings.
    Photo of rust forming in cracks Rust forming at microscopic cracks in this sailboat's bow pulpit railing, likely due to a combination of stress corrosion and fatigue.

Stray-Current Corrosion

Happens when electrical current is added to the formula for galvanic corrosion. This is not "electrolysis," either.

Pitting Corrosion

Can grow a tiny pinhole into a big, not always identifiable, perforation and its potential is often overlooked on stainless steel. This is insidious stuff that most affects metal surfaces that are hidden from view — for example, the area of a stainless-steel shaft that is totally isolated in a cutlass bearing.

  • Photo of swim platform support brackets and platform ladder bolts rusting Stainless-steel swim platform support brackets and platform ladder bolts rusting is an example of atmospheric and immersion corrosion aggravated by the platform's core (wood) being saturated from water intruding at the hardware fastenings.
    Photo of a footplate crumbling from corrosion The mast step footplate is crumbling from corrosion perpetrated by stray current from improper bilge pump wiring.

Poultice Corrosion

Like pitting corrosion, is hard to see easily because the corrosion takes place where the metal bears on a wet, porous material. An aluminum fuel tank supported directly by plywood that has become wet might suffer poultice corrosion.

Erosion Corrosion

The constant velocity of moving water gradually wears away the metal.

  • Photo of a bad installation of an aluminum fuel tank A bad installation of an aluminum fuel tank installed low in the bilge and exposed to bilge water splash. There is no drainage provision for tank surfaces and water is trapped within tank support structure, precipitating corrosion. When the tank was removed, poultice corrosion had destroyed the bottom panel.
    Photo of a rust stained valve handle Rust staining should have clued the boat owner to a problem with the valve control handle, which has now completely separated from the valve; shutoff will be impossible without another tool. Emergencies are not the time to go hunting for the tools.

Cavitation Corrosion

Damage can occur on any metal, anywhere in or on the boat that is exposed to the constant shock and explosive shock waves of air or vapor bubbles in water.

Stress Corrosion

Cracking is commonly seen in metal fittings that have been formed (controlled bending or working) — for example, swaged fittings in standing rigging, angle brackets, and such. The hairline cracks look like a tree without leaves, with cracks branching out from the main crack. One minute, these fittings are giving good service, and the next, they fail without warning and the mast comes down. Did I say "no warning"? You can find cracks if you look closely with a magnifier.

  • Photo of a stainless steel exhaust water-lift Stainless steel is not recommended for use in exhaust water-lift fabrication. Wet exhaust by-products are highly corrosive and this unit, installed in a boat built in the Far East, is leaking at every weld, and the drain plug of an incompatible metal has wasted away from galvanic activity. When the plug fails, exhaust water will leak into the bilge and flood the boat.
    Photo of a Stainless-steal prop nut Stern-drive gasoline engine exhaust system leaks at manifold and riser are causing rust to form on surfaces around gaskets. If water is leaking, so are toxic carbon-monoxide (CO) vapors.
  • Photo of a stainless-steel stanchion railing Rust and cracks observed at the welded ring on this stainless-steel stanchion railing are the result of weld sickness and stress corrosion.
    Photo of a stern-drive gasoline engine exhaust system Stainless-steel prop nut showing a large void that propagated from a tiny crevice under attack from crevice corrosion. When the nuts were separated, their facing surfaces looked like metal lace.

Corrosion Fatigue

Similar to stress corrosion and results from a combination of corrosive action and a recurring stress that causes a fracture to develop.

Crevice Corrosion

Any flaw in a metal can be subject to this form of corrosion when it's contaminated by stagnant water, specks of dirt, or other contaminants.

Notice that none of these terms are tagged "electrolysis." Corrosion and electricity are entwined. Mix water, metal mass, and electrical current and you can have corrosion, but that condition is not electrolysis and the word should not be used as a synonym for corrosion of any kind. Electrolysis is the breakdown of a chemical compound, like metal, in solution by passing an electric current through the solution. The process requires a source of direct current (DC), two electrodes, such as metals, and an electrolyte (seawater).

We've all seen the evidence that makes certain kinds of corrosion obvious, but what about the kinds of corrosion that are not so easy to differentiate? The physical evidence of corrosion activity is a symptom, and seeing the color and textures of the process at work is a call to action. If you see rust, you know something is corroding. To keep corrosion from ruining your day, you need to know what causes it and what it looks like. If you spot corrosion and can't identify the type or cause, it's time to call in an expert. 

Patricia Kearns worked for more than two decades as a marine surveyor and technical editor. This article first appeared in Mad Mariner's DIY Boat Owner Magazine.

— Published: June/July 2011

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