May 20, 2012

Polishing non-stainless steel

ONE OF THE FIRST TASKS that falls to boat owners each spring is to polish the stainless steel of pulpits, pushpits, and stanchions. The truth is that a lot of stainless steel isn't stainless at all. It often acquires a veneer of rust in the cold salty air of winter, a light brown stain that offends the mind as much as the eye because of its inherent promise to remain shiny and free of unsightly stains.

But even worse things happen to allegedly indestructible stainless steel under certain quite common conditions on boats. It can corrode and waste away as badly as any ordinary piece of mild steel. It seems almost paradoxical, but most types of stainless steel rely on a constant supply of oxygen to avoid corrosion.

On deck, or under water, uncovered stainless steel receives sufficient oxygen and stays bright. But if it's enclosed in a stern tube, covered with marine growth, or surrounded by wood, stagnant water or other material, it can be deprived of the oxygen it needs, and suffer from pitting. That's one good reason why regular stern glands should drip a little — to feed dissolved oxygen to the stainless steel propeller shaft — and that's also why it isn't always clever to use stainless steel for keel bolts if they're buried in long sections of damp wood.

According to marine author Nigel Calder, stainless steel is an alloy of several metals, one of which is chromium. When chromium is exposed to oxygen in air or water it forms an inert layer that protects the underlying metal.

Calder adds: "But if taken away from oxygen and surrounded by moisture, particularly salt water, the oxidized layer of chromium breaks down, leaving the stainless steel to rust and corrode much like ordinary steel — a situation commonly referred to as crevice corrosion."

That's why prudent mariners like you should look around with suspicion when polishing the pulpits and stanchions. Check the  stainless rigging screws, where salt water might wick down threads,  and the plastic-covered lifelines. Inspect swaged terminals and many other important stainless steel fittings, and watch them regularly for signs of corrosion. Unfortunately, all too often stainless doesn't mean stainless on a boat.

Today's Thought
Distrust justifies deceit.
— La Rochefoucauld, Maximes

As an airplane is about to crash, a female passenger jumps up frantically and yells: "If I'm going to die, I want to die feeling like a woman."
She removes all her clothing and cries: "Is there someone on this plane who is man enough to make me feel like a woman?"
The guy in front of her stands up and removes his pants. "Sure honey," he says. "Here, iron these!"

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

1 comment:

Matt Marsh said...

"Unfortunately, all too often stainless doesn't mean stainless on a boat."

Too true, John, all too true.

That's doubly so when dealing with that "400 series" martensitic SS that some cheapskate manufacturers have been sneaking into hose clamp screws, boiler innards, and other "you'd never think to look there" spots. Give this stuff a confined space and a bit of salt water, and it just disappears.

We're at the point now where you have to touch every single screw on every piece of hardware on the boat with a magnet to see what you're actually dealing with. (The good stuff, 300-series, is non-magnetic).