Seasonal maintenance: stainless steel

by • December 9, 2015 • older, Yacht MaintenanceComments Off on Seasonal maintenance: stainless steel2644

Stainless steel has many uses on board and looks great when freshly polished, but it’s not as indestructible as is commonly thought.

It’s often assumed that stainless steel is a bulletproof wonder material. It’s certainly used for many key elements of a yacht, including keel bolts, standing rigging, chain plates, propeller shaft, most of the fastenings and often stanchions and lifelines.

Never assume a stainless steel tank is impervious to problems.

However, it can be subject to a number of problems, the best know of which is crevice corrosion. The corrosion resistance of stainless steel relies on the presence of a microscopic surface oxide film. Once this is breached, an electrochemical cell becomes active, which propagates the damage. In addition stainless steel doesn’t accept repeated flexing very well – this quickly forms cracks in the crystalline structure. These, in turn, provide an ideal oxygen free site for the propagation of crevice corrosion.
Stainless steel used in marine applications will normally be 304 grade (also called A2) for use above the waterline and 316 (or A4) for use above or below the waterline.

The latter tends to have better resistance to crevice corrosion.

Microscopic cracks in which crevice corrosion can occur won’t be revealed by a visual check of standing rigging, however, they should show up in a dye test.
Stainless steel surfaces can also develop a rust coloured tarnish – this is particularly likely to show up after a rough passage or a ocean voyage in which the boat will have been subjected to salt spray for an extended period. Other situations in which discolouration may be encountered include contact with ferrous metals, or inadequate sealing between the fitting and the deck.

Usually washing with soapy water (use an ecologically friendly product) and a soft cloth is all that’s needed to remove any tarnishing and restore a bright polished appearance. If the damage is extensive cream cleaner or metal polish may be required, but avoid the temptation to attack stainless steel finishes with scouring pads or wire wool, which will visibly scratch the surface.

Never assume a stainless steel tank is impervious to problems.

Another application in which stainless steel has frequently been is for fuel, water and waste tanks. Unfortunately, its longevity is not guaranteed for these applications, with pinholes on the welded seams being a particular problem as the boat ages – don’t discount this possibility if you encounter problems with leaking tanks.

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