Stainless Steels

One of the main reasons why stainless steels are used is corrosion resistance but unfortunately even 'stainless' steel suffers from corrosion in some situations and environments. Sometimes quite dramatically. There are a number of different grades of stainless steels available and they have very different characteristics and so care must be taken to select a grade which will be suitable for the application. Generally in the marine environment we find grades 316 and 304 in use with 316 having considerably better resistance to corrosion than 304. 304 has slightly higher tensile strength than 316 and is sometimes used for rigging and other stainless grades may be used for high torque applications but generally these have lower corrosion resistance. Nitronic 50 (also called 22-15-5) and Aquamet 22 are both corrosion resistant and have high tensile strength but are very expensive and little used.

High chromium content stainless steels (with no nickel content) such as 445M2 may take the place of 316 as the preferred 'marine grade' in years to come. Some stainless steel alloys are formulated for easy machining but these generally have poor corrosion resistance.

‘Crevice corrosion’ of stainless steel occurs in an environment where the protective oxide layer on the surface breaks down and cannot be repaired due to lack of oxygen in the surrounding environment. This is most frequently encountered with seeping keel bolts (especially in the slightly acidic environment in wet timber) or in the threaded portions of rigging fittings.

This corrosion can be quite dramatic and lead to total failure of the component. Stainless steel propeller shafts sometimes corrode where crustaceans have attached themselves and, less commonly, in excessively tight bearings or packing glands where wear removes the oxide coating which cannot then reform due to lack of oxygen.

Pitting corrosion

is quite common in stainless steel water traps in exhaust systems. The relatively hot salt water mixed with the exhaust fumes create a highly corrosive environment and corrosion starts in the pitting on the inside of the welds. Once started the stainless steel is no longer ‘passive’ and corrosion can proceed quite rapidly. Often one sees staining and surface rust on fittings where these have not been suitably treated after welding. This can be eliminated by ‘passivation’ with dilute nitric acid or by applying a ‘pickling paste’ (a mix of hydrofluoric and nitric acids) and then rinsing and, ideally, polishing. Polishing does more than make the surface bright but does actually help to prevent corrosion and is recommended for rod rigging in particular.

Rigging

Stainless steel rigging can be subject to crystallisation or fatigue failure caused by excessive load, flexing, poor swaging and/or poor alignment. Most rod rigging failure takes place at the heads or terminals where it is most likely to be unevenly loaded. Badly swaged terminals are a common cause of rigging failure. Poor swaging equipment may cause the swage to bend slightly which leads to uneven strain on the wire rope and poor alignment. The use of toggles on chain plates and tangs allows the cable to align better with the terminals and minimizes fatigue. Excessively loose rigging is much more likely to fatigue than taut wire.

Often problems like this are the result of poor design and/or low quality material but a regular rinse with fresh water and occasional polishing helps prolong the life of even mediocre rigging and fittings.

1 x 19 wire rope is most commonly used for standing rigging
7 x 7 is good for lifelines and other semi flexible use
7 x 19 is best used for halyards or steering cables and similar where the rope has to pass over sheaves. Often low stretch/high tensile strength synthetic fibres are being used nowadays.

'Dyform' wire rope is a 1 x 7 compact construction with approximately 30% higher tensile strength (and weight) for a given diameter. Almost as strong as rod rigging (for its weight/diameter) and almost as resilient as 1 x 19.

Stainless rigging screws can seize due to 'galling' where the threads virtually weld themselves together when adjusted under load. Combination bronze and stainless rigging screws are a good combination since they are fairly 'galvanically' compatible and do not seize. Swageless terminals offer many advantages over press swaged fittings. They are less prone to failure and some can be opened for cleaning and inspection and then reused.

For more complete technical information on stainless steel alloys www.azom.com is a good place to start.

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