Everywhere on yachts we see stainless or stainless steel: pulpits, railings, hinges, propeller shafts and so on. This is mainly due to the fact that it is shiny, strong and affordable.
What is stainless steel actually (and what is its application on boats and yachts)?
Stainless steel is mainly iron mixed with chromium and nickel and sometimes other small amounts of substances, such as molybdenum, cobalt and others.
The chromium in the stainless steel oxidizes in contact with oxygen and lays a protective layer on our stainless steel. However, this chromium is not very resistant to acid and therefore (but also for other reasons, see below) nickel is added.
The amount of chromium and nickel in our mixture determines the type of stainless steel we obtain. Depending on the application, we find different types of stainless steel. For example, cutlery contains a lot of chromium but little or no nickel (nickel is expensive). That is why you will see that when cutlery is used in salt water, this cutlery quickly corrodes (blue water sailors sometimes wash the dishes in seawater). You will also immediately see that ‘stainless’ is not a very well chosen term. It may not ‘rust’, it still shines, but it does corrode.
Other reasons for using little nickel is to obtain higher tensile strengths, for example. These tensile strengths vary according to the alloy. For us, however, the type 316L is especially important, which is stronger (in tensile strength) than steel S235 (ordinary steel), but not much stronger than the better steel (S355) and much less than high strength steel such as Weldox. Stainless steel has less ‘elongation’ and thus breaks ‘more unexpectedly’ to put it simply. Therefore a hook or shackle in high strength steel, as always used in fishing, dredging, or industry, is much stronger than one in stainless steel.
How are the types of stainless steel for a boat identified?
The different types are identified by a number according to their iron, chromium, nickel and other content. For those who want to buy a boat: 304 and 316 are the most common (this is actually a designation coming from the USA). The carbon content in the stainless steel is also important. This must be kept low to prevent other forms of corrosion, for example after welding. This is referred to as 316L (low carbon content).
Other elements will be added in order to be even more corrosion resistant, such as molybdenum.
On bolts and nuts and other fastening materials, this is not indicated with, for example, 316L but in this case with A4. This A4 is the best stainless steel for fixing in yachting. A4 is 316L stainless steel with 16% chromium, 10% nickel and 2% molybdenum and a low carbon content.
Another widely used, cheaper species, is A2. This is stainless steel type 304 with 18% chromium, 8% nickel and widely used for propeller shafts.
The corrosion resistance is less at A2 stainless steel, but therefore not bad. However, if you buy a nut or bolt, the price difference is so small that you should choose the best: A4. If there is nothing on the nut or bolt, do not buy it.
Especially propeller shafts will now also see new types, types that are stronger, more resistant to corrosion and so on. They are not specially developed for yachting, but rather for industry (derricks, fishery, …). However, they can also be very useful in yachting because of their better properties. Because they are stronger, for example, lighter propeller shafts can be made (the diameter can then be smaller for the same strength as a 316 shaft for example).
The downside is the cost. The material is much more expensive and is usually more difficult to machine, so the fabrication of a propeller shaft, for example, will also be more expensive due to the additional work.
There are too many of these new species to mention: duplex, super duplex, hyper duplex, aquamet17, you name it.
In most cases in yachting 316L (i.e. A4) is best suited.
Which forms of corrosion are most common when using stainless steel?
1. Pitting and crevice corrosion
This is corrosion that occurs very locally due to the action of chlorine ions. This chlorine is found in salt (NaCl) and therefore in seawater. These chlorine ions break down the protective chromium layer. This breaking down of the chromium layer can also be done simply by lack of oxygen, such as between nut and bolt. Especially when water is present, galvanic corrosion occurs, as there are two different types of metal. These are an electrical connection and an electrolyte: iron with and without chromium oxide and water. You can read how this works in the article on electrolysis.
One element that reinforces this pitting corrosion is temperature. So beware of using stainless steel in the exhaust of the engine!
Chlorine ions are the big culprit here. So we strongly recommend that you do not throw disinfectant chlorine tablets into your stainless steel water reservoirs.
2. Intercrystalline corrosion
When welding stainless steel to ship steel, the composition of the metal on and around the weld changes. Particularly important here is that the carbon of the stainless steel (little) and the steel (a lot of carbon) connect to the chromium atoms. As a result, this zone becomes less corrosion resistant and also harder, i.e. more brittle. That is why fractures are often seen just next to the weld stainless steel. This can have major consequences, for example, if stainless steel reinforcements are welded onto the steel deck.
It is also possible in the rigging: the stainless steel cable can become brittle and suddenly break in contrast to steel for example. That is why, for example in industry or fishing, little or no stainless steel hooks are used in cranes. The same applies to the cables: cranes and winches are almost always equipped with steel cables.
3. Galvanic corrosion
Especially aluminium, which is much less ‘noble’ than stainless steel, is subject to a lot of corrosion when these two metals are bonded together. Where do we see this in particular? Of course on aluminium boats, but also on aluminium masts where fasteners on the mast are riveted with rivets without an insulating layer, on aluminium rudder shafts, and so on.
Sometimes electroplating has positive effects: for example, a stainless steel propeller shaft is less likely to develop pitting corrosion because the steel hull ‘protects’ it galvanically. That is why it is also advisable to galvanically protect fuselage shafts with GRP or FRP.
4. Stress and fatigue corrosion
Especially in warm climates, the combination of corrosion (due to salt on the staging, for example) and varying tension can lead to breakage. A staging, especially on long sea voyages, is constantly subject to varying loads. Old rigging can still look nice, but it should be regarded as suspicious and should be closely scrutinized on a regular basis. This is why some insurers demand that rigging be replaced every 10 years.
Even in very cold climates we have to be careful with staging: it shrinks due to the old temperatures and thus increases the tension. In Canada we experienced a temperature of minus 17° and 65 knots of wind, fortunately in the harbour. In such circumstances, it is crucial not to tighten the staging too hard. Additionally, you get all kinds of other unpleasant phenomena in metals at extremely cold temperatures.
How can we check what kind of stainless steel we are dealing with when buying a second hand yacht or boat?
First of all, all good quality stainless steel fasteners or shackles are marked A2 or A4. If there is nothing on it, treat it as suspicious and do not buy it. Another possibility is the use of a magnet. Stainless Steel 316 is NOT magnetic. 304 can be slightly magnetic (especially if it is twisted, such as a propeller shaft).
Attention: we have experienced that a D-shackle of diameter 12mm broke. A check revealed that the inside was brass, with a nice chrome layer on top. The seller had sold us this as ‘good quality stainless steel because it’s not magnetic’! We had bought this from a ship’s supplies dealer who is still active here in Belgium, so be warned.
Morality of the story: stainless steel on the water is a fantastic product, but be wary of its limitations as well as poor, unsuitable qualities.