Stainless Steel: It might just be the worst boat metal possible. At first glance a good grade of stainless steel looks like almost the perfect boat metal. Under most conditions it doesn’t rust, even if it does, it is usually just a touch of surface color that an engineer would call “rouging.” It is strong, hard, and shiny. Very shiny.
Unfortunately, the shininess of stainless steel hides a dirty secret. Catastrophic failure can occur at loads far less then the part was designed for, without warning, and on a part that looks to the naked eye to be as good as new. This ability to hide an approaching total failure behind a veil of “shiny” is what gets stainless steel my vote for the “worst boat metal possible.” Since stainless steel is used to make make many “mission critical” parts of a sailboat such as rigging and chainplates, this is not an insignificant problem.
This is a case study of a such a failure.
Harmonie is a 25 year old, 53 foot ketch. She has been sailed extensively during her life, chalking up two circumnavigations. She has been owned by knowledgable owners who have taken very good care of her.
While sailing offshore on a close reach in 16 knots of wind and light seas a loud “BANG” announced the problem. It didn’t take long to find it: The jib sheet car had failed, releasing the sheet from under the sheave. As far as I know this part was original to the boat, and is 25 years old. It had been visually inspected as part of the normal routine before this trip, and looked fine. How could a part that had worked for so long, fail under such relatively mild conditions?
The culprit is called “Crevice Corrosion,” or “Corrosion Cracking” or “Stress Cracking.” It is a complex problem, without simple solutions. Sometimes, with careful inspection, it can be seen happening, but often it is invisible to the naked eye until failure (as in this case.) Let’s have a close look at the failure.
Here is a photo of the good, and the bad. We’ll get to the ugly bit in a minute. This is a very heavily loaded piece of hardware. Loads on the line pulling upward on this block can approach 2 tons.
The normal configuration of this part is in the background, and the failed part in front. There are three load-bearing stainless parts. One piece bent upward into a “U” shape that holds the axel for the sheave, and two parts bent downward that transfer the load to a hinge pin. If we look closely at the two legs that have failed, we can see a difference. If you look closely you can see that one leg shows evidence of distortion under load with ripping and tearing of the metal, while the other leg looks like it just came apart with no stress, as neatly as a piece of paper cut with a razor.
A Closer Look…
This is the front leg of the broken part. The area circled in red is obviously fractured and torn metal. It is bent, rough, granular, and shiny. The rest of the failure face looks different. We get a better idea of what this means if we look at the other leg…
This looks rather odd for a stainless steel part that was separated only two days before. It is dull, grainy, even rusty in spots. It has an odd pattern of lines almost like tree rings.
If you look very closely you can see a very narrow line of shiny, fractured metal along the outer edge of the part, a fraction of a millimeter thick, which is slightly wider on the right side of the picture. The interesting thing is this tiny, little bit is the only part that could actually seen when visually inspecting the part. Everything looked fine, even though the effective width of the metal holding things together was reduced to less than 5% of its original thickness!
Why Did It Happen?
The following description is not intended to be an exact description of the chemistry that occurs, but to give you a feel for the issue. Stainless steel contains a significant about of chromium. Chromium reacts rapidly with oxygen in air or water to form chromium oxide, a very hard material that seals the surface of the stainless steel and prevents the oxygen from attacking the iron and other components of the alloy. The key part of this story is that without oxygen, chromium oxide can not form, and the alloy is susceptible to various different types of corrosion, especially attack by the chloride ions in seawater.
Imagine we have a tiny crack or other surface imperfection in a stainless part. The bottom of that crack is not well flushed with air or water, and becomes deficient in oxygen, and the very bottom of the crack begins to corrode and become weaker. If there is also stress on the part, the crack grows, and continues to corrode and grow, corrode, and grow… It might happen very slowly, but once started, it inevitably continues to grow until the part fails. The warmer, and saltier, the environment the faster the failure progresses.
The dull, rusty appearance of the fracture surface of our failed part shows us the extent of the crack before the part failed. Almost no metal was left to carry the load! The lines of “tree rings” shows the slow growth of the crack over time. Both legs of this part show the effects of this type of failure happening. It was more advanced in the rear leg because that is the one under greater tensile stress in normal operation.
The initial crack can come from lots of things. In this case, probably from stresses in the part when it was bent as part of the original fabrication. Stainless steel that is covered in some way that prevents access to oxygen can develop pits, and those pits can then form the starting point for cracks. Even something as simple as a piece of tape left on long enough, under the wrong conditions, can start this process.
Bolts are especially susceptible to this kind of failure because they are under constant tensile stress from being tightened, and the base of the threads form stress risers where cracks easily start. I have seen bolts securing the chainplates of a boat have the nuts literaly fall off from crevice corrosion.
Can It be Prevented?
The only real prevention for this kind of failure is in the initial design of the part when stresses can be minimized, and alloys chosen that are less susceptible to the problem under conditions that are expected to be encountered. Pretty much out of your hands as a sailboat owner.
If you have the opportunity to specify a stainless stell alloy for your boat parts, here is a list of common marine alloys in increasing order of resistanct to corrosion craking: 18-8, 304, 316, 316L. 316L is the best of the normally available alloys in this regard, but it is still not proof against this kind of failure. And proving that you never get something for nothing, it also has significantly lower tensile strength than 304. I have not found anywhere the manufacturer of this part spcifies the stainless alloy they used.
There are ways to inspect a part that make this kind of failure easier to see. Polishing the part and inspecting with a highpower (30-60X) loupe can help you find a crack as it starts, although they are so subtle it can take practice, and it can be exceedingly tedious to completely inspect a large part this way. There are penetrating dies, and magnetic flux tests, even x-rays. However, they require access to the part and usually full disassembly which was not possible in this case.
Stainless steel in life-critical installations on a boat in a marine environment needs to be carefully evaluated for the possibility of failure and the potential consequences. Regular dye testing or pro-active replacement on a conservative schedule are not unreasonable for parts whose failure could put the boat or the crew at serious risk..
Specific Lessons Worth Learning
In the very specific case here: These jib cars are a standard design from Antel. Looking at these, knowing their age, and the damage I see, it seems reasonable to me that a ten year life should be expected. Beyond that, you’re living on borrowed time. It seems unlikely that the failure of these parts could cause a situation that was immedietly threatening to the crew or the boat, but if it occured in the middle of the night, during a storm, halfway across the ocean it could be… very inconvenienet!
This kind of failure is why stainless steel just does not belong—ever—as the material of construction of an anchor chain or anchor. There is simply no way that every link in an anchor chain could ever be inspected closely enough to catch an incipient failure. This kind of problem does not happen with the kind of galvanized steels used in anchor chain. The only way to be safe would be to discard a stainless steel chain before it developed cracks like this, and I know of no way to predict its lifespan.
Given that corrosion cracking is so hard to detect visually, you should always be suspicious of anyone who does a rigging inspection and pronounces it “Good for another 10 years.” This is only one of several reasons that stainless steel rigging NEEDS to be replaced at reasonable intervals to avoid sudden, unexpected, failure, and the unpredictability of the problem is why people argue endlessly over what that interval should be.