Corrosion that may cost your rig

Some corrosion is easy to spot but stress corrosion cracking has the potential to weaken your rig with very little warning, says Vyv Cox

- By Vyv Cox

Some of the more common failure modes of the metals used in boats are generally quite well understood by yacht owners today, when internet searches and forums make topics that used to be solely the realm of specialists readily accessible to all. However, some means by which metal parts can break remain somewhat more clouded in mystery, few more so than stress-corrosion cracking (SCC). In a similar fashion to fatigue cracking, SCC is one of those failure mechanisms that can remain hidden for a long time, suddenly giving way when stress levels cross a certain threshold, which, as is the way of these things, is usually at the most inconvenient time for the sailor. To be technically correct, I’m discussing the version of it that occurs in chlorides (such as salt water), whereas another that occurs in sulphides can be ignored for this article.

Susceptibility of stainless steel

SCC, as the name suggests, is one of those failure mechanisms that relies on a combination of two factors. Either stress or corrosion alone would not cause the failures that are described in the article. Further to that, there is a threshold temperature below which the problem should not occur. Most grades of stainless steel are susceptible, and much research effort is expended in trying to improve the performance of this important class of alloys in high-chloride environments.