What kind of problems can you have with stainless steel?

Simply put; stainless steel is an alloy of iron, carbon and chromium, and in most cases
stainless steel is maintenance free and will last for years.  But it has some drawbacks, of
course.  It has a low modulus of elasticity, which means it doesn’t bend and flex well.  It
develops microscopic stress cracks, which are very hard to spot, and have a tendency to
cause sudden, unexpected total failure of the part.  The other problem is that stainless steel
protects itself by oxidizing its surface, (chromium oxide).  When deprived of oxygen, the
oxidation cannot occur and the ferrous component of this steel alloy begins to rust.  This is so
called crevice corrosion.  Typically found where a nut has covered the thread of a bolt, or a
bolt passing through a wooden bulkhead.  The SS wire inside a swaged fitting.  Behind a
chainplate, or inside a stress crack.  The result looks like some kind super-rat has been
gnawing the metal. (A word of caution, avoid painting stainless steel!)  You have to know
where to look and have a magnifying glass or microscope handy.  The magna-fluxing of
suspect pieces can show cracks imperceptible to even a strong lens.
How long will the rigging last on my sailboat?  Amount and type of use, length of season, and
climate will dictate. (See FAQ: Inspection of sailboats; Sails and Rigging).
I will perform a cursory exam of rigging at the deck level, but age and signs of stress or
crevice corrosion will lead me to recommend a thorough examination of all the standing
rigging.  I offer magna-fluxing of any component suspect of stress cracks.  This would include
swage fittings, chainplates, turnbuckles, tangs, prop shafts, propellers or any metal,
(especially stainless), that takes a lot of stress. Perhaps the best stainless steel for
underwater machinery is called Aquamet, an alloy of iron, carbon, manganese, silicon,
chromium, nickel, phosphorous, sulfur, molybdenum, vanadium and a few others.

Types of Stainless Corrosion

According to the DOD Technical Bulletin Corrosion Detection and Prevention there are 8
separate types of corrosion, with only a few having a major impact on stainless steel. Please
be advised the descriptions below are extremely brief and written in laymen terms. Before
acting on any particular application, qualified advice particular to such application should be
obtained.

1. Uniform Attack - also known as general corrosion, this type of corrosion occurs when there
is an overall breakdown of the passive film. The entire surface of the metal will show a uniform
sponge like appearance. Halogens penetrate the passive film of stainless and allow corrosion
to occur. These halogens are easily recognizable, because they end with "-ine". Fluorine,
chlorine, bromine, iodine and astatine are some of the most active.

2. Crevice Corrosion - this is a problem with stainless fasteners used in seawater applications,
because of the low PH of salt water. Chlorides pit the passivated surface, where the low PH
saltwater attacks the exposed metal. Lacking the oxygen to re-passivate, corrosion continues.
As is signified by its name, this corrosion is most common in oxygen restricted crevices, such
as under a bolt head. Swaged rigging fittings and chainplates are always suspect.

3. Pitting - See Galvanic Corrosion. Stainless that had had its passivation penetrated in a
small spot becomes an anodic, with the passivated part remaining a cathodic, causing a pit
type corrosion.

4. Galvanic Corrosion - Placing 2 dissimilar metals in a electrolyte produces an electrical
current. A battery incorporates this simple philosophy in a controlled environment. The current
flows from the anodic metal and towards the cathodic metal, and in the process slowly
removes material from the anodic metal. Seawater makes a good electrolyte, and thus,
galvanic corrosion is a common problem in this environment. 18-8 series stainless fasteners
that work fine on fresh water boats, may experience accelerated galvanic corrosion in
seawater boats, and thus it is suggested you examine 316 stainless.

5.  More rare- dendritic or sensitized stainless steel corrosion from overheating the alloy
during a welding process. The chromium in the alloy is lost and self protecting ability of the SS
is lost. See photos below of a welded shaft strut made of a lower grade, (less chromium), SS-
note the pitting and stress cracks near the weld. Welded SS tanks such as for waste water
must be watched carefully. Same for underwater welded SS hardware.
From the Surveyor's Notebook