Jack:

I had to replace 3 of my 6 chain plates back in 2001 the cracks were as follows
# A crevice corrosion between #1 & #2 bolt acros the ss strap (= no load failure as it was no where near a bolt, and had nothing to do with square holes.)
# B crevice corrosion starting at the bottom hole #1  and going down the length of the strap (= no load failure, and started with square hole edge I believe but was corrosion only.)
# C crevice corrosion between #3 bolt and the edge of the strap and a crack between #1 & #2 bolts At a 45Deg.  (the first maybe had load issues  but the second was just crevice corrosion between the hull and the plate.

So in summery If you passivate or electorplate the part you are much less likely to have issues+/-. (it is the corrosion that is the issue),   But some of the original tangs were not over built, and a small crack could develop into a big issue and as you pointed out the corners are very big stress risers.  One note: I believe the some of the square holes are punched which, if not relieved, can have extra stress.  Round holes should be ok, but you might think of some form of locking it (wire, weld, epoxy, etc) opps make sure the fix can be removed (grinder, cutter, etc.) if needed.

I hope this helps you out.
Norm

P.s. the square head does allows for a single person install and removal a +

Jack:

Thanks for the picture.

That is corrosion (crevice corrosion and opening up when stress was added)  Note it started on the outside of the strap and was headed for the hole, It would eventually fail.  So replacing it was a good call. 

See how the rest of the tang looks polished the the point of where crack is, looks corroded (this is from a concentration of iron (iron oxide) which leaches out the nickle and crome from the ss then the rust makes more iron oxide etc etc.  small micro cracks allow this to happen at a faster rate until they weeken the material so that stress and the swelling iron oxide can open them and move it even faster.

Jack you may want to inspect for this by the cotter pins?

This can be stopped if caught soon enough, by passivating or electro polishing the part = removing the free iron in the top 0.00001" of the material including micro cracks +/-  they make dye penetrant for finding these but the cost may out weigh the cost of the electro polish, if you don't have a spare can of the stuff around. If you are interested, it can be found at a weld supply shop+/-.

SS is not really Stainless:  just a bit of iron dust (grinding sparks, steel wool, etc) will stain the #$@ out of it. 

Oh: lime juice is a weak passivator

This information is great and unless the members object - I'll be adding it to the FAQ pages.

I do have slide show for members of "items that break" at I also would like to add Jacks photo to http://www.westsail.org/itemsthatbreak - there are some pretty graphic images to catch folks attention.

I recommend you email or call Bud for a quick response.

Jay  

Jack:

 I did use Buds plates for replacements in 2000 or some where around that time+/-
and am pretty happy with them still.

I did however want to sort of clear up a statement by Bud (he almost has it right )

The electropolish opens up (eats away the free iron by using electromagnetic forces and acid concentrated at the edges) and this process removes free iron form the surface as well, and most of the micro cracks (in real life you can never say all, just most).
 
1.) stainless steel needs air to keep the surface free from iron sticking to it. (keeps the surface chrome and nickel  rich =little corrosion +/-) = just rusting

The overall chemical equation for the formation of rust is

Iron + water + oxygen rust

4 Fe(s) + 6 H₂O(l) + 3 O₂(g) 4 Fe(OH)₃(s)

  the top layer and washing it away, leaving a shine and a surface of chrome and nickel.  Just think shine on the chrome bumper (it is just chrome plated steel)

2.) but crevice corrosion does not use this process (as surface corrosion mentioned above) but the crack is so small it holds iron and salt ions in it , which even in the open air are not washed away but are trapped in the microscopic crack, which leads to more corrosion etc. etc.    This is where the dye penetrent and magnafluxes are used to find these little buggers in critical applications (NASA, turbines, high pressure fittings and Nuk facilities, etc.).

Jack's picture is of the crevice type start (but the brown stuff is surface type).  You will note: the part was in the open air. I assume a surface flaw (crack) was the start and it just followed the steel grain tward the hole, but once there was room in the crack, the surface corrosion also kicked in to help eat the metal away. 

Buy having a tang prepared as Bud has explained, You will have the best chance of success. The 1.5" wide material gives you more time between when it starts and when it is a problem.  (I like extra time, I need all I can get)

Passivation will not chew on the edges of micro cracks and open them up as much as the EP process will, but it does clean up surface rust and gives it the Chrome and Nickel rich surface you want, but if you do have micro cracks, you will be doing it again in a year or two or replacing it.

3.) Nothing replaces a good inspection routine!

I did not mean to step on Bud, but I did want to explain why Jacks Picture shows the corrosion on the open air part.   I do hope this helps out.

Norm

P.s. ALWAYS try to keep Grinding dust and steel wool away from any Stainless Steel.

PPs  Looking at the picture I believe there is a micro crack about 1/4" below the crack we are talking about, take a look at Jack's Picture.

I just received a couple of images of the boomkin bolts which might explain some failures  and reinforces Bud's advise to check the bolts visually every time you haul the boat.

 
The top view - what you see on the outside ...

may look like this on the underside.

Thanks to Randy Leasure for the photos.