Stainless cracking. Metalurgists, puzzel this one

[dlj]

Originally Posted by thinwater

From the owner:


"All the ones (bolts, washers and nuts) I removed were 304 series (A2), it is isolated from the aluminium bracket with a glass reinforced acetal washer. This sort of crevice corrosion is quite common in saline environments where a water film can get trapped and not flushed. It is a significant area of interest in offshore oil and gas where the use of duplex steels (which have austenitic and ferritic phases) is common which are designed to be more resistant to this type of corrosion. One of this issues is there is no great way to tell if a fastener has crevice corrosion. PERN number, which is a ratio of chromium, molybdenum, nickel, and nitrogen content is often used to compare the resistance of steel to this type of corrosion. PERN is approx18 for A2 (304), 24 for A4 (316) and mid 30s for duplex. I notice the farrier sailing manual suggests no maintenance is required except flushing the bolts with fresh water if feasible."
_._,_._,_

In order to actually know what corrosion/mechanical mechanism(s) are causing these bolt heads to fail, they must be actually examined. The attached photos are insufficient to know. Crevice corrosion, a corrosion mechanism we love to hang all our corrosion failures on, actually can only be determined through metallurgical examination. It can't be positively identified soley through a visual examination of the fracture surface.

The images posted can't be blown up enough to make a positive ID of where the origins of those fractures are located. Identifying what mechanism caused the origin, even less so.

There are three fundamental "parts" of a fracture - 1) the origin, 2) propagation and 3) final fracture.

In order to know how best to replace those bolts, one must understand how the origin occurred. You also need to know what the propagation method is as that can also play into a better solution. Final fracture analysis mainly just helps make sure the base material was correct and can aid in finding the origin - typically diametrically opposed from the origin (not always)

For example, it could be that the fibers in the glass reinforced acetal washers are abrading the bolt right at the head causing a corrosion mechanism to take over through the propagation phase. In that case, it would be better to switch out the washers to a different type of washer that does not cause that abrasion. If you changed to 316 or a duplex alloy, they would both be subjected to the same abrasion and the new bolts may not have a much better chance of surviving. That would depend upon the propagation mechanism and if one of those alloys resisted that mechanism better. But, it would still be a better solution to remove the source of the origin.

There are numerous scenarios with different causes of both origin and propagation phases that play out differently. Without identifying the cause of the origin, you are just guessing how best to fix this.

dj

[Bowdrie]

I think if I had one of those boats, I'd be ordering some Titanium fasteners before I finished reading this thread.