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I replaced the washers on a house's foundation bolts (replacing old round washers with larger square washers).

On 6 of the ~30 bolts, the nut was rusted in place and the shaft was eroded where it sat in the sill plate. Trying to unthread the nut just twisted the bolt until it snapped. All the other nuts unthreaded without much effort (and some Liquid Wrench or WD40).

These 6 damaged bolts were the pairs at 3 of the house's corners. What would cause that? (I've replaced these bolts with UFPs, but am wondering if it's a sign of some other issue that should be fixed.)

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    speculation: for the same reason casseroles are more well-done at the corners; more exposure – dandavis Oct 17 '18 at 16:02
  • Interesting question, and I'm betting on @dandavis's answer. But, corrosion analysis probably isn't on-topic here. – Daniel Griscom Oct 18 '18 at 2:26
  • I like the casserole metaphor! I suppose a bolt near the corner does potentially get 2 different house-sides worth of rain. – Phil Esra Oct 18 '18 at 5:46
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Corrosion is a process of moisture and oxygen on the steel bolts. There is no special reason for the bolts to “rust” at the corners than any other location, unless 1) proximity to crawl space vents, 2) coincidental subsurface moisture (uphill locations) at those locations, 3) surface moisture (downspouts) introduced at those locations, 4) leaky siding joints at corners, etc.

  • Good points. I think #1 and #2 can be ruled out in this case. I'll check the downspout arrangement out. The corner joints are under stucco, so they can't easily be assessed; I guess I should get under the house during/after a big rain. – Phil Esra Oct 18 '18 at 5:53
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Were they all identical bolts, nuts, and washers? Since your failed ones were all at the corners, they could have come from a different batch than the others when the house was built, as perhaps the people who put them in did the corners first with one batch (maybe not up to the same standards or corrosion resistance) to just hold things in place. They then came back and did the middle ones later with possibly a different batch, or under different conditions, etc.

  • They looked identical, but it would be hard to say definitively. – Phil Esra Oct 18 '18 at 5:44

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