[rsync]

"Not sure if this was the issue, but some grades of stainless steel screws can't be used with treated lumber because of an increased corrosion risk."

If it's actually stainless, that is an approved connector for treated lumber. It's more expensive than galvanized, but much more resistant.[1]

It's plain old steel - untreated - that has very bad galvanic corrosion properties inside of treated lumber.

[1] http://www.finehomebuilding.com/2012/09/06/whats-the-differe...

[cma]

"All stainless steels may not be acceptable for use with preservative treated wood. Testing has shown that Types 304, 305 and 316 stainless steels perform very well with woods that may have excess surface chemicals. Type 316 stainless steel contains slightly more nickel than other grades, plus 2-3% molybdenum, giving it better corrosion resistance in high chloride environments prone to cause pitting such as environments exposed to sea water."

https://www.strongtie.com/products/product-use-information/c...