Bolts corroding much faster than steel members in wastewater clarifier 2

[Dennis59]

Hello,
I am investigating a very old clarifier mechanism in a gravity thickener tank. The unit was probably built in the 1970's. There are no shop drawings or records of construction. The mechanism is basically welded steel trusses made from small carbon steel angles, 1/4 inch thick. There are places where the members were bolted together, and many of these bolts are corroding away - just on the head side of the bolt. This in spite of the fact that the material in the steel angles is more or less intact. I have no idea what the bolts are made from - it is steel of some kind, not stainless. Have any of you experts seen a situation like this? Any ideas why the bolts might be corroding faster than the steel angles? If I were to replace the bolts, would 316 stainless be a good choice or would that induce corrosion in the steel angles? Anyone with experience with fiberglass bolts?
Thank you!

 

[TugboatEng]

It seems higher strength steels are more susceptible to corrosion. Consider using fasteners with a lower strength rating.

https://onepetro.org/NACECORR/proceedings-abstract/CORR11/All-CORR11/NACE-11420/119859

"It was found that the susceptibility of the steels to corrosion based on their weight loss were identical prior to 144 hours, after that is accelerated for high strength steel."

In my line of work often find welded mild steel structures will corrode much faster at the welds when submerged. 70ksi weld on A36 steel.

 

[EdStainless]

Don't use SS, it will cause the near by steel to corrode faster.
Replace with nice lower strength steel bolts.

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P.E. Metallurgy, consulting work welcomed

 

[Dennis59]

I have a little more information now - the owner was able to find some shop drawings of the clarifier mechanism.

The clarifier was rehabilitated in 2006. Only the old machine itself was saved and re-used. Everything else (center vertical truss, horizontal rake arms, sludge plows, center well, scum skimmer - everything) was completely rebuilt.

The 2006 bolts were hot-dip galvanized grade 5 bolts. All of the bolts I found with heavy corrosion are always completely submerged - near the bottom of the clarifier tank.

So we have a situation where some 3/4" grade 5 hot dip galvanized bolt heads have disappeared (one) or nearly disappeared (several) in just 15 years, always submerged.

I am planning to call for A307 bolts to replace the grade 5 bolts. A307 is more than sufficient strength-wise, and per TugboatEng's and EdStainless' comments, the lower-strength steel may survive longer. I plan to call for the areas with bolts to be painted with epoxy paint.

Does this new info change your opinions or thoughts at all?
Many thanks for any advice you care to offer.