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Corrosion Coating for Alloy Steel Pins in Live Tree

September 27, 2010

I am about to install a treehouse. It will be supported by 4340 alloy steel 220 ksi pins in live fir/hemlock trees. I need help with recommended corrosion protection. I am considering zinc (hot dipped galvanized), cadmium with chromate primer and epoxy top coat, or zinc rich primer with enamel top coat. The cadmium option is very pricey, and I am not convinced it would outperform zinc alone. I have noted that commercially available pins for treehouses are alloy steel with powder coating, so the corrosion environment must not be very severe. I'd like to have it last 20 years if possible. Please advise, my son is waiting for his treehouse.

Tim Ives
PE - Auburn, Washington

October 9, 2010

In my opinion, powder coating is your best choice. Make certain, though, it's a polyurethane or acrylic coating. Polyesters and epoxies degrade in sunlight.

Mike Burnson
- Chicago, Illinois, USA

October 28, 2010

Update: I have ruled out cad and zinc plating due to stress corrosion issues with alloy steel heat treated above 180 ksi. My plating plan now is 3 hour stress relief at 300 degrees F, cad-ti with chromate conversion coat per MIL-STD-1500 [affil. link], type II, class 1, 24 hour bake at 375 degrees F. I am considering finishing with 0.020 inches of coal tar epoxy. I figure this is about the best I can do, but still don't have a feel for the severity of the live tree corrosion environment. I would appreciate inputs regarding recommended alternatives.

Tim Ives
PE - Auburn, Washington USA

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