48076

Stainless steel vs. Zinc plated [Texas] 

March 6, 2008

My product is made of High density Polyethylene Plastic. The hardware we currently use to assemble the plastic pieces are 18-8 stainless. Our parts get buried in the ground anywhere from 6" to 48" deep. Most of the hardware is directly in contact with the plastic parts only. Some of the hardware is used to mount stainless steel or aluminum brackets to the plastic parts. We are considering changing some or all of our hardware to Zinc plated vs. the stainless steel. Can you give me any input on this plan. Would there be a major change in the life cycle expectancy of the hardware. Thanks for the help. Paul

John Millstone
product designer - Austin, Texas, USA

Zinc plated hardware is not really satisfactory for outdoor or buried use, John.

Ted Mooney, P.E.  finishing.com Brick, NJ

John, I once installed an outdoor lamp post with 1/4-20 zinc plated threaded rod, only to have anchor rods 3 feet in gound & concrete corrode out in 1 year. Dug it all out, what a job!!

I repeated the same with 18-8 stainless steel 1/4-20 threaded rod and Lampost anchors are still sound at 10 years +++ service. Stainless is the way to Go!

Tim Deakin
- North Tonawanda, NY

As others have pointed out, zinc plating / galvanized steel is not going to work well in direct burial applications.

Stainless steel will work underground, but since you can't do a soil analysis of every customer's location you may want to assume worst-case conditions and go with an expensive stainless grade such as 318. No stainless is cheap, they do tend to gall (the nut suddenly becomes one with the bolt, and can't be moved), and they're not as strong as most steels unless you go with one of the 400-series grades that can be heat-treated.

I do not know what your strength and shear requirements are, but given that your base material is HDPE, I urge you to take a look at Delrin fasteners. (Delrin is a Dupont trade name for their acetal resin.) Unlike the nylons, Delrin doesn't absorb water and swell a lot, pretty important for a buried application. It has some porosity, but not knowing your application I can't say whether that would be a problem or not.

David Doerschuk
- Beaumont, Texas, USA