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Metal finishing Q&As since 1989


Stainless steel vs. Zinc plated

Q. 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
March 6, 2008

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

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
March 6, 2008

A. John, I once installed an outdoor lamp post with 1/4-20 zinc plated threaded rod, only to have anchor rods 3 feet in ground & 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 Lamppost anchors are still sound at 10 years +++ service. Stainless is the way to Go!

Tim Deakin
North Tonawanda, New York
March 8, 2008

A. 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
March 10, 2008

Zinc plated (Fe/Zn 5) vs. 410 Stainless Steel screws

September 20, 2019

Q. Hello,

At the company I work for, we are using stainless steel screws to mount small plastic and metal enclosures that house pcbs inside fast food restaurants and remote antennas outdoors. We are looking into changing the screw to one that can be magnetic to ease installation of these devices and be resistant to corrosion (indoor/outdoor).

Two options we are looking into is Carbon Steel 1018-1022 zinc plated (Fe/Zn 5, SC 1, Type III Cr3) and 410 stainless steel (passivated) screws.

Which one has better corrosion resistance in an indoor and outdoor environment and will be magnetic? Other suggestions?

If the better option is Zinc plated steel, how long does the plating last?

Used in the US coast to coast.


Jesus Gutierrez
- Carlsbad, California, USA

A. Hi Jesus. The answer to your question, but an origin for more questions, lies in your description of the zinc plating as being "SC 1". Service Condition 1 is "Mild -- Exposure to indoor atmospheres with rare condensation and subject to minimum wear or abrasion". This is not satisfactory for "indoor/outdoor" use. It is possible to plate zinc for SC 2, SC 3, SC 4, or even SC 5. Even still, for general use indoors and out, on plastics and "metal" (which is presumably usually powder coated steel, but perhaps might occasionally be stainless, galvanized, or aluminum), I think stainless is a safer bet and less likely to stain walls and other objects the enclosures are mounted to.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
September 2019

Q. Hello Ted,
Thank you for your input! Since the 400 series stainless steels are magnetic, is there a specific grade that may be better than 410 for "indoor/outdoor" applications against corrosion?

Thank you again!

Jesus Gutierrez [returning]
- Carlsbad, California, USA
September 20, 2019

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