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Zinc-Plating vs Mechanical or Hot Dip Galvanized Coatings for Small Parts

2001

Q. We manufacture a small carbon steel fastener that is used in industrial plants (onshore) and also in many offshore (salt) environments, but not immersed in sea water at any time. Corrosion protection is wanted simply to reduce corrosive effects from outside / temperature extremes / rain / but other materials of manufacture, such as 316 stainless steel, or aluminum, or even copper-nickel, are stocked offered to suit other corrosive elements in the local environments.

Annual production of these parts is more than several million parts per year.

Assembly consists of three parts; top part is a simple stamped piece, approx. 1-1/2" x 2" square using 16 ga. p&o cold rolled steel; second part is a stamped/shaped piece piece, approx. 1" wide and 1-1/2" high having a u-shape, also from 16 ga. p&o steel; third part is a 1/4-20 hex head cap screw usually less than 2" long, which passes thru the top part and screws into a female thread cut into the bottom of the u-shape.

For corrosion protection, we have always had the parts galvanized before assembly into finished goods inventory.

We have experience with two methods of coating.

1. Hot-dipped galvanized coating does not seem to present a very suitable part, as there is uneven coating, usually lots of flash at the edges. For such a small part, the coating is not very attractive or appealing. Fit in final assembly, using a standard bolt and female threads in the u-shaped part, require re-tapping, which removes the coating on inside of the female threads, which is not repairable..

We understand centrifuge action after hot dip galvanize coating may reduce this problem, but we have not experienced acceptable quality from past (several) hot dipped experiences.

2. Mechanical galvanizing is acceptable to the trade, and provides a good looking assembly of all three parts. But residue from the process seems to build up in the threads, and requires re-tapping before assembly.

We are considering to begin using a zinc-plated cap screw, and continue with mechanical galvanizing of the two other parts. This seems to present a reasonable solution to our assembly problem, as re-tapping is not needed.

Does zinc-plating provide reasonable protection to corrosive elements as compared to mechanical or to hot dip galvanizing.

Do any other problems arise, as from the use of dissimilar coatings methods?

How do the two methods compare in salt spray tests?

Robert Howell
- New Orleans, LA, U.S.A.

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As a maintainer of a major subway system we are constantly struggling with suppliers of fasteners to provide us with fixings within our tunnels that can stand up to time. We are now trying some sherardized fixings. The reports I have read seems to all say that sherardizing is better than hot dipped.

Ray Chong
- Hong Kong
2001

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Ed. note: Hello Ray, here are some additional threads about Sherardizing on this web site:

Also, the March 2004 edition of Metal Finishing Magazine contains an article by Dr. Benu Chatterjee on sherardizing.

There are a number of used books about Sherardizing avail. at this link at Amazon.

! For high tensile fasteners, beware of hydrogen embrittlement, they can turn into sticks of chalk!

Interesting subject. I have seen floating dock gangways plated with something called ALU, it resembles hot dip galvanise but is much smoother. I am trying to find out more about it for our wind turbine masts but not having much luck. I think it is a Spanish or Belgium company that came up with this finish. SS would be great but it is so expensive. joe

Joseph Goodfellow
- Galway, Ireland
August 20, 2010