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Recently I saw some bolts described as "galvanized bolts." Are these bolts all hot-dip galvanized for corrosion protection?

Often when purchasing zinc-coated bolts, there is a misunderstanding about the difference between the different methods that can produce a zinc coating on steel. Many times a mechanically galvanized bolt will be described as a "galvanized" bolt, leading to the assumption that the bolt is hot-dip galvanized. There is a difference between the two methods; knowing the difference can be important in evaluating corrosion protection for a project.

Galv Bolts

Hot-dip galvanizing, by the nature of the process, guarantees a coating that is a mixture of iron and zinc. The coating is generally uniform on all surfaces, providing excellent protection at the corners and edges. The hot-dip galvanizing process creates a metallurgical reaction between the zinc and the steel resulting in a series of zinc-iron alloy layers. The pure zinc layer and the zinc-iron alloy layers are anodic to steel, providing sacrificial protection in the event the coating is scratched or damaged. This cathodic protection plus the barrier of the zinc coating give a double protection system for corrosion prevention.

The mechanically plated coating consists of a flash coating of copper, followed by tumbling the part in a barrel with glass beads and zinc powder. Coating thickness requirements contained in ASTM Specification B695 range from 0.2 to 4.3 mils. The coating has a density of about 0.45 oz/ft2/mil compared to the hot-dip galvanized coating density of about 0.6 oz/ft2/mil. The hot-dip coating has over 30% more zinc per unit volume than the mechanical coating. The corrosion protection is directly related to the amount of zinc in the coating.

The mechanically plated coating consists of rounded particles of zinc loosely bonded together. The bond between zinc and steel and zinc to zinc is considerably weaker than the metallurgical bond found in hot-dip galvanized coatings by several orders of magnitude. Edge, corner, and thread coating thicknesses are lower because of the deposition method of the mechanical process.

Corner

Although the mechanically plated fasteners appear to be reasonable products, there must be doubts regarding adhesion and the thinness of the coating at the sharp edges of nuts and bolts and thus the corrosion protection provided by this process. These faulty characteristics greatly affect the long-term corrosion protection of the fastener. Insist on hot-dip galvanized fasteners for reliable long-term corrosion protection.


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Comments

Madhavan

Thanks for the information

PS

This article is written with bias and does not give the complete picture. issues with Hot dip include hydrogen embitterment plating of higher grade bolts etc which is not possible with hot dipped.

Alana Fossa (AGA)

Hello PS. Please note we address the topic hydrogen embrittlement and high-strength bolts in multiple areas of our website to keep specifiers informed, such as our primary pages on steel selection for galvanizing. For more information: https://galvanizeit.org/design-and-fabrication/design-considerations/steel-selection We also have dedicated articles to this topic, such as: https://galvanizeit.org/knowledgebase/article/embrittlement-of-hot-dip-galvanized-steel-and-how-to-avoid-it

Frank Jiang

Dear Author, Thanks for the explicit comparison on HDG vs Mechanical Galv. Since you mentioned bolts and nuts will have tendency to rust on sharp edges with Mechanical galv. what will be the advice for washers? I'm asking because there are few authentic HDG washers on market, especially for smaller sizes (less than 1/2" etc), thanks!

Alana Fossa (AGA)

Hi Fank, Yes, any material with sharp edges will have the same concern.

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