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Hot Dip Galvanizing

Galvanizing is a process where there is a metallurgical reaction to bind zinc to steel.  All galvanizing is NOT the same however.  There are two types of galvanizing that protect steel from damaging corrosion at highly different rates.

The two types of galvanizing are batch hot dip galvanizing (HDG) and in-line galvanizing, or pre-galvanizing.  Some companies have branded a generic process of pre-galvanizing steel, they refer to it as “shielded”.  Do not be fooled by thinking the processes are the same.  

Hot Dip Galvanizing is the only process that meets ASTM 123 standards for corrosion protection of steel.  The process includes taking a finished fabricated steel member and sending it through a series of chemical baths.  The final process is dipping it in an 840° zinc bath.  In the bath there is a chemical bond of the zinc to the steel.  Once dipped there is a minimum of 3.9 mils (1 mil= 1/1000th of an inch).  The bonded material is now galvanized.  The final step in the process is a natural cycle of temperature and atmospheric change which causes a patina on the zinc providing the maximum protection.

The in-line (or pre-galvanized/ sold under the brand name Gatorshield) process is similar in that there is zinc fixed to steel.  The process is done before fabrication.  There is a continual coating of zinc on the steel before it is shipped to a fabricator.  The in-line process is similar, but there are only 0.9 mils (less than 1/1000th of an inch) of zinc on the steel.  The fabricator then cooks off this zinc as they weld the steel.  Once it is welded they take a zinc laden spray paint and cover the outside only of the welds.  This process is often inadequate and leaves pinholes in the area and begins to rust rapidly.   The inside joints are completely unprotected, leaving water vapor and dust to settle on raw steel.

For a more detailed and scientific explanation of galvanizing, please click here. 

There is a wealth of information available at the American Galvanizers Association, an independent, non-profit trade association.