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What is the production process for hot-dip galvanizing?

Posted by Popi Papadonta on Wed, Dec 21, 2011 @ 12:15 PM
 

Step 1: Surface preparation

Thorough cleaning is necessary as zinc will only adhere to clean steel. This gets accomplished by:

  • Degreasing: a hot alkaline cleaner is used to remove oil, grease, dirt, loose particles, and any other contaminates that may exist on the surface of the material

  • Acid pickling: an acid bath is used to remove mill scale and oxides

  • Fluxing (dry kettle): pre-fluxing in a zinc ammonium chloride solution to remove oxides and to prevent oxidation prior to dipping into molten zinc

Step 2: Immersion in bath of molten zinc

During this step, the steel is immersed in a bath of molten zinc at approximately 830˚F. The mix in the bath consists of ~98% pure zinc and 2% additives (Al, Bi, Ni). The zinc reacts with the iron in steel and it forms a coating which is metallurgically bonded to the steel. Also, the zinc flows into recesses and other areas difficult to access, coating all areas of complex shapes thoroughly for corrosion protection. Hot dip galvanizing is the only coating system that protects tubular steel on the inside. The zinc goes “in, around, over, under and through” the fabrication. 

hot dip galvanizingThe size of zinc baths varies from galvanizer to galvanizer and this restricts the size of steel that can be galvanized. When choosing a galvanizer, make sure that the size of the zinc bath is big enough to accommodate the size of your products such as large structural shapes. 

The final product is protected against corrosion as zinc creates a barrier between steel and the environment and also cathodically protects the base metal. 

Step 3: Finishing


After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, by vibrating or for small items, by centrifuging. The galvanized item is then cooled in air.



Step 4: Inspection

Steel is inspected after galvanizing to verify conformance with specs. 

 

Sources: Duncan Galvanizing Corp., American Galvanizers Association

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Comments

What is the difference between hot-dip galvanized steel fasteners and zinc-plated fasteners?
Posted @ Wednesday, January 04, 2012 2:22 AM by Carl Wang
Carl; 
 
 
Hot-dip fasteners generally have considerably more zinc on them than zinc plated fasteners and are designed for exterior as well as interior applications. Zinc-plated fasteners, on the other hand, have less zinc and are not particularly appropriate for exterior use.  
 
 
 
Howard A. Levine, FCSI, CDT 
 
Sr. Vice President 
 
Duncan Galvanizing Corp.
Posted @ Friday, January 06, 2012 8:55 AM by Popi Papadonta
We have designed a project with a large exposed steel trellis with a galvanized surface. Appearance is important. Members are round pipe and square tubes and I beams. The pipe columns all look uniform, slightly shiny or in other words "galvanized" The majority of the heavy pieces are mostly very dark dull gray but have some streaks of shiny galvanized coating. (The appearance is as if the beams were painted a dull, almost black color and then lightly over sprayed with streaks of silver spray paint. This is not the appearance we had in mind. At 8 stories in the air it is noticeable but not critical, we will be installing trellises on the second floor roof terraces. Apearance is really important here. I have done some research on the AGA website (Dr Galv, in particular) and have come to the conclusion that we are probably dealing with reactive steel (I have posted my findings atwww.mognot.com Staff Blogs/Tech Talk. I am looking for a reality check - is my thinking about reactive steel correct and if so (or if not), how do I make sure I get a nice, semi-shiny galvanized surface for heavy steel in the future? I understand that most of the time the appearance is not the critical component and that a tight specification addressing the issue will add cost to the project.
Posted @ Friday, February 24, 2012 7:06 PM by Jay Hyde
Jay; 
Today's steel is comprised of various materials including magnesium, chromium, silicon, and other elements not previously found in "your grandfather's steel" that prevent a uniform, aesthetically pleasing finish. It is important to note that you are still receiving the long-term corrosion protection from hot dip galvanizing that you have come to expect, but the aesthetic appeal is not there. It is also important to point out that all galvanized steel will ultimately weather out to the familiar gray patina once the zinc reaches the zinc carbonate stage (usually in 12-18 months). 
Having said that, I also realize that many projects will not allow the waiting time until the zinc reaches that stage. 
Dr. Galv has given some excellent suggestions as to how to overcome the problem. One additional thought would be to coat the galvanized steel with a non-pigmented clearcoat. While this would ensure a shiny look, the galvanized steel would have to be blasted after galvanizing to create an anchor profile for the clearcoat. This would remove the spangle layer and show the grind or blast marks but would provide a uniform finish and might be preferable to the irregular finish you are now getting. 
Hope this helps and good luck. 
 
Howard A. Levine, FCSI, CDT 
Sr. Vice President 
The Duncan Group 
Posted @ Monday, February 27, 2012 8:11 AM by Popi Papadonta
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