Thickness of Galvanized Coating

Different Standards determined minimum thickness of galvanized coatings on various items. By the nature of the galvanizing process, the minimum standards specified by JIS,ASTM,EN,or DIN are routinely exceeded by galvanizers. But, most of the factors that affecting the thickness and appearance of the galvanized coating are including the following items :  

• Chemical composition of the steel
• Steel surface condition
• Cold working of steel prior to galvanizing
• Bath immersion time
• Bath withdrawal rate
• The metallurgy of the galvanizing process.
• Steel cooling rate

    The chemical composition of the steel being galvanized is very important; for example, the amount of silicon and phosphorus present in the steel strongly influences the thickness and appearance of the galvanized coating
surface . A silicon level of 0.04 percent or greater or a phosphorous level of 0.05 percent or greater in the steel will generally result in thick coatings consisting primarily of zinc-iron alloys .The carbon, sulfur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness.

 

The effect of silicon on coating thickness

  Certain steel compositions tend to accelerate the growth of zinc-iron layers. This may result in a finished galvanized coating consisting entirely of zinc-iron alloy. Instead of the shiny spangled appearance, the galvanized coating will have a dark gray, matte finish. This dark gray, matte galvanized coating will provide
as much corrosion protection as a galvanized coating having the common spangled appearance.

As the galvanizing reaction is a diffusion process, higher zinc bath temperatures and longer immersion times will generally produce somewhat heavier alloy layers. Like all diffusion processes, however, the reaction proceeds rapidly at first and then slows as layers grow and become thicker. Continued immersion beyond a certain time will have little incremental effect . In galvanizing of silicon bearing steels containing more than 0.04 percent silicon, the diffusion process significantly changes.

The thickness of the outer pure zinc layer is largely depend on the rate of withdrawal from the zinc bath.
A rapid rate of withdrawal causes an article to carry out more zinc and generally results in a thicker coating.
Further, coating thickness measures must be taken a several points on each inspected article to comply with
the favorite standards such as ASTM or etc.