Why Some HDG Materials are more Shining than Others?
There are many Factors in how the finishes of Hot Dip Galvanized can be formed of which:
| 1. | The Chemical Composition of the Steel being Galvanized. |
| 2. | The Zinc Bath Chemistry and Process. |
| 3. | Exposure Conditions. |
Before we look into the factor effecting the finishes of the HDG Material, it is important to clear a very common 2 misconception in regards to surface finishes.
| 1. | Shining is better finishes than the dull gray finishes. |
| 2. | Dull Gray is better finishes than the shining finishes. |
The amount of Resistance the Coating can give, is not affected by how it looks but by its thickness. So Shining or Dull Gray, both can gives the same Corrosion protection if they provide the thickness and bonding required.
The Chemical Composition of Steel.
Reference from: http://www.galvanizeit.org/hot-dip-galvanizing/what-is-hot-dip-galvanizing-hdg/the-hdg-coating
The chemical composition of the steel being galvanized is very important. The amount of silicon and phosphorus in the steel strongly influences the thickness and appearance of the galvanized coating. Silicon, phosphorous or combinations of the two elements can cause thick, brittle galvanized coatings. The coating thickness curve shown in the figure below relates the effect of silicon in the base steel to the thickness of the zinc coating. The carbon, sulfur and manganese content of the steel also may have a minor effect on the galvanized coating thickness.
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The combination of elements mentioned above, known as “reactive steel” in the galvanizing industry, tends to accelerate the growth of zinc-iron alloy layers. This may result in a finished galvanized coating consisting entirely of zinc-iron alloy. Instead of a shiny appearance, the galvanized coating will have a dark gray, matte finish.
This dark gray, matte coating will provide as much corrosion protection as a galvanized coating having a bright appearance.
It is difficult to provide precise guidance in the area of steel selection without qualifying all of the grades of steel commercially available.
Zinc Bath Chemistry and Process
| 1. | Nickel and Aluminium are sometime added to improve the brightness of the Finishes. |
| 2. | Use of Quenching to Decreasing the Cooling time also affect the Looks of the Finishes. |
| 3. | Use of Passivation Agent like Sodium Dichromate or Phosphate |
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| The difference between the shiny luster and the dull luster. The appearance of the galvanized part does not make a difference in corrosion protection. The steel will be protrected for the same amount of time. |
Differences in the luster and color of galvanized coatings do not significantly affect corrosion resistance. The presence or absence of spangle has no effect on coating performance. The well-known spangled appearance found on galvanized products is a crystallization process dependent upon the zinc bath chemistry, the rate of cooling, the method of pickling, the steel chemistry, and the thickness of the coating. Dull gray or patchy matte gray galvanized coatings give a service life equal to bright or spangled coatings since the service life depends on the zinc coating thickness
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| The top rail (top) has a winding pattern of dull gray areas corresponding to the process used during the making of the tube. The stresses in the steel from processing affect the intermetallic formation and can result in this striped look. The corrosion protection is not affected thus these parts are acceptable according to the specification. |
Natural Exposure conditions.
Reference: http://www.galvanizeit.org/specification-and-inspection/inspection-of-hdg/types-of-inspection/appearance
It is natural that when the Zinc Coating after expose to weather will react and oxidized and a lot of time resulted in the White Stain.
Over time all Hot Dip Galvanized Steel will change to Matte Dull Gray Finishes. The Rate at which it changes depending on:
| 1. |
The Method of Storage after the Hot Dip Galvanizing.
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| 2. |
The Exposure of the Materials, Indoor or Outdoor?
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Although hot-dip galvanized (HDG) steel is specified for corrosion protection, differences in appearances can be frustrating. It is important to remember the causes of these differences are outside the galvanizers control, and do not affect the corrosion resistance of the coating.
Regardless of the initial coating appearance, over time as the galvanized steel weathers, variances in appearance will fade and the coating will become a uniform matte gray. When exposed to the atmosphere, galvanized steel naturally develops a protective zinc patina on the surface which provides the uniform soft gray appearance.
The time required for the change in appearance to occur is dependent on the exposure conditions. A great example of the transformation is the canopied walkway at Mark Twain Elementary in Riverside, CA. The original canopies installed had coating variances – bright and shiny on the same beam. After a few years, the structure was revisited to check the performance and the difference in appearance had evened out to a uniform matte gray.
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White Stain Does Not Affect on Quality.
Reference: http://www.hdgasa.org.za
White Storage Stain is not a reason for complaint or for rejection in terms of SANS 121. The Picture Below the Coating Thickness is not affected after White Stain is removed.
