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“Zinc Plating – The Corrosion-Prevention Workhorse”
- Sep 15, 2017 -

Zinc is a bluish-white metal, which, if mechanically polished, or electrodeposited with appropriate brighteners, somewhat resembles chromium in appearance.  However, the reflectivity of the polished surface is soon lost in most atmospheres.8   This quick tarnishing and corroding, is the property that makes zinc plating work so well in providing  “sacrificial” protection for steel.  To learn more about the sacrificial nature of zinc plating, please read the section on “Galvanic Series of Metals in Seawater” elsewhere in this report.

 

The relatively low cost, protective nature, and attractive appearance of zinc plating make it a popular coating for nuts, bolts, washers, metal stampings, and automotive components, fabricated parts for industrial applications, and also serves as an effective undercoat for paints. 

 

Electrolytic zinc coatings are used to protect and improve the appearance of ferrous metals, (i.e. iron & steel) as a corrosion barrier, and then as a sacrificial coating.  The application of chromate conversion coatings over zinc plating, and post-plate sealers, give additional protection against corrosion particularly under high humidity and moisture conditions.  For additional information on Chromate Coatings and Post-Plate Sealers, please read, “Heal Thyself! – How Chromates on Zinc Plating work”, elsewhere in this report.

 

In dry air, a protective layer of oxide soon forms on an untreated zinc surface, and subsequent attack is slow.  In moist air, zinc hydroxide forms first on the surface, and is then converted to zinc carbonate.  If the surface has not been chromated, the carbonate takes the form of a bulky, loose layer, often described as white rust, or wet storage stain.8  In confined spaces, zinc is attacked by organic acid vapors emitted by woods, plastics, and various insulating materials 8

 

Commercially, zinc is deposited in thick nesses ranging from 0.0001”- 0.0005”, depending upon the intended application and the corrosion protection required, the majority of which is 0.0001”-0.0003”, commonly known as “Commercial Zinc”.  Commercial Zinc has a high coefficient of friction, low strength, moderate abrasion resistance, poor impact resistance, brittle at room temperature, but malleable at 212-302°F. 

 

To relieve the potential for Hydrogen Embrittlement in hardened steels electro-plated with Zinc, a baking procedure after the plating is required to remove, or diffuse the hydrogen throughout the basis metal, reducing the risk of embrittlement.  For more information on Hydrogen Embrittlement, please read the section titled, “You Crack Me Up!  Hydrogen Embrittlement is No Laughing Matter”, located elsewhere in this report.


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