Electrolytic & Hot Dipped
Hot Dipped Galvanized is Electrochemical protection for Steel, however the derivation of the term ‘galvanizing’ has absolutely nothing to do with protecting steel from corrosion. The name comes from the Italian physiologist, Luigi Galvani, who identified the effects of electric current on the nervous system of dead frogs.
In the formative years of electrical science, zinc was the most widely used metal for producing galvanic electricity. When metals are in electrical contact, one metal (zinc) will give up electrons and oxidize while the current generated will prevent oxidation of the other metal (steel). This mechanism extends the structural life of products made from galvanized steel.
In 1837, a French scientist took out a patent in France for the process of dipping steel in molten zinc and provided the process with the name ‘galvanizing’ in honor of Galvani who died in 1798.
Hot Dipped Galvanized
Cold reduced sheet steel material in coil form (hot rolled pickled is sometime used) is uncoiled and passed continuously through the galvanizing line equipment. Several designs of continuous lines have been used. One of the types most frequently used today has a continuous anneal section. A second type has no anneal section in line, but uses preannealed substrate. In both types described, the strip passes through a pot of molten zinc.
Hot-Dipped Galvanized is produced to established minimum zinc coating weights in ounces per square foot. These coating weight will have a prefix of either “A” or “G” Generally “G” denote a free zinc coating whereas “A” denotes a zinc/iron alloy coating. (See table for ranges)
The free zinc type offers better corrosion resistance, but the alloy type is more easily spot welded. The corrosion resistance of the “G” type is directly related to the amount of zinc coating, i.e., G90 will resist corrosion to the base metal longer than G30.
The “A” coatings have a dull gray color with no spangle while the “G” coatings are bright/shiny and can have various sized spangle visible. This is the traditional galvanized appearance.
Hot-Dipped Galvanized can be chemically treated to inhibit the formation of zinc oxide or have a surface oil applied to retard storage stains. The “G” types are often coil coating with paint and are usually post formed by roll forming or embossing of the strip. Garage and entrance doors are examples of this practice. The “A” type is typically painted after all forming or bending is completed.
Both types, “A” and “G” are available on a full range of steel grades from CS (Commercial Steel) through EDDS (Extra Deep Drawing Steel). These grades have varying ability to be formed. Hot-Dipped Galvanized is produced to meet ASTM A 653 Specifications, “General Requirements for Steel Sheet Zinc Coated by the Hot-Dip Process”.
Additional information on Hot-Dipped Galvanize can be found at www.galvinfo.com
Electrolytic zinc coated steel (Electrogalvanized) is a cold reduced steel product which has been given a thin coating of zinc by electrolytic deposition. The coating weight refers to the total amount of zinc on both surfaces. Electrolytic zinc coating weights are generally lower than those for Hot-Dipped zinc coatings. Three coating classes are available for Electrolytic Zinc, “A”; “B”; “C”. (See table)
Electrogalvanized is not recommended for outdoor exposure unless it has been painted prior to exposure. This is done regularly for various types of doors and small buildings.
Electrolytic Zinc has a smooth, dull gray surface appearance. It is available on the full range of steel grades, Commercial Steel (CQ) through Extra Deep Drawing Steel (AKDQ) and is produced to ASTM specification A591 “Electrolytic Zinc Steel Sheets”. Chemical treatment (passivation) is available to retard zinc oxide or the surface can be phosphate coated (bonderized) to improve post painting adhesion.
Test Limits for Electrolytic Zinc Coated Sheet Steel (ASTM A591)
|Coating Class||Minimum Check Limit Triple Spot||Minimum Check Limit Single Spot|
oz. per sq. ft.
(Total Both Surfaces)
oz. per sq. ft.
(Total Both Surfaces)
Test Limits for Hot Dipped Galvanized (ASTM A525)
|Type||Coating Designation||Minimum Check
oz. Per sq. ft
oz. Per sq. ft
|“||G01||NO MIN||NO MIN|
|“||A01||NO MIN||NO MIN|
Note 1: The weight of coating in oz. per sq. ft. refers to the total coating on both surfaces. The coating designation number is the term by which this product is specified.
Note 2: As it is an established fact that the atmospheric corrosion resistance of mill galvanized sheet products is a direct function of coating weight (thickness), the selection of lighter coating designations will result in almost linearly reduced corrosion performance of the zinc coating. For example, the heavier galvanized coatings perform adequately in bold atmospheric exposure whereas the lighter weight coatings are often further coated with paint or a similar barrier coating for increased corrosion resistance. Because of this relationship products carrying the statement “meets ASTM A523 requirements” should also specify coating weight designation.