The following sections will explore the various zinc coatings available. Many people use “galvanizing” as a generic term for all types of zinc coatings; however, all zinc coatings are NOT the same. In fact, their physical, chemical, and corrosion resistance characteristics can be extremely different. The zinc coatings we will be examining are: metallizing, zinc-rich painting, sheet galvanizing, electroplating, and zinc (mechanical) plating.
Above is a photomicrograph of various zinc coatings. It depicts the various coating thicknesses and densities of zinc coatings. Refer back to this photomicrograph as you examine the different zinc coatings below.
The first coating depicted on the photomicrograph is a metallized coating. Metallizing or zinc spraying is accomplished by feeding zinc in either wire or powder form into a heating gun, where it is melted and sprayed onto the steel using a combustion of gasses and/or auxiliary compressed air to provide the necessary velocity. Metallizing can be done in the shop or field, is suitable for pieces too large for galvanizing, and is also an acceptable method of touch-up or repair of galvanized parts.
Metallizing (or zinc spraying) coating consistency depends on operator skill, and coating variation is always a possibility. Coatings may be thinner at corners and edges. Metallizing does provide cathodic protection, although there are no zinc-iron alloy layers. Metallizing is only 85% as dense as hot-dip galvanizing, and has a lower bond strength as it is a mechanical bond. So although a thicker coating may be applied, there is actually less zinc present. The black areas on the photomicrograph are the voids in the coating.
The properties of hot-dip galvanizing were discussed in the previous section, but it is important to note the denseness of the coating in comparison to the other zinc coatings. Also, remember hot-dip galvanizing is metallurgically bonded to the steel on the order of about 3600 psi.
The third coating on the photomicrograph is zinc-rich paint. Zinc-rich paints consist of zinc dust and either organic or inorganic binding material. Zinc-rich paint is often erroneously referred to as “cold galvanizing,” but zinc-rich paint’s characteristics and corrosion performance are very different from hot-dip galvanizing. Do not be fooled by claims of similar performance, which are based on salt spray testing. Salt spray tests do not accurately portray real-world conditions, and thus cannot accurately predict the performance of hot-dip galvanizing, which requires the natural wet and dry cycles to form the zinc patina, the critical component of galvanizing’s longevity.
Unlike other paint coatings, zinc-rich paints may exhibit limited cathodic protection; however, cathodic protection is only afforded if the binding material is conductive, or the zinc particles are in contact with each other and the steel. In the photomicrograph, the gray represents the zinc particles, and the black area is the binding material. Zinc-rich paints are also an acceptable touch-up and repair method for damaged galvanized coatings.
Continuous sheet galvanizing is often confused with after-fabrication hot-dip galvanizing. Both processes involve dipping steel in a tank of molten zinc, but in the continuous sheet process, the sheet metal is fed rapidly through the zinc, and thus picks up a minimal coating compared to hot-dip galvanizing. The most commonly specified galvanized sheet steels are G60 and G90. The numbers represent the weight of zinc per total surface area for both sides of the steel sheet. So, a G60 coating has about 0.30 oz/ft2 of zinc or approximately 0.54 mils per side. Continuous galvanizing, like hot-dip galvanizing, does provide cathodic and barrier corrosion protection, and has minimal zinc-iron alloy layers.
Electroplating (or Electrogalvanizing) generally refers to zinc coatings applied to steel sheet and strip by electrodeposition in a steel mill facility. There are no zinc-iron alloy layers, however, barrier and cathodic protection is provided. These coatings are very thin and mainly used for interior applications.
Zinc (mechanical) Plating
One final zinc coating, not depicted on the photomicrograph, is zinc plating. Zinc plating is similar to electroplating, but is used on small parts and fasteners. The parts are tumbled in a drum with zinc powder, proprietary chemicals, and glass beads. The glass beads peen the zinc onto the parts. It creates a mechanically bonded coating, and the thickness is regulated by the amount of zinc in the drum and the tumbling time. However, coating thickness may not be even, and difficult to reach corners and threads may not be coated completely.