Editorial Feature

Galvanized Coatings - Zinc Galvanizing vs. Zinc Plating

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Consumers at both domestic and industrial level are often left with no choice but to purchase steel products that are already galvanized. The standard of coating on such steels may not be clearly represented and even misrepresented in certain instances.

The manufacturer may make claims that cannot be proven in the field. In the case of other products, specifically those that are zinc-plated, the packaging contains descriptions such as galvanized that intentionally deceive buyers into believing that the product will be durable even if it is not.

An increasing number of products are being galvanized by high-speed, in-line galvanizing technology. This enables the application of a thin zinc coating to the steel at low cost. The thin zinc coatings are often coated using clear polymer topcoats to improve their storage life. In certain cases, it has been claimed that the application of these polymer topcoats considerably enhances the durability of the coating compared to traditional galvanized coatings. Manufacturers also claim that the addition of organic coatings to zinc-plated parts is also a common method that enhances the corrosion resistance of their products. This article analyzes the facts related to this claim.

Coating Characteristics

Zinc Plating

Zinc plating is formed by the electrolytic application of zinc by dipping clean steel parts in a zinc salt solution followed by the application of electric current. In this process a layer of pure zinc is applied, ranging from a few microns on inexpensive hardware components to ≥15 μm on good quality fasteners. The economical plating of components with heavier coatings is hindered by technical and cost issues.

In-Line Galvanized Coatings

Application of in-line galvanized coatings is performed during the manufacturing process of the open or hollow section where the cleaned steel section exits the mill and passes into the galvanizing bath. In this process, a coating of zinc is applied to the surface, the thickness of which can be controlled. In general, this coating, which is measured as coating mass in grams per square meter, is 100 g/m2 and above, with an average of about 175 g/m2.

Accelerated Weathering Testing

Conventionally, accelerated weathering testing of coatings has been performed in salt spray cabinets. This testing method has been largely reproached in relation to metallic coatings since it does not reflect like metallic coatings in atmospheric exposure conditions where the formation of stable oxide films gives the coatings their outstanding anti-corrosion performance. Although the evident performance of metallic coatings in salt spray tests will be considerably improved by the addition of polymer topcoats, this will not necessarily be reflected in field performance.

Finding the Facts

Recently, the South African Bureau of Standards carried out accelerated weathering trials of polymer-coated in-line galvanized coatings and compared them with traditional in-line galvanized and hot-dip galvanized coatings to assess the influence of the addition of polymer topcoats on durability. Given below is a summary of this report.

S.A.B.S. Report

The samples underwent Salt Fog, Damp SO2 Atmosphere, QUV Weatherometer and Hardness testing. The SABS report gave the following conclusion:

The results of the accelerated corrosion tests indicate that the expected life of the continuously galvanized and lacquer coated samples will not be essentially different from the commercially continuously galvanized sheet material. Test results demonstrate that the expected life exhibited by the standard hot-dip galvanized panels (zinc coating thickness approx. 100 microns) can be considered to be significantly superior to the continuous galvanized/lacquer samples. The lacquer coating appears not to be fully effective in inhibiting the onset of corrosion under damp conditions due to porosity.”

It is well known that the zinc/iron alloy layers of standard hot-dip galvanized coatings are hard in nature (in excess of 200HV—often harder than the base steel itself). Conventional hot-dip galvanized coatings, consisting of alloy layers with a soft zinc outer layer, therefore provide in essence a buffer stop coating which withstands knocks and abrasion. The soft nature of continuous galvanized lacquer coating (75 HV) coupled with the low coating thickness indicates that these coatings will not have the same ability to withstand rough handling compared to conventional hot-dip galvanized items.”

Poor Performance from Plated Coatings

Zinc-plated coatings are not appropriate for exterior exposure applications. Zinc-plated bolts and hardware fittings like gate hinges will not offer sufficient protection from corrosion and often do not last more than 12 months in exterior exposures in a majority of the urban coastal environments.

Very often, zinc plating has been employed in industrial coating applications, but with unsatisfactory results. In March 1998, Industrial Galvanizers commissioned its joint-venture galvanizing operation in Bekasi, Indonesia, PT Bukit Terang Paksi Galvanizing (BTG), to reprocess a large tonnage (nearly 400 tons) of cable trays that had been electroplated. The zinc-electroplated coating had failed before delivery to the project, leading to the rejection of the whole consignment.

The client placed a request for the zinc plating to be replaced with an extra-heavy, hot-dip galvanized coating. BTG could apply a 100-μm coating to the 3 mm thick cable tray sections, which is nearly 50% more than the required minimum standard for hot-dip galvanized coatings applied to steel of such thickness and more than 10 times the thickness of the zinc plating.

The appearance of zinc-plated products is attractive when new since the zinc coating is smooth and bright. By contrast, a hot-dip galvanized coating has a less smooth and duller surface. Typically, nearly 10 times the zinc is applied to small parts during the hot-dip galvanizing process compared to zinc plating. A shiny, bright, smooth zinc finish on builder hardware (nuts, bolts, gate latches, hinges, post shoes) suggests a plated coating that will not offer sufficient corrosion resistance and will hardly offer more than 12 months protection in a majority of the coastal population centers.

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