Editorial Feature

Galvanized Coatings - Zinc Galvanizing vs. Zinc Plating

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Consumers at both industrial and domestic levels usually have no choice but to buy already galvanized steel products. The coating standard of such steels may not be represented explicitly, and even misrepresented in some cases.

Claims made by manufacturers may not be proven in the field. In the case of other products, especially those that are zinc-plated, the packaging comes with descriptions (such as “galvanized”) that deliberately mislead consumers into believing that the product will be sturdy even if it is not.

More and more products are being galvanized using high-speed, in-line galvanizing technology. This allows a thin zinc coating to be applied to the steel at a lower cost. The thin zinc coatings are usually coated using clear polymer topcoats to increase their storage life.

In some cases, the application of these polymer topcoats has been claimed to significantly improve the durability of the coating compared to conventional galvanized coatings. Moreover, manufacturers claim that organic coatings are commonly added to zinc-plated parts to improve the corrosion resistance of their products.

This article investigates the facts associated with this claim.

Coating Characteristics

Zinc Plating

Zinc plating is applied electrolytically by immersing clean steel parts in a zinc salt solution and applying an electric current. A layer of pure zinc is applied in the process, where the thickness of the layer ranges from a few microns for low-cost hardware components to ≥15 μm for fasteners of superior quality. Technical and cost-related problems hamper the cost-effective plating of components with heavier coatings.

In-Line Galvanized Coatings

While manufacturing hollow or open sections, in-line galvanized coatings are applied by passing the cleaned steel section that exits the mill into the galvanizing bath. During this process, the thickness of the zinc coating applied to the surface can be regulated. The coating mass of this coating is typically 100 g/m2 or more, with an average of approximately 175 g/m2.

Accelerated Weathering Testing

Traditionally, salt spray cabinets are used to perform accelerated weathering testing of coatings. This testing technique has been reproached to a considerable degree when it comes to metallic coatings. This is because it does not reflect like metallic coatings under atmospheric conditions where the formation of stable oxide films gives the coatings their exceptional anti-corrosion performance.

Although the noticeable performance of metallic coatings in salt spray tests will be significantly enhanced by the incorporation of polymer topcoats, it is not necessary that this will be reflected in field performance.

Finding the Facts

The South African Bureau of Standards (SABS) recently performed accelerated weathering trials of polymer-coated, in-line galvanized coatings, and compared them with conventional in-line galvanized and hot-dip galvanized coatings, to evaluate the impact of the addition of polymer topcoats on sturdiness. A summary of this report is given below.

SABS Report

The samples went through Damp SO2 Atmosphere, Salt Fog, QUV Weatherometer, and Hardness testing. The SABS report provided 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 suitable for applications exposed to outdoor atmosphere. Zinc-plated bolts and hardware fittings, such as gate hinges, will not provide adequate protection from corrosion, and usually do not last beyond 12 months in outdoor settings such as urban coastal environments.

Zinc plating has been used quite often in industrial coating applications, but with poor results. In March 1998, Industrial Galvanizers launched its joint-venture galvanizing operation in Bekasi, Indonesia—PT Bukit Terang Paksi Galvanizing (BTG)—to reprocess a large tonnage (approximately 400 tons) of cable trays that had been electroplated.

The zinc-electroplated coating had failed before the project could be delivered, resulting in the rejection of the entire consignment.

The client made a request for the zinc plating to be substituted with a heavier, hot-dip galvanized coating. BTG could apply a 100-μm coating to the 3-mm thick cable tray sections, which is approximately 50% greater compared to the mandatory minimum standard for hot-dip galvanized coatings applied to steel of such thickness, and over 10 times the thickness of the zinc plating.

The appearance of zinc-plated products is appealing when they are new, since the zinc coating is bright and smooth. In contrast, a hot-dip galvanized coating has a duller and less smooth surface. Characteristically, when compared to zinc plating, approximately 10 times the zinc is applied to small components during the hot-dip galvanizing process.

A bright, shiny, smooth zinc finish on builder hardware (hinges, nuts, gate latches, bolts, post shoes) indicates a plated coating that will not offer adequate corrosion resistance and will barely provide more than a year’s protection in most of the coastal population areas.

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