By Frank Goodwin, International Lead Zinc Research Organization Inc., Durham, North Carolina

Operating either as-cast or with an outer coating, zinc die castings can serve in both functional and decorative applications. A relatively dense metal, zinc has a substantial and durable feel. The metal’s hardness, self-lubricating properties, dimensional stability and high modulus make it suitable for working mechanical parts, such as gears and pinions. Zinc also accepts a broad assortment of finishes, from chemical conversion treatments to electroplating and sprayed and baked polymers. When a finish is properly selected and applied to diecast zinc, almost any desired aesthetic can be achieved. Zinc castings can be made to look like solid gold, weathered brass, stainless steel and even leather.

As shown on this kitchen mixer, zinc die castings can be made to look like stainless steel when the right surface finish is applied.

After a designer has determined whether a particular zinc die casting requires a finish, he or she must assess the component’s end-use requirements and choose the most effective among a variety of options. A recent test has ranked nine popular finishes from least to most effective.

This Is Not Only a Test

In an investigation sponsored by the International Lead Zinc Research Organization to evaluate the performance of nine modern protective and decorative finishes commonly used on zinc die castings, researchers looked at two performance criteria—the ability of a finish to protect the underlying zinc against corrosion, and the ability of the finish to maintain its initial aesthetic properties upon exposure to corrosive conditions.

The researchers used a bolt boss plate that incorporated flat and curved surfaces, rounded and sharp edges, through-holes of various diameter-to-depth ratios, and interior and exterior corners. These test panels were drilled and tapped, and stainless steel screws were added after finishing but before test exposure.

To determine the best testing method for the finishes, researchers conducted a preliminary comparison. Diecast zinc panels coated with a chromate conversion process, a powder epoxy and an automotive grade of copper-nickel-chrome electroplate each were subjected to three different accelerated corrosion tests—the standard ASTM B-117 salt spray test (500 hours), the newer ASTM G85 mixed salt cyclic fog test (500 hours), and the CAMRI test, which exposes panels to a constant 100% relative humidity at 122F (50C) with weekly misting of the panels with a solution of 1% sodium chloride and 1% sodium sulfate. Panels subjected to this test were exposed for up to six months, with interim evaluation at one and three months.

The preliminary phase found that while the two ASTM tests produced white rust on portions of some panels, the CAMRI test produced mixed corrosion products and physical failures not seen in either of the ASTM tests. More importantly, the appearance of the corrosion and the localized physical failures of the finishes appeared similar to what actually occurs on coated zinc panels after extended exposure to corrosive environments.

Based on these observations, it was decided to use the CAMRI test method for the overall study. While the six-month test clearly gave the most dramatic failures and corrosion, all of the six-month failures also were observed in the three-month inspection. It was decided, therefore, to run the final tests using the CAMRI protocol for three months. All test sample pictures shown in this report are from the final phase using the CAMRI protocol.

To put the CAMRI test in perspective relative to actual field exposure, studies found that a three-month test in the laboratory was equal in nature and severity to roughly one year of actual exposure at a severely corrosive chloroform plant site on the U.S. Gulf Coast or three to five years at inland chemical plant sites.

Two zinc alloys, No. 3 and No. 5, were tested with each of the nine types of zinc finishes. There was no significant difference between these two alloys as far as resistance to corrosive atmospheres. All panels were tested “as finished,” with no heat treatment.

Scoring of the finishes (Table 1) was done by two CAMRI corrosion technologists who inspected the panels together and agreed upon scores for individual panels. The scoring was based on a maximum value of 10, with points deducted for observed imperfections in the finish. Each panel was judged against an unexposed panel with the same surface finish, along with unfinished control panels that had gone through the same test exposure. While scoring the panels, it became apparent that some finishes allowed localized breakdown but still maintained the aesthetic properties of the coating, while other finishes protected the entire surface of the sample but became quite unsightly and would be judged for many applications as having failed. Each of the scores was on a scale of 0-10, ranging from no better than the unfinished control to visually perfect.

Table 1. Performance Ranking of Finishes 

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