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Selecting a Surface Finish for Zinc

The majority of zinc diecast applications are not exposed to corrosive environments, so the aesthetic requirements of the part define which finish should be used, which in some cases means no finish at all.

But for applications where the service environment is aggressive, such as marine hardware, external automotive parts and items for use outdoors at industrial sites, corrosive attack can result in white rust, black staining or flaking and peeling of a finish due to corrosion of the underlying zinc. For such severe environments, the manufacturer and designer must select corrosion-resistant finishes.

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 

 Finish category and description  Zamak 3  Zamak 5  Average   Preserving Aesthetics 
 E-chromate conversion, no sealer  5   6   5.5  6
 E-chromate conversion, no sealer  6   4  5  4
 E-chromate conversion with sealer  7  5  6  6
 C-clear chromate (no Zn) with sealer  2  1  1.5  4
 C-clear chromate with sealer  3  2  2.5  2
 C-Zn plate, trivalent chrome, JS 500 sealer  4  4  4  3
 G-"dual nickel" Cu/Ni/Cr plate  5  6  5.5  7
 G-"marine" Cu/Ni/Cr plate  8  8  8  9
 F-Zinc-tin mechanical plating  9  9  9  6
 H-powder epoxy 10-7011 on non-blasted panel  8  9  8.5  9
 H- powder epoxy 10-7011 on grit-blasted panel  9  8  8.5  9
 H-polyester powder coating  9  9  9  8
 D-4510 liquid polyester coating  5  4  4.5  5
 I-urethane resin e-coat with nanoparticles  9  10  9.5  10
 I-urethane resin e-coat without nanoparticles  9  9  9  9
 I-urethane resin e-coat without nanoparticles  10   9  9.5  10
 D-low friction coating, phenolic resin  4  5  4.5  5
 D-low friction coating, polyamide-imide resin  3  3  3  4
 B-copper-Cu/Sn/Zn electroplate  6  6  6  1
 A-zinc black coating  1  1  1  2
  How They Fared

Nine types of coatings are described below, along with the results of the CAMRI test. They are listed from least to most effective at resisting corrosion and maintaining its initial aesthetic properties.

Zinc Black. A relatively thick black phosphate film is imparted to the casting to protect against humidity and moderately corrosive atmospheres. The finish is not usually proposed as a stand-alone corrosion barrier, but rather as a paint pre-treatment. Unlike the smooth, dense blacking that is used widely on steel guns and tools, the blacking on zinc is dull and somewhat powdery in consistency. Zinc blacking by itself did not offer significant protection in this test and was largely dissolved or washed off by the periodic wetting of the panels with mixed salt solution.

Chromate Conversion Coatings. These chemical immersion treatments (trivalent chromium and hexavalent chromate conversion) produce a thin protective film on the zinc surface. They are intended primarily to protect parts during storage, in mild (e.g. indoor) environments or, like zinc black, to provide an optimum surface for adhesion of subsequent paint or other organic finishes. Conversion coatings are sometimes followed by a sealer or lacquer to enhance their performance and extend the range of their applicability. The hexavalent chromate conversion coatings, with or without sealer, performed much better than did trivalent chrome or “clear” chromate finishes.

Copper-Tin-Zinc Electroplate. This proprietary process forms a dull, silvery finish on the zinc. It offered fair protection to the zinc, but the finish itself developed an unsightly, sometimes black, splotchy appearance. The overall thickness of the finish in this case was about 0.04 in. (1 mm).

Sprayed and Baked Liquid Coatings. This includes a broad spectrum of different chemistries, including epoxies, polyesters, phenolics and urethanes. The test included low friction fluoropolymer coatings not primarily intended for protecting against corrosion. The coatings were applied at thicknesses of approximately 0.04-0.08 in. (1-2 mm) and provided only moderate protection. Some also discolored or became generally unsightly. Many thicker industrial sprayed and baked organic coatings are on the market that would have performed better in this test.

Copper-Nickel-Chrome Electroplate. One of the workhorse finishes for outdoor corrosive applications for many years, it begins with a thin layer of cyanide (non-acid) copper flash to protect the zinc against the acidity of subsequent baths. Next is a thicker layer of acid copper plate, which serves to make the surface more uniform and assures good electrical conductivity. This is followed with one or more layers of nickel, which provides a continuous corrosion resistant barrier. Finally, one or more layers of chromium are applied to give the desired shiny, silvery appearance and to protect the nickel against mechanical forces such as wear and erosion. Electroplating has one disadvantage vs. non-electrical processes in that it is difficult to put plated metal into interior corners and holes. This can be largely overcome by using conforming anodes, but these make the process more expensive. A two- and three-nickel-layer system both was tested. The first is commonly referred to as automotive grade, and the second is sometimes called marine grade, as it is used for more stringent applications. A noticeable improvement emerged with the three-nickel system. Both systems showed an incidence of local failures at inside corners, presumably indicative of thinner plating applications at those locations.

Mechanical Plating. This general category of finishes involves placing parts in a drum with the desired mixtures or metal powders and a chemical activator and tumbling the parts until the desired thickness of coating builds on the part. It is possible in this way to coat with an alloy of almost any desired metal, though generally some combination of zinc and another metal is used. This process has a distinct advantage over electroplating in that materials can be applied very uniformly on all surfaces, including on interior corners. The use of different metal combinations also offers different aesthetics. In this test, a zinc and tin alloy was employed with a coating thickness of 0.08 in. (2 mm), including a topcoat of trivalent chrome and clear sealer. The finish yielded to some slight discoloration and whitish stains, but otherwise the sample was unchanged.

Epoxy and Polyester Powder Coatings. These polymeric coatings are applied as powders in a dry electrostatic process and subsequently fused in an oven. This process offers environmental and personal hygiene advantages over wet sprayed and baked coatings because there are no solvents to drive off. Because the powder application is usually an electrostatic process, sprayed powder coatings also provide better buildup at the edges than do wet-sprayed polymers. On the down side, for this same reason, it is difficult to get coating materials into deep recesses and interior corners, although this problem was not observed with the sample geometry used for these tests. In fact, no local failures of these coatings were observed at interior corners. In these tests, both the epoxy and the polyester powder did much better than had the sprayed liquid coatings. While powder coatings are excellent corrosion protecting barriers, these powder coatings, at 0.12-0.16 in. (3-4 mm), were much thicker than were the liquid coatings evaluated in the test.

Electrophoretic Urethane Coatings. Also known as “e-coats,” the three electrophoretic coatings evaluated here all did exceptionally well, despite measuring only about 0.03-0.04 in. (0.8-1 mm) in thickness. One of the finishes tested contained ceramic nanoparticles to give added resistance to abrasion and mechanical wear, as well as a black color. The nanoparticles did not show any measurable effect on corrosion resistance compared to the regular urethane resin e-coats.  MetalcastingDesign.com