Surface finishes produce a variety of dazzling aesthetic properties, but the corrosion-resistance of the finishes can be just as important in certain applications. Before and after photos are shown of each of the nine finishes tested.

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