Perhaps the first thing to ask oneself when specifying or designing a sheet-metal item is whether a finish is actually needed. If the item is made of stainless steel, it probably will not need a coating to protect it from corrosion, and a brushed, bright, or even mirror finish may be acceptable appearance-wise. Aluminum, brass, or copper, depending on where they will be used, may or may not require coating for protection. The question does not apply for steel, of course, which will rust without protection.
If a coating will be needed, the choice comes down to organic coatings, conversion coatings, and plating.
Organic coatings can be paint or powder. Paint has relatively low cost, it’s simple to apply (spray — even with spray cans; in small quantities, brush or dip) and it’s available in a wide variety of materials and colors. On the other hand, paint generally requires priming, if solvent-based it can emit volatile organic compounds (VOCs), it usually requires a dedicated spray booth with air filtration, and it can take time to dry. On top of that there can be sagging and non-uniformity between vertical and horizontal surfaces, and it may be difficult for spray to reach into recesses and pockets.
Powder coating can be more durable than paint, it releases no VOCs, it covers in one coat, it cures more quickly than paint, and any overspray can be gathered up and recycled. On top of that, it provides a uniform appearance between horizontal and vertical surfaces.
“We powder-coat in house,” says James Hayes, Applications Engineer at Protolabs. “So it’s a lot cheaper from our perspective.” And, he adds, “It’s more durable.”
On the other hand, it requires specialized equipment to apply, and it can develop an orange-peel effect when applied in thin coatings. Sometimes this effect is desirable, as the resulting surface texture can help hide fingerprints and blemishes.
Conversion coatings involve changes to the chemistry of the metal itself. They can be used to provide corrosion resistance, enhance appearance, or act as a primer coat for paint.
Anodizing, used mostly on aluminum and zinc, is an electrochemical process that deposits an oxide coating on the surface of the metal, which provides a certain amount of corrosion resistance and can be dyed in brilliant colors—frequently used in consumer items. Anodized coating can be brittle, however, and exhibit poorer wear resistance than some alternatives.
Chromate conversion coating is used on steel, aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys. It creates an iridescent yellow-green film that resists corrosion, can be used as a primer for paint, and retains electrical conductivity. Over the past few decades, the use of some forms of chromate has been restricted by environmental regulations such as RoHS on Cr(VI) (hexavalent chromium), which is toxic.
Phosphate conversion coatings range from the simple phosphoric acid spray used to prevent rust on sheet steel before painting to zinc, and manganese phosphate coatings and even the Parkerizing applied on some automotive parts and on WWII firearms. Since phosphate coatings are porous, they must be further protected from corrosion, usually by paint.
Electroplating involves the electrolytic deposition of one metal onto another, to improve its appearance and resistance to wear or corrosion. Almost any substrate, ferrous or nonferrous, can be plated with a dizzying array of metals, including zinc, copper, silver, gold, cadmium, chromium, nickel, and tin. Plated surfaces may be used as-is or finished with paint or powder coating.
Zinc protects the underlying steel because it acts as a sacrificial anode; in a wet environment the zinc and steel form an electrolytic cell — a one-cell battery, if you will—creating a current that eats away at the zinc and protects the steel. This is the reason that replaceable zinc sacrificial anodes are attached to the underwater parts of boats, to protect propellers, rudders, and other parts from corrosion.
Zinc can be applied by dipping sheet steel in molten zinc (hot-dip galvanizing). The resultant coating is thick and protective, but rough-looking. Galvanealing, a variation of this process that involves further heat treating, produces a smoother and more paintable finish. Both are used for outdoor applications where the greatest corrosion resistance is required.
Electroplating the zinc gives a thinner but more attractive surface. Zinc electroplating can be done with an acid (chloride) or basic (alkaline) process. The acid process is quicker to do, while the alkaline process results in a smoother finish. Both are readily paintable.
Factors in the choice
The choice of finishes starts with the reason for finishing in the first place: aesthetics, environmental resistance, or other reasons. Then the choice becomes what’s available for the type of substrate used, cost for setup, materials and labor, turnaround time, and regulatory issues. We strongly recommend consulting with applications engineers to help make the best choice for your situation.
It’s also important to see what your manufacturing contractor does in-house and what must be outsourced. Depending on the vendor and desired process, this may or may not add to lead time: “We do have reliable vendors,” says Hayes. “That doesn’t necessarily increase our lead time, whereas other finishes, could take two weeks to get delivered to the shop.”
So when talking to a vendor, Protolabs strongly recommends consulting with applications engineers to help make the best choice for your situation.