Zinc plating is considered to be the go-to metal plating process throughout the world. Industry professionals know that’s because zinc coatings offer superior corrosion inhibiting benefits and it provides aesthetically pleasing finishes at a reasonable price. In fact, zinc has been electroplated onto parts for over 200 years thanks in part to Alessandro Volta’s invention of the electrochemical battery back in 1800.
This finish has lasted as the standard for so long mainly because of its reliability in the plating process, its availability, and most importantly, its cost-effectiveness. This process endures in spite of regulatory pressures like RoHS, REACH, ELV, and other international laws and directives.
The final product is aesthetically pleasing because it’s finish is typically very bright and quite uniform in thickness. Zinc is normally provided in a supplementary chromated condition, ranging in color from blue bright, clear to yellow iridescent, olive drab and black. The corrosion resistant plating process is excellent on yellow chromates reaching an impressive 96 hours of neutral salt spray to white corrosion.
Older formulations of clear chromates normally reach 12 hours of salt spray resistance before white corrosion. However, the newest generation of clear, trivalent chromates perform extremely well too. And red rust remains a function of the coating thickness.
Zinc is used in a variety of industrial applications—but especially in the manufacture of automotive parts, stampings, and fasteners. Additionally, it provides an excellent base for painting and other coatings.
In the 1980s, the plating industry responded to tightening regulations along with the need for better corrosion protection to replace more toxic processes like cadmium and hexchrome chromates by initiating a new wave of research. This led to the development of hexavalent chrome-free passivates and zinc alloy coatings that substantially improved on zinc electroplating while providing environmentally friendly alternatives over previous processes.
Chromate is a conversion coating used on top of the zinc, which is sacrificial to zinc plating. Resistance to corrosion is superior with the new generations of chromates, typically reaching 48 to 120 hours of neutral salt spray to white corrosion. Yellow trivalent chromates perform equally well, however, unlike hexavalents used in the past, trivalent yellows are colored yellow using dyes.
Dyed systems normally result in poor adhesion during subsequent processes like paints, powder coats, adhesives, and other coatings. Thicker film or high-performance chromates have been tested to exceed 240 hours to white corrosion. Hexavalent chromates are still out there, however availability is limited because most platers don’t offer hexavalent chromate options for zinc mainly due to the strict environmental and health regulations. Also, requirements like RoHS and ELV directives make the use of hexavalents less popular today.
Many chromated and zinc-plated parts have topcoat seals applied. These silicate-based organic topcoats or lacquers have two key functions:
- They provide additional corrosion resistance to the chromate and zinc system by adding a barrier to the chromate; and
- Depending on the formulation, it offers some additional protection to bare surfaces like deep holes and tube IDs.
Another benefit these seals provide involves the modification of torque tension properties–giving finished parts a consistent coefficient of friction. This is especially important in robotic/automated assembly environments where torque tension and torque retention are critical requirements for safety.
Zinc plating is a complex process requiring a high level of expertise that only metal finishing companies like Aerospace Metals LLC can deliver consistently. Specialized equipment and machinery is required like rectifiers, plating stations, ancillary tanks to properly dissolve the zinc anode, and reservoirs. The most critical factor in achieving a top-notch finish and maximum corrosion resistance is substrate preparation.
Steel parts normally get a cathodic, or negative charge applied to them to attract zinc metal ions from the plating bath to the part surface. When these ions make contact with steel surfaces with the negative charge, the ions are transformed into metallic zinc and they become electrochemically bonded to the steel surface, creating a zinc deposit. Chemicals are then added to the plating bath to make the finish bright, uniform, and smooth.
Parts with scale may require descaling first before the plating process in order to promote ideal plating adhesion and to produce an aesthetically pleasing finished part. Heat-treated parts with heavy scale, welded fabrications, hot rolled steel, and thermal deburred parts are examples of parts processed in this manner. Customers have several choices for descaling, including pickles, alkaline or acid descalers, and media blasting or tumbling. Media blasting is the preferred and it’s usually the most costly method for scale removal on hardened steels.
It should be noted that most parts don’t require pretreatments. Instead, they are typically processed through the plating line directly from the containers they were shipped in. Alloys like tin zinc offer better corrosion protection and lubricity, zinc nickel plated parts provide up to 10 times the galvanic protection on steel compared to zinc, while offering superior resistance to other chemicals like chlorides for saltwater exposure or fertilizers for agricultural applications.
As you can see, although there are many benefits to the zinc plating process, it is not ideal 100% of the time. For example, zinc plating should not be used on critical steel parts exposed to temperatures in excess of 500° F. Zinc plating is also not recommended in marine or seawater applications, or in tropical environments because it can aid in the formation of bulky corrosion. Finally, I’ve learned in my 30+ years of metal finishing experience, you should never apply a zinc coating on equipment with moving parts that make contact with one another. Zinc plating is also not good on products or equipment stored in confined environments with a lot of moisture or condensation.
When it’s done properly as we do at Aerospace Metals, zinc electroplating is affordable and the optimal choice for applications in almost every industry. The key as a finisher is to be sure to get all of the specs needed to properly pretreat, plate, and post-treat parts. For manufacturers seeking finishers, be sure that you provide the finisher with everything needed to utilize best practices that yields your desired level of quality in zinc plating finish for your parts.
Here’s a quick review of zinc plating benefits:
- Superior protection against corrosion.
- Low cost. Zinc can be found in abundance so it’s more cost-effective than when using precious metals like gold or palladium.
- Strength. Even though zinc is relatively lightweight, zinc coatings increase the strength of the substrate.
- Applying a coating or zinc or zinc-nickel will not place too much stress on a metal part or component—good for longer wear.
- Flexibility. Zinc or zinc-nickel works in a wide range of bath chemistries, providing greater flexibility for customizing the plating outcome. Zinc also works for both rack and barrel plating processes.
- Aesthetically appealing. Zinc plating improves the appearance of an iron or steel part or component. Colors can also be used during post-treatment to help you customize the final appearance of the finished product.
- Ductility. Zinc is easy to contour to the shape of the underlying substrate. That’s because zinc can be stretched into long, thin strands without breaking.
- Tolerant to high temperatures. Zinc can handle temperatures of up to 120° F, which helps to lower cooling costs.
- Environmentally friendly process. Zinc plating is considered in the industry to be relatively eco-friendly, especially when compared to its cadmium counterpart. Zinc is also recyclable.
Zinc Nickel Plating
I’d like to close this month’s blog with a quick review of zinc nickel–a popular finish for many industries with high-corrosion and high-performance needs. Zinc nickel was first developed as a replacement for cadmium plating.
Zinc nickel is an alloy plate made up of electro-deposited zinc and nickel. The finish is functional, but when it comes to aesthetics, this alloy can be treated with a vast array of colors—but only after chromating the alloy from its natural light pale yellow to dark purple hues. This finish makes for a wonderful base, ready for painting and/or other coatings.
When a chromate conversion coating is applied, this finish offers some of the best corrosion resistance available, in excess of 1,000 hours salt spray when tested per ASTM B117.
The zinc nickel process is growing rapidly in the electroplating industry. That’s because it offers a lot of great benefits that appeal to manufacturers in the automotive, fastener, casting, and heavy equipment industries. Zinc nickel plating is also becoming a popular choice in the defense industry as a substitute for the more environmentally unfriendly cadmium plating process.
In brief, the zinc nickel alloy offers you:
- Superior corrosion resistance with most deposits at a 12-to-15% alloy range producing a coating that can achieve 250+ hours to first white corrosion and 1,000+ hours to first red corrosion; and
- Hardness, heat resistance and high-alloy benefits as well.
The Vickers hardness measure is 400 to 500 HV and zinc-only deposits have a hardness of roughly 100 HV. Handling and assembly can often result in the destruction of softer coatings like zinc and zinc flake. So zinc-nickel’s combination of hardness and overall corrosion protection makes this alloy ideal for fasteners or components that require handling or assembly. Zinc-nickel is also frequently used for stampings and fasteners for barrel applications. It’s used a lot on brake component castings in rack applications.
Another benefit of the zinc-nickel alloy deposit is its overall heat resistance. Zinc-nickel deposits can withstand baking temperatures upwards of 200°C for four hours while still achieving 800 hours of neutral salt spray protection to first red rust.
Aerospace Metals LLC specializes in both zinc and zinc nickel plating. With 30+ years of professional experience, Paul Fredericks and his team of professionals have helped clients with achieving their metal finishing application goals by using a customized approach. His network of metal finishing professionals offer cutting edge technology and equipment. They have what it takes to produce high-quality metal products to exact specifications.
Visit our website at High-Quality Metal Finishing Services – Aerospace Metals to get a free, no obligation quote today!