Benefits of Zinc Plating on Perforated Tubes
Just as a comedian is often funnier with a sidekick, steel performs better with a layer of zinc. Steel, other than the stainless kind, needs some kind of protective coating if it’s not to corrode. Painting and powder coating have their place, but in many applications zinc plating is the way to go. By preventing corrosion it allows the use of less expensive materials, but that’s not the only benefit. Here’s a closer look.
Meet Zinc: The Corrosion Resistor
On its own zinc is a silvery white metal with few uses. It’s neither as strong as steel or as light as aluminum and melts at 787°F (419°C,) which is about the same as lead. Zinc does have some strengths though. It’s ductile, it adheres well to other metals – like steel – and it oxidizes.
In iron and steel oxidation is a problem. As oxygen bonds to iron the resulting ferric oxide, (rust, to most of us,) lifts off the surface in flakes, revealing fresh metal beneath. In a moisture-rich environment, which accelerates the process, a piece of mild steel can corrode away completely in months or even less. Zinc however oxidizes more like stainless steel and aluminum.
Stainless and aluminum both form an oxide layer on the surface, but here there’s no flaking to expose fresh metal. The oxide layer is actually self-healing because if scratched the newly exposed surface forms another protective oxide barrier. (In the case of stainless it’s the chromium that creates the surface layer of oxide.)
Zinc corrodes in a similar way. In the presence of water zinc forms zinc hydroxide. This acts as a skin that prevents further oxidation. If left undisturbed the zinc hydroxide eventually bonds with carbon dioxide to create relatively hard zinc carbonate. The result is a surface that strongly resists corrosion.
A second way in which zinc provides corrosion resistance is as a sacrificial metal. When dissimilar metals are in contact the presence of water creates a voltage differential that greatly accelerates corrosion. (Maritime applications are especially susceptible.) In these circumstances zinc corrodes faster than any almost other metal, in effect sacrificing itself so the other metal is preserved.
Zinc Plating for Corrosion Resistance
A coating of zinc is often referred to as galvanization, after the galvanizing process. Steel can be galvanized in two ways: by hot-dipping and through electroplating. Dipping produces a thicker and more corrosion-resistant coating but tends to obscure fine surface features. There’s also a risk of hydrogen embrittlement on high-strength steels.
Electroplating with zinc creates a relatively thin, though still corrosion-resistant, surface layer that’s well-bonded to the substrate. As zinc is ductile, the underlying metal can be bent after plating without cracking the coating.
The electroplating process has two other attractive characteristics. First, it offers the ability to manage surface appearance. By controlling the plating variables it’s possible to produce anything from a bright, reflective finish to one that’s quite dull. And second, it’s less hazardous and more environmentally friendly that many other coating processes.
Zinc Plating and Perforated Tubes
Filtration is done many ways, but perforated tubes often play an important role. Fluid passes through the holes in the wall while solids are excluded. Those fluids could be oils, acids, steam or even just water. Water though is exactly what corrosion needs to get started.
When using perforated tube in watery applications – agriculture is one example – corrosion is a potential risk. Switching from mild to stainless steel prevents the problem but carries a cost penalty: not only is stainless a more expensive material but it’s harder, and thus more costly, to work. Thus zinc-plated mild steel tubing can be a more economical solution.
In some applications the appearance of the perforated tube is important. One example is when it’s used as a guard or shield around a hot pipe. Again, stainless is an option, albeit a pricey one. Zinc plated tube is often a good alternative as the finish is easily tailored to the application. Zinc has temperature limitations but holds up much better than paint or plastic (powder) coating.
Protection and Decoration
Mild steel is ductile, inexpensive and readily formed into complex shapes. However, it does love to rust, which limits its use for perforated tube in wet applications. Stainless steel is always an option, although it’s not so easily worked and costs more. (Other corrosion-resistant materials are even more costly.)
An alternative that’s sometimes overlooked is zinc plated mild steel perforated tube. The zinc provides long-lasting corrosion resistance without taking away from mild steel ductility and low-cost advantages. A further benefit is in how careful plating process management can create surface finishes ranging from bright to dull.
As sidekicks go, zinc isn’t the funniest or most interesting, but it’s hard to deny it does a great job on perforated tubes!
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