We’ve all seen those unsightly brown streaks down the sides of painted metal structures. They look ugly but are of greater importance, they signify corrosion is taking place somewhere. That could mean leaks, structural failure, or contamination of what’s inside. In every case there are clean-up and possibly replacement costs involved, all of which a business could do without. These costs are avoided, or at least deferred, through a combination of metal selection, the metal finishing process or treatment selected, and corrosion-resistant alloys like stainless steel and aluminum. In this blog, we will concentrate on corrosion-resisting metal finishing processes.
If we’re going to fight corrosion through metal finishing, we need to understand what we’re dealing with. Corrosion is what happens when oxidation is able to advance. Oxidation is something all metals do: it’s when surface atoms form attachments to oxygen in the atmosphere.
With some metals, once the surface layer has oxidized the process stops. Aluminum is a prime example.
In many ferrous metals, iron oxide creates flakes that lift off the surface – what we call rust. This lifting exposes fresh metal to the air, letting corrosion continue until the metal has been completely transformed into iron oxide. (Stainless and high-carbon steels are less prone to this, thanks to the alloying metals like chromium and nickel that they contain.)
A form of corrosion particularly prevalent in wet environments is galvanic corrosion. This occurs when two different metals are electrically connected. That connection is often produced by seawater which forms a circuit with a cathode and an anode. In this situation, the anode corrodes quickly while the cathode is untouched.
The main ways to prevent corrosion are:
- Keep oxygen from reaching the surface of the metal
- Modify the surface chemistry to limit oxidation
- Isolate, or insulate, connections between different alloys (to prevent galvanic corrosion)
Another option is to harness the galvanic effect. This entails adding some sacrificial metal whose purpose is to corrode preferentially.
Preventing galvanic corrosion is largely a design issue. Here we’re going to focus on coatings and surface modifications to prevent unchecked oxidation.
Coatings for Metal Finishing
While paint is usually the first coating that comes to mind, powder coating, galvanizing, and plating are other options.
In metal fabrication paint is usually applied by spraying or through a process called electro-deposition or e-coat. Spraying is done through a nozzle and needs a lot of solvents. E-coat is an immersion process. Both use electrostatics to attract charged particles onto the metal. Both also require thorough cleaning of the metal before coating.
Powder coating is similar to paint spraying but uses epoxy particles. The coated structure goes into an oven after coating to melt the powder just enough to create a smooth layer.
With both paint and powder coating it’s possible to have almost any color you might want, (although unusual hues will likely incur additional costs.)
Both paint and powder coating provide years of protection against corrosion. Eventually, though, they will fail. What happens is, moisture in the atmosphere gradually finds a path through the coating. Over time, and especially with repeated expansion and contraction, that pathway grows and the metal beneath the coating starts to oxidize and rust.
Galvanizing is a steel treatment that has been around a long time and persists because it works. The process involves dipping steel into liquid zinc, which forms a bright, spangled layer over the surface. In service this acts like a cathode, corroding quickly and so protecting the steel.
Plating is another electrochemical process where a thin layer of a corrosion-resistant metal is applied to the surface of the structure. Chrome and copper are two common forms of plating, but have fallen out of favor due to adverse health and environment impacts and stringent regulation.
Surface Chemistry Modifications
The main metal finishing processes falling under this heading are anodizing and passivation. Anodizing is an electrochemical process that effectively supercharges natural surface oxidation. It’s used for nonferrous alloys like aluminum, magnesium, and titanium.
Adding dyes to the anodizing bath creates attractive colors on the surface of the metal. Unlike paint or powder coat, these are part of the surface and cannot be scraped off.
Passivation is a finishing treatment applied to stainless steel. Its purpose is to remove any iron from the surface that isn’t bound to other corrosion-resisting metals like chromium. (Fabrication processes often leave trace quantities of so-called “free” iron on the surface of stainless steel which can initiate corrosion.)
Passivation involves immersion in a mild acid, which mops up this free iron. Unlike the other metal finishing treatments covered here, it doesn’t change the surface but instead cleans it to prevent corrosion.
Why Protect Against Corrosion?
The two factors to consider here are cost and appearance.
Metal finishing adds cost so it’s reasonable to ask whether it’s worth doing. Let’s answer that by looking at the costs of corrosion and the benefits of treatment processes.
Ultimately, corrosion shortens the life of metal structures. As a result, they need replacing sooner than would be the case if they were better protected. If the required life is relatively short it might not be worth investing in an expensive metal treatment process. However, as corrosion progresses rapidly, especially in damp and humid atmospheres, it’s almost always worth providing some kind of coating.
A second consideration is appearance. Corrosion looks ugly and sends a negative message to customers and anyone else who sees your equipment. Conversely, spending a little to avoid those brown streaks and stains creates a positive impression.
A bonus is that finishing is a branding or image-building opportunity. Finish your structure in red, white, and blue stripes if you like, or make it purple or bright orange.
Sustainable Metal Finishing Practices
As mentioned previously, some processes and substances used pose environmental concerns. Paint spraying involves VOCs for example, and chrome plating is notorious for health and safety issues.
Cleaning, required to achieve a good bond between coating and substrate, has long been a difficult area. Trichloroethylene (TCE) in particular is now linked with a host of problems. However, there are solutions.
Powder coating in particular avoids the use of VOCs. Anodizing is a process with low levels of greenhouse gas emissions. What’s more, preventing corrosion supports moves towards a circular economy: metal can’t be recycled when it’s rusted but when it’s pristine it’s readily remelted and reused.
A famous example of galvanic corrosion is the Statue of Liberty. Yes, the copper skin developed a green patina, but rust in the frame was a bigger issue. This was found early in the 1980s at locations where the iron frame came into contact with the copper skin.
A particularly tragic case is that of the Silver Bridge in West Virginia. In 1967 this collapsed without warning, killing 46 people. The cause was later traced to a corroded eyebar in the suspension system.
For another example, consider major oil leaks that occurred in pipelines in Alaska. These were found to result from corrosion of the pipe itself.
More generally, boats are extremely susceptible to corrosion. For example, it’s believed that the 2002 sinking of the tanker Prestige, which led to the spill of 20 million gallons of crude oil, was caused by corrosion. It should also be noted that many grades of stainless steel will corrode in coastal atmospheres.
Help With Selecting the Right Metal Finishing Technique
The questions to ask are:
- How long do you need protection to last?
- What appearance do you need?
- What is the potential cost of corrosion?
Your answers to these will guide you toward the most appropriate metal finishing process. For more information, contact us for a discussion.