Corrosion prevention should be considered at the start of your metal fabrication project and not left as an afterthought. There are many ways of protecting against corrosion, but only some of them can be applied once the fabrication is finished and installed on site. This blog discusses the options available and gives you the information you need to select the right corrosion-prevention technology for your fabrications.
Know the Enemy
Corrosion is what happens when a metal oxidizes. This is a process where air or waterborne oxygen atoms bond with surface atoms of metal.
When this happens to ferrous metals, oxidation of the iron leads to a progressive breakdown – what we call rust. Rust is a huge problem because it flakes off the surface, can contaminate fluids or materials it’s in contact with, looks ugly, and eventually completely destroys the metal.
With many other metals oxidation just forms a protective surface film. Aluminum, copper, and stainless steel are all examples. If this film is scratched the newly exposed metal quickly oxidizes, effectively healing itself.
A particular form of corrosion occurs when dissimilar metals are held in contact in the presence of an electrolyte. Water is an electrolyte, with seawater being especially powerful, and it facilitates the flow of a small electrical current between the two metals. This is known as galvanic corrosion, and it can quickly wreak havoc.
Established Methods of Protecting Against Corrosion
People have battled corrosion for a long time, probably since the dawn of the Iron Age. Minimizing its progression and impact entails one or more of the following strategies:
- Design: Select corrosion-resistant materials, isolate dissimilar metals to slow galvanic corrosion, and add a sacrificial anode (an approach often used to protect pipelines).
- Surface modification: Add something to the surface that resists corrosion, or is sacrificially consumed in preference to the structural material.
- Barrier coatings: Encase the metal in a coating that prevents oxygen from reaching the surface.
Unfortunately, each of these has limitations.
Corrosion-resistant materials tend to be more expensive than ferrous alternatives. Isolating dissimilar metals adds cost and can reduce structural rigidity. (Plus, the isolating material may break down over time, as happened in the case of the Statue of Liberty.) Sacrificial anodes are gradually consumed and must be replaced.
The most well-known form of surface modification is hot-dip galvanization. Used for over 200 years, this entails immersing ferrous metal in a bath of molten zinc. The zinc layer, which has a distinctive spangled appearance, becomes a sacrificial anode, oxidizing in preference to the underlying steel.
Another approach is to apply a thin layer of corrosion-resistant metal over the surface by electroplating. Chrome has been applied this way for a long time, for decoration as well as protection. Nickel is another corrosion-resistant metal suitable for plating.
Immersion processes like plating and galvanizing are limited in the size of parts they can handle. While small fabrications might fit in the tank, corrosion protection must be applied to larger structures while they’re in component form. As the coatings increase overall dimensions slightly, unless allowed for during design, this can create assembly problems. There’s also a risk of damaging the coating while inserting fasteners. Welding is generally not practical either.
These are divided into temporary and permanent. Temporary coatings are rust-preventative fluids that are sprayed brushed on, or applied by dipping. There are many options; most apply a wax or oil coating that provides short-term protection.
For permanent barrier coatings, consider paint or epoxy powder. These are usually applied after welding and assembly (and need a clean surface to bond to). The main application method is electrostatic spraying, where the paint or powder particles are given one charge and the fabrication the opposite charge, to ensure complete coverage. Electrostatic spraying reduces waste but can suffer from increased coating thickness on sharp projections.
One limitation of epoxy powder is the need to bake the particles into a solid coating. This requires an oven, which limits the maximum size of fabrication that can be handled. Another problem is that moisture will eventually penetrate to the underlying steel and cause corrosion. Consequently, paint and powder coatings must usually be removed and reapplied periodically.
Newer Corrosion Prevention Technologies
Corrosion is expensive and potentially dangerous, so despite the many existing ways of preventing it, work has continued to develop new and improved methods. The main ones to be aware of are:
- Electrical methods
- Laser treatment
E-coat is a combination of paint dipping with an electrostatic charge to draw the paint particles onto the surface. E-coat is very good at coating both internal and external surfaces, which is why it’s widely used in the automotive industry. However, as an immersion process, maximum fabrication size is limited by the tank dimensions.
Various electrical corrosion prevention methods have been developed over the years. In each case, the goal is to counter the oxidation process where the metal gives up electrons to oxygen. These all entail applying a small electric current to the metal structure in an effort to substitute for electron depletion.
Lasers are being used to prevent corrosion in two ways. One is to apply a cladding of a corrosion-resistant material, usually a metal, although ceramics are under investigation too. The alternative, and still very experimental, method is nanosecond laser carbonization. This seeks to form a thin carbonized layer on the surface that prevents oxidation.
Environmental Drivers for Corrosion Prevention Technologies
Many of the methods once widely used pose health and safety risks and environmental problems. As well as producing hazardous waste, chrome plating is now known to be potentially harmful. Cadmium plating, once widely used, is now banned in Europe and permitted in only a few special cases in the US.
In recent years paints have gone solvent-free, which creates challenges in achieving good adhesion. (Silicone contamination, which can come in on hand cream, is a particular problem.) And powders, while more benign than other processes, still require careful control.
For these reasons, much of the research into corrosion prevention focuses on environmentally friendly coatings. Various organic and inorganic materials are being trialed and work is underway on “nano” additives that improve the protection provided by paint.
The bottom line is, that when it comes to safety and sustainability, corrosion prevention is still a difficult area. The best approach is to consult with a fabricator or corrosion specialist.
Ask Your Metal Fabricator About Corrosion Prevention
Corrosion has been a problem for as long as man has been fabricating tools and structures from metal. That’s why corrosion prevention has almost as long of a history. Today there are many methods to choose from, although none are perfect. That’s why you should consider corrosion at the design stage, evaluate the pros and cons, and consider the tradeoffs, before pushing ahead with design and fabrication.
At Wiley, we’ve been in the metal fabrication business a long time and we understand what happens to structures that don’t receive appropriate protection. If you’re unsure of the best way of preventing corrosion in your application, ask us. We can talk about material choices, coating options, and more. Contact us today.