PROTECTING STEEL FROM RUST
Of all of the possible choices for structural metals, the most commonly used is carbon steel. It has a reasonably high strength-to-weight ratio, is easy to form into structural shapes, can easily be welded, and stands up well to stresses due to movement such as earthquakes and high wind and water loads. The one downside is that in the presence of moisture, the iron in the carbon steel eventually corrodes to iron oxides (commonly called rust) which are mostly crystalline and fall apart under stress.
Ever since the invention of the steel-making process, there have been numerous approaches to protecting the steel surface from moisture in order to preserve its structural integrity. One of the most difficult wet environments to protect against is seawater. The salts in seawater are chemically very active and so they accelerate the rusting process. This is the challenge that we chose for Tesla’s Nanocoating. For reference, it is useful to know about other protective methods and their positives and limitations.
One of the most common and easy-to-apply protectants is paint. It is best applied on a clean, dry surface which is not always practical in a marine environment. Due to its viscosity, paint tends to suffer from a tendency to pull away from edges and angles. This just means that in areas that have non-linearities, the paint naturally does not build up to a uniform thickness. The thinner areas are vulnerable to early breakdown. This same phenomenon can occur in places where there is weld splatter or at seams in the steel. Paint is what is known as a barrier coating. Other barrier coats include plastic coating and fused powder coating. These latter approaches are fine for treating railings and pipes before installation, but are virtually impossible to repair in place if they are damaged in service. Epoxies are one type of barrier coating that meets many of the characteristics that are desirable for use in marine environments. They set up relatively quickly and can be applied with common painting tools. Most formulations require three layers, and each must be fully set before the subsequent layer can be applied.
Most other techniques for protecting steel are intended to keep rust from developing, by creating a rust-resistant environment right at the surface of the steel. In addition to a barrier the next two most common methods are to use cathodic protection or a corrosion inhibitor. Galvanic protection is commonly used in conjunction with a barrier in the form of galvanizing, sacrificial zinc rich coatings or impressed current systems. To be effective an inhibitor must always be present on the surface of the steel. Unfortunately, this is not really practical for steel that needs to operate in marine environments, but for the sake of completeness, this technique does have its uses. Other methods can include applying an oil layer on the surface, keeping the steel in a clean, dry place or applying a dry coating layer.
A new entrant into rust prevention is a packaging pouch that encloses steel parts (gears for example) in a sealed rust-inhibiting vapor. During shipment and storage, the vapor causes a passive oxide layer to build up on the part so that by the time it is ready to use, it can be installed without any special preparation. Of course, this technique is unsuitable for a field repair and appears to be intended for relatively small parts. The limited information available does not address its suitability for steel in a marine environment.
The last set of techniques involves the actual metallurgy and electrochemical properties of steel. One of these techniques is to dip the steel in zinc, a process known as galvanizing. This works, in large part because the zinc has free electrons to donate to the oxidation process, so it is preferentially consumed instead of the steel. This is known as cathodic protection. It works well for freshwater environments and is often found in metal lampposts or traffic signs. Galvanizing is a thin layer on top of the steel and once the zinc is consumed, the steel will eventually start to corrode and there is no easy field repair – only replacement. The other metallurgical approach is adding chrome and nickel to the steel during manufacturing, which turns it into stainless steel. This is a fine solution and is widely used in marine environments. However this is a very expensive approach and is usually just reserved for specific fittings, railings, brackets and other small parts.
WHAT’S THE TARGET MARKET?
It was decided for a variety of reasons, that it would make sense to tackle one of the toughest environments for steel structures – offshore platforms.
Not only do they have lots of steel structure that needs protection, there are challenges due to saltwater exposure, as well as wind and wave environments that can damage surface coatings.
Given the varied success rates of steel protection techniques we also knew that it would take a new approach to show significant improvement over conventional techniques. But it was also an opportunity to make a big difference. There are already zinc-based primers available which do an adequate job of protection in typical inland exposure to weather and freshwater regimes. However the downside is that over time, due to galvanic action, the zinc gets consumed and there is no longer an electron path to protect the steel. In addition to the galvanic action causing zinc depletion, offshore platforms are often moving heavy equipment, such as pipe and supply containers which can cause physical damage to surfaces. What was needed is a tough coating that also provides electrical continuity to allow the galvanic action to continue even when there has been a surface disruption.
ENTER CARBON NANOTUBES
Carbon nanotubes have been produced in laboratory environments for over 30 years. During this period, their properties have been explored and several methods were developed to mass produce these materials. Physically, they are similar to graphene, which is a sheet of pure carbon, a single atom thick, with the carbon atoms arranged in a hexagonal structure. Nanotubes are graphene in the form of a tube approximately. They are now mass produced by a variety of companies and the economies of scale allow them to be sold by the ton. They have a wide variety of useful characteristics, but for corrosion protection, three in particular make it stand out. First, it is one of the strongest materials found in nature, secondly it is electrically conductive, and thirdly, it tends to self-align as it moves. These three properties are what make it superior to all other anti-corrosion approaches to protect steel.
HOW DO CARBON NANOTUBES WORK?
With its high strength characteristics, it is very resistant to deformation and abrasion which overcomes one of the core weaknesses of zinc primers. CNTs, with their high electrical conductivity and the small size of nanotubes, the coating is essentially zinc particles embedded in a conductive, reinforced epoxy matrix. This ensures that a conductive path is always available for galvanic action. The self-aligning property is a unique character for any material. What it means for corrosion protection is that the nanotubes will “seek out” other nearby nanotubes to connect to. The result is a “self-repairing” characteristic from the combination of zinc and the carbon nanotubes. Nanotubes will maintain alignment as the coated area flexes due to stress or other physical changes. It turns out that this miraculous material has just the right attributes that help solve the daunting problem of protecting steel in tough environments. Tesla Nanocoating has hit the mark with the combination of unique attributes of the available technologies to out-perform other coating solutions for steel in the offshore market. Based on our reception in the market, our customers seem to agree.
