THE PHENOMENA OF RUST
Most people don’t think about rust or steel corrosion on a daily basis. Their only contact might be some old steel plumbing in their house that has rusted through that needs replacement or steel panels on their car rusting through, especially if they live in a location that experiences icy winters where salting the roads is common. What most people don’t think about on a regular basis are the effects of corrosion on the steel rebar that holds up our highways, the steel trusses that keep our bridges safe, and the steel tanks and pipes that help maintain a sanitary supply of water. Association for Materials Protection and Performance (AMPP) estimates that corrosion costs represent $2.5 trillion.
Understanding the nature of corrosion is straightforward as it only requires four simple elements to make it work. Those four are: anode, cathode, metallic pathway and an electrolyte. Steel is manufactured with three of those four components, the only thing missing is the electrolyte, most commonly water. If you could encapsulate a piece of perfectly dry steel in a container that was full of a dry, inert gas, like nitrogen, it would not rust. It would be missing the key component of the electrolyte. Of course, this is not a practical solution for structural elements. Because corrosion is an electrochemical process, the electrolyte, combined with the metallic pathway acts as the conductor between the anode and cathode. This electrical connection allows the anode to donate electrons to the cathode, thereby creating corrosion. Movement of electrons is an electrical phenomenon so rust development can be affected by changes in electrical potential as well. This is why salting wintertime roads can increase rusting and why exposure to seawater also increases the rate of rusting. In this case salt (also known as sodium chloride) happens to dissolve in water, making the solution far more conductive. Higher conductivity in the electrolyte transfers electrons far more efficiently, thereby accelerating rust.
JOB NUMBER ONE IS A CLEAN AND DRY BARRIER
The Tesla NanoCoatings products are designed to work in one of the most aggressive environments for corrosion; offshore production platforms. The basic ingredients in Tesla NanoCoatings primer formulation that work together to combat rust are an epoxy polymer, zinc powder and carbon nanotubes. Each of these contributes a set of properties that add up to the most highly effective barrier against rust for this harsh environment. The goal is to create a barrier where the steel and epoxy coating come together that protects the surface and prevents the ingress of moisture. As you could imagine, when warm dry days are available for maintenance on a platform, the crews want to put protective coatings on as much area as possible. Time is of the essence. Before we invented the NanoCoating formulations, it took up to three coats to produce an effective barrier. Those traditional three coat systems also have long intervals in between coats that slow productivity. By contrast the Tesla NanoCoating is a two-coat wet-on-wet process that only needs two coats with 30 minutes between coats. This can double or even triple the productivity compared to other solutions.
As you can imagine, making sure that the coating has good adhesion is critical to an effective barrier. This is where Carbon Nanotubes’ (CNT) strength comes into play. These nanometer-sized structures are one of the strongest materials known to man. In addition, they self-align into rebar-like structures, that gives them flexibility and adds strength to the coating. In pull tests, the coating can withstand pull forces of 5,500 pounds per square inch. The barrier is so strong it effectively acts more like a plating than a coating.
MAKING IT LAST
Having engineered a coating that is an effective barrier against moisture and that has a high adhesion to steel is critical to longevity. It also exhibits a character known as edge retention. This property means that the epoxy/nanotube coating doesn’t retract from sharp edges like most protective coatings for offshore service. Traditional 3-coat offshore paint systems often fail at edges and corners because of the low edge retention resulting in thin coating. Tesla’s excellent edge retention is a function of the right epoxy mix with the CNTs. The CNTs also have a tensile strength up to 50 times greater than steel making them much more highly resistant to impact and abrasion. All of this translates to great adhesion and excellent resistance to mechanical damage.
ZINC AND CARBON NANOTUBES
Zinc and Caron nanotubes add tremendous rust resisting characteristics to the barrier, once it is in place. We’ve already mentioned how the carbon nanotubes add strength, flexibility, adhesion and edge retention properties. They have two more properties that help with corrosion resistance: high electrical conductivity, and a self-aligning property. To understand how these help we need to first talk about zinc. Not surprisingly, different metals have different levels of reactivity. We already know that rusting is both a chemical and an electrical process. As it turns out, when you have a good electrical connection between zinc and iron in the presence of seawater, the zinc is much more reactive and preferentially corrodes in place of the steel. This dramatically slows down the rate of rust production on the steel side while increasing the consumption of zinc at the other end. This method of protecting against rust is referred to as galvanic action or cathodic protection.
Putting this all together, the zinc powder is in a conductive nanotube matrix that ensures there will always be an electrical path between the zinc embedded in the epoxy/CNT matrix and the steel. Even if the barrier layer is damaged, the carbon nanotube network connects the steel surface to zinc in the remaining coating film, maintaining the integrity of the galvanic action.
When taken as a whole, this is one of the most effective, quickest to apply, rust and damage resistant coatings for steel available today.