What is Weathering Steel?
If you are searching for shipping containers for sale, you have or you will have heard the term “weathering steel” or “Cor-Ten Steel.” This is the type of steel used in the manufacturing of shipping containers, but why the two names you ask? In this instance it is like the difference between Kleenex and tissue. Cor-Ten steel is the trademark name United States Steel used in the 1930’s when weathering steel was developed for use on the railroads. Today, as other steel makers produce a similar product they refer to it a weathering steel.
Weathering steel is a very tough and dependable product that was developed to eliminate the need for painting. Weathering steel controls the rate at which oxygen in the atmosphere can react with the surface of the metal. Iron and steel both rust in the presence of air and water, non-weather-resisting steels have a relatively porous layer, which tends to hold moisture and promote corrosion. In time, this rust layer will delaminate the metal, exposing the surface and causing a continuous cycle of corrosion damage to the carbon steel or rust as we know it.
Weathering steel exhibits superior corrosion resistance over regular carbon steel as a result of the formation of a stable rust like appearance if exposed to the weather for several years. Once the rust layer forms the protection begins! Weathering steels are now being used in a variety of applications, in addition to shipping containers, weathering steel can be found in bridges, rail cars, transmission towers, and chimneys. It is also very popular with sculptors and as an architectural feature in some buildings.
Using weathering steel does present some challenges. Ensuring that welds weather at the same rate as the other materials require special low alloy welding rods and welding techniques; fasteners must also be fabricated from weathering steel. Weathering steel is not rustproof in itself, if water is allowed to accumulate in pockets or dents, those areas will experience higher corrosion rates, so provision for drainage must be made or dents on horizontal surfaces must be repaired. Weathering steel is sensitive to salt-laden air environments. In such environments, it is possible that the protective patina may not stabilize but instead continue to corrode.
Obviously all shipping containers are painted, so why use a product that specifically does not need to be painted? As we all know, containers live a tough life, any painted finish will inevitably be damaged due to loading and offloading and the containers rubbing against each other as they are stacked eight units high on the moving vessel bringing them across the ocean. Any damage to the paint finish in the harsh conditions will eventually show a little rust (as designed) but the corrosion will end there and the container will live out a long serviceable life.