Many people are under the false belief that shipping containers are bulletproof and rustproof.
While many general purpose shipping containers are made from Cor-ten steel, this does not mean that they are rustproof.
Failing to protect your shipping containers from rust can spell big trouble when using them to construct a shipping container building.
In this article we’re going to look at what causes shipping containers to rust, how to prevent this rust and how to treat rust if it occurs.
What Materials Are Used in the Manufacturing of Shipping Containers?
To understand why shipping containers can rust, we first need to understand what materials are used in making shipping containers. Shipping containers can be manufactured from a variety of steels. General purpose shipping containers, such as the 20-foot and 40-foot, standard height and high cube containers, are made from Cor-ten steel (also known as Weathering steel).
Cor-ten steel was created using a group of steel alloys with the aim being to create corrosion resistant steel.
In the right environment, Cor-ten steel can display a high resistance to corrosion, however, it isn’t entirely corrosion proof. It can still rust and corrode.
What Causes Shipping Containers to Rust?
Cor-ten steel has a high level of corrosion resistance, but only slows corrosion. It doesn’t eliminate corrosion completely.
Cor-ten steel has an enhanced corrosion resistance due to the development of a protective oxide film on the metals surface which slows down further corrosion.
However, for this oxide film to develop, it requires interchanging wet and dry cycles. The wet cycle causes the corrosion process to begin. The dry cycle creates the oxide layer which gives the container its nonporous state.
The more wet/dry cycles the container is exposed to, the greater the oxide layer is.
As we know, for containers to rust, they need to come into contact with both water and oxygen. When the containers come into contact with both oxygen and water an oxidation reaction occurs.
This reaction between the steel, the water, and oxygen causes hydrated iron (III) oxide to form, which we can visibly see as rust.
A container which is constantly exposed to sea air that is full of salt will rust much faster than one in a climate with alternating wet and dry cycles. This is because it’s speeding up the oxidation reaction and also restricting the growth of the oxide film. Worst of all for rusting is when the containers have surface water lying on them.
An ideal climate for a shipping container is one which has alternating wet and dry cycles, as this promotes the growth of the oxide film which limits corrosion.
However, a dry climate is also suitable, but clearly isn’t ideal since it will lack the wet cycle.
Shipping containers can also rust as a result of being damaged during transit and movement. When a container is damaged, any protective coating on the container can also be damaged or removed which allows the oxidation reaction to occur.
When purchasing a used container you should check the container thoroughly for rust. Pay attention to any points of contact during transportation, including the sides, top and bottom when they are stacked together. Check carefully for any areas that could have been damaged. Also check areas that are heavily used, such as the doors. Pay special attention to the underneath of the doors and to corners. The underside of the doors can rust away because they sometimes allow water to collect beneath them for long periods of time.
The importance of inspecting the shipping container before you make your purchase cannot be overestimated. Following our checklist will be beneficial when you get ready to purchase your container.
Types of Shipping Container Rust
There are two types of rust on shipping containers which you need to be aware of: structural and non-structural.
Non-structural rust can be easily treated and while it is displeasing to look at, it isn’t too much of a concern.
The real concern for shipping container building owners is structural rust. This is rust which has gone beneath the surface level and can actually reduce the container’s structural integrity.
In some extreme cases, the rusting is so extensive that it’s best to cut your losses, discard the container and start again.
How to Prevent Shipping Container Rust
Since we can all agree that “prevention is better than cure”, the majority of your effort (and money) should be spent preventing rust from occurring on your shipping containers, rather than waiting for rust to occur before you do anything about it.
There are three key ways which you can prevent rust from building up on your shipping container: pre-construction, maintenance and location.
This means that you plan your build so as to ensure that your shipping containers aren’t exposed to severe weather any more than is necessary.
For instance, if you lived to the east of a mountain range and the weather blew in from over the mountains, you’d want to protect the side of your containers that will be hit the hardest.
You could plant trees or other large vegetation around your container home to block much of the rain and weather from hitting the containers.
Another option would be to clad the exposed sides of your shipping containers to prevent the Cor-ten steel from being exposed.
Maintenance Rust Prevention
We recommend that you notice problem areas early as a way to prevent extensive rust corrosion. Regularly walk around your container building to inspect it.
During this visual inspection, notice whether anything has fallen on your container and damaged the protective coating. Look for any standing water. The most common place for standing water will be on the roof. When inspecting the roof, pay special attention to any dents. Unless repaired, dents can collect rain water which will speed up the corrosion process.
Location Rust Prevention
As previously mentioned, areas that have salt laden air, high rainfall, humidity, or persistent fog don’t make ideal locations for shipping container buildings.
This is not to say that you can’t build in these areas using shipping containers. These types of locations require that you you shouldn’t build shipping container homes without applying additional protection to the containers. We’ll discuss this more later.
So, you can prevent a lot of rust on your containers just by selecting the right location and climate.
Shipping Container Rust Treatment
There may be times when you have not been able to prevent the rust from forming on your shipping containers. This could be for a number of reasons. Maybe you’ve just purchased the containers used and they’re already rusty.
Whatever the reason, though, the rust should be removed. The container then needs to be protected again to prevent further damage that could change the issue from a non-structural problem to a structural problem.
One of the best, but most expensive choices, is to sandblast the rust away and then seal and paint the container. To ensure that you’ve removed the rust, you need to go down to bare metal where you can no longer see the rust. Once the rust is removed, it is essential that the container needs to be sealed and painted.
Thankfully, much of the time, sandblasting and completely refurbishing the container is excessive.
Often, you can remove small patch(es) of rust using a wire brush and sand paper. Start by removing the rusted areas with the wire brush first. Use the sand paper to finish. Once the rust has been removed, use vinegar and rub this gently on the cleaned areas and leave to dry. After the vinegar has dried, use marine grade DTM (direct to metal) paint and cover the cleaned area. For a 20-foot container you will need four gallons of paint to cover it and eight gallons to cover a 40-foot container.
While shipping containers are made from Cor-ten steel, they are not corrosion or rustproof.
Cor-ten steel only slows the rate of rust developing.
The best way to maintain your shipping containers is to prevent rust from forming on your containers in the first place.
This can be largely achieved through maintenance and regular inspection.
However, small patches of rust on your container can be treated individually without having to refurbish the entire container.
Have you had any issues with rust and corrosion? Leave your comments or questions below.