Find here step-by-step instructions on rust protection of container houses. Corrosion is the biggest enemy of your shipping container home!
Frame: Rust & Corrosion Protection
Frames of Customized Container(s) houses are made from GI (Galvanized Iron) steel. The Galvanization process protects the steel from rusting (oxidation). It is done by applying a layer of zinc coating. In conclusion: It can be achieved either by a hot-dip process or electro-coating.
The hot-dip galvanization method
The hot-dip galvanization method immerses the iron (steel) in a bath of molten zinc. Both metals undergo accelerated chemical reactions at high temperatures, creating an impenetrable layer of zinc-steel alloy on steel’s surface. It effectively isolates the steel from atmospheric exposure.
Benefits of Hot-dip galvanized method.
- Hot-dip galvanization is the most cost-effective and robust rust-protection process so it is suitable for outdoor applications.
- The layer of the inter-metal alloy makes an integral part of the steel element.
- In contrast to any paint-coating protection, it’s not prone to chipping, and the typical “wear & tear” process destroys the coating’s integrity.
- Depending on the thickness of the protective layer and the surrounding environment, the galvanization process guarantees rust and corrosion protection for decades.
A 1 mil (25μm) thick hot-dipped zinc coating will offer protection for about 15 years in industrial and tropical marine zones. It provides protection for at least 30 years in a rural environment.
- The galvanized frame components are usually covered by paint (baked varnish). On top of aesthetic effects, it also adds an extra layer of protection against rust.
Drawbacks of the galvanizing process
- The electro-galvanization process is more expensive.
- It produces a coating of well-controlled thickness and uniformity.
- But the protective layer is thin, and so much steel should not be exposed to weather elements.
Siding: Rust & Corrosion Protection
The walls and roofs of customized containers houses are usually made from sandwich panels. They consist of a thick thermal insulation layer (Rock wool, PU, etc.) sandwiched between two galvanized steel sheets. The thickness of steel sheets depends on the manufacturer.
- Typically it will be around 0.5 mm (20 mils).
- The demanding exterior wall is thicker – around 0.8 mm (30 mils) and the interior one thinner – around 0.4mm (15 mils).
Structure of wall panels
Wall panels are made from coiled pre-painted galvanized iron (PPGI). The galvanization of coiled sheets is performed in a continuous hot-dip process. It’s much faster than traditional (manual) hot-dipping used for structural elements. However, it produces a substantially thinner layer of zinc coating – typically about 18 μm (slightly less than ¾ mil). For adequate rust protection in outdoor applications, PPGI sheets must be painted!
Undoubtedly, exterior sidings are the weakest parts of customized container houses. Besides the reduced steel thickness (compared to CSC) and thinner zinc coating. But also to abrasions that may eventually break the rust protection coating (galvanization and paint) and expose the raw steel.
The life span of shipping containers
According to manufacturers of customized shipping container houses, their estimated lifespan reaches about 20-25 years. It does not mean that at the end of this period, containers will collapse. What you may rather see will be damage in the form of rust and possibly areas of deeper corrosion.
Important for rust protection of container houses:
Periodic inspections and proper maintenance can largely extend the lifespans of customized containers houses.
Rust and Corrosion Protection of Cargo Containers
Industrial-grade Cargo Shipping Containers (frames, walls, roof, and doors) are mostly made from CorTen steel. CorTen stands for CORrosion resistance and TENsile strength steel. It is an original material developed by US Steel Corporation, belonging to the class of “weathering” steel. Such steel has “self-protecting” properties.
CorTen’s working principle
During the initial phase of weathering (aging), on the surface forms a strongly adhesive layer of Corten’s oxide. It’s “patina” in quite pleasant “coppery” color that bonds with the Corten. It effectively isolates its internal layers from exposure to the atmosphere, preventing this way the progress of rust. If properly aged, this natural protection will last indefinitely. Any areas of exposed steel due to mechanical damages will be self-healed thanks to the “weathering” properties of the Corten.
Eliminate the protective coating or painting
Theoretically, such steel eliminates the need for protective coating and painting!
Well, it sounds too good to be true. In fact, to achieve such a state of protection, the initial aging must take place in a favorable environment. A calm environment means no exposure to water, rain, or moisture, absence of aggressive factors like salty, humid moisture, etc.
Practically, it is difficult (if not impossible) to implement the perfect natural weathering process. It takes a few years to build a protective Corten’s oxide coating!
Rusting of CorTen
It is then fair to say that Corten rusts are slower than steel. Exposure to seawater or salty moisture (Hcl) “weakens” the protective barrier, accelerates the rusting process and leads to deeper corrosion.
Rust and corrosion can also damage CSC made from CorTen steel. Overseas transport is exposed to saltwater as well as hot and salty moisture. Cargo shipping containers are not in the position to get through the appropriate weathering process. But they are also continuously exposed to an aggressive environment.
Some companies may use ceramic or powder coating to protect the steel from rust and corrosion for more demanding applications. The process is more expensive. However, it provides a much more damage-resistant protective layer compared to the paint.
Rust and Corrosion: Background Notes
For many of us, Rust and Corrosion are the same “animals.” While both are chemical processes, there are some important differences between them.
It is a complex process affecting not only metals but also other substances like ceramics polymers. It occurs when you expose those materials not only to air (oxygen) but also to other chemicals like sulfates, carbon dioxide, etc.
The most common examples are metallic corrosion. Almost everybody is familiar with darkening silver or famous greenish patina developing on copper roofs (reaction with copper oxide).
It is an example of one specific type of corrosion. Rusting is a chemical reaction between oxygen and iron (or iron alloys like steel). Its result is a thin, reddish film of iron-oxide (Rust) formed on the surface of the iron. Corosian does not make strong bonds with the iron, so in the initial phase of rusting, it can be easily washed.
Rust protection of container houses in a more advanced phase
Rust forms a flaky coating that easily chips, brakes-off, and exposes deeper layers of iron to the oxygen. In contact with dry oxygen, rusting is a relatively slow process. However, moisture speeds it up, while the presence of salt (seawater and salty moisture) substantially accelerates it.
The hot-dip galvanization process creates a layer of an inter-metal compound called alloy (in this case, iron-zinc). The microscopic view of its cross-section allows identifying a few different metallic layers. It ranges from pure zinc through layers of a zinc-steel alloy of varying concentrations of zinc to pure steel. This protective layer is impenetrable because the zinc-steel alloy is not the traditional coating. It actually makes an integral part of the steel. On top of that, on the outer layer of zinc, it will eventually develop a firm, non-reactive layer of zinc-carbonate.
Manual hot-dip process
The “manual” hot-dip process is carried out by immersing suspended on-chain/wire/hook “steel” in the bath of molten zinc. It should be there until the internal temperature of the steel component and the temperature of the bath becomes equal. The typical thickness of zinc coating on industrial structural elements is between 2 to 6 miles (50 to 150 μm).
Molten zinc bath
In continuous hot-dip galvanization, the uncoiling metal sheet passes through the bath of molten zinc by 8 feet per second. Then after drying, it is re-coiled back. The zinc-steel alloy created during this process is usually much thinner (< 20 μm (< 1 mil)). It must be kept in reasonable proportion to the thickness of the sheet of steel (around 0.5mm (20 mils)).
For outdoor application
When is used for outdoor applications (roofing, siding, etc.), such steel needs to be, which results in so-called PPGI steel? In order to avoid any deterioration. Pre-painting comes shortly after the galvanization.
The final layer of paint on PPGI sheets will be applied at the latest stage of the final product. The paint provides as an extra layer of protection and for aesthetics (color).
We hope this answered most of your questions related to the rust protection of container houses. If you still have some questions, please feel free to contact us.