Metal racking: What finish offers greater protection against corrosion?
In warehouses, racks are exposed to changes in temperature, humidity, continuous transport of goods and operators. These factors can cause deterioration of the components that make up the structures. This article analyses the advantages and disadvantages of the different methods and finishes available on the market to protect the metal profiles from the effects of corrosion and oxidation.
The components that are part of the racking should be protected against oxidation. Scratches or lack of continuity in the coating generate more vulnerabilities. For this reason, both the type of finish employed and the pre-treatment applied are very important, which act as a second barrier protection against corrosion.
Different methods and systems coexist on the market:
Galvanised consists of covering the metal with zinc to protect and insulate it from corrosion.
There are two options:
Painted profiles are coated with a paint that acts as a physical barrier against the onslaught of oxidising environments, thanks to the anti-corrosive effect of the pigments and resins used.
There are two systems:
Below, the article examines the main advantages and disadvantages of these protection systems for metal components.
The finish of pre-galvanised elements is procured by the immersion of the steel coil in a bath of molten zinc in a continuous process. It is an operation carried out by the steel manufacturers themselves. It creates a coating with a thickness of between 14 and 19 µm.
This system entails certain advantages:
However, this system also involves some drawbacks:
Hot galvanisation consists of introducing the piece, once made, in a receptacle of 450 °C molten zinc. A zinc layer is created with a thickness varying between 60 and 90 µm.
This process ensures maximum protection of profiles and a very long, maintenance free product life. In general, their use is widespread in urban fittings and structures found outside (electric poles, signs, barriers). Being an immersion process, the entire piece is coated and its mechanical strength (against blows, impacts, abrasions, loading and unloading, etc.) is higher.
However, it presents more than a few disadvantages:
The behavior of zinc varies depending on the environment where it is found. In humid environments, it generates zinc salts which are highly hygroscopic and moisture absorbent. It is a snowball or loop effect: the wetter it is, the more salts are generated. Instead, in a dry environment the zinc is passivated; this means that a film forms on the metal surface which protects it against external factors. So the zinc becomes more resistant and stable, a constant exchange of wetting and drying cycles is needed, meaning this metal is optimal for outdoor use. Conversely, if the profiles are permanently in a humid environment, that is when the zinc salts appear, starting the oxidation and deterioration.
The relative humidity of the frozen storage is, in theory, practically zero, only being found in the entrances and exits of the warehouse. That is why galvanised racking is a perfect fit. However, humidity is higher, even necessary, in cold storage. For this reason, the installation of galvanised racks in this kind of cold storage is a mistake: passivation will protect the entire structure for a few months, but once it loses its effectiveness, the zinc will decompose forming zinc salts and disappear.
In fact, during tests in a salt fog chamber, it can be seen that the behaviour of hot-rolled galvanised material is much worse. By not being passivated, its deterioration occurs in under 100 hours and pre-galvanised sheet metal at around 150 hours. This test is applicable when determining the comparative anti-corrosive quality of various types of coatings.
It is a procedure that distributes liquid or powder paint on parts for a double effect. On the one hand, the electrostatic attracts the sprayed paint until depositing it on the piece. And, secondly, the cinematic effect unleashes paint at high speeds.
The process consists of the following phases:
Indeed, its application is very simple and requires less investment compared with other systems. An air transport chain is needed to hang the pieces, cabins to apply paint and a polimerisation oven. In smaller facilities, any stage of the process can be modified and re-purposed with ease, and the colour changed at any time.
It is an optimal system for closed profiles or flat sheet metal with one or two folds. The anti-corrosive quality depends on the type of paint used, the preliminary procedure before painting and the thickness of the layer.
However, the electrostatic painting has several disadvantages in open profiles. The paint does not reach the interior parts, nooks and corners with the same thickness, due to the Faraday effect. Consequently, the distribution is not uniform and there are always areas with less paint or that are unpainted.
The coatings are electrical insulation. Physically, the differences in thicknesses on the distinct areas of the same piece produce potential variations and generate the passage of currents inside the piece (thus, facilitating corrosion). Furthermore, the fact of having a reduced thickness results in an inferior physical barrier against corrosion.
This is an immersion process that allows the pieces to be coated with paint. It is performed by cathodic electro-deposition: the cations (positively charged ions generated by the dissociation of paint molecules), immersed in an aqueous solution, adhere to the profiles creating a uniform layer.
The process involves the following phases:
Listed below are its many advantages:
The cataphoresis process requires a considerable and sustained expenditure. However, it goes without saying that the finish is superior and ensures greater safety in relation to the process of galvanisation and electrostatic painting.
For over 30 years, Topeasy Industry has applied this process to the uprights their racking frames consist of, due to their critical importance in any installation: they are subject to interior microclimates, in contact with the floor, bear heavy loads, can suffer blows, etc.
The anti-oxidation strength of cataphoresis is much higher than other finishes on the market, as can be seen in the comparative chart.