Advantages and disadvantages of the surface treatments applied to Corten steel

The commercial designation Corten is usually referred to a low-alloy self-weathering steel category in which, by means of a controlled addition of certain alloying elements such as Cu, Cr, Ni and P, it is possible to modify the nature of the oxide layer (patina) that is formed in its surface due to the atmospheric exposure. This patina provides the steel special characteristics that increase its corrosion resistance. 

The denomination Corten (or COR-TEN) is the name of the trademark originally registered by United States Steel Corporation with which this type of steel was developed. Metonymically, this name has become the familiar designation for the whole family of "steels with improved resistance to atmospheric corrosion" (or "Weathering Steels”), which is their actual designation. In Spain the standard UNE EN 10025-5 is the one that rules the "technical delivery conditions for structural steels with improved atmospheric corrosion resistance". 

The formation of this outer layer of oxide on the surface is known as passivation and it occurs spontaneously when exposing the metal to atmospheres with enough levels of oxygen. This is where the term "self-weathering" comes from. The patina is formed as a thin film of reddish-orange hue, very adherent and waterproof. Indeed, these properties are the ones that prevent the oxidation of steel from growing into the metallic structure of the material. This layer is self-regenerable, that is, if any superficial damage that deteriorates this protective layer of oxide occurs, this coating will regenerate in a natural way.

The Corten steel, in a usual decorative application, shows its a redish hue due to its natural oxide layer already settled because of atmospheric exposure.

The main advantage of Corten steel lays in its null maintenance, as opposed for example to a paint coating. Indeed, over the years, the surface oxide layer of weathering steels becomes more and more stable, unlike paint coatings, which gradually decompose due to the aggression of the atmospheric agents and therefore need a continuous maintenance. 

This improved resistance to corrosion also makes Corten steels an attractive material for diverse architectural applications, urban furniture, decorative structures, etc. In this type of applications the aesthetic appearance becomes essential and a reddish uniform and stable color is sought after from the beginning of the installation of the material. But these needs are not always in correspondence with the natural weathering process of the Corten steel and the environmental conditions, such as atmospheric factors (rain, sunstroke, dew, salinity, pollution, etc.) can delay or accelerate the formation of the patina. Also, the weathering can evolve to different shades of color, getting darker or clearer, or even showing different hues by zones.

Also, a usual problem related to naked Corten steel that generates many claims is that the oxide is water soluble, especially in the preliminary oxidation stages. That way, the rainwater that washes the surface of the steel drags certain contents of dissolved oxide that provoke the formation of reddish or rust stains in the water deposition zones. These stains are hard to remove (especially from concrete surfaces). This is a very well-known problem that gets softer progressively over time as the patina ages and get a more stable structure.

Corten steel in an architectural application in fences. Some of its disadvantages are visible: heterogenous coloring because of atmospheric conditions and oxide stains.

"Some circumstances require a solution for the disadvantages that are implicit to Corten steel." 

These treatments are normally based on the application of an oxide-activation product, which achieves the desired patina hue, followed by a rust stabilizing product known as the "stop bath". It should be meant that the effect of these treatments is not definitive and that their duration depends on the conditions to which the material is exposed. Therefore, if not periodically renewed, it will lose its effect and steel will recover the natural aging process of an untreated steel.

Some treatments recommend a third stage that consists on applying a varnish after the stabilization of the oxide. The application of this varnish involves modifying the anti-corrosion model of Corten steel, that onward will behave like a painted steel, where the effectiveness of the protection will depend exclusively on the capacity of the varnish layer to maintain the water tightness of the structural assembly, isolating completely the surface of the steel from moisture. 

If the varnish layer gets damaged or some areas are let unprotected allowing the entrance of moisture, feature blisters will be created, which will increase progressively until the layer of varnish gets detached, chipping of or exfoliating. This is the usual deteriorating process of any coated metal structure and the same model applies to the varnished Corten steel, with the peculiarity of the presence of a highly hygroscopic patina under the paint layer that will increase the speed of degradation.

Exfoliation degradation in a Corten steel structure that has been treated with a protective varnish layer. The degradation model is comparable to the blisters that appear in painted steel structures as captioned in the right picture.

A significant work line in the activity of the IK4-AZTERLAN Metallurgy Research Centre is centered in the analysis of any kind of failure methods and pathologies regarding corrosion in metallic components. It is a relatively common issue to address claims based on "deficiencies" of installed Corten steel, which are just a reflection of the average behavior of the same, mostly regarding “problems” such as stains or uneducated tonality. But, even more usual than these are the problems related to defective surface treatments of Corten steel, either because they are inappropriate, or because they have been poorly applied. When these occur, far from constituting a suitable solution, they tend to increase the complexity of the problem.

Based on the experience of IK4-AZTERLAN, the practice of applying a layer of varnish on the Corten steel is considered especially inadvisable due to the risk it entails as a lasting solution. Environmental conditions affect the varnish layer, which will gradually degrade over the time. It is a short-term solution that prioritizes the aesthetic function of steel (avoid stains and stabilize the appearance of the patina) but it invalidates the corrosion resistance properties of the steel itself and makes it an inadvisable solution, from a durability and integrity point of view.