In architecture, stainless steel is a frequently used material for its physical characteristics (weld-ability, deformability, corrosion resistance, etc…) and for its look. The surface finishing is a decoration given to the surface of the stainless steel through a series of thermal, electro-chemical and mechanical treatment. Depending on the kind of the treatment, different surface finishing can be obtained for various applications. EN 10088 part 2 classifies them through a combination of numbers and letters describing how operations are carried out but leaving out the different fields of application.
1D surface finishing
This surface finishing is obtained by hot rolling and annealing. This thermal treatment reduces internal stress and segregation and creates a stable crystalline structure. The steel scrap is removed throuh a pickling. The reflectivity is very low because the roughness is very high. This finishing is used in building sector because the thick sheets are not visible and the aesthetic factor is not relevant.
Stainless steel 1D for structural applications
This finishing is achieved through cold rolling, heat treatment and pickling. It has a smoother surface than 1D finishing but not like 2B or 2R. It has opaque appearance and low reflection. It is ideal the engineering industry and its is used in bulding only on less visible parts.
2D finishing coil for industrial applications
This finishing is obtained by a 2D finishing and a rolling with glossy rolls (skin-pass) that gives it a milky gray appearance. This finishing is the most popular and is used for sofisticated decorations. Iits reflection is much higher than that of 2D making it a less opaque and cleaner. This finish can be used for exterior coatings of tank, buildings, cabinets, kitchen, and industrial equipment.
Vibrating separator with 2B finishing
Tank with 2B finishing
2R finishing (BA)
BA designation means “Bright Annealed”. This is a finishing of sheets subjected to heat treatment of annealing, to degreasing and to recrystallization in controlled atmosphere (hydrogen) after cold rolling to avoid undesired oxidation. Thanks to the total absence of oxygen, this annealing treatment causes stainless steel to exit from the oven in the same conditions as it entered without any surface alteration. The reflectivity is so high and the surface so glossy that guarantees less contamination of impurities. The surface is much easier to clean. It is used in pharmaceutical and food industries to avoid contamination and in the building sector to make structural forms stand out.
Pharmaceutical tank with 2R finishing
Starting from this finishing, other kinds of finishing can be obtained by polishing and brushing the surface. In this phase, it is common to use abrasive materials that can be lubricated (polishing) or dry (satin). Both perform engraving on the steel to a certain depth. However the roughness is different: the polished effect has a roughness (Ra) lower than the satin. Very often, tests are carried out to carefully evaluate the surface finishing needed.
2G finishing surface
This finishing is achieved trough cold rolling and treatment with rolls at different roughness. The sheet is ground in a single direction, making the surface rough and very opaque. It is often possible to specify the grain of the emery ribbon or the desired roughness. G means Ground. Usually only one face of the material is treated. This finish is frequently used on furniture and kitchen accessories.
Stainless steel electrical panel with 2G finishing
Stainless steel shower panel with 2G finishing
This finishing is normally obtained starting from 2B finishing. The sheet is ground with abrasive grain belts 80-100 meshes, obtaining coarse satin finishing. It is usually never used in this state, but is an intermediate stage to obtain 2k finishing.
The most common brushed finishing is obtained through 2J, 2D or 2B by grinding the surface with a finer grain abrasive, with an average grain dimension of 120-180.
It has a brilliant satin-like appearance, is commonly referred to as “satin” and has a standard roughness of 0.5 micron.
During the grinding operation, the metal surface is appropriately lubricated.
Depending on the types of abrasives, it is possible to obtain different surface coloration. The silicon carbide abrasive tapes give a slightly bluish appearance, while aluminum-based (aluminum oxide) gives a tending to gray color. This finishing is used where it is required an high corrosion resistance to atmospheric conditions.
Stainless steel with 2K finishing
This finishing is achieved trough a very coarse finishing, arriving to a surface that is polished with “cloth” rotating brushes impregnated with extra fine “paste”. The result is a glossy and the roughness is very low. It is impossible to see the grinding lines and the surface is like “perfect mirror”.
Cloud Gate with 2P finishing
Decorative finishing are obtained through molding processes or rolls that have embossed motifs. The pattern is molded on the surface, improving the aesthetic appearance. The treatment enhances the stiffness, reducing the cost for further treatments.
The types are essentially two:
Only one side decorated plates are classified as 2M;
Sheets with both decorated sides are classified as 2W.
In public areas, such as underground and shopping malls, there is a high percentage of risk that the metal is subject to impact and scratches. With these superficial decorations, it is easy to hide the damage.
Different 2M finishing
Tank with 2M finishing
This finish produces homogeneous, opaque, milky, non-directional surfaces. The shoot-peening is a treatment where the surface is affected by stainless steel spheres, ceramic grains, aluminum oxides, or glass fragments. Carbon steel balls or sand particles are not recommended because they contaminate the surface and could cause corrosion. Shoot-peening process deforms the surface microscopically, increasing the mechanical strength of the product (hardening). Shoot-peening increases the roughness of the surface, increasing the coating adhesion.
The coloration of stainless steel is caused by the growth of a layer of chromium oxide through a chemical process. The oxide layer is transparent and free of pigments, thus the coloration is generated by the absorption of a part of the white light (interference wavelength). The process allows to obtain colors such as bronze, golden, red, violet, blue and green corresponding to a film thickness increase of 0.02 μm to 0.36 μm.
Because of its colorlessness, the chromium oxide layer is not sensitive to aging by ultraviolet light and, since the coloring process does not provide pigments, it can be processed after treatment without breaking the film. Since the passivated film is transparent, the finish of the underlying substrate influences the final appearance. For example, an opaque finish creates a soft and opaque color, while a mirror finish produces a brilliant look. Unlike painted surfaces, the color does not tend to vanish with exposure to sunlight. However, if the surface is damaged by scratches, during fabrication, or corrosion, it can not be repaired. In fact, the stain can be removed by abrasion, so it should not be applied if there is a risk of accidental or voluntary damage, such as public areas or in presence of abrasive particles transported by the wind. Pickling, acid attacks and electro-polishing remove the color from the surface. Colored steel is very much used for outdoor coatings and design where it is very important to highlight a part of the product.