For most people, stainless steel is that bright, shiny metal used everywhere from kitchens to operating rooms to decorative exteriors of buildings. And, if asked about the finish, most would answer something along the lines of “mirror smooth.” In reality, of course, a stainless steel item has a finish that best serves its purpose. Beyond a mirror-like, reflective finish, stainless steel can be manufactured with a number of decorative or utilitarian treatments.
Collectively, these are known as Surface Finishes, but surface roughness, surface texture and surface topology are also accepted terms for the texture (or lack thereof) of the metal. The finish can also be further broken down by additional characteristics including waviness, lay, friction, surface roughness and profile. The finish chosen for a structure or component is a balance between the cost of achieving and maintaining the final piece efficiently against the requirement for optimal quality and reliability. Aesthetic appearance can also be a factor in many applications.
Making the Grade
Within the stainless steel industry, the types of finishes are broadly grouped into “grades”. These grades range from 1 through 8 with increasing quality of finish. There are sub-types within each grade but there are a number which are the most common.
- #1 is hot-rolled, annealed (toughened via a gradual cooldown during the manufacturing process) and then pickled (treated with pickling acid to remove surface impurities). The metal is not mechanically polished so the surface is very rough and non-uniform in appearance.
- #2B is more reflective and has been described as resembling a cloudy mirror. Unlike most other finishes, there is no mechanical polishing involved. Instead, the metal is passed through polished rollers on the last thickness pass, producing a dull, but very smooth and uniform finish that is very difficult to reproduce by other means.
- #2BA has a highly reflective surface. “BA” stands for Bright Annealed. It receives its annealing treatment in an atmosphere-controlled furnace to prevent scaling and oxidation.
- #2D also has a uniform, dull silver-gray appearance. It is cold rolled and heat treated, but lacks the final pass through the polish rollers that #2B receives. It is used when visual appearance is not important.
- #3 is a brushed finish. It is a finish consisting with short, parallel lines made using 100-120 grit abrasives in a process referred to as grinding or roughing.
- #4 is perhaps the most widely utilized finish, being used for a pair of distinct purposes. Known as Brushed, Directional or Satin finish, #4 is ground with 120-230 grit abrasives to give it fine, directional and uniform lines. For architects, it gives low gloss and heightens the appearance of flatness. It also allows welds to be hidden. It is also the finish most frequently used for kitchen and restaurants – often for the work surfaces and equipment, as well as decorative elements. Using the finer grits, #4 Sanitary is used in the dairy and pharmaceutical industries.
- #7 is very shiny and reflective. It is created by taking panels with a #4 finish and buffing it to a high shine. Even though it has good reflectivity, the original grit lines can be seen at a distance.
- #8 is the bright, mirror finish. It is the most reflective mechanically polished possible. It is produced by using successively finer grits buffed to a high shine. Custom architecture, design and decor along with optics use this for high end projects.
Finishing Processes
Certain finishes are achieved using a variety of means beyond grinding and polishing:
- Mechanical Polishing is the technique used to produce a uniform finish in grades of measurable surface roughness generally using abrasive pads between 60 and 600 grit sizes. Measured as Ra, the surface finish can be graded into grades such as Ra≤30u in.
- Glass Beading produces a satin texture. It is useful for creating a finish equivalent to a #2B that can both hide welds, or produce an artistic pattern to enhance appearance.
- Electroplating adds a layer of another metal to the stainless steel surface. While stainless steel’s corrosion-resistance means that electroplating is seldom done to protect it, it is done for aesthetic reasons such as to ensure paint adheres, or to add a precious metal. It can also be done to make parts easier to solder or to enhance durability.
- Electropolishing is the reverse of Electroplating. During this process, small amounts of metal are removed to make it smooth on a microscopic level. This can improve the metal’s resistance to contamination by foreign objects or bacteria.
On a related note, Passivation does not fall into the category of “surface finish” as it does not affect the surface profile, its roughness nor its physical appearance. Instead, involves removing iron ions from the surface of the stainless steel using a chemical treatment. This renders the stainless steel chemically inert on a molecular level, preventing corrosion and interaction of the contents of the vessel or container.
Ultimately, the finish of a stainless steel part or structure is directly dependent on its use and appearance frequently counts on an even footing with utility.
Sometimes, the best surface is the one that falls in the middle of the range of grades. A famous example is the stainless steel surface of the Walt Disney Concert Hall in Los Angeles. Much of the Hall’s exterior is clad in flowing, curving stainless steel given a matte finish. However, there were initially portions that had a mirror-like finish. In the California sun, several of these panels arrayed in a parabola, heated adjacent sidewalks to temperatures as high as 140° as well as reflecting light and heat into adjacent apartments. To correct this, the panels were mechanically polished to reduce the amount of reflectivity. The resulting finish did not lessen the beauty of the structure and it is an iconic feature of the city’s art culture.