Stainless steels are naturally reflective. Oxidation does not develop very rapidly and the surface remains very smooth. Having a tight, smooth surface enhances the corrosion resistance of stainless steel. The more the surface is polished, the better the long-term performance.
During the production of stainless steels, the mill surface of the thick plate is cleaned of scale using strong acids. This process, known as pickling, dissolves the heavy oxides and free iron carbides that rise to the surface of the hot plate.
Further reduction of the stainless-steel thickness is performed on cold semi-polished rolls. These rolls impart a smooth, reflective sheen on the surface of the metal. This initial sheen is the base for all subsequent finishes. The surface is designated as a No. 2B finish. Dull sheens can also be developed using rolls that have a dull surface. These initial finishes are known as No. 2D
The more specular No. 2B finish can be further enhanced by special annealing processes. Annealing the stainless steel in a controlled atmosphere will create a mirrorlike surface known as Bright Annealed, designated as No. 2BA. The No. 2BA can be the base surface for glass bead, No. 8 and No. 9 mirror surfaces, as well as fine satin finishes. The No. 2BA will provide a consistent color and surface for the more refined surfaces.
The reflective character of the various stainless-steel finishes can be divided into three categories as shown in Figure 2.4. The Reflective Finishes can be described as those that reflect light similar to a mirror. A bright light will reflect as a "hot spot." The angle of incidence equals the angle of reflection. Very little scattering of light occurs. The No. 9 finish is the equivalent to a mirror on one scale while the No. 2B finish is somewhat smoky.
Disturbing the even reflective surfaces with minute surface fractures or indentations, which scatter the reflected light slightly, produces the Diffused Reflective Finishes. Because these finishes are typically applied over the Reflective Finishes, they possess a brightness, an almost glowing behavior when in strong light.
The Low Reflective Finishes possess a dull reflection. Light is effectively scattered by the rough surface. and these surfaces appear flat in most light.
Because of its chrome content stainless steel reflects 49 percent of the visible wavelength of light. It is much more heavily weighted toward the blue wavelength and captures well the tone of the sky. On cloudy days, stainless steel will appear with very little luster. This is due in part to the scattering effect of the clouds, which reduces the blue segment of the wavelength of light reaching the stainless-steel surface.
Because of the specular nature of stainless steel surfaces, slight variations in plane can affect the relative color. Moving around the surface changes the angle of view from one surface of a plate or panel relative to another. The more direct the reflection, the lighter the color. The panel that is slightly askew will appear darker in strong light. A stainless-steel surface can look dark from one angle of view, then light in color from a different angle of view. The difference can be only a few degrees out of plane. This faceted reflection is common in stainless steel thin-plate surfaces. This is also why "oil-canning" tendencies are greater in reflective metals such as stainless steel. The relative high and low points in a stainless-steel surface reflect light back to the viewer at varying angles, which create apparent visual distortions in the surface.
When fabricated and installed correctly, the reflection is not distorted by the undulating surface. Light washes over the diffused reflective surface in straight lines. The mirror reflective surfaces show straight lines as straight images and not curved images.
Stainless-steel surfaces, particularly the diffused finishes such as No. 4 satin, angel hair, and glass-bead-blast surfaces, reflect the colors and shadows of the surrounding environment, but in a more scattered, subdued fashion.
Visual defects such as chatter are more visible in reflective stainless-steel surfaces than in most other materials. Chatter defects are caused when the polishing belts slip during the application of the finish. Whether the finish is a linear satin finish, such as a No. 4 or No. 3, or a mirror finish, slight reflective differences caused by the slipping of the polishing belts will create a series of visual lines of distorted reflection. The viewer cannot feel the distortion, but it is apparent when viewing the surface at an acute angle. These distortions are not repairable.