1. Natural appearance of stainless steel
Stainless steel exhibits a natural, robust and shiny appeal, and its color softly reflects the hues of the surrounding environment.
2. Basic types of surface processing
Approximately five types of surface processing can be applied to stainless steel. They can be combined to produce a wider range of end products.
These types include: rolling surface processing, mechanical surface processing, chemical surface processing, mesh texture surface processing and colored surface processing.
There are also some specialized surface treatments.
Regardless of the type chosen, the following steps must be followed:
The. Consult the manufacturer to determine the surface processing required, ideally preparing a sample as a standard for mass production.
B. When used on large areas (such as composite boards), make sure the substrate coil board or coil used is from the same batch.
w. In many architectural applications, such as elevator interiors, fingerprints, although removable, are unsightly. If a fabric-textured surface is used, it will become less noticeable. Stainless steel with a mirror finish should never be used in these sensitive areas.
d. When choosing surface processing, consider the manufacturing process. For example, to remove weld seams, it may be necessary to sand the weld and restore the original surface processing. Standardized plates are challenging or even impossible to meet this requirement.
It is. Some surface treatments, sanding or polishing have directional patterns, known as unidirectional. If this pattern is applied vertically rather than horizontally, it is less likely to collect dirt and is easier to clean.
f. Regardless of the precision processing used, additional process steps are required, thus increasing costs. Therefore, the choice of surface treatment must be made carefully. Architects, designers and manufacturers need to understand the surface processing of stainless steel. Through friendly cooperation and mutual exchange, the desired result is guaranteed.
g. Based on our experience, we do not recommend using alumina as an abrasive unless it is handled very carefully during use. Silicon carbide abrasive is preferable.
3. Standard surface finish
Various surface finishing techniques are often represented by numbers or other classification methods, all of which are included in relevant standards such as “British Standard BS1449” and “American Iron and Steel Institute Stainless Steel Producers Committee Standard”.
4. Laminated Surface Finish
There are three basic types of laminated surface finishes for sheets and strips, represented by their production processes.
#1: It involves hot rolling, annealing, pickling and descaling. The resulting steel surface has a matte, somewhat rough finish.
Nº2D: Superior to the No.1 finish, it also has a matte surface. It involves cold rolling, annealing, descaling and a final light rolling with a blind roller.
#2B: This is most commonly used in construction applications. It shares all processes with 2D, except a final light cold rolling with a polished roller after annealing and descaling. The surface is slightly shiny and can be polished.
No.2B bright annealed: This results in a reflective surface, rolled with a polished roller and finally annealed in a controlled atmosphere. Bright annealing keeps its surface reflective without forming an oxide layer. Since there is no oxidation during the bright annealing process, no additional pickling and passivation are required.
5. Polished surface finish
N°3: Represented by 3A and 3B.
3A: The surface is evenly sanded with a grit size of 80-100.
3B: Matte polished, resulting in straight and uniform striations. Generally obtained by a single grind with a grain size of 180-200 on a 2A or 2B plate.
#4: Unidirectional surface finish with low reflectivity, widely used in construction applications. The process involves initial polishing with coarse grit, followed by grinding with 180 grit.
Number 6: This is an improvement over No.4, involving polishing the No.4 surface with a tamping brush in a medium of sand and oil. This finish is not included in “British Standard 1449”, but can be found in American standards.
#7: Known as gloss polish, it is applied to finely sanded surfaces that still have sanding marks. Typically, a 2A or 2B plate is polished using a fiber or fabric polishing wheel and the corresponding polishing paste.
#8: This is a mirror polished surface with high reflectivity, often called a mirror finish because it reflects images clearly. Stainless steel is continuously polished with a fine grain, followed by a very fine polishing paste.
In construction applications, it should be noted that this surface may leave fingerprints in high traffic areas or places frequently touched by people. Of course, fingerprints can be erased, but sometimes they can affect the aesthetics.
The surface finishes described in “official” standards and literature are general introductions.
Samples are the most direct way to represent surface finish types.
Precision polishing or metallurgy manufacturers will provide samples of various surface finishes and users should discuss with them.
6. Surface Roughness
The categorization of laminated surface finishing and polished surface processing indicates the achievable level; Another effective representation is the measurement of surface roughness.
The standard measurement method is called CLA (Center Line Average), where the gauge moves transversely along the surface of the steel plate, recording the amplitude of peak-valley changes.
The lower the CLA number, the smoother the surface. The final results of the different levels can be deduced from the surface finish and CLA numbers in the table below.
| CLA surface finish | Micron |
| 2B | 0.1-0.5 |
| 2A | 0.05-0.1 |
| 2D | 0.4-1.0 |
| 3 | 0.4-1.5 |
| 4 | 0.2-1.5 |
| 8 | 0.2-1.5 |
| FOOT | Half the basic amount |
EP = Electrolytic Polishing, which can approximately reduce the amplitude of the peak-valley change to half of the original surface.
7. Mechanical Polishing
Points to note:
We must remember that grinding operations with sandpaper or sand belt essentially belong to polishing and cutting operations, leaving very fine lines on the surface of the steel sheet.
We have encountered problems using alumina as an abrasive, in part due to pressure issues.
Any grinding parts of the equipment, such as sand belts and grinding wheels, should not be used on materials other than stainless steel before use, as this would contaminate the surface of the stainless steel.
To ensure the consistency of surface processing, new grinding wheels or sand belts must first be tested on similar scrap materials for comparative purposes.
8. Electrolytic Polishing
This is a metal removal process in which stainless steel, as the anode in the electrolyte, removes metal from the surface layer when electrified. This process is commonly used in the processing of parts, as their shapes are difficult to polish using traditional methods.
It is often used on the surface of cold-rolled steel plates because its surface is smoother than hot-rolled steel plates.
However, electropolishing makes surface impurities more noticeable, especially titanium and niobium stabilized materials where granular impurities cause discrepancies in the weld zone.
Small weld scars and sharp edges can be removed by this process. This technique focuses on dealing with protrusions on the surface, prioritizing their dissolution.
Electrolytic polishing is achieved by immersing stainless steel in a heated liquid, with the liquid ratio involving many proprietary technologies and patented techniques. The electropolishing effect of austenitic stainless steel is excellent.
9. Textured surface processing
Stainless steel can be crafted into a variety of patterns. The benefits of adding patterns or textured surface processing to steel sheets include:
1. Reduction of “oil canning”, a term that describes the visible irregularity of glossy materials, such as large decorative panels, that remain difficult to completely flatten even after leveling by stretching or tension, leading to oil canning.
2. Patterned texture can reduce glare when exposed to sunlight.
3. Small scratches and small dents are less noticeable on patterned panels.
4. The strength of the steel plate is increased.
5. Provides more options for architects.
Patented patterns include linen (used in London's Euston Tower), embossed, pearlescent and leather patterns.
Wavy and linear patterns can also be used. Textured surfaces are especially suitable for interior decoration, such as elevator panels, counters, wall panels and entrances.
For external applications, it is important to ensure that stainless steel can be cleaned with rainwater or hand washing to avoid corners where dirt and airborne impurities can accumulate, preventing corrosion and maintaining aesthetic appeal.
10. Satin surface processing
Satin surface processing is one of the most common surface treatments and involves polishing the surface of a steel sheet that has been subjected to polishing or bright annealing with a nylon belt or brush.
11. Glass bead or sandblasting
For interior applications, such as elevator interiors, mixed surface treatments are popular.
This mixed technique involves creating a matte surface by blasting with glass beads, followed by masking treatment, and then applying a plastic film to create a polished surface treatment, resulting in a mixed matte and polished surface. Stainless steel shot can also be used for similar processes.
The glass balls or shot to be used must never have been used on other materials, especially carbon steel, as carbon steel particles can settle on the surface of the stainless steel and easily cause corrosion. Ceramic granules can also be used as blasting material.
12. Colored stainless steel
The colored stainless steel process was successfully developed by the International Nickel Company (INCO) in the 1970s, and many companies have licenses to use this technology.
As explained previously, stainless steel does not rust due to its surface layer of passivating chromium oxide.
The coloring process takes advantage of this layer to produce specific colors. Because stainless steel uses this ever-present layer, it doesn't fade or require regular maintenance like paint.
Colored stainless steel can also undergo shaping treatment, and even sharp curves will not negatively affect the color.
Regarding corrosion resistance, tests indicate an improvement after applying this process. The process is closely related to operating time, as different durations result in color variations.
The sequence of color changes is brown, gold, red, purple and green.
A key feature of this process is that the final appearance reflects the original surface of the material: a mirror or polished surface will produce a strong metallic shine, while a matte surface will produce a dull color.
Process procedure:
The process involves immersing stainless steel in a tank of solution.
Ideally, the solution should contain 250 grams of Cr2O3 per liter, or 490 grams of sulfuric acid per liter, with a temperature range of 80-85°C.
The soaking time depends on the desired color, but should not exceed 25 minutes.
After rinsing the steel plate with clean cold water, it is placed in a liquid with a concentration of 250 grams/liter of chloric acid and 2.5 grams/liter of phosphoric acid for cathodic treatment at room temperature for about 10 minutes.
The current density is 0.2-0.4 A/dm2. To avoid damage, the hardening treatment is carried out immediately after the coloring treatment, followed by rinsing in hot water and drying.
13. Mixed surface processing
Patterns can be added to colored stainless steel. The proprietary technology developed includes the use of corundum sand belts to remove “protruding” parts, thus combining the natural beauty of the steel sheet with the color of the pattern.
This surface does not easily retain fingerprints and is particularly suitable for interior decoration. Polishing manufacturers can provide surface processing samples.
14. Engraving Surface Treatment
Patterns are printed on the surface of the steel sheet using overlapping techniques, subsequently submerging the sheet in ferric chloride solution (grade 0) to remove non-overlapping sections, forming beautiful patterns on the stainless steel surfaces.
Stainless steel wire drawing is a metal machining process, currently the most popular surface treatment technique in the stainless steel and aluminum products industry. It creates a drawing effect on stainless steel and aluminum products.
Generally, stainless steel wire drawing has several effects: straight line, snowflake and nylon.
Straight lines are uninterrupted top-to-bottom patterns typically achieved using a stationary wire drawing machine with the workpiece moving back and forth.
The snowflake pattern, currently the most popular, is made up of consistently placed dots and can be achieved with bugle sandpaper.
The nylon pattern is made up of lines of varying lengths. Due to the soft texture of the nylon wheel, it can polish uneven areas to achieve the nylon pattern.
Stainless steel wire drawing requires careful attention to procedures and techniques, generally combined with wire drawing machines to repair and restore product scratches and welds, ultimately achieving an overall artistic effect of wire drawing.
The various surface treatments possible with stainless steel expand its fields of application – different surface treatments result in unique stainless steel surfaces, each with its own unique character in application.
In architectural applications, the importance of surface treatment of stainless steel lies for several reasons. Corrosive environments require smooth surfaces because they are less likely to accumulate dirt.
Dirt buildup can cause rust or even corrosion of stainless steel. In spacious halls, stainless steel is the most commonly used material for elevator decorative panels.
Although fingerprints on the surface can be removed, they affect the aesthetics, so it is better to choose a surface that prevents fingerprints.
Hygienic conditions are vital for many industries such as food processing, catering, brewing and chemical industries.
In these applications, surfaces must be easy to clean daily, often using chemical cleaning agents. Stainless steel is the best material in this regard.
In public places, stainless steel surfaces are often vandalized, but one of their main features is that they can be cleaned – this is a significant advantage of stainless steel over aluminum. Aluminum surfaces tend to retain marks that are often difficult to remove.
Cleaning the stainless steel surface should be done in the direction of the stainless steel grain, as some processed grains are unidirectional.
Stainless steel is best suited for hospitals or other areas where hygiene conditions are critical, such as food processing, catering, brewery and chemical industries.
This is not only because it is easy to clean daily, sometimes with chemical cleaning agents, but also because it is not prone to bacterial growth. Tests show that stainless steel's performance in this aspect is on par with glass and ceramics.
