Why would an engineer specify surface finish?
Service Detail from Eagle Stainless Tube & Fabrication, Inc.
Stainless Steel Surface Finishes
Stainless steels are a versatile alloy of iron widely used throughout the industrial world in countless applications. The alloy is well-known for its high corrosion resistance due to its minimum 10.5% chromium content by weight. But it is more familiar as that shiny, silver-like surface found on many household products in the bath and kitchen.
Why do Surface Finishes Matter?
The importance of stainless steel surface finishes is in providing an added layer of protection against oxidation, corrosion, rusting, and staining. Finishes can meet design needs, too, leaving aesthetically
pleasing appearances for architectural or other decorative applications. Some finishing services may require commercially cleaning and capping or thermocouple cleaning and capping stainless steel material already in service.
Stainless steel surface finishes will largely depend on the intended post-fabrication application. To help in deciding which secondary process is best-suited for your company’s needs, here are six common stainless steel surface finishes for consideration.
Sandblasting is an abrasive blasting finishing technique that uses various blasting materials to achieve a specific finish to a metal’s surface. Common sandblasting media included steel shot, silicon carbide and glass beads which are shot at high speeds through a machine nozzle over a surface until it is sanded smooth. Sandblasting can be done to remove a previous finish, to remove rust or stains, or to prepare a surface to receive a new coat of paint. Glass beads are an excellent choice for stainless steel applications that require a softer, brighter finish, and it has the added benefit of being used multiple times.
Electro-polishing is a stainless steel surface finish used to polish, passivate, and deburr metal parts. The process uses an electrochemical solution to remove the outer layer of a metal surface, leaving a uniform layer free of embedded contaminants, micro cracks, pits, and various surface imperfections that could compromise the longevity and performance of a part. Electro-polishing can be applied to a wide range of stainless steel grades and provides a lasting bright finish.
As a surface finish, belt polishing leaves stainless steel with a decorative, aesthetic, or high luster mirrored finish. The process is used on various stainless steel grades and can be applied to meet spec-required finishes for products and parts. It is generally used for appliances, visible structural parts, architectural adornments and the like.
Black oxide finishing is a blackening conversion coating used on a variety of ferrous metals and alloys, among them, stainless steel. The process is formed by a chemical reaction when material is immersed in a high temperature solution. The application provides stainless steel with a surface finish that adds mild corrosion resistance, but is ideally applied to meet environmental conditions that minimize light reflection. It is also used to leave surfaces with an aesthetic appearance.
Centerless Ground Finish
Centerless ground finishing is a technique that offers a similar appearance to stainless steel as polished finishes do. The main difference is that the ground finish shows visible circumferential scratch lines as a result of the grinding stone. The finishing details may vary from smooth to coarse depending on the spec-requirements. The technique also aids in achieving tight tolerances and even correcting dimensional problems that may arise in structural applications, shafts, threaded rods, fasteners, and the like.
Passivation of stainless steel treats the metal surface with an acid solution to further add to surface corrosion resistance. The technique acts to remove any free iron from the metal’s surface before that iron can react to water and oxygen in the air that, subsequently, can cause rust oxidation. As a post-fabrication process, the method also maximizes the natural corrosion resistance of other alloys found in stainless steel. Once the surface is treated, other alloys that comprise the material such as chromium or nickel react with oxygen to form an oxide layer that protects the underlying metal surface from corrosion. There are two methods used to treat stainless steel surface finishes, citric acid passivation and nitric acid passivation.
Eagle Offers Surface Finishing Services
Stainless steel is often chosen for its aesthetic appearance, so make sure to specify the surface finish you need. The most common is the shiny, silvery finish manufacturers refer to as “bright”; alternatively, you can request a “pickled” or dull finish. Polishing and centerless grinding operations can be performed as secondary processes to produce desired RMS finishes as needed.
Produces a matte finish with various grit sizes. 60 / 100 / 180 / 220 / Glass Bead
Produces a high bright finish
Produces a high luster finish using grits from 180 to 640
Black Oxide Finish
Minimizes light reflection
Centerless Ground Finish
Commercially Clean & Capped
Thermocouple Clean & Capped
Passivation (Chemical Cleaning)
Treatment of stainless steels after fabrication with oxidizing chemicals known as chemical cleaning or passivation. If iron particles or other substances have become embedded in the surface during fabrication or polishing operations, they must be removed. Otherwise, these minute particles may promote discoloration, rusting, or even pitting. Besides dissolving such particles, the oxidizing action of the bath also tends to enhance corrosion resistance of stainless steels by fortifying the natural passive surface film.
This processing should be the final operation on a stainless steel part. It is generally done by immersing in a nitric or citric acid solution and then rinsing in clear deionized water and drying. If immersion of the stainless steel piece is impractical due to size, the acid solution may be applied with a suitable swab and removed by rinsing with water.
Nitric acid is recommended because it will dissolve any iron particles and leave the stainless steel unaffected. It is necessary that the surface of the steel be free of scale, heavy grease and oil if the chemical cleaning treatment is to be effective.
Eagle has the knowledge, the expertise and the right equipment to give your finished parts the “look” you want.
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Custom Tubing Designed for Your Needs
Making a difference in your design… from simple tubing to complex designs.
At Eagle, there’s nothing more important than producing a customized solution which exceeds our clients’ expectations and satisfies the unique specifications of their application. Our expertise with stainless steel custom tubing gives us the ability to help you design the product that will fit your project.
Complex designs need more than just stainless steel custom tubing. Secondary operations such as cutting to precision lengths, CNC and EDM machining, laser welding, bending, and precision grinding can transform a simple tube into just the part you need for your design.
We welcome the opportunity to share our expertise in developing product concepts and helping you solve your engineering problems. With decades of experience in the metals industry, you can count on Eagle as a resource for designing and fabricating your metal products.
Eagle Stainless Tube & Fabrication Inc.
Eagle Stainless Tube & Fabrication Inc.supplies stainless steel tubing to the medical, aerospace and high tech industries. Stainless steel, nickel alloys, aluminum, and titanium.
- Cut to length
- Precision machining
- Laser cutting & welding bending
- Electro-polishing and passivation.
- Re-draw to specialty & millimeter sizes in both round, square and rectangular.
- Grinding to precision tolerances