Surface Finish Guide

🔍 Curious about why engineers and designers specify surface finish for stainless steel tubing? 🤔

Stainless steel is often chosen for its aesthetic appearance, so make sure to specify the surface finish you need. Finishes can be applied to improve functionality and for aesthetic appearance. 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 (root mean square) finishes as needed.

Surface Finishing Services

Surface Finish Sand Blast

Production sandblasting center for achieving various degrees of matte surface finishes.

Sand Blasting – Produces a dull matte finish with various grit sizes. 60 / 100 / 180 / 220 / Glass Bead

Sandblasting is an abrasive blasting finishing technique typically used when you are looking to roughen the surface finish or to create a dull/ matte finish. Generally this is used to make the part more abrasive for molding so that the plastic will adhere to the metal tube. This process is achieved by placing parts in a sandblast cabinet and they are exposed to a stream of sand or grit at high speeds through a machine nozzle. A variety of sand or grits are available. The two most commonly used are silica carbide and aluminum oxide. The grits are available in size from 60 grit to 120 grit depending on the desired roughness.

Electropolishing – Produces a high, bright finish. Electropolishing is a process where the tubes are submerged in a tank and an electric current is used between a cathode and anode with the tubes in between. This electric current will then remove a small layer from the surface of the metal leaving behind a highly polished and clean surface.  It will eliminate imperfections, microburrs and contaminants.  This process not only enhances corrosion resistance but also ensures the utmost cleanliness, making it vital for applications where hygiene, biocompatibility, and precision are paramount.

Belt Polishing – Produces a high luster finish using grits from 180 to 640. The material can have the smoothness/ brightness improved by conditioning the tubing using a belt polisher. The higher the number the finer the grit and the smoother the surface finish.  Belt polishing allows for precise control over the amount of the material removed.  This control is valuable when you need to acheive specific dimensional tolerances or to maintaint the tubes structural integrity.
e.g., 100-180: Medium grit levels provide a balance between material removal and surface refinement. They result in a moderately smooth finish, making them suitable for applications that require both functionality and a somewhat polished appearance.
e.g., 220-320: Fine grit levels are chosen for achieving smoother and more refined surface finishes. They are ideal for applications where appearance and moderate material removal are equally important.
e.g., 400-600: Very fine grit levels produce highly polished surfaces with minimal texture. They are often used in applications demanding a high-quality, mirror-like finish, such as decorative or precision components.

Black Oxide Finish – Minimizes light reflection. Black Oxide finishing is a blackening conversion coating used on a variety of ferrous metals and alloys, including, stainless steel. The process is formed by a chemical reaction when material is submerged 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.
Some surgical and medical instruments feature black oxide finishes to reduce light reflections during procedures and to protect against corrosion in sterilization processes. Aerospace parts, such as fasteners, gears, and hydraulic components, often have black oxide finishes to enhance wear resistance and reduce light reflection in critical systems.

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, and fasteners.
Specifying a coarse centerless ground finish enhances adhesion and grip in components, making it ideal for applications requiring secure attachment and improved handling.

Commercially Clean & Capped
When stainless steel tubing is commercially cleaned it undergoes a cleaning and preperation process that removes contaminants, improve surface finish, meet industry-specific standards and ensure the tubings sutiability for its intended application.  “Capping” refers to sealing the ends of the tubes to maintain the purity of the contents and prevent contamination.

Thermocouple Clean & Capped
Thermocouple cleaned tubing undergoes a rigorous cleaning process to ensure it is free from contaminants that could affect temperature sensors or thermocouples attached to it.  The cleaning process “Capping” refers to sealing the ends of the tubes to maintain the purity of the contents and prevent contamination.

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 the corrosion resistance of stainless steel 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 while leaving 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.

How is Surface Finish Measured?

RA (roughness average) and RMS (root mean square) both measure surface finish. They are analogous. They use a slightly different path but both arrive at the same conclusion.

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Eagle has the knowledge, the expertise, and the right equipment to give your finished parts the “look” you want. To inquire about a specific project or for any questions, please request a quote from us today.