Electropolished and Mechanically Polished Tubing, Part 1
This two-part article summarizes the important points in the electropolishing paper and previews Tverberg's presentation at InterPhex later this month. Today, in Part 1, we examine the importance of electropolishing stainless steel tubing, electropolishing techniques, and analytical methods. In Part 2 we present a preview of the latest research on passivating mechanically polished stainless tubing.
Part 1: Electropolished Stainless Steel Tubing
The pharmaceutical and semiconductor industries require large quantities of internally electropolished stainless steel tubing. In both cases Type 316L stainless steel is the alloy of choice. Occasionally, 6 percent molybdenum stainless alloys are used; alloys C-22 and C-276 are important for semiconductor manufacturers, especially when hydrochloric acid gas is used as an etchant.
Electropolished tubing is specified for so many applications because of:
- Surface smoothness, with fewer sites for trapping impurities. For the pharmaceutical industry this means a purer product without danger of bacteria growing in surface defects; the semiconductor industry is extremely sensitive to "killer particles" -- small pieces of debris that break free from a pit or crevice and lodge on wafers before vapor deposition, resulting in defective chips.
Easy characterization of surface defects that would otherwise be masked in the maze of surface anomalies found in more common materials.
At the material level, the most prominent feature of electropolishing is the remarkable change in the stainless steel surface chemistry. Electropolishing, especially when followed by a nitric acid passivation treatment, imparts a passive layer in which the chromium to iron ratio is raised from 0.25:1 for the base alloy to 1.5:1, and sometimes even higher, for the electropolished surface. With the passive layer comes another feature: deep oxygen penetration accompanying chromium enrichment. Electropolished surfaces have a depth of oxygen enrichment in the 100 to 400 Å range, which corresponds to 30 - 120 atoms in depth. Mechanically polished surfaces, by comparison, have penetration of 25 - 50 Å, or 8 to 15 atoms in depth.
Chemical inertness of the passive layer results from both the chromium and iron being in the 3+ oxidation state rather than as zerovalent metals. Mechanically polished surfaces, even after long, hot nitric acid passivation, still retain high levels of free iron in the film. This factor alone gives electropolished surfaces a big advantage in long-term stability.
Another major difference between the two surfaces is the presence (in mechanically polished surfaces) or absence (in electropolished surfaces) of alloying elements. Mechanically polished surfaces retain the basic alloy composition with only slight depletion of the other alloy elements, whereas electropolished surfaces contain essentially only chromium and iron.
Making Electropolished Tubing
To produce a smooth electropolished surface requires starting with a smooth surface. This means starting with very high quality steel, fabricated for the best weldability. During smelting sulfur, silicon, manganese, and deoxidizer elements (e.g. Al, Ti, Ca, Mg) and delta ferrite need to be controlled. The strip must be heat treated to dissolve all second phases that may originate during solidification of the melt, or that may form during high-temperature processing.
Likewise the type of strip finish is most important. Three commercially available cold rolled strip surface finishes are specified in ASTM A-480: 2D (air annealed, pickled, and finished with dull rolls); 2B (air annealed, pickled, and finished with polished rolls); and 2BA (protective atmosphere bright annealed and finished with polished rolls).
Roll forming, welding, and bead conditioning all must be carefully controlled to produce as round a tube as possible. Even the slightest weld undercut or weld bead leveling line will be visible after polishing. Likewise, roll-forming marks, weld roll-down patterns, and any mechanical damage to surfaces will be evident after electropolish.
Following heat treatment the tube ID must be mechanically polished to remove surface imperfections arising from the strip and the tube forming process. This step is where strip finish selection becomes critical. If the rugosity is too deep, more metal must be removed from the tube ID surface to produce a smooth tube. If rugosity is shallow or non-existent, then less metal must be removed. The best electropolish finishes, generally in the range of 5 micro-inches or smoother, are obtained from tubing that is longitudinally belt polished. This type of polishing removes the most metal from the surface, usually in the range of 0.001 inch, thereby removing grain boundaries, surface defects, and forming imperfections. Swirl polishing removes less material, produces a "smeared" surface, and generally gives a higher Ra (average surface roughness) in the range of 10 - 15 micro-inches.
Electropolishing
Electropolishing is simply electroplating in reverse. The electropolishing solution is pumped through the ID of the tube while a cathode is pulled through the tube. Metal is removed preferentially from the highest points on the surface. The process "wants" to plate the cathode with the metal dissolved from the tube interior, which is the anode. It is important to control the electrochemistry to prevent cathode plating, and to keep the proper valence for each of the ions.
During electropolishing oxygen is generated at the anode, or stainless steel surface, and hydrogen is generated at the cathode surface. Oxygen is a critical component in creating the special properties of electropolished surfaces, both to increase the depth of the passive layer and to produce a true passive layer.
Electropolishing occurs under a so-called "Jacquet" layer which appears to be polymerized nickel sulfite. Anything that prevents formation of the Jacquet layer causes a defective electropolished surface. Typically this is an ion, such as chloride or nitrate, that prevents formation of nickel sulfite. Other interferants are silicone oils, grease, wax, and other long-chain hydrocarbons.
Following electropolishing the tubes are water-rinsed, then further passivated in hot nitric acid. This additional passivation is necessary to remove any residual nickel sulfite and to improve the surface ratio of chromium to iron. Following passivation tubes are rinsed with process water, placed in hot deionized water, dried, and packaged. If clean room packaging is required the tubes are further rinsed in deionized water until a specified conductivity is met, then dried with hot nitrogen gas before packaging.
The most common techniques for analyzing electropolished surfaces are Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis). AES, which uses electrons generated near the surface to generate a specific signal for each element, gives an elemental profile as a function of depth. XPS uses soft X-rays that yield binding energy spectra which allow differentiation of the molecular species by oxidation number.
Surface Profile vs. Surface Appearance
Similar surface roughness values do not mean the surface appearance is the same. Most profilometers today can report a number of different surface roughness values, including Rq (also called rms), Ra, Rt (maximum difference in the minimum valley and maximum peak), Rz (average maximum height of profile) and several others. These expressions are derived through various calculations made from a single traverse of the surface with a diamond stylus. Within this traverse a portion is electronically selected, called the "cutoff," on which the calculations are based.
It is possible to use a combination of different calculated values, for example Ra and Rt, to better describe a surface, but there is no single function that can differentiate between two different surfaces with the same Ra value. The ASME publishes a standard, ASME B46.1, that defines the meaning of each calculated function.
For more information contact: John Tverberg, Trent Tube, 2015 Energy Dr., PO Box 77, East Troy, WI 53120. Tel: 262-642-8210.
Edited by Angelo DePalma
Go to Electropolished and Mechanically Polished Tubing, Part 2