This two-part article summarizes the key points of the article on electropolishing and previews Tverberg’s presentation at InterPhex later this month. Today, in Part 1, we will discuss the importance of electropolishing stainless steel pipes, electropolishing techniques, and analytical methods. In the second part, we present the latest research on passivated mechanically polished stainless steel pipes.
Part 1: Electropolished Stainless Steel Tubes The pharmaceutical and semiconductor industries need a large number of electropolished stainless steel tubes. In both cases, 316L stainless steel is the preferred alloy. Stainless steel alloys with 6% molybdenum are sometimes used; alloys C-22 and C-276 are important to semiconductor manufacturers, especially when gaseous hydrochloric acid is used as an etchant.
Easily characterize surface defects that would otherwise be masked in the maze of surface anomalies found in more common materials.
The chemical inertness of the passivating layer is due to the fact that both chromium and iron are in the 3+ oxidation state, and are not zerovalent metals. Mechanically polished surfaces retained a high content of free iron in the film even after prolonged thermal passivation with nitric acid. This factor alone gives electropolished surfaces a great advantage in terms of long-term stability.
Another important 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 main alloying composition with little loss of other alloying elements, while electropolished surfaces contain mostly only chromium and iron.
Making electropolished pipes To get a smooth electropolished surface, you need to start with a smooth surface. This means that we start with very high quality steel, manufactured for optimum weldability. Control is necessary when melting sulfur, silicon, manganese and deoxidizing elements such as aluminum, titanium, calcium, magnesium and delta ferrite. The strip must be heat treated to dissolve any secondary phases that may be formed during melt solidification or formed during high temperature processing.
In addition, the type of stripe finish is the most important. ASTM A-480 lists three commercially available cold strip surface finishes: 2D (air annealed, pickled, and blunt rolled), 2B (air annealed, roll pickled, and roll polished), and 2BA (bright annealed and shield polished). atmosphere). rolls).
Profiling, welding and bead adjustment must be carefully controlled to obtain the most round tube possible. After polishing, even the slightest undercut of the weld or a flat line of the bead will be visible. In addition, after electropolishing, traces of rolling, rolling patterns of welds and any mechanical damage to the surface will be obvious.
After heat treatment, the inner diameter of the pipe must be mechanically polished to eliminate surface defects formed during the formation of the strip and pipe. At this stage, the choice of stripe finish becomes critical. If the fold is too deep, more metal must be removed from the surface of the inner diameter of the tube to obtain a smooth tube. If the roughness is shallow or absent, less metal needs to be removed. The best electropolished finish, typically in the 5 micro-inch range or smoother, is obtained by longitudinal band polishing of the tubes. This type of polishing removes most of the metal from the surface, typically in the 0.001 inch range, thereby removing grain boundaries, surface imperfections, and formed defects. Whirling polishing removes less material, creates a “cloudy” surface, and typically produces a higher Ra (average surface roughness) in the 10–15 microinch range.
Electropolishing Electropolishing is just a reverse coating. An electropolishing solution is pumped over the inner diameter of the tube while the cathode is drawn through the tube. The metal is preferably removed from the highest points on the surface. The process “hopes” to galvanize the cathode with metal that dissolves from inside the tube (i.e., the anode). It is important to control the electrochemistry to prevent cathodic coating and to maintain the correct valency for each ion.
During electropolishing, oxygen is formed on the surface of the anode or stainless steel, and hydrogen is formed on the surface of the cathode. Oxygen is a key ingredient in creating the special properties of electropolished surfaces, both to increase the depth of the passivation layer and to create a true passivation layer.
Electropolishing takes place under the so-called “Jacquet” layer, which is a polymerized nickel sulfite. Anything that interferes with the formation of the Jacquet layer will result in a defective electropolished surface. This is usually an ion, such as chloride or nitrate, which prevents the formation of nickel sulfite. Other interfering substances are silicone oils, greases, waxes and other long chain hydrocarbons.
After electropolishing, the tubes were washed with water and additionally passivated in hot nitric acid. This additional passivation is necessary to remove any residual nickel sulfite and to improve the surface chromium to iron ratio. Subsequent passivated tubes were washed with process water, placed in hot deionized water, dried and packaged. If clean room packaging is required, the tubing is additionally rinsed in deionized water until the specified conductivity is reached, then dried with hot nitrogen before packaging.
The most common methods for analyzing electropolished surfaces are Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) (also known as chemical analysis electron spectroscopy). AES uses electrons generated near the surface to generate a specific signal for each element, which gives a distribution of elements with depth. XPS uses soft X-rays that create binding spectra, allowing molecular species to be distinguished by oxidation state.
A surface roughness value with a surface profile similar to the surface appearance does not mean the same surface appearance. Most modern profilers can report many different surface roughness values, including Rq (also known as RMS), Ra, Rt (maximum difference between minimum trough and maximum peak), Rz (average maximum profile height), and several other values. These expressions were obtained as a result of various calculations using a single pass around the surface with a diamond pen. In this bypass, a part called “cutoff” is electronically selected and calculations are based on this part.
Surfaces can be better described using combinations of different design values such as Ra and Rt, but there is no single function that can distinguish between two different surfaces with the same Ra value. ASME publishes the ASME B46.1 standard, which defines the meaning of each calculation function.
For more information contact: John Tverberg, Trent Tube, 2015 Energy Dr., PO Box 77, East Troy, WI 53120. Phone: 262-642-8210.