Stainless electropolishing

After purification, 316L grade stainless steel is used. This is resistant to corrosion and is electropolished and passivated to reduce roughness, which may act as sites for bacterial growth and future corrosion. 

Tungsten was selected because of its superior strength and hardness relative to stainless steel and titanium. The tungsten pins are electropolished... 

Cass and Fraser (private communication) and Fraser et al. (1998) have used another technique, collection in an internally electropolished stainless steel canister, to... 

Gas Samples for Subsequent Laboratory Analysis. Collection of air samples for later analysis in the laboratory is a common technique used for aircraft sampling. Whole air sampling for stable compounds (CO, CO , ha-locarbons, and low-molecular-weight hydrocarbons) is usually accomplished by filling a container to a pressure of about 2 atm (203 kPa) with a metal bellows pump. Alternatively, containers may be evacuated in the laboratory to a low pressure and filled during flight by simply opening a valve at the appropriate time. Containers are typically constructed of stainless steel that has been electropolished or treated in some way to reduce surface activity... 

The finish on metallic materials such as stainless steel, whether it be a refined mill finish, polished to a specific grit, or an electropolished treatment, should... 

As explained previously, electrodissolution in ionic liquids is a simple and efficient process, particularly in chloride-based eutectics. Type III eutectics based on hydrogen bond donors are particularly suitable for this purpose. However, it has been noted that the polishing process only occurs in very specific liquids and even structurally related compounds are often not effective. It has been shown that 316 series stainless steels can be electropolished in choline chloride ethylene glycol eutectics [19] and extensive electrochemical studies have been carried out. The dissolution process in aqueous solutions has been described by two main models the duplex salt model, which describes a compact and porous layer at the iron surface [20], and an adsorbate-acceptor mechanism, which looks at the role of adsorbed metallic species and the transport of the acceptor which solubilises... 

Similar electropolishing experiments were carried out using different grades of stainless steel (410, 302, 304, 316 or 347) and it was found that the mechanism of metal dissolution and the oxidation potentials for the metals were very similar. The slight exception was the 410 series steel (which has no Ni, unlike the 300 series steels which have 8-14%). The 410 steel required a more positive oxidation potential to break down the oxide in the ionic liquid whereas once the oxide was removed the... 

Fig. 11.5 AFM image of a 316 stainless steel sample in which one side has been electropolished while the other has been masked with lacquer.

This technology was scaled-up to a 1.3 tonne plant by Anopol Ltd (Birmingham, UK). Results have shown that the technology can be applied in a similar manner to the existing technology. The ionic liquid has been found to be compatible with most of the materials used in current electropolishing equipment, i.e. polypropylene, nylon tank and fittings, stainless steel cathode sheets and a titanium anode jig. 

Water is completely miscible with the spent ionic liquid but the resulting mixture leads to a completely transparent liquid and almost all the metal complex is precipitated to the base of the cell. The water can be distilled from the mixture to leave a dry ionic liquid which has lost only ca. 15% ethylene glycol, mostly in the form of the metal complex. The residual concentration of each metal in the ionic liquids was less than 5 ppm. Hence, not only has it been demonstrated that electropolishing can be carried out in this non-corrosive liquid, but also that the liquid can be completely recycled and all of the metal can be recovered. Figure 11.6 shows a variety of stainless steel pieces electropolished using the choline-based ionic liquid. 

The only large scale tests that have been carried out using ionic liquids were in collaboration between Anopol Ltd and the University of Leicester for the electropolishing of stainless steel. Figure 11.14 shows a 1.3 m3 tank that was constructed from polypropylene with polypropylene, nylon and polyethylene fittings and run as a pilot plant. It has a standard 3 kW heater to maintain the liquid at 50 to 60 °C. Tank agitation was achieved by recirculation of electrolyte via eight banks of inductor nozzles [68],... 

Fig. 11.16 Recycling of ionic liquid (1 ChCl 2 ethylene glycol) used to electropolish stainless steel (a) used liquid containing Fe Cr and Ni salts, (b) as (a) with 1 equiv. v/v added water, (c) as (b), after gravity filtration and subsequent removal of residual water by distillation.

In addition to electrodeposition, ionic liquids and DESs can be used in electropolishing, which aims to remove the roughness from metallic surfaces to increase optical reflectivity for high-tech applications. For example, a eutectic mixture of ethylene glycol and choline chloride has been used in the electropolishing of various stainless steel alloys. This method is preferable to current industrial procedures that use a corrosive mixture of phosphoric and sulfuric adds. 

To ensure the cleanliness of the ceramic membrane gas filters, the housing typically made of stainless steel should have electropolished internal wetted surfaces and welded parts free of flux materials. In addition, considerations need to be given to the materials of construction for housing (e.g., stainless steel) and seals. 

Fig. 2 Before electropolishing the mechanically polished surface of stainless steel appears rough as viewed under lOOOx magnification on scanning electron microscope.

Air Sample Collection. Air samples are collected in stainless steel (SS-314) bottles that are internally electropolished by using the Summa process. Bottles with internal volumes of 800 mL, 1.6 L, and 35 L are used. The bottles are fitted with a purge T and Nupro SS-4H4 bellows valves. A typical sample bottle and purge T are shown in Figure 1. The bottles are cleaned by heating to 150 °C under high vacuum (20 mtorr) for several hours and then filled with zero air (AADCO 737A) to prevent contamination until used in the field. 

Figure 1. Stainless steel bottle (1.6 L) internally electropolished by Summa process, fitted with purge T and two SS-4H4 Nupro valves, and used for cryogenic air sample collections.

Mott and Bott illustrated the effect of different materials on the accumulation of Pseudomonas fluor-escens biofilms on the inside of tubes under identical operating conditions (see Fig. 9). The differences between the effects of the materials occur for two reasons roughness and surface electrical properties. The quality of the surface, in terms of roughness, on which microorganisms attach, can affect the biofilm accumulation as discussed earlier. The effect of roughness is illustrated in Fig. 9 by the difference of biofilm accumulation between electropolished and as received 316 stainless steel. The rougher stainless steel is seen to be more hospitable to biofilm growth. 

Most HPLC columns are made of 316 grade stainless steel, which is austenitic chromium-nickel-molybdenum steel, USA standard AISI, resistant to the usual HPLC pressure and also relatively inert to chemical corrosion (chloride ions and lithium ions at low pH being important exceptions). The inside of the column should have no rough surfaces, grooves or microporous structures, so the steel mbes must be either precision drilled or polished or electropolished after common manufacturing, e.g. by drawing. 

Vessels (316L Stainless Steel or Hastelloy, electropolished)... 

In the range of electrode potential more positive (more anodic) than the pitting potential, the pitting corrosion occurs in the presence of chloride ions and the metal dissolution at a pit, initially hemispherical, proceeds through the mode of electropolishing, in which concentrated chloride salts in an occluded pit solution will control the pit dissolution. It is likely that the polishing mode of metal dissolution proceeds in the presence of a metal salt layer on the pit surface in the salt-saturated pit solution. It was experimentally found with stainless steels in acid solution [54] that the pit dissolution current density, pit, is an exponential function of the electrode potential, E (Tafel equation) ... 

Nickel in perchloric acid solution [Fig. 10(a)] has three peaks at about E = — 425 mV, E = - 200 mV, and E = - 50 mV. Similar peaks are observed on an electropolished nickel electrode [Fig. 10(b)]. The nature of the solution phase plays a role although, in chloride-containing solution, two peaks appear at E = -425 mV and E = -275 mV, but at more positive potentials a fall in capacity occurs, indicating probable formation of a layer on the electrode. On the other hand, chromium in perchloric acid solution has a quite different peak structure with a number of peaks [Fig. 10(d)]. Iron in the same solution has a single peak at about E = — 475 mV and active dissolution starts at E = — 400 mV. The stainless steel, on the other hand, seems to show features of nickel and chromium. At potentials from E = — 700 mV to E = 0 mV, the curve is similar to that of nickel while at more positive potentials, it is similar to chromium. 

Fig. 10. Double layer capacity-potential curves for different metal rotating-disc electrodes (45 Hz rotation speed) in the given acid solutions, (a) Nickel in 1M HC104 (b) electropolished nickel in 1M HC104 (c) nickel in lM HC1 (d) chromium in 1M HC104 (e) iron in 1M HC104 and (f) stainless steel (304 L) in 1M HC104.

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