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    Electrolytic polishing

    A method that is used in particular for the preparation of microscopy samples. For example, a sample of pure tin, a sample of single-phase alloy or a binary Sn-Pb alloy is used as the anode, placed in an appropriate etching solution and then current is generated between the anode and a suitable cathode. Depending on the voltage and current density, metal is separated from the surface from time to time, creating a surface that is ready for microscopy. The etching is, however, most intense at the edges of the sample. This method is applicable to certain alloys only because in samples with two or more phases, it is difficult to process all with the same removal rate which may lead to an incomplete polishing. If the structure consists of one or more phases, a completely uniform removal of the surface is impossible even when applying special electrolytes or specific conditions and samples may become too unpolished or too strongly etched. Even if two different phases contact each other in the electrolyte solution, a galvanic reaction occurs between them with difficulty and this reduces the chances of achieving a quality finish that would match mechanical polishing. Mechanically polished samples as well as samples polished electrically with success are then electrically etched. The sample is placed into the prepared etching solution and processed with a stronger current than during polishing. If only a certain phase of the alloy is of interest, electro-polishing is easier to monitor and quantitatively adjust than chemical etching. The former has the advantage of being able to separate crystal grains, grain boundaries, single phases, more layers or certain metallic components from the surrounding matrix. Furthermore, as opposed to mechanical polishing, it leaves no cracks on the polished surface. If large amounts of similar samples need regular examination, electropolishing is overall more effective than manual polishing and etching. Suitable polishing fluids and conditions for Sn alloys are listed in the following table. Three of the electrolyte fluids are chlorine-based and highly explosive at high temperature; extra caution is advised. Since two samples are seldom identical, electrolysis conditions must be adapted in each case.

    Electropolishing solutions for tin-based alloys
    Components Cathode Voltage (V) Current density A/dm2 Time Temperature (°C)
    Perchloric acid (194ml)
    Acetic anhydride (806ml)
    Sn 25-40 9-15 8-10 min. <25
    Perchloric acid (content 1,60) Al 50-60 40 10-15 s. <35
    Perchloric acid (63ml)
    Acetic acid (300ml)
    Water (12ml)
    Sn 20-30 9-15 10mMin. 25
    Fluoroboric acid (40ml)
    Sulfuric acid (2ml)
    Water (58ml)
    stainless steel 15-21 400-700 3-5 s. 20-45
    Electropolishing solutions for tin-based alloys