AISI 316 stainless steel, especially in its low carbon version (L), is widely used in the food, pharmaceutical, bioprocessing and personal care product industries for machine parts that are in contact with the product, raw materials or product intermediates during manufacturing due to its excellent mechanical properties, corrosion resistance, machinability and weldability [1]. The surface properties of these machine parts are of critical importance for its functionality and safety and must comply with specific standards, such as ASME BPE [2].
The finishing process of the machine parts, which constitutes the final phase of their manufacture, must provide continuous surfaces with low roughness and without alteration of the microstructural properties of the material, to avoid stagnant zones that favor the formation of bacterial colonies and/or corrosion issues. The finishing process most commonly used in the industry is mechanical polishing with fine abrasives. However, mechanical polishing is difficult to implement on parts with complex geometry.
Electropolishing is another applicable finishing process [3, 4, 5, 6]. This is based on the principle of the electrolysis, due to the action of a current flow between a cathode (auxiliary metal plate) and an anode (the piece to be polished), within a solution used as an electrolyte. Treatment times are relatively short (between 2 and 30 minutes). In principle, it can be applied to pieces of any shape or size, including structurally weak ones, since there is no contact or stresses on them [7]. Surfaces with a uniform surface texture can be obtained, free of residual stresses of mechanical or thermal nature, easy to clean and with low roughness [8]. With respect to mechanical polishing, it presents several advantages, due to the substantially less time and labor input, the high repeatability in the surface characteristics and the absence of scratches and incrustations of abrasive particles.
Many theories try to explain the electropolishing process such as Jacquet, Elmore, Edwards and Hoar theories among others [7]. However, due to the process is governed by different mechanisms and depends on many variables, no single theory is capable of covering all their particularities. At the same time, these theories are considered of key importance for a better understanding of the process. The main factors that influence the electropolishing process are electrolyte composition and temperature, cathode-anode gap, bath agitation (or workpiece rotation), initial roughness, applied voltage (or current density) and polishing time [9].
The electrolyte composition is highly dependent on the material to be polished. Electrolytes normally used for stainless steel electropolishing are acids, such as sulfuric acid, phosphoric acid, perchloric acid or their mixtures in a certain solvent, such as water or alcohol [9, 10]. AISI 316L presents an active-passive-transpassive behavior in most of these electrolytes. Generally, the best electropolishing effect is achieved in the passive region [11]. However, for many austenitic stainless steels in phosphoric–sulphuric acids electrolytes, the best electropolishing results are achieved in the transpassive region, under oxygen evolution regime [12, 13]. Unfortunately, there is no one-fit-all parameter set for all electropolishing setups and the best conditions still has to be established experimentally.
For the best of the author’s knowledge, there is no information available regarding the influence of electropolishing process on present phases, hardness and waviness of AISI 316L nor about the influence of certain process conditions, such as cathode geometry on roughness evolution. On this basis, the aim of this work is to study the electropolishing of AISI 316L in an electrolyte composed of sulfuric acid, phosphoric acid and water and evaluate the influence of the electropolishing process parameters (voltage, temperature, time, cathode material, cathode geometry and cathode-anode distance). The effect of initial roughness and specimen geometry is also evaluated. Once the best process conditions have been determined, the influence of electropolishing process on the surface characteristics (phases, hardness, topography, roughness and waviness) and corrosion behavior is evaluated.