The changes in surface composition of stainless steel electrolytic tubes before and after electropolishing are key to understanding the process's effectiveness and improving material properties. Electropolishing selectively dissolves surface asperities through electrochemical action, achieving a smooth and bright surface. This process is accompanied by significant adjustments in surface composition.
Before polishing, the surface of stainless steel electrolytic tubes may contain residual oil, oxides, and minor defects from machining. These components not only affect appearance but also reduce corrosion resistance. For example, metal debris from machining, residual cutting fluid, and the oxide layer formed by long-term exposure to air all alter the surface chemical composition. The surface may contain a high proportion of iron oxides and adsorbed impurities, resulting in an uneven distribution of composition.
During the electropolishing process, components such as phosphoric acid and sulfuric acid in the electrolyte react with the tube surface. Phosphoric acid dissolves the metal while forming a protective phosphate film, inhibiting excessive corrosion. Sulfuric acid increases the solution's conductivity and promotes anodic dissolution. Due to the high current density, surface asperities are preferentially dissolved, allowing metal ions such as iron and chromium to enter the solution. Chromium, a key element for stainless steel's corrosion resistance, may temporarily decrease its surface content due to selective dissolution, but subsequent processing will redistribute it.
As polishing progresses, the surface composition gradually becomes more uniform. Strong oxidants such as chromic anhydride in the electrolyte promote the formation of a passive film on the surface, primarily composed of chromium oxide. This film not only fills microscopic defects but also enhances corrosion resistance by enriching chromium. Simultaneously, additives such as glycerol form complexes with phosphoric acid, further refining the surface grains and achieving a more uniform composition distribution. At this stage, the surface iron content decreases relatively while the chromium content increases, forming an oxide film structure dominated by chromium and supplemented by iron.
The compositional changes on the surface of polished stainless steel electrolytic tubes are primarily reflected in the composition and structure of the oxide film. The proportion of chromium oxide in the oxide film increases significantly, forming a dense Cr₂O₃ layer that effectively isolates the corrosive medium. Small amounts of electrolyte components, such as phosphates, may remain on the surface, but these can be removed through subsequent water washing and passivation. Ultimately, the surface composition is primarily chromium oxide, supplemented by small amounts of iron oxide and residual additives, forming a stable, corrosion-resistant chemical structure.
Changes in surface composition directly impact the performance of stainless steel electrolytic tubes. Chromium enrichment and the formation of an oxide film significantly improve the tube's corrosion resistance, particularly in environments containing chloride ions. Furthermore, the uniform composition distribution reduces the risk of microscopic galvanic corrosion and extends service life. Furthermore, the smooth surface reduces contaminant adhesion, further reducing the corrosion rate. These changes make electropolished stainless steel electrolytic tubes more suitable for use in hygiene-critical applications such as medical and food processing.
From a process perspective, electropolishing optimizes surface quality through composition adjustment. Selective dissolution of micro-asperities, chromium enrichment, and the formation of a passive film all rely on precise control of electrolyte composition and process parameters. For example, the ratio of phosphoric acid to sulfuric acid influences the dissolution rate and film quality, while the chromic anhydride concentration determines the effectiveness of the passive film. By adjusting these parameters, customized control of surface composition and performance is possible.
The surface composition changes of stainless steel electrolytic tubes before and after electropolishing are the result of a combination of electrochemistry and materials science. From initial residual impurities and uneven composition to chromium enrichment and oxide film formation after polishing, this process not only improves appearance but also significantly enhances corrosion resistance and service life. Understanding these changes is crucial for optimizing the electropolishing process and improving the performance of stainless steel electrolytic tubes.
