Polishing Levels of Metallic Chemical Reactors

Polishing is a critical process for metallic chemical reactors, especially those used in pharmaceutical, food, beverage, and high-purity chemical industries, because it directly affects surface roughness (Ra), which influences cleanability, corrosion resistance, and product contamination (e.g., microbial growth, product adhesion).

Here is a detailed description of the common polishing levels, often specified in terms of Ra in micro-inches min or micrometers mm.

Polishing levels are generally categorised into mechanical (abrasive) and electrolytic (electropolishing) finishes.

1. Mechanical Polishes (Abrasive Finishes)

These finishes are achieved by using abrasive materials like sandpaper, belts, or wheels, starting with coarser grits and progressively moving to finer grits.

2. Electropolishing (EP)

Electropolishing is an electrochemical process that removes a microscopically thin layer of the metal surface, resulting in a smooth, featureless, passive, and ultra-clean finish. It is typically performed after a mechanical polish (e.g., 180 or 320 grit) to remove the microscopic "peaks" left by the abrasive process.

🔍 Key Considerations

• Surface Roughness (Ra): This is the arithmetic average of the absolute values of the profile deviations from the mean line. Lower Ra means a smoother surface. The required Ra is the main driver for the polishing cost.

• Directional vs. Non-directional: Mechanical finishes often have a directional grain (lines), which can sometimes trap product. Electropolishing produces a non-directional or featureless finish, which is better for cleanability.

• Passivation: After mechanical polishing and especially after electropolishing, the metal (typically stainless steel) surface must be passivated. This chemical treatment removes iron particles embedded during finishing and forms a stable, protective chromium oxide layer that prevents corrosion.