Sanitary pump materials and surface roughness: AISI 316L, Ra and polishing

Parts in direct contact with the fluid (pump casing, impeller, cover, shaft) in sanitary pumps are typically manufactured from austenitic stainless steel AISI 316L. Compared with 304, grade 316L adds approximately 2–3% molybdenum, which improves pitting corrosion resistance against chloride ions found in rinse water, brine solutions, dairy products, or CIP fluids. The suffix “L” (low carbon) denotes a reduced carbon content that limits grain boundary sensitization after welding, making welds less susceptible to localized corrosion — an important factor since sanitary pumps contain numerous hygienic welds.

For fluids with higher corrosivity or elevated chloride concentrations, some applications consider higher-grade alloys such as duplex; however, for the majority of food & beverage, pharmaceutical, and cosmetics process lines, 316L meets both hygiene and durability requirements. One important point: material grade is only half the story — even with 316L, a rough surface will still accumulate deposits and microorganisms, so surface roughness must be controlled in parallel.

Ra (roughness average) is the arithmetic mean surface roughness, measured in micrometres (µm). The lower the Ra value, the smoother the surface. On rough surfaces, microscopic pits become hiding places for product residues and microorganisms, shielding them from the mechanical and chemical action of cleaning solutions. Reducing the Ra of contact surfaces is therefore a direct way to lower the risk of biofilm formation and to make CIP more effective.

In hygienic practice, Ra ≤ 0.8 µm is commonly cited as the reference threshold for product-contact surfaces; certain pharmaceutical or aseptic applications may require an even lower Ra with additional electropolishing treatment. It should be noted that specific Ra values depend on the design documentation and certification level of each pump range — when selecting equipment, always compare the process line’s requirements against the surface parameters published by the manufacturer.

Ra values in the table are indicative reference figures; official parameters follow the design documentation of each pump range.

Mechanical polishing uses cloth wheels, abrasive belts, and polishing compounds with progressively finer grit to flatten machining marks and bring the Ra of contact surfaces down to hygienic levels. This is the common approach for pump casings and impellers in sanitary centrifugal pumps. Electropolishing dissolves an extremely thin layer of metal in an electrolytic solution, preferentially removing microscopic surface peaks so the surface becomes smoother; it also enriches the chromium-oxide passive layer, enhancing corrosion resistance.

In sanitary pumps, both techniques are often combined: mechanical polishing creates the base finish, followed by electropolishing for surfaces requiring a higher standard, such as aseptic applications. Smoothing is also applied to welds and corner radii — locations that easily retain product if left with sharp edges. A smooth surface also slightly reduces hydraulic friction and eliminates surface defects where microorganisms could attach, but uniform treatment across the entire wetted path is necessary for the hygienic benefit to be realized.

316L material and smooth surfaces must be accompanied by non-metallic components and hygienic architecture. Product-contact seals use food-grade elastomers such as EPDM, FKM, or silicone, selected according to operating temperature, CIP/SIP chemicals, and fluid type. Hygienic design aims for crevice-free and self-draining principles: no fluid pockets, rounded internal corners, and pipework sloped toward drain points so the pump retains no product between batches.

Only when all three layers — material, surface finish, and design — are controlled in a coordinated manner can a pump be cleaned in place (CIP) without disassembly; and for pharmaceutical applications, it can be sterilized in place with steam (SIP) or configured for aseptic duty.

CSF Inox (Italy) manufactures sanitary pumps for food & beverage, pharmaceutical, and cosmetics applications, with wetted parts made from AISI 316L, polished product-contact surfaces to reduce roughness, and quick-release hygienic fittings. The flagship sanitary centrifugal range CS/CSK/CSD features polished surfaces, flow rates up to 500 m³/h, heads up to 100 m, and pressures up to 10 bar across 29 sizes; the CSK variant is the aseptic version for more stringent hygiene requirements, while the CSD is a related variant in the same family.

For fluids containing solids or fibres, the CR range uses an open impeller and large inlet ports (up to 150 m³/h). Applications requiring certification can reference the CSA (EHEDG/3-A certified, up to 300 m³/h) or the self-priming pump ASH (3-A) when air-entraining suction is needed, such as during CIP return. When evaluating materials and surface roughness, the key reference points are the surface parameters and certification level of each range versus the process line’s requirements — not simply the steel grade.