If both the level of chlorides and the temperature exceed amounts that are approximately double these values, the material will require greater resistance to the crevice and pitting corrosion that may result from chlorides concentrating locally. In this case, 316 stainless steel should be used to improve corrosion resistance; this grade is recommended for components such as valves, pump casings, rotors, and shafts, while its low-carbon equivalent AISI-316L is recommended for pipework and vessels due to its enhanced weldability. Alternatively, titanium may be appropriate.
As temperatures approach 150ºC, even 316 stainless steels may suffer from stress-corrosion cracking where regions of high stress are exposed to high levels of chloride. If this is the case, 410, 409, and 329 stainless steel, or even Incoloy 825, may be required for their high strength and/or high corrosion resistance, although they may be more costly.
Surface Texture and Finish
Any surface that is textured, polished, or ground should have a finish that is free of cracks and crevices. Both 3-A SSI and EHEDG have adopted an industry-recognized method for determining an acceptable food contact surface, the roughness average or Ra value. The Ra value is found using a profilometer with a diamond-tipped stylus to measure peaks and valleys in a surface. According to 3-A SSI, ground or polished stainless steel must meet a No. 4 ground surface, while unpolished surfaces must meet a No. 2B or mill finish.
EHEDG also has guidelines for surface finish:
- “Large areas of product contact surface should have a surface finish of 0.8 µm Ra [32 µinch Ra], or better, although the cleanability strongly depends on the applied surface finishing technology, as this can affect the surface topography.
- “It should be noted that cold-rolled steel has a roughness of Ra = 0.2 to 0.5 µm [8 to 20 µinch Ra] and therefore usually does not need to be polished in order to meet surface roughness requirements, provided the product contact surfaces are free from pits, folds and crevices when in the final fabricated form.
- “A roughness of Ra >0.8 µm is acceptable if test results have shown that the required cleanability is achieved because of other design features or procedures such as a high flow rate of the cleaning agent. Specifically, in the case of polymeric surfaces, the hydrophobicity, wettability and reactivity may enhance cleanability.”
It is also important to remember that food equipment should be designed and fabricated in such a way that all food contact surfaces are free of sharp corners and crevices. All mating surfaces must also be continuous (e.g., substantially flush). Construction of all food handling or processing equipment should allow for easy disassembly for cleaning and inspection. Where appropriate, equipment such as vessels, chambers, and tanks should be self-draining and pitched to a drainable port with no potential holdup of food materials or solutions.
Another consideration is internal angles. As 3-A SSI sanitary standards state, “all internal angles 135° or less should have a minimum radii of 1/4 inch (6.35mm).” The standards allow for smaller radii where needed for function, within certain specifications.
EHEDG guidelines stipulate a preference for corners with a radius equal to or larger than 6 mm; the minimum radius is 3 mm. Sharp, 90° corners must be avoided. If used as a sealing point, however, corners must be as sharp as possible to form a tight seal at the point closest to the product/seal interface. In this situation, a small break edge or radius of 0.2 mm may be required to prevent damage to elastomeric seals during thermal cycling. If any of these criteria cannot be met due to technical and functional reasons, the loss of cleanability must be compensated for in some way, and the effectiveness of this compensation must be demonstrated by testing. In addition, both 3-A SSI and EHEDG require joints to be smooth, continuous, and butt type.
Perhaps the biggest difference between 3-A SSI and EHEDG is in the testing of process manufacturing equipment. For 3-A approval, the certified conformance evaluator uses personal knowledge and experience to conduct a detailed physical evaluation of the equipment, engineering drawings, and documentation associated with the equipment to be verified. This is very much “opinion engineering.”