Which surface materials such as Hastelloy are recommended for corrosive chemicals in powder mixers?
The appropriate choice of material depends above all on the medium, concentration, temperature, halide content, pH value and mechanical load. For powder mixers handling corrosive chemicals, stainless steel, higher-alloyed steels, nickel-based alloys or, in special cases, linings and coatings come into consideration depending on the application.
Suitable material groups
Austenitic stainless steels
For moderately corrosive media, 1.4301 or better 1.4404 or 1.4435 are frequently used. Materials similar to 316L are suitable for many organic acids, dilute mineral acids and slightly chloride-containing media, but reach their limits with strongly chloride-containing or very aggressive chemicals.
Duplex and superduplex steels
Where higher mechanical strength is required alongside corrosion resistance, duplex materials can be sensible. They often offer better resistance to stress corrosion cracking and pitting than standard stainless steels, particularly in chloride-containing or slightly acidic media.
Nickel-based alloys
For strongly aggressive media, nickel-based alloys are often the most robust choice. Hastelloy C-276 or Hastelloy-C22, for example, are frequently used where high resistance to oxidising and reducing chemicals, chlorides, acids or changing media conditions is required.
Titanium, zirconium and tantalum
These materials are used with particularly critical media, for instance with very aggressive acids, high temperatures or special purity requirements. They are technically very capable, but markedly more expensive and more demanding to fabricate.
Coatings and linings
Where solid metal solutions are not economical or not sufficiently resistant, linings or coatings can be sensible. Typical options are fluoropolymer systems such as PTFE, PFA or ECTFE, enamel coatings and technical ceramics.
These solutions are often very resistant chemically, but must be checked for abrasion, temperature cycling, mechanical load and repairability.
Surface treatment matters
Besides the material itself, the surface quality is decisive. Low roughness, clean weld seams, electropolishing and proper passivation improve corrosion resistance and facilitate cleaning.
A practical selection rule
- For moderately corrosive media, 1.4404 or 1.4435 are often sufficient.
- For chloride-containing or markedly more aggressive chemicals, duplex, superduplex or nickel-based alloys are usually better.
- For extreme conditions, titanium, zirconium, tantalum or coated or lined systems may be necessary.
The final selection should always rest on an analysis of the media, resistance testing and consideration of the real process conditions.
How amixon® solves surface finish and material selection for hygienic powder processes
Surfaces: ground, low in adhesion, cleanable
At amixon®, the mixing chamber and mixing tool are welded free of crevices and ground smooth, so that product residues adhere as little as possible and no areas that are difficult to clean can form. The surface quality is specified according to the application – from industrially ground versions to highly polished surfaces for GMP-critical processes such as those in the pharmaceutical or infant formula industry. Since every amixon® apparatus is a one-off manufactured to the URS, the required Ra value is defined and documented project by project. The surface roughness can be selected project-specifically, typically with Ra values of 0.8 µm, 0.6 µm, 0.4 µm or 0.2 µm; beyond that, amixon® can lap to a mirror finish and electropolish.
Smooth, homogeneous surfaces markedly reduce build-up. Together with the fine addition of liquids, the mixing chamber remains as free of deposits as possible even during moistening processes.
A material range for different product chemistries
Depending on the application, amixon® processes various materials, including mild steel, wear-resistant steels, austenitic stainless steels, duplex stainless steels and highly corrosion-resistant nickel-based alloys such as Alloy 59 or Hastelloy-C22.
For strongly abrasive materials, hardened tools and suitable wear protection solutions are additionally available.
Constructional complement
At amixon®, surface quality always acts together with freedom from dead space. The mixing tool is supported only at the top and the product-contact areas have minimal dead space. Large inspection doors with a hygienic seal also facilitate access, cleaning and inspection. With free-flowing materials, residual discharge can be very high, which reduces the quantity of residues that have to be cleaned at all.
Manufacture in Paderborn as the quality foundation
amixon® develops and manufactures exclusively at the Paderborn works with a high depth of manufacture and uses components from Germany exclusively. As a certified welding company with European, Japanese, Korean and American qualifications, amixon® designs every apparatus as a one-off on the basis of the operator's URS. Quality control remains entirely in-house without gaps and every specification is verifiable down to component level.
During the manufacturing process, amixon® can carry out Cabot tests on clad components. Two identical clad plates – Hastelloy C22 or Alloy 59, each 6 mm thick, explosion-clad or roll-clad onto a 25 mm carbon steel boiler plate – are butt-welded after the weld preparation has been milled. Welding is carried out with low heat input and many individual passes so that the noble and the base material mix metallurgically as little as possible. A sample is cut out of the weld and milled down to three millimetres from the base-material side. This sample is then heated to varying degrees in an acid bath over extended periods; the test objective is that it does not corrode.
This manufacturing autonomy also secures long-term supply, since every component can still be re-manufactured reproducibly decades later.
Qualifiable and documented
For regulated environments, amixon® supplies plants ready for qualification: every apparatus is a one-off manufactured in-house to the URS with quality control without gaps. amixon® assists on request with DQ, IQ and OQ. Documentation and execution follow the GMP and hygiene standards required in each case. On request, the mixers are designed for requirements conforming to EHEDG, FDA hygiene guidelines, 3-A Sanitary Standards, USDA, GMP, ATEX or ASME and are integrated into the operator's validation concept.
Verification in the customer's process environment
In advance, amixon® examines together with the customer whether the specific formulation delivers the expected results. For this purpose, welded test pieces of the materials under consideration are installed in the customer's process plant. The trials take place under real conditions. Metallurgical examinations of the test components form a robust basis for decision-making before the investment.