What is Ra ≤ 0.8 µm and why does it matter?

Ra is the arithmetic mean roughness (Average Roughness). Ra ≤ 0.8 µm means the tank's internal surface is smooth enough to prevent bacterial retention and product residue build-up. This standard is required by the food, sugar and pharmaceutical industries. The value is measured using a contact profilometer.

How often should a stainless steel tank be passivated?

Chemical passivation is mandatory after every welding repair. As a preventive measure (without repair), it is recommended every 3–5 years, or whenever signs of corrosion are detected. Some logistics operators require a passivation certificate annually.

Can a food-grade Feldbinder tank be repaired without a certified welder?

No. Any welding repair on a food-grade tank must be carried out by a welder certified to EN ISO 9606-1 for austenitic steel, with full process documentation (WPS qualification). Repair by an uncertified welder disqualifies the trailer from food-grade transport operations.

11.06.2023

Stainless Steel in Food-Grade Trailers: Cleaning and Ra

Name: PHS Magnum
Address: ul. Kościelna 9, Chorula, Opolskie, 47-316, PL
Telephone: +48 602 716 551
Price range: $$

DAF silo trailer for bulk material transport

Summary

Passivation of stainless steel in Feldbinder trailers – the Cr₂O₃ passive layer is destroyed by welding, iron contamination during grinding, and chlorides. Without it, the steel corrodes and cannot be approved for contact with foodstuffs. Passivation methods: 30% HNO₃ (large surfaces, 20–30 minutes) or Avesta 401 gel (welds and hard-to-reach areas). Required after every welding repair. Ra ≤ 0.8 µm measured by profilometer – the certificate is required by TDT and food-grade logistics operators (Südzucker, Nestlé, Tchibo). PHS Magnum carries out passivation and issues cleanliness and Ra protocols.

Stainless Steel in Feldbinder Food-Grade Trailers

Why stainless steel is not truly indestructible

Stainless steel is only “stainless” when its surface oxide passive layer (Cr₂O₃) is intact. That layer:

Forms naturally on exposure to atmospheric oxygen
Is DESTROYED during welding (high temperature)
Is DAMAGED by chlorides (road salt, sea water, certain detergents)
Is DAMAGED by abrasive cleaning with incorrect tools (carbon steel brushes instead of stainless steel)

Without the passive layer, stainless steel corrodes – not as quickly as carbon steel, but corrode it will.

Types of corrosion in stainless steel tanks

Crevice corrosion: Occurs in gaps and corners of the tank. Characteristic of poorly ventilated areas where liquid or product stagnates.

Prevention: correct weld geometry (no sharp internal corners), polishing to Ra ≤ 0.8 µm, regular cleaning.

Pitting corrosion: Localised pits initiated by chlorides or contamination. Visible as black or brown spots on the surface.

Prevention: avoiding chlorides, passivation, and not leaving water standing in the tank.

Weld sensitisation: During welding, chromium precipitates out of the steel matrix (sensitisation), forming chromium carbides in the heat-affected zone. The area around the weld loses its corrosion resistance.

Prevention: welding with “L”-grade alloys (304L, 316L – very low carbon content), rapid post-weld cooling, or solution annealing.

Passivation at PHS Magnum – the process

Tank cleaning: alkaline detergent (pH 10–12), rinse with deionised water
Visual inspection and surface condition assessment
Acid pickling (where required: HNO₃ + HF, or HNO₃ 30% alone) – removal of contamination and corrosion products
Passivation: 30% nitric acid for 30–60 minutes (or Avesta 401 passivant for 15–30 minutes)
Rinse with deionised water at pH 6–8 (rinse-water pH verified)
Ra measurement by profilometer at multiple points on the tank base and side wall
Passivation protocol: date, material, chemicals used, Ra results, technician signature