On carbon steel dishwasher rack coating lines, Rilsan Fine Powders T GREY 49 AC2 PA11 is deposited after a seven-stage pretreatment sequence: alkaline degreasing, rinse, acid pickle, rinse, zinc phosphate, rinse, and forced-air drying. The steel rack is preheated in a gas-fired convection oven until the core reaches 270 °C to 320 °C. The fluidised bed of fine powder with a particle size D50 in the 30–50 µm range then forms a continuous sintered film of 300–450 µm on wire intersections and 200–300 µm on straight wire runs. The grey 49 AC2 colour is dispersed as a pigment concentrate; dispersion quality influences gloss under ISO 2813 and affects visual acceptance on visible front rails, but it does not define the corrosion barrier. The relevant end-use performance is resistance to alkaline dishwasher detergents at 60–75 °C, combined with impact resistance from loading of ceramic plates and cutlery.

Failure records from production lines show that edge coverage below 200 µm at welded wire intersections is the most frequent rejection mode. The defect appears as rust bleed within 800–1,000 h of intermittent exposure to 1.0% alkaline cleaning solution at 65 °C. The condition is detected by ASTM D610-08 rust grading and by cross-cut adhesion per ISO 2409. To prevent edge pullback, parts are rotated during extraction from the fluidised bed, and oven set points are raised by 10–15 °C when ambient relative humidity exceeds 60%. The PA11 film absorbs approximately 1.8–2.0% water at saturation according to ISO 62; this limited water uptake reduces dimensional plasticisation compared to PA6 and PA66, but prolonged exposure above 75 °C in strongly alkaline formulations still hydrolyses surface amide groups and reduces gloss retention. For dishwasher baskets sold in the EU, food-contact compliance is assessed under Regulation (EU) No 10/2011, and in the United States under FDA 21 CFR 177.1500 for nylon resins used in repeated-use articles. The final film is inspected under 2,000 lux illumination before dispatch.

What Happens to Impact Resistance After 2,000 Hours of Accelerated Salt Spray?

Accelerated corrosion testing of marine and offshore hardware coated with Rilsan Fine Powders T GREY 49 AC2 PA11 follows a two-stage procedure. Steel substrates are blast-cleaned to Sa 2½ per ISO 8501-1, then zinc phosphated to a coating weight of 1.5–3.0 g/m². The PA11 powder is applied by electrostatic spray or fluidised bed immersion and sintered into a final dry film thickness of 300–500 µm. Test coupons are scribed to base metal and exposed in a neutral salt spray chamber operated per ISO 9227:2022. After 2,000 h, the acceptance criterion for a C5-M marine environment under ISO 12944-6 is commonly scribe creep not exceeding 3 mm and no blistering greater than grade 2(S2) per ISO 4628-2. The sintered PA11 matrix delays cathodic delamination because its low water transmission reduces the electrolyte path to the zinc phosphate interface.

The practical conflict is the trade-off between impact flexibility and corrosion barrier thickness. Thin films below 250 µm retain high impact resistance under ISO 6272-1 but show early scribe creep on sharp edges. Films above 500 µm improve barrier properties but can crack when a dropped steel shackle imposes local strain rates above the yield point. The PA11 grade has a low glass transition temperature near 45 °C, which preserves flexibility at low service temperatures, but impact strength decreases when the film has been oxidatively aged. In qualification reports, the practical lower service temperature is often set at -40 °C, below which crystalline phase stiffening and thermal shock on coated valve flanges require case-specific testing. High-voltage holiday testing is performed at 5 kV using a direct current detector per NACE SP0188. The grey pigmented formulation is visually checked for cathodic disbondment after salt spray by removing the film mechanically and examining the zinc phosphate layer. Published data for this specific configuration is limited outside supplier validation reports, so the 2,000 h exposure should be supplemented by cyclic ageing per ISO 12944-9 for offshore atmospheric service.

Fluidized Bed Sintering Window for Zinc Phosphated Steel

In fluidised bed coating cells, the deposition rate and film consolidation of Rilsan Fine Powders T GREY 49 AC2 PA11 are governed by substrate peak metal temperature, immersion dwell, and bed air dew point. The polymer is a semicrystalline PA11 with a melting endotherm near 186 °C measured by ISO 11357-3. Sintering requires the substrate to remain above the crystalline melting point long enough for particle coalescence and air escape. Common set points range from 280 °C to 340 °C for thin-wall steel and from 320 °C to 380 °C for cast ductile iron. Bed immersion time varies from 2 s for low-mass wire goods to 15 s for heavy valve bodies. The fluidisation air must be dried to a dew point below -40 °C; higher moisture contents create powder clumping, uneven fluidisation, and surface defects described as orange peel. The extracted part is post-heated to complete levelling. Inadequate post-heat leaves residual particle boundaries visible under 20–40× microscopy after an acetone wipe per internal control.

Process parameters are adjusted by part thermal mass and not by colour; the grey 49 AC2 pigment has a negligible effect on melt viscosity compared with natural PA11, but pigment dispersion can shift gloss and opacity. The following table lists starting conditions from production-scale fluidised bed trials.

The main process defect is blistering from residual moisture or phosphate hydration. When parts exit the phosphate stage with incomplete drying, vapour expands through the melt and creates dome-shaped voids. This failure is prevented by maintaining the phosphate line final dry-off oven above 110 °C for at least 10 min. A second defect is edge pullback on sharp corners, caused by lower thermal mass at the edge; increasing preheat by 15–20 °C for high-edge-count parts reduces this condition without degrading the bulk film. The upper temperature limit for this PA11 grade is approximately 400 °C, beyond which thermal oxidation discolours the grey film and reduces tensile strength after ageing. Thermal oxidation is monitored by ISO 11357-6 oxidation induction time and by colour change measurement under ISO 7724.

Immersion service in chlorinated potable water subjects the Rilsan Fine Powders T GREY 49 AC2 PA11 film to simultaneous hydrolytic attack of the amide linkage and oxidative attack by free chlorine. The coating is used on butterfly valve bodies, pump impellers, pipe couplings, and actuator housings in water treatment and distribution systems. The PA11 film provides a barrier between the cast iron or steel substrate and treated water. The relevant test method is ISO 175:2010 for immersion in chemical fluids, supplemented by NSF/ANSI 61 for potable water contact where jurisdiction requires certification. The practical continuous service temperature for PA11 in treated water is typically limited to 60 °C. At 65–70 °C, hydrolysis of the polyamide backbone accelerates and film elongation measured by ISO 527-3 declines after prolonged exposure. Free chlorine above 1 mg/L at 60 °C leads to surface oxidation and microcracking; above 5 mg/L, published data for this specific configuration is limited and case-by-case immersion testing under ISO 2812-1:2017 is required.

Adhesion on cast iron is controlled by blast profile and phosphate conversion. A surface profile of 40–80 µm measured per ISO 8503-2 is generated before zinc phosphating. The sintered film must be continuous at the mating faces of the valve body to prevent crevice corrosion. In field inspections, the least durable area is the seating face of a butterfly valve, where repeated contact removes the coating and exposes the substrate. The PA11 grade is not recommended for continuous immersion in strong inorganic acids above 10% concentration or in polar solvents with solubility parameter close to the polymer; qualification for these media follows ISO 2812-1:2017.

When Substrate Peak Metal Temperature Drops Below 220 °C

For cast ductile iron components with heavy wall thickness, the peak metal temperature achieved during preheat can fall below the lower sintering limit of 220 °C. The condition occurs when non-contact infrared pyrometer readings are taken after the part has cooled during transfer from oven to fluidised bed. The PA11 particles deposit, but the latent heat is insufficient to drive full coalescence. The resulting film exhibits a rough, under-sintered structure with high void content. This condition is different from orange peel; it appears as a matte, particulate surface that can be scraped with a metal edge under low force. The film adhesion measured by pull-off per ISO 4624 drops below 5 MPa because the particle-particle boundary remains weak. The corrective action is to increase oven set point by 30–40 °C or to delay the transfer path so that the part reaches the bed at the required surface temperature. Heavy castings may require an intermediate infrared tunnel to restore surface temperature before immersion.

The lower limit is critical when coating components that combine a thick flange with a thin wall. The thin wall sinters correctly while the flange remains below the coalescence temperature. This produces localised under-sintering at the flange face and can be mistaken for adhesion loss. Thermal profiling with attached thermocouples rather than spot infrared readings is required to identify the cold zone. The minimum measured peak temperature should be recorded at the thickest section, not the average of the part. Where preheating above 220 °C is not possible due to substrate metallurgy, the powder process is not suitable; a solvent-borne polyamide coating may be evaluated instead. No crosslinking occurs in PA11 sintering, so under-heating cannot be corrected by extended post-heat at the same temperature.

Evaluating Disinfection Wipe Scars and Outgassing on Grey PA11 Hospital Coatings

Medical and laboratory furniture coated with Rilsan Fine Powders T GREY 49 AC2 PA11 must withstand repeated disinfection with quaternary ammonium compounds, hydrogen peroxide vapour, and alcohol-based wipes. The sintered PA11 surface resists stress cracking when exposed to 70% ethanol under ISO 2812-1 contact for 24 h, but prolonged contact with phenol-based disinfectants above 2% concentration can cause surface softening and gloss loss. The grey pigmented surface is inspected for colour change under ISO 7724 and for adhesion after 1,000 wipe cycles using a saturated disinfectant pad. Surface roughness is maintained below Ra 1.6 µm to reduce visual dirt retention while not becoming polished enough to produce specular contrast under surgical lighting.

Electrostatic spray deposition is preferred for hospital bed frames because it applies the powder in a thin, smooth layer of 150–250 µm over tubular steel. The grey 49 AC2 shade is formulated to hide scratches and sanitizer residues more effectively than light colours. In high-contact areas, a top layer may be applied without compromising adhesion; dry film thickness above 350 µm increases edge cracking risk when the frame is struck by mobile medical equipment. For surfaces exposed to hydrogen peroxide vapour, the PA11 film must be degassed after sintering to reduce residual monomer and volatile components; the part is post-cured at 120 °C for 30 min to reduce outgassing. The process is monitored by residual volatiles testing per internal specification. Not all PA11 fine powder grades meet the low-outgassing requirement; published data for this specific configuration is limited and qualification is performed on finished parts.