Astro Pak does not consider drying nor dry hold time necessary for a passivation process to be complete. The passive layer on stainless steel is not removed during a chemical passivation process as defined in ASTM A967-25 Section 3.2.3, therefore the passive layer does not need to be re-formed following the chemical process. Passivation, by definition, only removes surface contaminants on the stainless steel. The passivation chemistry Astro Pak utilizes does not have enough chemical potential to dissolve the chromium oxide that composes the passive layer, so no atoms from the base metal below the passive layer are dissolved either. This is supported by the fact that no measurable amount of chromium nor nickel is found in Astro Pak’s passivation waste stream. Because the passive layer is not removed nor reformed during the chemical process of passivation, no oxidation process, including drying, is necessary to ensure the passive layer exists. This is supported in Section 3.2.3.2 of ASTM A967-25, quoted below:
ASTM A967-25
“The formation of the protective passive metal oxide film on a stainless steel, also called passivation in a more general context, will occur spontaneously in air or other oxygen-containing environments when the stainless steel surface is free of oxide scale and exogenous matter... The presence of exogenous surface contamination, including dirt, grease, contaminant iron from contact with steel tooling, and so forth, may interfere with the formation of the passive metal oxide film. The cleaning of these contaminants from the stainless steel surface will facilitate the spontaneous passivation by allowing the oxygen uniform access to the surface. The passive metal oxide film may be augmented by chemical treatments that provide an oxidizing environment for the stainless steel surface, for example the removal of alloyed iron from the surface, or providing a rich oxygen source to accelerate the formation of the passive metal oxide film after iron has been removed from the surface...”
Section 3.2.3.2
The last sentence of the quote above mentions an oxidizing environment can augment the passive layer, this is what drying does. Extended drying can increase the thickness of the passive layer, it is not responsible for initially forming the passive layer. Increased passive layer thickness is beneficial, though not critical for stainless steel to have the corrosion resistance it is designed for. In fact, high purity passivation specifications prioritize the chromium enrichment level, as measured by the chromium-to-iron ratio, over the passive layer thickness. SEMI F19, the specification on surface finish requirements for stainless steel in semiconductor applications, exhibits this prioritization. The two highest quality grades defined in the document, High Purity and Ultra-High Purity, have the same passive layer thickness requirement of 15 Angstroms. The Ultra-High Purity Grade requires a minimum chromium-to-iron ratio of 1.5 whereas 1.0 is the minimum for High Purity components.
Astro Pak’s recommended substitute for extended drying is circulation with hydrogen peroxide or a peracetic acid sanitizer. These oxidizing chemicals increase the passive layer thickness, help reduce bioburden, and are performed far quicker than extended drying, so system downtime is reduced.
Further oxidation treatments or drying time following passivation increase the thickness of the passive layer, but they are not necessary to form the passive layer.