Type three rouge is iron oxide of the specific compound magnetite, Fe3O4. This product is formed at high temperatures and is most often found in steam environments. Type three rouge is the darkest colored iron corrosion product. It looks either black or blueish purple and typically has a glossy texture. Magnetite is a far more chemically stable compound than ferric oxide, the primary compound of type one and two rouge. For this reason, many users suggest that type three rouge can be considered a protective layer. Astro Pak does not share this sentiment and recommends removing type three rouge to prevent system contamination or failure.
Type three rouged stainless steel can be found with one of two surface characteristics, glossy or a powdery / matte finish. Both the glossy rouge and powdery rouge are magnetite, requiring strong chemicals to dissolve. The powdery rouge is a contamination concern because it can be picked up by the process fluid and migrate further into the system. System owners report that wiping off the powdery rouge reveals the glossy rouge underneath, which suggests type three rouge has both migratory (type one rouge) and in-situ grown (type two rouge) characteristics.
Although type three rouge is chemically stable, it is not passive. The glossy surface of type three rouge has a near-immeasurable rate of further corrosion, provided no discontinuities in the oxide exist. Should a discontinuity occur in the glossy rouge through thermal shock or mechanical action, catastrophic localized corrosion may occur. The even larger concern for glossy type three rouge is what it can hold. The microscopic surface of type three rouge is far rougher than the bare stainless steel the rouge originated on. Figure 1 is a micrograph of the clean and passive surface of stainless steel used in a clean steam system. Figures 2a and 2b show a surface from the same system after type three rouge has grown.
Figure 1. (a) Astro Pak received photo of a clean stainless steel surface at 500x. Roughness is generated from the metallic surface finish, not oxidation or impurities.
Figure 2. (a) Astro Pak received photo of type three rouge crystals on a stainless steel surface at 1000x and (b) 450x. Fig. 1b from John C. Tverberg, Rouging of Stainless Steel in High-Purity Water, Corrosion: Environments and Industries, Vol 13C, ASM Handbook
The increased roughness allows more contamination to settle on the surface. Contamination can include silicates or other impurities that make it through distillation during steam generation, plus bacterial growth when steam is not actively flowing. The microscopic rough pockets on the rouge surface provide shelter for bacteria to grow into a complete biofilm if enough downtime occurs. A biofilm, which is contamination in and of itself, will capture more contamination than the rouged and clean stainless surfaces.
It is true that a type three rouged system typically does not corrode further. However, this claim is based on the contingency that the rouge surface never breaks. And although the rouge layer has chemical stability, its likelihood to capture, grow, and migrate contaminants is significantly higher than the bare stainless surface. Astro Pak does not recommend leaving type three rouge alone due to the increased concern for depositing contaminants into process streams.