"The effects of rust on the carburization behavior of AISI 8620 steel have been experimentally investigated. AISI 8620 steel samples were subjected to a humid environment for time of 1 day to 30 days. After the exposure, a part of the samples was cleaned by acid cleaning. Both cleaned and non-cleaned samples have been carburized, followed by quenching in mineral oil, and then tempered. To determine the effect of rust on gas carburizing, weight gained by the parts and the surface hardness were measured. Surface carbon concentration was also measured using mass spectrometry. Carbon flux and mass transfer coefficient have been calculated. The results show that acid cleaning removes the rust layer effectively. Acid cleaned samples displayed the same response to carburization as clean parts. Rusted parts had a lower carbon uptake as well as lower surface carbon concentration. The surface hardness (Rc) did not show a significant difference between the heavily rusted sample and clean sample. It has been observed that the carbon flux and mass transfer coefficient are smaller due to rust layer for the heavily rusted samples. These results are discussed in terms of the effects of carbon mass transfer on the steel surface and the resulting mass transfer coefficient."