To accurately compare S235J0 and DX51 in terms of their mechanical and chemical compositions, we would need to refer to the relevant international standards (e.g., EN, ASTM) or manufacturer datasheets. Without specific information about the standards or datasheets being referred to, it is not possible to provide an accurate comparison. However, in general, S235J0 is a structural steel grade and DX51 refers to a galvanized steel grade. They may have different mechanical and chemical properties, depending on the standards and specifications they adhere to.