To verify the chemistry of secondary side of CANDU steam generators, Millipore filters are used to sampling from condensing extraction pump, feed water header and blow down of steam generator. These filters retain the corrosion products as very fine particles and are used as samples in chemistry water control. X-Ray diffraction (XRD) is the most used technique that can provide information about the phase composition of analyzed samples. Because the crud layers on the Millipore filters are very thin, to identify the corrosion products Grazing Incidence X-Ray diffraction (GIXRD) technique was used. The following compounds have identified: magnetite (Fe3O4), hematite (Fe2O3), and iron oxide hydroxide (FeOOH). It was observed a brown-reddish background specific to hematite and iron oxide hydroxide, especially for filters extracted from condensing extraction pump. The black colour of crud present on filters extracted from feed water header and blow down of steam generator shows the presence of magnetite.