It is known that some kind of inclusions account for pitting formation in steel tube. Therefore the identification of this sort of nonmetallic inclusions in 13CrV steel was the main idea of this work. The new method for estimation dirty steel was introduced. It involves using scanning electron microscopy and software for detecting dimensions, chemical composition and location every inclusion on the investigated surface. Microstructure of certain inclusions was investigated on nonmetallic inclusion cross-section made cutting ion beam of scanning electron microscope AURIGA. It was found that in nonmetallic inclusions of this steel were two-phase. The first phase is MgO·Al2O3 spinel inclusion, the second phase is calcium and manganese sulphide. These two parts of one inclusion bear against. This sort of inclusions influences on corrosion processes in steel. It was established that sulphide part of inclusion was thermodynamically unstable in environment with chlorine ion. Therefore calcium and manganese sulphide dissolves and forms pitting.