<p>A study is presented on the composition and properties of hydrogen sulfide and carbon dioxide corrosion products. X-ray diffraction and electron microscopy were used to evaluate the danger of internal corrosion processes. Differences were found between the structures of the iron sulfide deposits obtained upon hydrogen sulfide corrosion in aqueous and gas phases. We analyzed the effect of the composition of the operational media on the formation and protective properties of stoichiometric and non-stoichiometric siderite products of carbon dioxide corrosion. The deposits were obtained using new procedures for corrosion testing simulating the operational conditions at natural gas processing plants. Examples are given for the use of siderite crystal structure parameters found by analyzing the X-ray diffraction measurements to determine the extent of danger of localized steel corrosion.</p>