In the present study, the High Score Plus software (Malvern Panalytical, 2014), combined with the PDF-4+ database release 2023 (ICDD, 2018), was used to perform phase identification from all the powder XRD data sets of 0.5 g by weight of the crystalline deposits from various units of refineries and gas plants. Subsequently, the Rietveld method with the generalized spherical harmonic description for preferred orientation correction [Von Dreele (1997). Journal of Applied Crystallography 30: 517–25; Sitepu (2002). Journal of Applied Crystallography 35: 274–77; Sitepu et al., (2005). Journal of Applied Crystallography 38: 158–67; Sitepu (2009). Powder Diffraction 24: 315–26] were used to determine texture and crystal structure refinement of scale deposits (calcite – CaCO3) from the boiler equipment at a gas plant and quantitative phase analysis of (i) iron oxide corrosion products from the boiler tube, (ii) synthetic mixtures of 87.0 wt% by weight of barite (BaSO4), 10.0 wt% by weight of hematite (Fe2O3), and 3.0 wt% by weight of quartz (SiO2), (iii) iron oxide corrosion products from the affected equipment parts in a refinery, (iv) vanadium oxide (V2O5), sodium vanadium oxide (NaV2O5), sodium vanadium sulfate hydrate (Na2V(SO4)2⋅H2O), and mackinawite (FeS) compounds found in the ash deposits from an external surface of the boiler tubes in a refinery, and (v) iron sulfide corrosion products found at the affected equipment in the sulfur recovery unit. The results revealed that the phase identification of powder XRD data is an excellent tool to determine the nature, source, and formation mechanism of crystalline deposits – part of the scale and corrosion products formed by the processes in the various units of refineries and gas plants. The quantitative Rietveld analysis results serve to guide the engineers at the refinery and gas plants to overcome the problems by applying the right procedures. For example, for iron oxide corrosion products, at a high temperature, magnetite will coat the iron/steel to prevent oxygen reaching the underlying metal. At low temperature, lepidocrocite formed and with time it transformed into the most stable goethite. Akaganeite is formed in marine environments. Additionally, for iron sulfide corrosion products, pyrophoric iron sulfide (pyrrhotite – FeS) results from the corrosive action of sulfur compounds (H2S) and moisture on the iron (steel). Additionally, for the crystalline ash samples from an external surface of the boiler tubes in a refinery, if sodium and vanadium compounds appear, the fuel oil is poor. For the boiler crystalline deposits, if a hematite phase appears, it means that the boiler feed water contains dissolved oxygen; and if the metallic copper appears among the crystalline deposits, it indicates erosion in the boiler tubes, and therefore, special precaution is required to prevent the plating out of copper during cleaning operations. Finally, for crystalline deposits from the steam drum equipment at the sulfur recovery unit, if magnetite has a high quantity, it indicates the presence of dissolved oxygen in the boiler feed water.