A study on non-destructive detecting method for carbon particles inside metal structures based on X-ray imaging with contrast agents

The non-destructive detecting of carbon deposits inside metal structures is of great significance for the study of coke accumulating behavior and lifetime assessment of aerospace engines. However, due to the carbon is difficult to be clearly imaged in metal channels under X-rays, there is still a lack of high-resolution and low-cost detecting methods. In this work, firstly, theoretical analysis of X-ray attenuation in graphite and different mediums was conducted. It was found that the linear attenuation coefficient lac of carbon, air and RP-3 fuel are small and similar compared to that of stainless steel matrix material. This explains the difficulty for X-ray visualization of carbon in metal structures. Subsequently, this work proposed a novel approach for X-ray imaging of carbon deposits inside metal structures, utilizing high-density salt solution as contrast agent. The imaging characteristics of different solutions of sodium metatungstate (SMT), ammonium metatungstate (AMT), and sodium iodide (NaI) with different concentrations were compared as experimental contrast agents. The results demonstrated that under the irradiation of high-energy X-rays, the SMT and AMT solutions generated image with significantly higher resolution compared to the traditional medical contrast agent NaI solution. This superiority is attributed to the high density rho and the large effective atomic number Zeff of metatungstate solutions. Among them, the Xray absorption intensity of saturated SMT solutions exceeded that of 304 stainless steel, and the imaging contrast index Fct was increased by 124% compared to air. This work provides a feasible method for the non-destructive detecting of carbon deposits in metal structures.