Surface Modification of Inconel 625 in Nitrate Environment

The influence of nitrate (NO3−) concentration on the corrosion resistance of the Inconel 625 (superalloy) was investigated. The surface of Inconel 625 was chemically modified by oxidation in an alkaline sodium nitrate(V) solution. The surface and microstructure of specimens were observed by a scanning electron microscope (SEM). The mechanical properties of Inconel 625 were characterized by microhardness (HV) measurements. The corrosion tests of materials were carried out by using the electrochemical method in the acidic chloride solution. The adsorption of the (MemOn)ads layer effectively separates the Inconel 625 surface from contact with the aggressive corrosive environment. The microhardness (HV10) value increased (about 13%) with the increase in nitrate concentration. A more-than-five-times-lower corrosion rate (CW) value was obtained for the Inconel 625 sample, whose surface was modified in an alkaline solution with the highest NO3− concentration. Chemical modification improves the structure and surface topography of the superalloy. After exposing Inconel 625 to an oxidizing environment (1.00 M NO3−), the surface coverage degree (SC) was 80%.