X-ray photoelectron spectroscopy study of the passive films formed on thermally sprayed and wrought Inconel 625

There is a well known performance gap in corrosion resistance between thermally sprayed corrosion resistant coatings and the equivalent bulk materials. Interconnected porosity has an important and well known effect, however there are additional relevant microstructural effects. Previous work has shown that a compositional difference exists between the regions of resolidified and non-melted material that exist in the as-sprayed coatings. The resolidified regions are depleted in oxide forming elements due to formation of oxides during coating deposition. Formation of galvanic cells between these different regions is believed to decrease the corrosion resistance of the coating. In order to increase understanding of the details of this effect, this work uses X-ray photoelectron spectroscopy (XPS) to study the passive films formed on thermally sprayed coatings (HVOF) and bulk Inconel 625, a commercially available corrosion resistant Ni-Cr-Mo-Nb alloy. Passive films produced by potentiodynamic scanning to 400mV in 0.5M sulphuric acid were compared with air formed films. The poorer corrosion performance of the thermally sprayed coatings was attributed to Ni(OH)2, which forms a loose, non-adherent and therefore non-protective film. The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides. Keywords

Corrosion behaviour of crystalline and amorphous forms of the glass forming alloy Fe43Cr16Mo16C15B10

The corrosion behaviour of both crystalline and largely amorphous forms of the Fe-based glass forming alloy, Fe43Cr16Mo16C15B10 alloy was investigated. Two different methods were used to induce transformation to the amorphous form of the alloy: laser melting and HVOF spraying. Both methods produced largely amorphous material, however the high brittleness of the alloy makes it susceptible to cracking during laser treatment, hence this technique is not suitable for largescale application. Potentiodynamic scanning showed that in 0.5M H2SO4 and 3.5% NaCl electrolytes both amorphous forms of the alloy had better corrosion resistance (lower current densities for -200 to +1000mV SCE) compared to the crystalline material. The laser treated material and HVOF coating performed similarly in 3.5% NaCl. In 0.5M H2SO4 the HVOF coating had a lower current density than the laser melted material for almost all of the potential range -300 to +1000mV SCE. The improved corrosion behaviour of the largely amorphous material is attributed to its homogeneity, and particularly to the elimination of the Mo-rich phase that underwent preferential corrosion in the crystalline form of the material

The effects of microstructural features on the performance gap in corrosion resistance between bulk and HVOF sprayed Inconel 625

It is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5M H2SO4 to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction. Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 were compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance. Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20 - 40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples