This invention relates to a method for applying a Ni-based alloy surface coating to enhance wear and corrosion resistance of components such as industrial components. The invention also relates to a Ni-based powder for application by high velocity oxyfuel deposition to impart wear and corrosion resistance.
For many components it is desirable to impart wear and/or corrosion resistance to the component surface by deposition of an alloy having enhanced resistance to these phenomena. For example, printing rolls are subject to both abrasive wear and complex corrosion by printing inks and printing substrates. And paper mill rolls are subject to abrasive wear and complex corrosion by paper bleaches and other chemicals.
High velocity oxyfuel (HVOF) deposition is an alloy deposition technique which utilizes an explosive reaction between oxygen and a fuel, such as propylene, to propel an alloy powder onto a target surface at supersonic speeds. HVOF yields coatings with high bond strength resulting from the force with which semi-molten powder particles strike the substrate surface. Such coatings typically have a microstructure consisting of splats, which are formed upon impact of the semi-molten particles on the substrate surface at high speeds. Each individual splat generally retains the original chemical composition of the particular semi-molten powder particle from which it is formed.
Metal powder formation processes typically produce powder having a given bulk composition, such as 16% Cr, 16% Mo, 4% Fe, 4% W, and balance Ni. However, the bulk powder is made up of individual powder particles, many of which have compositions varying from the bulk composition. For example, for standard alloys such as the foregoing, some particles are relatively rich in Ni, others relatively rich in Mo, some relatively rich in Cr, and still others relatively rich in Fe. The chemical compositions of the various individual powder particles are therefore heterogeneous. The varying compositions are believed to be due to violent action of high-pressure gas blowing on the molten metal stream during atomization.
This heterogeneity is tolerable in forming wrought and cast structures for which such powders are designed, because the melting of the alloy powder in the casting, or other high temperature operation eliminates such heterogeneity, and the individual particles lose their separate identities when they are melted as part of an overall bulk of material. However, with HVOF deposition, the deposit consists of a series of splats, and no overall molten bulk is ever formed. Accordingly, powder chemistry heterogeneity manifests itself as heterogeneous surface chemistry in the HVOF build up. Certain areas of an HVOF coating are therefore left vulnerable to corrosive attack, as they lack the optimal surface chemistry, that is, the design chemistry, of the alloy. For example, high-Fe content splats can be more subject to corrosion than splats having the design chemistry. Corrosion has been observed on substrates with HVOF coatings made from traditional alloy composition powders, with the ultimate result being separation of the coating from the substrate once the corrosive medium reaches the base metal.