Fluoride Ion Cleaning As a Pre-Braze Process

Fluoride Ion Cleaning (FIC), synonymous with Hydrogen Fluoride (HF) cleaning is an extremely effective process for the removal of metallic oxides from metal alloys.  It was developed by a major jet engine manufacturer as a cost effective repair process for nickel based airfoil components such as turbine blades and vanes.  Today it is widely used to prepare nickel and cobalt based supper alloys for braze repair / Activated Diffusion Healing (ADH) on jet engines and industrial gas turbines.

Conventional processes such as Hydrogen Cleaning were effective for a wide range of stainless steels, cobalt and nickel based alloys.  However hydrogen is not very effective on alloys containing significant amounts of aluminum and titanium.  These two metals severely oxidize to form complex spinels on hardware surfaces that penetrate deeply into existing cracks.  Only cleaning methods utilizing the fluoride-ion technique are currently capable of removing these deeply imbedded oxides.  FIC utilizing HF gas offers a simpler, more precise and consistent alternative to other more complex techniques. These reactions are illustrated by the following:

                        1.     6HF  +  Al₂O₃     à     2AlF₃  +  3H₂O

                        2.     4HF  +  TiO₂     à     TiF₄  +  2H₂O

                        3.     6HG  +  Cr₂O₃     à     2CrF₂  +  F₂  + 3H₂O

In addition to removal of the oxides present on the surface and within cracks, surface depletion of elements such as titanium and aluminum also occurs which enhances brazeability by removing oxide reformers.  These reactions are illustrated by the following:

                                     1.      6HF  +  2Al      à     2AlF₃  +  3H₂

                                     2.     8HF  +  2Ti     à     2TiF₄  +  4H₂

 The depletion reaction is a function of the reaction temperature, the concentration of HF and alloy composition.