Fluoride Ion Cleaning as a pre-braze process
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:
- 6HF + Al2O3 -> 2AlF3 + 3H2O
- 4HF + TiO2 -> TiF4 + 2H2O
- 6HG + Cr2O3 -> 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 braze ability by removing oxide reformers. These reactions are illustrated by the following:
- 6HF + 2Al -> 2AlF3 + 3H2
- 8HF + 2Ti -> 2TiF4 + 4H2
The depletion reaction is a function of the reaction temperature, the concentration of HF and alloy composition.