FLUX REACTIONS IN BRAZING OF STAINLESS-STEEL

Presence of tenacious chromic oxide film at the surface may impart cor rosion resistance properties to stainless steel but it proves detrimen tal to good wetting which is pre-requisite to any joining process such as soldering and brazing. Fluxes promote wetting by removing the oxid e film and protecting the surface from reoxidation. Flux - metal and r eoxidation reactions have been studied in the present work by designin g experiments to assess the loss in weight and changes in the appearan ce of the base plate due to heating at different conditions in the tem perature range of 700 to 1000-degrees-C. The commercial grade brazing flux did not protect the surface from reoxidation as the surface got c harred and became unwettable within a few minutes of the flux applicat ion. There was little reaction occurring between the flux and stainles s steel when a strip was dipped inside the molten flux bath in a cruci ble. The areas to which the braze filter metal consisting of copper - silver alloys of different compositions spread on a stainless steel su bstrate using synthetic fluxes were measured. Best wetting results wer e obtained when a mixture containing 90 % boric acid and 10 % potassiu m chloride was used as a flux in the temperature range of 800 to 1100- degrees-C. In many experiments distinct bands appeared at the edges of the droplets and the same could be attributed to the chemical reactio ns or interactions occurring at the liquid metal - flux and flux - bas e metal interfaces. The chemical and electro-chemical nature of the re actions were studied by setting up a cell apparatus in the laboratory. A thermodynamic model of the reactions occurring in the brazing of st ainless steel using chloride based fluxes has been proposed.