 The fluxes often consist of corrosive chemicals such as ammonium chloride and zinc chloride in a binder such as petroleum jelly.If too much flux is applied to the joint then the excess will melt and run down the bore of a vertical tube or pool in the bottom of a horizontal tube.Where the bore of the tube is covered in a layer of flux it may be locally protected from corrosion but at the edges of the flux pits often initiate.