The Deterioration of Molten Salt Bath for Electrolytic Boronizing and its Countermeasure

Abstract

It is well known that the electrolytic boronizing is, for industrial purposes, more advantageous than other boronizing processes such as gas, pack, and salt bath processes. There is one problem in this process, however, that the boride layer tends to become less thick in a given time of processing with the repetitive use of molten borax (Na2B4O7) bath in a heat-resisting steel pot.In this study the cause of the deterioration of the molten salt bath were investigated and the methods to prevent it were established.The results obtained are summarized as follows:(1) The deterioration of the molten salt bath for electrolytic boronizing is considered to be due to the dissolution of iron into the bath from heat-resisting steel pot.( 2) It is found that the deterioration can be effectively prevented by providing the passage of electric current through the bath to the pot which is to be a cathode.This method is a sort of cathodic protection.The deterioration can be considerably reduced by the addition of ZrO2 or MgO to the bath. (4) The boronizing power can be restored by the removal of the iron-rich slag on the bath surface formed by the addition of a small amount of Ca, Mg, Al, Si, Zr, Ti or others into the deteriorated bath.