Control Pin Refractory Reaction in High Magnesium-Aluminium Melts

Pin hole defects observed in 5182 can lid were found to contain Al-Mg-O spinel inclusions with traces of silica. The lack of calcium and phosphorous in the inclusions suggested they were not from usual refractory sources but could be from dense fused silica (DFS) pins and downspouts reacting with magnesium in the melt. Pin immersion tests, thermodynamic analysis, and SEM (scanning electron microscopy) examinations were carried out to study the mechanism. The thermodynamic analysis and the immersion tests showed that at low magnesium content, MgO forms as the reaction product but at higher magnesium levels MgAl2O4 forms. It seems likely that the reaction products take up more volume than the silica which would cause further spalling of the fused silica and separation from the BN coating. The importance of examining and maintaining the integrity of the BN coating was highlighted to prevent a melt-to-pin reaction. It is speculated that lower porosity pins would be more resistant to attack.