A case study of a stress corrosion cracking failure in an AA5083 mold material used for curing rubber compounds

Resumo

In this study the failure analysis of an AA5083 mold material, used for curing rubber compounds, was carried out. The problem revealed itself as the formation of bubbles on the mold surface during curing process and caused quality problems in the final product. Preliminary microanalysis studies showed that there was an accumulation of both Mg and Cl in the bubble. Several samples were taken out from the inner/outer part of the mold material and examined using scanning electron microscope and energy dispersive X-ray spectrometer. It was concluded that (i) the contamination by Cl ions was due to the cutting oil used to open vent holes during mold cleaning, (ii) the outer part of mold material had finer α-Al grains whereas the inner part in contact with the compound had coarse grains due to the thermal effect of curing process, (iii) β-phase (Al₃Mg₂) formed at the grain boundaries after a sufficient exposure to temperatures above 50°C, (iv) the grain boundaries had Mg-rich phase showing anodic behavior, while the grains had Al-rich phase showing cathodic behavior, (v) the cracks initiated and propagated through the grain boundaries as stress corrosion cracking which resulted in the separation of grains and the accumulation of gases appeared as bubbles due to thermo-mechanical aging and effects of corrosive medium.