EFFECT OF THE ADDITION OF CR AND NB ON THE MICROSTRUCTURE AND ELECTROCHEMICAL CORROSION OF HEAT-TREATABLE AL-ZN-MG ALLOYS

The effect of the addition of Cr and Nb on the microstructure and the electrochemical corrosion of the weldable, high-strength and stress co rrosion cracking (SCC) resistant Al-5%Zn-1.67%Mg-0.23%Cu alloy (H) has been studied. Combined additions of the alloying elements, J (with Nb ), L (with Cr) and O (with Cr and Nb) and different heat treatments, S T (cold-rolled), A (annealed), F (quenched), B (quenched and aged) and C (quenched in two steps and aged), to obtain different microstructur es and hardness have been performed. To correlate the electrochemical corrosion with the microstructure of the specimens, corrosion potentia l (E(cor)) measurements in different chloride solutions were performed and optical microscopy, SEM, TEM and EDX were applied. In chloride so lutions containing dissolved O-2 or H2O2, the present alloys were pola rized up to the pitting attack. It was shown that the E(cor) measureme nts were very sensitive to the alloy composition and heat treatment, i ncreasing in the order H < J < L < O < Al (for a given heat treatment) and F < A approximate to ST < B < C (for a given alloy). The MgZn2 pr ecipitates of the annealed (A) and cold-rolled (ST) specimens were dis solved in chloride solutions containing oxidizing agents and pitting a ttack was shown to develop in the cavities where the precipitates were present. In the specimens B and C, the compositions of the precipitat e free zones was found to be equal to that of the matrix solid solutio n and preferential intergranular attack was not evident, this being in agreement with their SCC resistance. The addition of Cr and Nb increa sed the pitting corrosion resistance. The effects of Cr and Nb were ad ditive, that of Cr being predominant, either, in the E(cor) shift or i n the increase in the pitting corrosion resistance.