Passive Corrosion Protection of Al-Zn-Mg alloy in Seawater

摘要

In the article the chosen problems connected with corrosion of the maritime construction materials with particular consideration of the passive anticorrosion protection using high strength aluminum alloy and its welded joints were presented. Also currently used methods of anticorrosion protection of maritime constructions with use of aluminium and its alloys in form of protective coatings and sacrificial anodes were described. The first part of the article presents the corrosion resistance of the basic material of the tested alloys series 7xxx (Al-Zn-Mg alloys). In the further part of the article the results of the own researches concerning the passive protection of the Al-Zn-Mg alloy and its joints of the AlZn5Mg2CrZr alloy (mark 7020M) with the systems of protective paints and sacrificial anode cathodic protection were presented. 7020M alloy is a modification of the chemical composition of the alloy 7020 (AlZn4,5Mg1 alloy). In the 7020M alloy the increase of corrosion resistance and strength was obtained by increasing the total Zn+Mg7% and adding Cr and Zr. The effectiveness of the anticorrosion protection has been determined by a comparison of the mechanical properties deterioration of the protected and unprotected welded joints that were set under the influence of stress and artificial seawater. The welding process is a type of heat treatment consisting of: heating, supersaturation and cooling at room temperature. This type of heat treatment called natural is the most susceptible to corrosion in seawater. Each type of welded joint means increased corrosion susceptibility. The system of epoxy paints provides more efficient protection from the effects of stress corrosion for the welded joints of 7020M alloy than the system of polyvinyl paints. Electrochemical potential of 7020M alloy determined on microstructure depends on chemical composition and heat treatment. This group of materials, which are characterized by the electrochemical potential lower than -1000mV vs. SCE, can be used in sacrificial anodes for the tested alloy.