CORRELATION BETWEEN ELECTROCHEMICAL PROPERTIES OF THE WELDED JOINT OF ALUMINIUM ALLOY OF THE AL-MG-CU-SI SYSTEM AND ITS RESISTANCE TO LOCAL CORROSION

Abstract

The study of the correlation between the electrochemical properties of a welded joint of an aluminum alloy of the Al-Mg-Cu-Si alloying system and its resistance against local corrosion. The technique. Potentiometry, polarization curves techniques, accelerated tests of resistance against intergranular corrosion according to GOST 9.913 were used. The measuring equipment analytical VRRL 200 scales, potentiostat PI-50.1 was applied. Electrochemical properties in a solution of 3% NaCl of a welded joint of an aluminum alloy of the Al-Mg-Cu-Si alloying system obtained by a free arc were studied, its resistance against intergranular corrosion was evaluated. In accordance with the results of electrochemical studies it have established that
the  potential  of  the  weld  and  the  heat-affected  zone  are  more  noble  in  comparison  with  the  base  metal. Taking into account the larger area of the base metal in the structure in compare to the weld (the weld area is  no  more  than  10%  of  the  base  metal  area),  in  general,  satisfactory  resistance  of  the  welded  joint  to uniform corrosion can be expected. The maximum depth of intergranular corrosion of the heat-affected zone of  the  welded  joint  of  the  Al-Mg-Cu-Si  alloy  is  about  0.350 µm, which is more than the depth of intergranular corrosion for the base metal — about 0.086 µm. The increasing of the depth of breaking down of  grain  boundaries  in  a welded  joint confirms  the fact  of the  softening  of the  heat-affected zone  during welding. Scientific novelty. A correlation between the electrochemical characteristics of the welded joint of an aluminum alloy of the Al-Mg-Cu-Si alloying system obtained by a free arc, in particular, the corrosion potential  and  the  anodic  dissolution  current  in  the  active  potential  region  and  resistance  against  to intergranular  corrosion  was  established.  It  is  shown  that  an  increasing  of  the  depth  of  intergranular corrosion of the heat-affected zone correlates with the large currents of the anodic dissolution of this zone. Practical  significance.  It  has  been  established  that  the  thermal  welding  cycle  promotes  the weakening of the heat-affected zone of the welded joint of an aluminum alloy of the Al-Mg-Cu-Si alloying system, obtained by a free arc. It leads to a decreasing of the resistance against intergranular corrosion.