Galvanic corrosion behavior of AA5083-H131 in contact with 4340 steel under atmospheric exposure conditions

AA5XXX alloys are widely used to the cabin of ships and vehicles across Department of Defense, because they have a good combination of strength and ductility, weldability and corrosion resistance. Their use can decrease the weight of structures, thus increasing speed, range and fuel economy.  These aluminum alloys are generally joined via welding and bolts.  The bolts are generally made from AISI 4340 steel due to its high strength.  There is thus a concern that galvanic interactions will exacerbate corrosion of the AA5083-H131 leading to pitting, intergranular corrosion and intergranular stress-corrosion cracking of the aluminum alloy if it is sensitized.  The resistance of the thin electrolyte films that form under atmospheric exposure will limit the distance over which the influence of the steel fastener will be important.  The aim of this study is to determine the influence of various factors affecting atmospheric galvanic corrosion of AA5083-H131 due to coupling to 4340 steel quantitatively.  Factors of interest include the loading density of salt, the relative humidity, the size of the fastener, and the degree of sensitization of the 5083-H131.  Results from modeling and measurement of the potential distribution around the steel fastener will be presented.  Scanning Kelvin Probe measurements allow the potential distributions around the fastener to be determined.  In addition, cross-sectional metallography is used to determine the severity and the range of IGC damage as a function of the identified factors.  The implications of these modeling and experimental results on the prediction of IGC and IGSCC damage will be presented.