Abstract
The caustic etching behaviour of the automotive AA6111 alloy with different aging conditions that simulate the various levels of heat input within the heat affected zone during the welding heating cycles has been investigated. The alloy dissolution rate was found to substantially increase with the pre-aging temperature, which enhances precipitation of the cathodic Q-phase and results in depletion of the alloy solid solution in magnesium and silicon. On the other hand, enhancing the kinetics of precipitation at high temperatures induces a more uniform distribution the Q-phase precipitates, which minimizes the potential difference between the interiors and boundaries of the adjacent grains. This was observed to reduce the severity of the surface topography developed by subsequent caustic etching surface pre-treatment.
Highlights
In the automotive industry, chemical surface pre-treatment of the aluminium alloy body panels in hot caustic soda solutions is one of the most critical steps in the coating application process [1]
The homogenous distribution of high population density of the Q-phase precipitate in the matrix of the highly over-aged AA6111 alloy resulted in a uniform etching of the alloy in sodium hydroxide solution
The dissolution rate of the automotive AA6111 alloy during the conventional caustic etching and acidic desmutting surface pre-treatments increases with the alloy preaging condition
Summary
Chemical surface pre-treatment of the aluminium alloy body panels in hot caustic soda solutions is one of the most critical steps in the coating application process [1]. Caustic etching of magnesium-containing aluminum alloys is known to enrich their outer alumina film with magnesium [9] [10], which is related to the faster migration of the magnesium species relative to that of Al3+ ions and to the higher stability of the magnesium oxides/hydroxides in strong alkaline environments. This phenomenon is reported to reduce the dissolution rate of the underlying aluminum substrates [11] [12]. Previous studies have shown that the caustic etching rate decrease with the silicon contents [8], and significantly increase with the copper contents [13] of the aluminum alloys
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