Abstract
The prevention of porosity in hot dipped aluminum coating on the steel sheet is of great importance since in many environments steel is anodic to aluminum and will corrode at an accelerated rate at any discontinuity in the coating. The occurrence of pinholes may be caused either by the character or cleanness of the steel surface, or by factors associated with the aluminum bath. The origins of pinholes to be associated with the aluminum bath have been investigated in this report and the following results are obtained: (1) Among the factors which may lead to pinholes, aluminum oxide on the aluminum bath has a very remarked effect. (2) Aluminum oxide is dissolved gradually with an addition of fluoride (NaF-AlF3 system) to the flux and the great improvement of surface appearance is attained with an addition of about 20% of fluoride. (3) Occurrence of pinholes decreases with the decreasing quantity of aluminum oxide using the same flux in the same quantity. (4) The porosity decreases with an increase of the dipping time. The appearance of the coating is also improved by the shaking of the sheet in the aluminum bath. (5) In the presence of the flux, no accelerated rate of the diffusion of aluminum to the base metal is observed.
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