2. The purpose of adding Si to the coating bath
(excerpts from JIS G3314)
Aluminum is apt to form alloy with iron and as the coating bath temperature is considerably high, Al-Fe alloy layer is apt to develop during production. This alloy layer has an adverse effect on formability and thus for such applications as heat-resisting equipment that requires forming process, it is necessary to suppress the development of the alloy layer to the minimum during production, for which silicon add to aluminum effective.
A common practice is to add approximately 5-11% of silicon to molten aluminum during production.
ASTM Type 2 (hereinafter called Type 2) made in the U. S. includes products manufactured through a process that does not add silicon but these products are seldom formed into roofing of the building and primarily used in applications not subjected to high temperature.
Photo 1 shows, as an example, cross sections of layers from different coating bath compositions. As can be noted, "Pure aluminum type" has a much larger growth of layer than "Al-Si alloy type".

Photo-1. Cross sections of layers from different coating bath compositions (X500)