Water Staining Due to Storage of Semi-products

Experience has shown that aluminium semi-products such as sheets and profiles can develop surface alterations during transportation or storage, when stacked one on the top of the other without insets. This alteration shows up as grey or white spots that often appear as aureoles. If two stacked sheets or profiles are stained, their contour is usually symmetrical and the same on the two opposing faces. 

These spots are not actually corrosion, but only an alteration of the visual properties of the oxide layer caused by a complex reaction of the oxide layer with water trapped between 

While this surface alteration has no effect on the mechanical characteristics of the metal and causes no subsequent corrosion in service, it nevertheless permanently alters the siuface appearance. After surface treatment such as pickling, and also after anodising, more or less marked contrasts between the stained zones and the rest of the surface will remain. These semi-products are thus difficult to use in applications where surface aspect is paramount, i.e. where the surface appearance is the very reason for using aluminium. 

In the case of cast (Properzi) drawing stock, these spots, which may be accompanied by superficial corrosion, can be the cause of rupture during subsequent cold drawing operations [5]. 

On semi-products to be welded, these spots need to be removed carefully in the welding zone by strong brushing with a stainless steel brush. 

---