Common Defects: Pits

Pits can take on a variety of shapes and forms and can be caused by metal quality, the environment the parts were exposed to while they were stored before processing, excessive chlorides in process tanks, bottlenecks resulting in extended immersion in rinse, stray currents and galvanic effects, soft anodize, inadequate cleaning (bright dip), etc. It is important to look at the shape of the pit, how they are arranged or distributed on the part, and to determine whether all parts on a load are affected. Another useful exercise is to determine whether a coating is present at the pit location.

Shape

Shallow “star” shaped pits over entire surface suggests chlorides in the anodize bath or corrosion occurring in the desmut rinse. Moat and castle shaped pits on bright work may be due to poor cleaning or aluminum phosphate precipitation (icing) on the part in the bright dip.

Arrangement / Distribution

Pits following the metal grain or extrusion direction may be caused by particles in the metal. Pits that are distributed over the entire part are likely caused by the finishing process. Galvanic effects, rinse tank staging, chlorides, etc. Fingerprint, spacer bar, one side only – these kinds of arrangements suggest the origin was in the handling or storage of the part before it got to the anodize line. If only a few randomly located parts on a load have pits, the source is likely related to the metal quality.

Coating Present

If the pit has a protective coating then you can assume that the pit originated before the anodize step. The reverse also holds true.