Preventing Iron Contamination on Non Ferrous Parts

The Symptom

Aluminium or zinc parts leave the blast machine looking clean, and days later carry brown spots. The spots are not the part corroding. They are particles of the media embedded in the surface, rusting.

The Cause

Carbon steel media is iron. Every impact drives a small amount of it into a softer surface. On a steel part that does not matter. On aluminium, zinc, brass or stainless it does.

The Fix

• Switch the alloy. Cast Stainless Steel Shot makes the casting shiny and highlights the metal colour, with no rusting and no discolouration.
• Or go softer still. Zinc Shot is denser than aluminium and softer than steel, so it deburrs without wearing down the surface.
• Or match the substrate. Aluminum Round Shot on aluminium work gives the obvious white effect with nothing foreign left behind.
• Clean the machine down. Residual steel media in the hopper and separator will contaminate the first non-ferrous batch through it.

The Step You Get Back

With stainless media the surface is bright after processing and no further acid treatment is required. That removes the pickling tank, the chemistry and the effluent from the process.