Colloidal silica’s slightly alkaline nature can act as an electrolyte, accelerating galvanic corrosion between the base metal and filler. Switching to fine alumina abrasives may improve results, and I also recommend using diamond polishing on cross-section samples is generally a good choice for achieving a high-quality, scratch-free finish, and it can help reduce the risk of galvanic corrosion compared to colloidal silica. Sometimes, the issue isn't just due to polishing. Another common problem is oxidation—bare aluminum can oxidize rapidly, leading to a dull surface or appearance if it's not properly sealed or treated.

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Thankyou. Chanakan Arampongpun

To solve this, you must eliminate or inhibit the electrolyte. Here are the most effective methods, ordered from most recommended to least.

After grinding with SiC papers (you can use water for grinding, but clean thoroughly afterward), switch to diamond suspensions for all subsequent polishing steps (e.g., 9-µm, 3-µm, 1-µm).

• Use a high-quality oil-based or glycol-based lubricant/extender compatible with your diamond suspension.
• Between steps, clean the sample with ethanol or isopropanol, not water. Dry it immediately with compressed air.

Or use Alcohol-Based Suspensions or Lubricants