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Corrosion of steel by anodised shaft-seal.

2000

My company is taking up the manufacture of shaft-seals comprising a PTFE lip-seal mounted in an aluminium-alloy case. The completed assembly is contracted out for anodisation. Our client reports some corrosion of the shaft upon which the seal is mounted. On disassembling an anodized assembly we note a deposit of some sort trapped between the case and the PTFE, which we suspect could be residue from the anodizing tank. When an anodized assembly is immersed in a minimal amount of distilled water the liquid shows an acidic pH of 4.5 - 5.0.

Would much appreciate expert comment on this situation. For example, what is the chemical nature of anodizing solutions, and are they likely to be corrosive to steel? Better still, any suggestions for overcoming this problem! Norman Sibley
- Athens, Alabama

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2000

Greetings Norman -

It depends if your parts are being finished in chromic or sulfuric acid anodizing. Sulfuric acid anodizing can leave an acidic residue behind on hard to rinse parts (which your's sound like). The sulfuric acid residue would indeed be corrosive to steel. The residue left behind by chromic acid anodizing would not be corrosive (chromic acid can be used to passivate the surface), but any chrome residue could be a potential health issue. Hope this helps.

Victor Waldman
Modern Metal Finishing - Naugatuck, Connecticut

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2000

Victor,

Many thanks for your quick response!

Sounds as though sulfuric acid could be the anodizing solution used, and that more thorough rinsing is needed. We will also investigate whether chromic acid anodizing would be acceptable to all parties.

Appreciate your help,

Norman Sibley
- Athens, Alabama

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