Cast Aluminum Porosity After Anodize | Vacuum Impregnation Q&A
Quick Answer: Impregnation can technically be performed after anodizing cast aluminum (355 T6), but it is not recommended. The preferred approach is to strip, impregnate, and then re-anodize to prevent long-term corrosion risk.
Question
Q: I have some cast aluminum (355 T6) parts in question that have been anodized but have issues with porosity. Can impregnation be done on these parts to correct the condition in an anodized state? Would we need to re-anodize after impregnation?
Answer
A: The reason you’re seeing porosity after anodize is that during processing the pores adsorbed the anodize electrolyte — namely sulfuric acid. And if your parts are dyed, we’re sure the acid is leeching the dye, making the pores even more noticeable.
The parts can be impregnated-post anodize ... however, this type of approach is less effective (if not frowned upon) as:
- (1) The entrapped electrolyte and chemicals inhibit the impregnant resin from penetrating as deep as it might have otherwise penetrated.
- (2) Impregnating post-anodize entraps those acid ‘corrodents’ in the part and should they be released later on, this can result in significant corrosion.
That said, they can be impregnated after anodize but we wouldn’t recommend it. Better to strip, impregnate and re-anodize, but there may be dimensional consideration that would be impacted.
We would also recommend a mask for bushing holes or threaded features to prevent them from being stripped.
Technical Context & Related Terms
This discussion applies to cast aluminum components including 355 T6 alloy parts commonly used in structural and industrial applications. Processes referenced include vacuum impregnation, anodizing, and corrosion control in aluminum alloys.
Related finishing and materials topics include:
- Impregnation After Anodize
- Cast Aluminum Porosity
- Chromic Acid & Sulfuric Acid Anodizing
- Passivate / Passivated Surfaces
- Mil-Spec and MIL-STD Requirements
- Black Anodized Aluminum
- Silver Plating
- Nickel Thickness Considerations
If you are evaluating impregnation or anodizing solutions for cast aluminum components and need technical guidance, contact our engineering team for application-specific recommendations.
