Ano
Article · Process
What Does Sealing Do in Anodizing?
Sealing closes the pores in the fresh anodic coating. It locks dye in and substantially improves corrosion resistance, at a small cost in surface hardness. That trade is why Hard Coat parts are sometimes specified non-sealed.
Why the Coating Is Porous to Begin With
The anodic layer grows as a structure of dense oxide columns with pores running through it. That porosity is not a flaw; it is what makes color possible, because dye is drawn down into the pores and ends up inside the coating rather than on it.
Left open, though, those pores are also paths for moisture and contaminants to reach the metal, and any dye in them can leach out.
What Sealing Does About It
Sealing closes the pore structure, typically in hot deionized water or a nickel acetate solution. Two things follow. Dye is locked in permanently, which is why sealed anodized color does not bleed or wash out. And corrosion resistance improves substantially, because the paths to the substrate are shut.
The cost is a small reduction in surface hardness, since the sealing reaction slightly alters the outermost coating structure.
How Sealing Is Done, and How It Goes Wrong
The common methods are hot deionized water, which hydrates the oxide so it swells and closes its own pores, and a nickel acetate bath, which does the same job at a lower temperature with a chemical assist. Both are controlled processes with real temperature, time and chemistry windows.
Two failures show up on finished parts. A seal bloom, a white powdery haze, indicates the seal bath chemistry or temperature drifted, and it is a cosmetic defect a customer notices immediately. An under-seal is invisible and worse: the part looks correct, then fails a salt spray test or bleeds dye later, because the pores never actually closed.
That second failure is the argument for buying finishing on process discipline rather than on price. Nothing about an under-sealed part looks wrong when it arrives.
Why Anyone Would Skip It
Wear resistance. On a Type III Hard Coat where the entire point of the part is abrasion resistance and corrosion is not the primary concern, a non-sealed coating keeps the hardest surface the process can produce.
It is a genuine trade rather than a preference, and the print usually states it. If yours is silent on the seal state, we raise it before running instead of choosing for you.
Common Questions
Should My Part Be Sealed?
If corrosion protection matters, yes. If the part exists to resist wear and lives somewhere dry, non-sealed Hard Coat gives better wear performance. The drawing usually decides; when it does not, tell us what the part does.
Does Sealing Change the Color?
It can shift the appearance slightly as the coating structure alters, which is one more reason a batch should be dyed and sealed together so parts match each other.
How Do I Know if a Part Was Properly Sealed?
You generally cannot tell by looking, which is the problem. A sealed coating resists staining and dye bleed and performs in salt spray testing; an under-sealed one looks identical and fails later. Where it matters, specify the testing.
Can a Part Be Sealed Later?
Sealing is most effective on a freshly anodized coating, so it is part of the process rather than a separate later step. Tell us the requirement with the request.
Does Sealing Take Long?
It is a controlled step measured in tens of minutes rather than a quick rinse, and it sits inside the same 3 to 5 day standard turnaround as everything else. It is not a step worth rushing, because an under-sealed part looks perfect and fails later.
Is Unsealed Anodizing Less Protective?
Against corrosion, yes, meaningfully so. Against wear, no, it is better. They are different properties and the choice depends on which one the part needs.
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