Industry Insights · Technology · Sep 28, 2026

Sealing Technology Trends — Hot Water, Nickel Acetate and Low-Temperature Sealing, and the Shift to Nickel-Free Sealing

Sealing, which closes the pores of anodic coatings, has evolved from hot water sealing to mid-temperature nickel acetate sealing and nickel fluoride cold sealing. This article reviews, based on the literature, the research trend toward low-temperature and nickel-free sealing driven by energy use and nickel regulations.

Anodic coatings have a porous structure of vertically aligned pores tens of nm in diameter; left as is, contaminants and moisture penetrate the pores and reduce corrosion resistance and dye fastness. Sealing closes these pores through hydration swelling or metal salt precipitation. ISO 7599 assesses sealing quality with the ISO 3210 phosphoric-chromic acid immersion mass loss test (limit of 30 or 40 mg/dm²), and QUALANOD also applies a limit of 30 mg/dm².

Comparison of Sealing Methods

Method	Principle	Characteristics
Hot water (boiling water) sealing	Coating surface hydrates to boehmite, narrowing the pores	No chemicals required; high energy consumption; dye bleed may occur
Mid-temperature nickel acetate sealing	Hydration plus nickel hydroxide precipitation	Less bleed; nickel wastewater treatment required
Nickel fluoride cold sealing	Pores closed by nickel and fluoride precipitation at room temperature	Low energy; long treatment time and aging required
Nickel-free low-temperature sealing (including research stage)	Alternative metal salts such as zirconium fluoride	No nickel wastewater; commercial use expanding

A Products Finishing article explains that hot water sealing tends to cause dye bleed, that nickel acetate sealing causes less bleed at lower temperatures, and that dual sealing combining the two is used. In hard anodizing, omitting sealing is the rule under the specification when wear resistance is the main purpose (MIL-PRF-8625 3.8.2, 6.1.3: sealing reduces wear resistance).

Research on Low-Temperature Sealing

Jo et al. (Materials, 2020) compared nickel acetate sealing at 95 °C, nickel fluoride sealing at room temperature and a room-temperature sealing solution mixing the two chemicals. The authors note that hot water-based sealing consumes a large amount of electricity to boil water, and describe room-temperature sealing as economically advantageous despite longer treatment times. Once sealing was complete, the corrosion current density decreased by more than two orders of magnitude compared with unsealed coatings.

The Shift to Nickel-Free Sealing

Nickel compounds are classified in Korea's exposure standards notice at TWA 0.1 mg/m³ for soluble nickel compounds and as carcinogen category 1A, which places a heavy burden on the work environment and wastewater treatment. Hao et al. (Rare Metals, 2020) performed room-temperature sealing with a nickel-free solution of potassium fluorozirconate with additives, evaluated the sealing effect by the phosphoric-chromic acid mass loss method, and reported that fluorozirconate provides a good sealing effect. A notable feature of the study is the confirmation, by cross-sectional SEM and Zr distribution analysis, that zirconium precipitates inside the pores.

Points to Note in Application

Sealing quality is checked periodically by dye drop (ASTM B136) or mass loss (ISO 3210) testing and revalidated when chemicals are changed.
Data on dye fastness and long-term weathering of nickel-free sealed coatings remain limited, so validation is required for each application.

EF Technology controls sealing conditions to meet customer sealing criteria (ISO 3210, ASTM B136, etc.) and is reviewing the applicability of low-temperature and reduced-nickel sealing.

References

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