Surface Finishing Tutorial

#318 Chromate Films on Zinc Plating

Category : Electroplating

November17, 2017

The chromate treatment over zinc plating is designed to add gloss finish or color tones, such as pale yellow, yellow, and green, while drastically enhancing the corrosion resistance by forming anti-corrosive films over the zinc-plated surfaces.

(1) Grades, types, and symbols of chromate films

[Table 1] Grades, types, and symbols of chromate films

Grade	Type	Symbols for grades & types	Film mass per unit area (g/m²) Note)	Common shades of color
Grade 1	Gloss	CM1 A	0.5 or less	Transparent, or a tinge of blue at times
Grade 1	Pale yellow	CM1 B	1.0 or less	Slight interference pattern
Grade 2	Yellow	CM2 C	More than 0.5 but not greater than 1.5	Yellow interference pattern
Grade 2	Green	CM2 D	Greater than 1.5	Olive, green, bronze, brown, etc.

*	Using a different type of solution will generate black chromate films. The film mass per unit area for the black chromate films should be determined and agreed between the parties concerned.

(2) Mass measurement method for chromate films

For the measurement, it is necessary to prepare a test piece consisting of the surface area shown in [Table 2] (24 hours but no more than 14 days elapsed from the chromate treatment), and the one-liter solution that is made by dissolving 50 g of sodium cyanide and 5 g of sodium hydroxide into pure water. The mass is measured by a reduction of weight after soaking the test piece into this solution for one minute, while cathode current density stays at 15 A/dm2 to dissolve the chromate film.

[Table 2] Total surface area of test pieces

Estimated film mass per unit area (g/m²)	Minimum total surface area (cm²) of test pieces
Less than 1	400
1 or more and less than 10	200
10 or more and less than 25	100
25 or more and less than 50	50
50 or more	25

(3) Corrosion resistance of chromate films

The minimum hours shown in [Table 3] must be satisfied after conducting a neutral salt spray test consecutively.

[Table 3] Corrosion resistance of chromate films

Symbols for grades & types	Minimum hours (h) without generating white corrosion product
CM1 A	6
CM1 B	24
CM2 C	72
CM2 D	96

(4) Notation of chromate films

Ep-Fe/Zn 25/CM2 D

Electro-galvanizing deposited at 25 µm or higher on a steel substrate, topped with Grade 2 green chromate film

Ep-Fe/Zn 8/CM1A

Electro-galvanizing deposited at 8 µm or higher on a steel substrate, topped with Grade 1 gloss chromate film

Reference
This standard applies to chromate films on electroplated coatings of cadmium as well. In recent years, chromate films containing hexavalent chromium have been linked to environmental problems. As a result, chromate films free from hexavalent chromium were developed. This new alternative will become the mainstream in the future.

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