In today's society, many automotive exteriors adopt decorative electroplating technology, which not only adds a sense of fashion to automotive lamps but also enhances the overall grade of the vehicle.
However, because electroplated parts on automobiles are usually exposed and subjected to various environmental tests, the electroplated exterior parts require very strict process and evaluation standards.
Generally, automotive lamps are selected according to the service conditions, as shown in Table 1 below:
Table 1
Grade | Service condition | Applicable parts
U-type | Severe conditions | Housing parts
S-grade | Strict conditions | Housing parts
A-grade | Normal conditions | Housing parts
B-grade | Mitigated conditions | Internal parts
The following is a detailed description of the chromium plating process and decorative chromium plating test methods for a Japanese rear tail lamp of a certain company.
I. Electroplating type
The electroplating type used for Japanese automotive headlamp decoration is S-grade, with strict service conditions. The applicable example is the exterior parts, and the material type is plastic.
II. Electroplating quality
1. Electroplating method
The electroplating method uses electrolysis on the substrate to attach elements such as copper, nickel, and chromium to the surface to impart a metallic luster.
In addition, the nickel layer is further subdivided into several electroplating methods.
The lamp is made of double nickel or triple nickel + dispersion stress (nickel seal) and microporous chromium.
In the case of double nickel and triple nickel, the thickness of the semi-bright nickel layer shall not be less than 1/2 of the total nickel layer thickness.
2. Minimum thickness of electroplating
Thickness measurement is based on the microscopic method, and can also be measured by an electrolytic film thickness gauge.
In addition, a fluorescent X-ray film thickness gauge may be used for chromium alone.
In principle, the measurement area of the coating thickness should be the minimum thickness.
However, when the plating shape causes uneven plating thickness, the measurement area may be changed through agreement between the parties.
Except for zinc alloy and aluminum alloy substrates, other substrates may be nickel-plated instead of copper.
The copper thickness should ensure appearance and thermal cycling.
Although substrate conditions such as shape, size, and molding conditions vary, the minimum copper layer thickness to be measured should be not less than one time the nickel layer thickness.
The minimum thickness of the nickel plating and the minimum thickness of the chromium plating have different regulations depending on the service environment.
3. Appearance
Visual inspection is performed under indoor light at a distance of 500 mm from the test surface.
The coated surface shall be smooth and glossy, free from stains, bumps, damage, cuts, exposed material, blisters, and dullness. However, it is not applicable to parts that are invisible on the whole vehicle.
Limit samples are determined by mutual agreement. The limit is the degree at which the vehicle appearance is visually observed at 50 cm intervals under sunlight and appearance defects are not obvious.
4. Corrosion resistance
Salt spray test (SST): conducted continuously according to JIS Z 2371 or ASTM B117.
For S-grade and A-grade, no flying rust test is performed, and no rusting occurs.
For plastic materials, the salt spray test (SST) is not required.
The Corrodkote test is conducted according to JIS H 8502 or ASTM B380. The method for preparing the Corrodkote paste may be selected as follows: weigh 2.5 g of copper nitrate (Cu(NO3)2·3H2O), dissolve it in a volumetric flask, and accurately dilute to 500 ml with distilled water; weigh 2.50 g of ferric chloride (FeCl3·6H2O).
Dissolve it in a second volumetric flask and dilute to 500 ml with distilled water (when not in use, the ferric chloride solution should be stored in the dark and stoppered with a rubber or glass stopper) (the ferric chloride solution should not be stored for more than 2 weeks).
If stored for too long, instability will occur. Weigh 50.0 g of ammonium chloride (NH4Cl) dissolved in a volumetric flask, and accurately dilute to 500 ml with distilled water. Then accurately measure 7.0 ml of copper nitrate solution, 33.0 ml of ferric chloride solution, and 10.0 ml of ammonium chloride solution, pour the above solutions into a pouring beaker, add 30.0 g of china clay, and stir with a glass rod. The Corrodkote paste should be used immediately.
No rusting or discoloration is observed at 50 cm intervals.
However, for U-grade, the localized corrosion rate shall not exceed 10%.
When the applicable area is less than 25 cm², the overall corrosion rate shall not exceed 10%.
The Corrodkote test for the S-grade plating is 48 hours or longer.
Film corrosion resistance test (CASS): conducted according to JIS H8502 or ASTM B368. At 50 cm intervals, observe no rusting (surface rust), discoloration, etc. The corrosion resistance test for the S-grade plating is 60 hours or longer.
5. Thermal cycling
For S-grade, the substrate is made of plastic. When hot and cold cycling is performed according to the following test method, the number of hot and cold cycles is 4 or more.
After the test, the effective surface shall not swell, peel, or split.
Thermal cycling test method: the conditions in Table 2 constitute 1 cycle. In principle, after electroplating, the plate is left for 48 hours or more and then cycled. Note (10) is determined by mutual agreement.
Table 2
6. Adhesion
The adhesion of the plastic substrate is tested according to the following test methods, and peeling between the substrate and the plating layer does not occur easily.
However, the acceptance criteria vary depending on the test method and are determined by mutual agreement. The method is based on JIS H 8630 Annex 6 or ASTM B533.
In principle, 48 hours after electroplating, the test is performed as follows:
(1) Use a sharp edge to cut through the coating surface; the blade should contact the substrate.
(2) The coating film at the sharp-angled part cracks from the crosscut side.
(3) Pull the split film in the direction perpendicular to the electroplated surface.
The adhesion of the plastic substrate is 10 mm, and the peel speed is 10 mm. The test is performed at a peel speed of 30 mm/min, and the value is generally higher than 9.8 N/cm.
The coating thickness must be strictly determined (e.g., by the 3σ method). That is, the average plating thickness at the general measurement location (e.g., the centerline of the X control chart) is 30% to 50% greater than the specified value.
7. Ductility
According to ASTM B490, the ductility of the bright nickel coating should be 0.1 or higher. The ductility of the semi-bright nickel coating should conform to ASTM B490 and should be higher than 0.4.
III. Process conditions
1. Double nickel
(1) The lower layer is made of semi-bright nickel plating, and the upper layer is bright nickel plating, forming a double nickel coating.
(2) The sulfur content in the coating is 0 to 0.005% for semi-bright nickel plating, and 0.05% to 0.07% for bright nickel plating.
2. Triple nickel
(1) Between the same semi-bright nickel layer and bright nickel layer as in double nickel plating, an intermediate nickel plating of 0.6 μm or more is applied.
(2) The sulfur content of the intermediate nickel layer is 0.1% or more.
3. Microporous chromium plating (ordinary chromium)
(1) In the nickel layer beneath the chromium plating, fine non-conductive particles are co-deposited during nickel plating, and the chromium layer produces a large number of micropores.
(2) The micropores exceed 2000/cm².
Due to China's vast territory and varying weather conditions, exterior automotive electroplated parts are put to a severe test. How to improve the management of electroplating quality is also one of the key issues for automakers and parts manufacturers alike—namely, conducting stricter experimental evaluations of various possible usage environments, maintaining the vibrant colors of electroplated components, and making them a highlight of the vehicle.