December 2017 Archives

[Table 1] shows the electroplated steel sheets.

[Table 1] Electroplated steel sheets

Compared to hot-dip galvanized steel sheets, electro-galvanized steel sheets are produced with less plating deposited. For its evenly deposited surfaces, better workability and aesthetic finish, electro-galvanized steel sheets are adopted in various fields.For applications requiring even superior corrosion resistance, increasing the amount of plating deposited is one of the options. However, doing so will adversely affect the workability and weldability.
To resolve such issue, electro-galvanized alloy plating was developed.This type of steel sheets has enhanced corrosion resistance, superior painting, weldability, and stamping workability with less electroplating deposited.Zn-Ni based and Zn-Fe based plating is the most common type of alloy plating.Zn-Ni based plating contains 10 to 13% of nickel. Compared to pure zinc, this material can withstand extremely corrosive conditions. Even without the extra coating, the corrosion products having the superior anti-corrosion properties can prevent the corrosion to progress further.To improve the corrosion resistance further, composite films with organic coating applied over plating have been developed recently.
Steel sheets with Zn-Fe based plating applied contain 10 to 30% of steel. The main purpose of using steel is to improve the corrosion resistance after plated sheets are treated with additional coating.Double-layer zinc alloy plated steel sheets are formed by flash-plating Fe-Zn or Fe-P over the Fe alloy layer. The double-layer sheets have superior electrodeposition than the Zn-Fe based steel sheets.

A variety of surface-treated steel sheets that are available can be classified by the production method, such as hot dipping, electroplating, painting, and a combination of these. Each type of surface-treated steel sheets will be introduced hereinafter.

[Table 1] shows the types, characteristics, and major applications of hot-dipped steel sheets.

Hot-dip galvanized steel sheets constitute more than 80% of the total production volume of hot-dip galvanized steel sheets. Since not many of them are used as-is, they are usually coated prior to use.For those that are adopted without the coatings, they used be treated with chromate in order to prevent generation of white rust after being plated. Since the introduction of chromium-free antirust coating that is compliant with green procurement, chromium is no longer the antirust treatment option.
As [Table 1] clarifies, the molten and alloyed aluminum coated steel sheets are adopted for components requiring enhanced heat resistance.

(1)Method of adhesion test

JIS defines the following two methods of adhesion test of decorative electroplating applied over plastics:
1. Method of adhesion test, 2. Simplified method of adhesion test

(2)Method of adhesion test

This testing uses a tension tester to measure the adhesion strength of plating quantitatively when the plating film is peeled off from the substrate. This method is also known as the peeling test method.

1)Sample preparation

1. Cut off a sample from copper-plated components that are at least 50 µm thick. The area can be any size but must be free from distortion and must have consistent properties.   2. Use the components that are treated with heat at 80°C for one hour or those that had been plated over 48 hours ago.   3. As shown in [Fig.1], make a parallel incision (dotted lines) of approx.10 mm wide on the plating surface using a sharp knife.   4. Peel off the plating film for approx. 5 mm long roughly at 10 mm wide from the edge. Lift it up and peel off 20mm more using pliers.To reinforce the peeled-off portion, paste fiber-containing tape over it.

2)Test operation

(3)Simplified method of adhesion test

This test is a simplified method of measuring the adhesiveness when the plating is separated from the substrate using a spring balance. Using a spring balance having the minimum scale of 20 g, secure the sample on the balance and attach the fixture for the test sample as shown in [Fig.3]. Then, peel off the plating.Read the value when the plating is separated.

(1)Appearance

The appearance of decorative electroplating on plastics must be free from detrimental flaws that could affect the usage, including uneven luster or color, tarnishes, spots, swelling, scratches, pits, peeling, cracks, and exposure of substrates or base-coat plating.

(2)Number of micropores and microcracks on chromium plating (Cr mp, Cr mc)

The number of pores and cracks on microporous/microcracked chromium plating is as follows:

They are counted by applying copper sulfate plating over the chromium plating surface and enlarging the distribution pattern of copper deposited over the plating surface.

(3)Cold temperature cycle test

This is a stress test designed to measure the adhesiveness of plating using the difference of thermal expansion rates between plastic substrates and plating films.[Table 1] shows the test conditions.The samples are exposed at certain temperatures for specified time in the controlled test equipment.Here is an example.
-20℃(1h)→20℃(0.5h)→80℃(1h)→20℃(0.5h)
After this testing, the plating should be free from notable defects, including swelling, peeling, cracks, deformation, and so on.

[Table 1] Cold temperature cycle test

(4)Corrosion resistance test

In addition to CASS test, Corrodkote test, and neutral salt spray testing, the following two corrosion resistance measurements are available for this type of plating: 1. Nitric acid aeration test where samples are exposed in nitric acid vapor; 2. High-temperature and humidity testing where samples are exposed in the high-temperature and high-humidity environment (temperature of 50±2℃ and relative humidity of 95±5%) to evaluate the plating's corrosion resistance.
[Table 2] shows the testing hours of the first three testing methods.

[Table 2] Corrosion resistance test hours

Apply the base-coat plating comprised of nickel or copper-nickel over plastics, and then apply the decorative plating comprised of chromium, gold, silver, tin-cobalt alloy, tin-nickel alloy, tin-nickel-copper alloy, and so on.

(1)Grades, symbols, and minimum thickness of decorative plating

[Table 1] Grades, symbols, and minimum thickness of plating

(2)Plating categories, types, and symbols

[Table 2] Plating categories, types, and symbols