The formulation of hard chromium plating from trivalent chromium electrolyte and its related process have been intensively studied in this work. Through optimized conditions, the coating hardness can achieve more than HV 0.1900 without any treatment and HV 0.11700 after heat treatment for five minutes at 300 °C, and the thickness of hard chromium coating was about 100 μm. The hard chromium coatings show good adhesion on the carbon steel and low alloy structural steel. The corrosion resistance of hard chromium coatings was enhanced by the adding nanometer materials into trivalent chromium plating coatings. More than 120 hours salt spray corrosion resistance can be achieved with 40 to 50 microns thickness of trivalent chromium plating coatings.
Hard chromium as the functional electroplating coatings has been widely used for equipment manufacturing, such as automotive, hydraulic components, industrial roll and heavy-duty machine tools, etc. [
The hard chromium plating coatings with 60 microns thick have been reported, and the various factors associated with the trivalent chromium performance and related process have been investigated [
In this work, we studied the hardness and corrosion properties of trivalent chromium hard chromium coating. We found that its hardness and corrosion resistance were evidently improved, and that the possible factors responsible for these results were also be proposed. In this article, extensive research has been studied on the improvement of the trivalent chromium hard chromium corrosion resistance and its stability of the plating solution in the mass production process.
The volume of plating solution is 200 L, anode and cathode are DSA insoluble anode and 45# carbon steel bars with Φ32 mm and 20 cm in length, respectively. The area of covering cathode is 1.6 dm2. The current, potential, temperature, time and current density are 50 A, 12 V, 45˚C, 120 min and 30 A/dm2, respectively. The thickness and micro-hardness of thin film were measured by Portable Thickness Gauge (Fischer, mop) and micro-hardness Tester (Shanghai Jiving Precision, HV-10001S, the load is 100 g), respectively. The salt spray test and the measurement of grain size were carried out by Salt Spray Tester (Dungun Xebio, XB-OTS-90) and Metallographic Microscope (Instrument Manufacturing Co., 4X-CIS), respectively. The differential scanning calorimetry (DSC) of thin film was measured by NETZSCH-Gerätebau GmbH (STA-449C).
Adopt the method of continuous plating, until the solution was unable to plating, then adjust back to the normal usage situation, study the stability of the solution performance. Adopt the method of continuous plating, until the solution was unable to plating, then adjust back to the normal usage situation, study the stability of the solution performance. It must be ensuring that a new process in production can be stable for a long time. Plating test of trivalent hard chromium solution was carried out as the following steps. Solution: 2 cups of 1.6 L; Anode: DSA insoluble anode, 150 × 60 mm × 2 pieces; Cathode: copper bar, Φ 8 mm, immersed in the plating solution length is 8 cm, covers an area of 0.2 dm2; I = 6 A, J = 30 A/dm2, T = 40˚C. The rectifier is described as
By electroplating copper bar continuously, the only trivalent chromium was added in the solution, the destruction test was carried out, until it experienced 150 ampere-hours/L, the performance of plating bath was as shown in the Hull specimens (left), after adding additional agent, Hull specimen (right) showed the plating recovered well, the plating solution showed good restore ability (
By continuous electroplating copper bar while adding additional agent, we assess the terminal life of the trivalent hard chromium solution.
No. | Instrument | Manufacturer | Type |
---|---|---|---|
1 | DC Rectifier | Fushun Yingke | YK-3050 |
No. | Plating time (Ah/L) | Current density (A/dm2) | Tank pressure (V) | |
---|---|---|---|---|
1 | 0 | 30 | 28 | |
2 | 100 | 30 | 23.4 | |
3 | 200 | 20 | 19.9 | |
4 | 300 | 10 | 19.7 | |
5 | 430 | 5 | 19.8 | |
from left to right, from top to bottom, followed by electroplating time 0, 100 Ah/L, 200 Ah /L, 300 Ah/L, 430 Ah/L. Longer duration of electroplating, high area of the plating gradually becomes poor. When electroplating time is 430 Ah/L, on high area of the Hull specimen, the burning phenomenon is serious. And the working current density can only reach 5 A/dm2, reaching the limit of plating solution life.
The loading of micro-hardness tester is 100 g (HV0.1), force duration for 10 s. The specimens with trivalent hard chromium coatings were heat treated at temperature 100˚C, 200˚C and 300˚C respectively, heat preservation for 2 hours. After cooling to room temperature, the hardness tests were carried out. The differential thermal analysis for the specimen was carried out.
The metallographic image along the cross section of coating was shown in
The average thickness of coatings is about 20 microns; the micrographs of specimens are shown as following figures (Figures 4-7). After heat treatment, the picture shows images (400×) after the determination of hardness. The upper layer is trivalent coating, the lower layer is the base material (45 # steel).
No. | T/˚C | Without heat treatment | 100˚C | 200˚C | 300˚C |
---|---|---|---|---|---|
1 | Average coating hardness | 991 | 1050 | 1769 | 1803 |
2 | Substrate average hardness | 187 | 212 | 210 | 226 |
By continuous electroplating copper bar while adding additional agent, we assess the terminal life of the trivalent hard chromium solution.
NO. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
---|---|---|---|---|---|---|---|---|---|---|---|
Sodium sulfate (g/L) | original solution | 10 | 30 | 50 | 70 | 100 | 150 | 200 | 250 | 300 | 350 |
Tank pressure | 25.2 | 28.0 | 26.4 | 25.3 | 24.8 | 23.3 | 22.1 | 21.3 | 21.3 | 21.3 | 21.7 |
from top to bottom, followed by electroplating time 0, 100 Ah/L, 200 Ah /L, 300 Ah/L, 430 Ah/L. Longer duration of electroplating, high area of the plating gradually becomes poor. When electroplating time is 430 Ah/L, on high area of the Hull specimen, the burning phenomenon is serious. And the working current density can only reach 5 A/dm2, reaching the limit of plating solution life. From
When the content of the sodium sulfate by artificially adding is 100 g/L, the coating has no obvious change. On the other hand, the tank pressure obviously decreased with the increase of sodium sulfate. When the content is 200 g/L, the tank pressure no longer decreased with the addition of sodium sulfate. When adding sodium sulfate content in the 150 g/L, the burning and the mist phenomenon of the high coating area became more and more serious with the increase of sodium sulfate. Too much quantity sodium sulfate is the main reason of the decrease of the tank pressure and the burning phenomenon of the high coating area.
When plating solution arrive the limit life span, it does not represent that the plating solution should be scrapped. Since too much sodium sulfate is the main
reason of the burning phenomenon of the high coating area, then we can remove redundant sodium sulfate to restore the plating solution by cooling crystallization when reaching the limit of the life. The Hull specimens are shown as
Trivalent chromium plating hard chromium is amorphous and nanocrystalline, but when the plating stress accumulated to a certain extent, the coating can produce the through cracks. At this time, the electrolyte is easy to through the crack into the basement, which resulting in the formation of micro cell corrosion. The thicker coating is easier to cause through cracks because of the greater stress of coating. So, the increase of the thickness not only can not improve the corrosion resistance of trivalent hard chromium plating, instead, it may lead the decrease of the corrosion resistance because of the wider crack. If nano-meter particles exist in trivalent chromium plating coatings, maybe it can disperse the stress and reduce the cracks, so as to improve the corrosion resistance of coatings. Based on the above reasons, we designed a list of tests to improve corrosion resistance performance of the coatings.
The salt spray data are shown as
After adding LX in trivalent chromium plating, salt spray of the coatings increases gradually with the increase of coating thickness as
The influence on the trivalent chromium coatings corrosion resistance of different
No. | Plating solution | Work piece (45 # steel) | Thickness | Salt spray |
---|---|---|---|---|
1 | trivalent chromium bath | Polish rods | 10 μm | 0.5 h |
2 | hexavalent chromium bath (micro cracks agent) | 24 h | ||
3 | trivalent chromium bath | Roughen bars | 40 - 50 μm | 1 h |
4 | hexavalent chromium bath (micro cracks agent) | 72 h | ||
5 | hexavalent chromium bath (without micro cracks agent) | Polish rods | 10 μm | 2 h |
6 | Roughen bars | 45 μm | 30 h |
No. | Coating thickness (μm) | Salt spray |
---|---|---|
1 | 10 | 0.5 h |
2 | 20 | 12 h |
3 | 28 | 40 h |
4 | 43 | 70 h |
5 | 53 | 90 h |
6 | 68 | 105 |
7 | 92 | 120 |
8 | 10 | 0.5 h |
No. | LX content | Salt spray resistance property |
---|---|---|
1 | 0.2% | 2 h |
2 | 0.5% | 4 - 6 h |
3 | 1% | 6 - 8 h |
4 | 1.5% | <24 h |
5 | 2% | 24 - 48 h |
6 | 3% | 32 - 55 h |
7 | 5% | 48 - 60 h |
8 | 7% | 96 - 120 h |
9 | 10% | coating scorc hed |
10 | Hexavalent chromium (micro cracks agent) | 72 - 96 H |
Note: LX is a nanometer material which we made by ourselves.
contents LX (nanometer additive) (roughen iron bars, thickness in 40 - 50 μm) are shown in
The hard chromium coating was prepared from trivalent chromium solution, and then the chromium coating was carried out differential thermal analysis. The DSC curve is shown as
The trivalent chromium pilot bath is shown as
More than 100 microns thickness of trivalent chromium plating coatings has been achieved. Moreover, hardness can be more than HV0.1950, with iridium tantalum DSA anode. After heat treatment, the hardness can be more than HV0.11700. The trivalent hard chromium solutions are stable, easy maintenance and continuous plating 430 Ah/L. With nanometer additive, the corrosion resistance of trivalent hard chromium coatings shows obvious improvement.
Li, J.Z., Li, Y.J., Tian, X.H., Zou, L., Zhao, X., Wang, S.F. and Wang, S.G. (2017) The Hardness and Corrosion Properties of Trivalent Chromium Hard Chromium. Materials Sciences and Applications, 8, 1014-1026. https://doi.org/10.4236/msa.2017.813074