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 Table 1.

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 (Figure 1).

By continuous electroplating copper bar while adding additional agent, we assess the terminal life of the trivalent hard chromium solution. Table 2 shows the change of tank pressure and current density in different electroplating time,

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 Figure 2, the thickness of trivalent hard chromium coating is over 100 microns. The hardness of trivalent hard chromium coating was HV_0.1950. With the increase of temperature, macroscopic crack on the surface of the specimen will be more obvious, may be due to release internal stress of the coating. With the increase of heat treatment temperature, the hardness of coating gradually rises. The hardness value rises significantly up to HV1800 when heat treatment temperature was 200˚Cand 300˚C.Maybe when temperature rises, amorphous or nanocrystals of chromium changes to the crystalline state, so that the hardness increases greatly. The hardness of basic material went down slightly fast as the up of temperature, but not obvious. It may be having the wide industrial application. The result was shown as Table 3 and Figure 3.

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).

By continuous electroplating copper bar while adding additional agent, we assess the terminal life of the trivalent hard chromium solution. Table 4 shows the change of tank pressure and current density in different electroplating time, and the Hull Specimens corresponding as shown in Figure 3: 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. From Table 3, we can know that, the tank pressure of plating solution decreased gradually with the increase of concentration of sodium sulfate. The influence of sodium sulfate was observed by artificially adding the sodium sulfate to the plate solution and the results shows in Table 4 and Figure 8.

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 Figure 9 before and after cooling crystallization of solution.

Figure 9 shows that coating is recovered very well by cooling crystallization process, and plating solution can continue to be used. One side, the destruction test showed that the plating solution has the good restore ability. On the other hand, plating solution has a long terminal life which can reach 430 Ah/L by adding additional agent. In the meantime, plating solution can still be restored when it reached the terminal life by removing the enrichment of conductive salt sodium sulfate. Tank pressure of the plating solution will gradually decline along with the increase of current efficiency, and the enhancement of dispersion and cover ability, so plating solution after working a period of time is better than the new plating solution. So, we can give the conclusion that the stability of trivalent chromium plating solution is very good.

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 Table 5. The salt spray of trivalent chromium coating and hexavalent chromium coating are respectively 0.5 h and 24 h when the thickness is 10 μm with polished bars which is simulation of roll. When the thickness is 40 - 50 μm with roughen bars, which is simulation of the technology of roll, the salt spray of trivalent chromium and hexavalent chromium coating (adding micro cracks agent) are respectively 1 h and 72 h. Without the micro cracks agent, the salt spray of 10 μm of the hexavalent chromium coating is only 2 h and 45 μm is 30 h. It shows that the thicker is the coatings of hexavalent chromium plating, the better is corrosion resistance. However, the thickness of trivalent chromium coatings does not affect the corrosion resistance at all.

After adding LX in trivalent chromium plating, salt spray of the coatings increases gradually with the increase of coating thickness as Table 6 was shown. But if the coating is too thin, salt fog resistance cannot be improved. The salt spray of original plating solution plating (without LX) does not change with the Change of the thickness of the coating.

The influence on the trivalent chromium coatings corrosion resistance of different

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 Table 7. In the solution with magnetic stirring, the salt spray of trivalent hard chromium coatings increased gradually (up to 96 - 120 h) with the increase of LX content.

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 Figure 10. The first peak at about 300˚C may show that the coating has occurred crystal transfer. It is from amorphous to crystal. The second peak at about 600˚C maybe show that some polymorphism of the coating occurs.

The trivalent chromium pilot bath is shown as Figure 11, it has about 200 liters of solution. And the worms, disk hub parts and rear axle with trivalent chromium hard chromium coatings are shown as Figures 12-14.