The corrosion resistance of piston rod of automobile shock absorber is one of the main factors affecting the quality and life of automobile shock absorber. The selection of electroplating process parameters of piston rod determines the quality and corrosion resistance of chromium plating layer. The corresponding relationship between the current density of piston rod of automobile shock absorber and the microstructure of chromium plating layer was found, and the optimized current density parameters were obtained through salt spray test. The comparison of salt spray tests of piston rod with different surface qualities shows that the corrosion resistance of connecting rod can be significantly improved by superfinishing the piston rod surface of automobile shock absorber before electroplating.
The piston rod of automobile shock absorber is the heart of its suspension system. If the corrosion resistance is not good, it will lead to the early failure of the piston rod and oil leakage. This kind of automobile shock absorber damage caused by piston rod corrosion accounts for a high proportion. Therefore, it is of great practical significance to improve the corrosion resistance of shock absorber piston rod.
The surface of the piston rod of the shock absorber is plated with hard chromium to improve the corrosion resistance and wear resistance of the connecting rod. Because chromium can easily form a passive film on the surface of connecting rod, the corrosion resistance of piston rod of shock absorber can be greatly improved. Different from other metal coatings, chromium coating has microcrack structure. Micro-cracks have two effects on corrosion resistance: on the one hand, because there are a certain number of micro-cracks in the chromium plating layer, a small amount of damping liquid can be stored in the automobile shock absorber when it is used, which avoids "dry friction" and prolongs the service life of the product; On the other hand, because of the strong activity of crack edge, it becomes the storage place of corrosion medium in the process of electrochemical corrosion. Under the action of alternating stress, corrosion fatigue will also occur, which will accelerate the corrosion rate of the coating, change the potential sequence between the chromium plating layer and the substrate (No.35 carbon structural steel), and accelerate the corrosion of the substrate. Therefore, the micro-cracks in chromium plating layer have an important influence on the corrosion resistance of piston rod, and the number, distribution and surface defects of micro-cracks are closely related to the electroplating process and pretreatment of connecting rod.
Machining technology of piston rod
The connecting rod is made of 35 # steel, and the processing technology is: cold drawing, turning, continuous medium frequency induction quenching, pre-grinding the excircle, pre-grinding the excircle, fine grinding the excircle, ultra-finishing, chromium plating, dehydrogenation tempering and ultra-precision grinding. In order to study the relationship between surface quality and corrosion resistance of connecting rod, superfinishing technology was added before chromium plating.
Plating equipment is a German imported electroplating line.
Bath composition:
Chromic acid 225-275g/l,
2.5-4.0g/L sulfuric acid,
Cr2O3 3~ 10g/L，
dw-032 10-20ml；
The temperature of the plating solution is 55-58 DEG C;
For 20 min, the cathode current efficiency is 26%-36% and the current density is 40-90A/dm2.
technical characteristics
1, and the cathode current efficiency is as high as 35-45%. The deposition speed is fast, which can save about 40%-50% of the electricity bill.
2. Cathode without fluorine, rare earth and low current corrosion.
3. The lead and tin anodes will not be severely eroded, and special anode materials are not needed. (due to the addition of anodic corrosion inhibitor)
4. Good finish and high electroplating ability. High dispersion ability, the four corners of the inner mouth of square parts can be evenly chrome plated. The coating thickness at the corner is close to the surface thickness.
5. The hardness of the coating is very high, reaching over HV 1000- 1200.
6. Micro-cracks can be generated, and the number of micro-cracks can reach 800-2000 /cm (adjusted as required), thus improving the corrosion resistance. The piston rod can resist salt spray for more than 500 hours. The shock absorber rod is resistant to salt spray for more than 500 hours.
7. Wide range of current density, which can be as high as 90 amps/square decimeter. According to the calculation of large current, the current efficiency can reach 45%, and the plating speed is increased by 1.2 times.
8. The adhesion between the coating and the substrate is excellent, and even the chromium layer of small parts will not fall off. The chromium-plated layer is really high in hardness, good in wettability and extremely wear-resistant.
New plating solution, add15-20ml/L.
Supplement: KAH: 50-100ml/L.
Using this additive to chrome plate 2-3 wires, the hardness of chrome plating layer is 750 ~ 980 HV, and through 750h salt spray test, the multi-layer electroplating is completely overcome and the cost is solved.