1. Rough substrate surface before plating: If the substrate itself is rough after turning or grinding, electroplating will "replicate" this defect. The substrate should be finely ground or polished first to ensure a surface roughness Ra ≤ 0.4μm before plating; chemical polishing or electrolytic polishing pretreatment can also be used to improve uniformity.
2. Improper control of electroplating parameters: Excessive current density, unstable plating solution temperature, or imbalanced composition (such as excessive trivalent chromium or an unbalanced sulfuric acid ratio) will lead to coarse crystals. The plating solution ratio needs to be adjusted to maintain the CrO₃ concentration at 250–300 g/L, control the anode-cathode area ratio at 1:5 to 1:10, and use pulse electroplating technology to obtain a denser coating.
3. Missing post-treatment steps: Failure to perform rolling, ultra-fine grinding, or mirror polishing after plating can easily lead to a high Ra value. Roller burnishing can be used to reduce surface roughness through cold work hardening, while improving wear resistance and fatigue strength. For high-precision requirements, ultra-fine grinding or mechanical polishing is recommended to reduce Ra to below 0.2.
4. Pinholes, pits, or nodules in the plating layer are mostly caused by suspended impurities in the plating solution, incomplete degreasing, or gas retention. The plating solution should be filtered to remove particles; pretreatment should be strengthened to ensure no oil or oxide scale; the fixture design should be optimized to avoid air pockets and ensure smooth gas discharge.
Rework Recommendations
1. Slightly exceeding the limit (Ra slightly higher than 0.8μm): Grinding and polishing can be performed directly to restore Ra to below 0.4. This is suitable for general guide shafts or support rods.
2. Obvious roughness or tool marks: It is recommended to remove the plating and re-plat. First, remove the original plating by acid pickling or reverse polarity method, then re-plat according to specifications to ensure process stability.
3. Localized defects (such as pitting, pinholes): Localized grinding and touch-up plating can be performed, repairing only the affected area, saving time and costs.
✅ Note: For high-precision applications such as hydraulic system piston rods and automated equipment guide shafts, it is not recommended to directly use chrome-plated rods with excessive roughness, as this will accelerate seal wear, causing leaks or operational malfunctions.
Preventive Measures:
1. Set up inspection points at each stage before, during, and after electroplating to achieve full-process quality control;
2. Filter the plating solution at least once a week and regularly test CrO₃, sulfuric acid ratio, and trivalent chromium content;
3. Prioritize suppliers with online testing capabilities and complete quality control systems to ensure batch consistency.
