Common defects in the quality of nickel-tungsten coatings are more pitting, rough coatings on the side and spherical surfaces with white particles, severe unevenness between the central coating and side, corners and R, and even micro cracks at the corners and R.
1. pitting
Pockmarks are tiny white and black dot-like defects on the coating. They have various shapes, some are obvious and regular, easy to find, and relatively easy to control; a small amount of “invisible” to the naked eye can only be inspected with instruments.
Cause Analysis:
The mold matrix is caused by blisters, pores, pitting, adhesions, etc.; the mold adheres to sand particles during sandblasting, or the sand quality is poor, or the sand quality is caused by impurities, oil, foreign matter, dust, etc.; the mold is degreasing Adhesive emulsions, impurities in the water during cleaning, etc.; compressed air, pure water and its pipelines, tools and impurities in the environment used in liquid preparation; undissolved salts in the plating solution and activation solution Particles, acidified electrode metal objects, etc. can cause pitting.
The above-mentioned pitting can be seen directly, and some of the thin layer of adhesion on the surface of the mold, after analysis, is mainly the carbon compound in the contaminated sand before plating. It is firm, strong adhesion will gradually become acid and alkali resistant, anti-vibration high viscosity jelly. It is not easy to remove during degreasing and cleaning. Once it is neglected, it will be covered by the plating layer when it flows into the electroplating process.
2. Rough coating
Cause Analysis:
The basic composition of the plating solution is nickel sulfate, sodium citrate, sodium tungstate, etc., as well as salts and organic complexes. The test found that the defects of mold plating are mainly related to the impurity content in the plating solution. The purity of chemicals is not high, the foreign matter in the plating solution, the frequent use of the plating solution, and the metal impurities Cu, Fe, Cr, Co content in the plating solution exceed the allowable range, etc., can cause the coating to be rough, pitting, and white. Particles.
3. The thickness of the coating is severely uneven
The center and side, corners, and R areas of the coating are severely uneven, and even micro cracks appear at the corners and R areas.
Cause Analysis:
After the plating solution is used for a long time, the probability of defects in the coating increases. The white particles on the side and spherical surface, the micro cracks at the corners and R are particularly obvious. The test found that when the impurities Cu, Fe, Cr, and Co content in the plating solution were significantly increased to 20, 20, 20, and 50 mgl, the plating quality was significantly reduced.
The test also found that the distribution of the electroplating current directly affects the uniformity of the coating. The current density at the edges, corners, and R of the mold is obviously higher than that of other parts, the corresponding metal deposition is more, and the coating thickness is large. Under normal circumstances, the difference in thickness between the center and the side, corners, and R of the plating layer is about 4μm. Uneven current distribution will cause the coating thickness difference to reach more than 15μm. In severe cases, the coating will fall off due to excessive local metal deposition.
In addition, the shape, structure, and material of the mold also affect the quality of the coating.