I believe that many people have black spots on the surface of bolts during salt spray testing. Today, I will analyze the causes and countermeasures:
“Our bolts are being plated with blue zinc, white zinc, color zinc, or other electrodeposited coatings. Customers will require a salt spray test at 48H/72H to ensure no abnormalities.”. The appearance of white rust during the salt spray test indicates that the electrodeposited layer (zinc layer, etc.) has been oxidized – zinc oxide is a white powder, while the appearance of red rust indicates that the material itself has begun to oxidize – we have all seen maroon iron oxide.
Many times during salt spray inspection, black spots or spots appear before the white rust appears
The reason for forming this black spot is that the impurities on the surface of the electrodeposited layer are oxidized, which is not the oxidation of the zinc layer or the material, but the result of the oxidation of impurities in the electrodeposited layer.
So how did the impurities in the plating come from? Mainly from the electroplating solution (electrolyte) in the electroplating tank. Electroplating solution contains metal ions, electrolytes, and other additives. There should be no impurities, but due to repeated application of the plating bath, debris on the surface of the plated workpiece, falling metal, and even unwashed oil stains remain in the plating bath. If the plating solution is not cleaned or replaced in a timely manner, impurities will adhere to the surface of the workpiece along with the plating ions. Each batch of electroplated products has more or less different levels of impurities, and the black spots reflected in salt spray inspection are also different. Some products that are invisible to the naked eye are considered to have no black spots, while others are very obvious.
How to prevent excessive impurities in electroplating solution? Clean the electroplating tank in a timely manner, replace the electroplating solution, and thoroughly clean the workpiece before plating to prevent oil and debris. “Some batches of our electroplating products have been inspected and found to be very good without black spots, while others have black spots. Obviously, this is related to the timing of the electroplating plant’s replacement of the electroplating solution. The newly replaced electroplating solution has no black spots, while the products from the electroplating tank with excessive impurities have black spots.”.