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Why do black spots appear in the salt spray test of electroplated products?

When the workpiece or screw plated with blue zinc, white zinc, color zinc or other electroplated coatings is tested by neutral salt spray, the general user’s drawing will require how many hours without white rust and red rust, such as 48 hours without white rust and 72 hours without red rust. The appearance of white rust indicates that the electroplated layer (zinc layer, etc.) has been oxidized – zinc oxide is white powder, while the appearance of red rust means that the material itself has begun to oxidize – iron oxide has been seen, maroon.

In fact, we often find that black spots or spots appear before white rust appears in salt spray test, which is a bit embarrassing, as shown in the figure below:

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Black spots after salt spray test

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Salt spray test black spot

Causes of black spots or spots

The reason for this black spot is that the impurities on the surface of the electrodeposited coating are oxidized. It is not the oxidation of the zinc layer, let alone the oxidation of the material, but the result of the oxidation of the impurities in the coating.
So how did the impurities in the coating come from? It mainly comes from the electroplating solution (electrolyte) in the electroplating tank. The electroplating solution contains metal ions, electrolytes and other additives and should not have impurities. However, due to the repeated use of the plating tank, the sundries on the surface of the plated workpiece, falling metal and even unclean oil stains remain in the plating tank. If the electroplating solution is not cleaned or replaced in time, Impurities are reduced on the surface of the workpiece with the plating ion. The accumulation of such impurities is not a sudden change from qualitative change to quantitative change, but a process of gradual increase. In other words, there are more or less impurities in each batch of electroplated products, but the degree is different, and the black spots reflected in the salt spray test are also different. Some are invisible to the naked eye and are regarded as no black spots, while others are obvious as shown in the figure above.
At present, it is difficult to determine what kind of organization this black spot is and what its component molecular formula is. There are many kinds of impurities in the electroplating solution. It seems that there is no authoritative explanation for which component oxidizes into black spots in the salt fog. It is collectively referred to as impurities.
Interestingly, electroplating manufacturers and suppliers sometimes interpret it as “black spots caused by high current”. Black spots may appear if the current is too high, but there is no need for electroplating manufacturers to deliberately increase the current. Therefore, this explanation may be a kind of “good faith prevarication”, which does not need to be too serious.

Solutions for black spots or spots

How to avoid excessive impurities in electroplating solution? It is necessary to timely clean the electroplating tank, replace the electroplating solution, and fully clean the workpiece before plating to avoid oil and debris. The frequent replacement of electroplating solution will lead to the rise of cost. Electroplating plants (dare not say all, most of them) usually wait for impurities to be replaced. Therefore, there is also a phenomenon that some batches of our electroplated products are tested well without black spots, and some batches have black spots. Obviously, this is related to the timing of the electroplating plant to replace the electroplating solution. The products from the newly replaced electroplating solution have no black spots, while the products from the electroplating tank with too many impurities naturally have black spots.
As a metal parts manufacturer, such as our company, electroplating is basically outsourced to electroplating manufacturers. If it is required that “our products require the replacement of electroplating solution”, the electroplating manufacturers are faced with operational confusion. It is a big job to replace the electroplating solution once, let alone whether or when to replace it.
Does the black spot in the salt spray test mean that there is also poor antirust performance in the atmospheric environment?
I haven’t made a large-scale observation record myself, but I did some tracking for the needs of my work. From limited experience, the workpieces with black spots in salt spray test show no obvious signs of early rust in daily use compared with those without black spots. The possible reason is that impurities are only trace components in the coating and do not affect the protection of the coating to the workpiece material.
This conclusion only argues for the workpiece with black spots. If there are large areas of black spots in the test, it is recommended to return plating from the perspective of insurance.
As a user, how to judge black spots?

How to judge the black spots found by users as incoming materials?

There is no unified standard. Some enterprises think that this situation is normal, and they are qualified if there are not very serious black spots. Some enterprises even don’t consider black spots. Some enterprises do not accept black spots.
My personal suggestion based on the actual situation of the industry is that the black spots do not form patches (i.e. black spots), and the proportion of defects in the total area does not exceed 2.5%. It is recommended to be judged as qualified. Of course, the defects mentioned here only refer to the black spots (black spots) proposed in this paper, and how to consider white rust or red rust.

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