The chromium plating layer, especially the gun barrel chromium plating layer, has strict requirements for thickness uniformity. Because of the poor dispersion ability of chromium plating solution, for large workpieces, especially hard chromium plating with long plating time and large current, the thicker the chromium layer is, the simpler the current is distributed unevenly, resulting in defects such as taper, Ovality and "dog bone" shape. This paper introduces the influence of anode, internal resistance and trivalent chromium on the thickness uniformity of chromium plating layer.
1. Influence of anode and plating part layout
1.1 anode position
Generally, the uniformity of the thickness of the chromium plating layer depends on the relationship between the anode and the plated part. In general, the closer the distance between the anode and the plated part, the more consistent the chromium plating layer is with the geometry of the plated part. However, when the outer diameter of the anode and the plated part are not similar, if the distance between the anode and the plated part is increased, the current can be evenly distributed on the anode surface; If the distance between the anode and the plated part is too close, the current distribution is not uniform. When electroplating outer diameter workpieces, the distance between anode and plated workpieces shall be controlled within 10-25 cm. If the anode is too far away from the plating part, the solution resistance increases and the power loss increases. The following points shall be achieved during operation:
(1) the plated parts with simple shape can be closer to the anode; The distance between the irregular shaped plated parts and the anode shall be as large as possible.
(2) the plated parts with different sizes cannot be plated in the same bath to prevent the current from spreading in a severe uneven state.
(3) the distance between the different parts of the plated parts and the anode should be as equal as possible, so that the resistance of the solution is similar and the current is evenly distributed.
(4) position the plated part (especially the shaft) in the respective "box type" composed of four anodes, so that the current can be evenly distributed on the surface of the plated part to prevent ovality.
1.2 anode is too long
The lead alloy anode used for hard chromium plating should be shorter than the plated part (especially the shaft). The top of the anode is slightly lower than the top of the plated part, while the bottom of the anode is higher than the bottom of the plated part, so as to prevent the current at the excessively long part of the anode from spreading to the plated part and showing a "dog bone" shape, that is, the upper chromium plating layer is too thick, the center is slightly thin, and the lower part is relatively thick.
The length of chromium plated anode is usually determined according to the depth of the plating bath. Unless the special anode designed and manufactured with the finalized product is adopted, the lower part of the anode is generally 15 cm from the bottom of the bath and the upper part is 5 cm higher than the liquid level. For anodes in this condition, if the products are electroplated (generally shaft parts), the anode surface beyond the plating part (top or bottom) can be wrapped and insulated with plastic tape; If it is an amorphous product or a workpiece with inconsistent length, a maintenance cathode can be added to the upper and lower parts of the plated part.
2. Influence of longitudinal dispersion of internal resistance
2.1 solution resistance
The longitudinal dispersion of the solution resistance causes the hydrogen bubbles generated in the chromium plating process to continuously overflow from the barrel, driving the solution to move from bottom to top and constantly update. At this time, the mass concentration of the upper solution is low and the number of bubbles is large, so that the vertical dispersion of the solution resistance is uneven.
2.2 polarization resistance
Because of the gradient of the number of bubbles in the plating solution, the more upward the bubbles occupy, the faster the exchange rate of the solution in the upper part of the cathode than in the lower part. This non-uniform state of the activity rate also constitutes a gradient state in which the longitudinal dispersion of the polarization resistance is gradually small up and large down.
2.3 anode resistance
Steel wire lead anode is generally used for barrel chrome plating. The potential drop caused by the resistance of the steel core anode makes the voltage between the cathode and anode on different sections in the barrel different (for example, the voltage at the upper part is higher than that at the lower part), forming a gradient from top to bottom. This condition makes the longitudinal dispersion of current on the cathode surface uneven.
3. Effect of trivalent chromium
The mass concentration of trivalent chromium in chromium plating solution has a significant effect on the longitudinal dispersion of internal resistance. Practice has proved that with the addition of trivalent chromium in the plating solution, the longitudinal dispersion unevenness of internal resistance is gradually deleted. When the mass concentration of trivalent chromium is high, the bottom-up flow rate of the solution in the barrel becomes slow, which is obviously caused by the slow gas generation rate. The slow flow rate of the solution increases the mass concentration gradient of the solution, and the longitudinal gradient of the solution resistance increases; At the same time, the replacement rate difference between the upper and lower cathode surface solutions is reduced, resulting in the longitudinal gradient of polarization resistance decreasing. The increase of the longitudinal gradient of the solution resistance and the decrease of the longitudinal gradient of the polarization resistance are conducive to the formation of the gradient of the resistance (including the anode resistance) from the top to the bottom, so as to improve the uniformity of the longitudinal distribution of the current.
In the process of chromium plating, the basic chromium chromate film is continuously formed on the cathode surface, while the chromium sulfate ion continuously dissolves the basic chromium chromate film on the cathode. The process of film formation and dissolution is replaced on the cathode surface. When the dissolution rate of the film is slow, the hydrogen separation is faster; When the dissolution rate of the membrane is faster, the hydrogen separation is slower. It is believed that when the mass concentration of trivalent chromium is high, the amount of chromium sulfate ions in the solution increases, which accelerates the dissolution of the basic chromium chromate film on the cathode, thus slowing down the separation of hydrogen. It may be that the resistance to be overcome when trivalent chromium is oxidized to hexavalent chromium is smaller than that when oxygen is separated. Therefore, when the mass concentration of trivalent chromium is added, the reaction rate of trivalent chromium oxidation to hexavalent chromium is accelerated, and the generation rate of oxygen is correspondingly suppressed.
In view of this, the mass concentration of trivalent chromium in the chromium plating solution should be maintained above 10 g / L (the process specification is within the range of 8-12 g / L) for the chromium plating of the barrel (especially the elongated barrel). When the mass concentration of trivalent chromium is low, the deposition rate of the upper chromium layer is fast and the lower part is slow, forming an uneven chromium plating layer with a thick upper layer and a thin lower layer. However, when the mass concentration of trivalent chromium is too high (generally above 14 g / L), the deposition rate of the upper chromium layer is slow and the lower chromium layer is fast, forming an uneven chromium plating layer with thin upper layer and thick lower layer.
In addition, ferric ions also have an effect on the process of film formation and dissolution replacement on the cathode surface, which is similar to that of trivalent chromium. That is to say, both ferric ions and trivalent chromium affect the hydrogen generation rate. Therefore, the mass concentration of ferric ions shall be less than 8 g / L in the process.
(1) the uniformity of the thickness of the chromium plating layer generally depends on the relationship between the anode and the plated part. Therefore, the distance between the anode and the plated part should be reasonably controlled during operation.
(2) the longitudinal taper of the chromium plating layer thickness of the barrel is closely related to the mass concentration of trivalent chromium in the chromium plating solution. In order to obtain an ideal chromium plating layer, the mass concentration of trivalent chromium should be strictly controlled within a certain range.
Source: fast hard chromium additive http://www.newstar-china.com
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