Alkaline galvanized common faults and corrections

- Nov 28, 2017 -

(1) coating rough, passive film is not bright. The temperature of the bath is too high; the cathodic current density is too large, the zinc content in the bath is too high or the content of sodium hydroxide and DPE is too low; solid particles or excessive metal impurities in the bath can cause such failures.

Analysis of the failure, the first observation of the phenomenon, if the coating is rough coating to the top, there may be solid particles in the bath; if the rough edges and tips of the parts more serious, the current density may be too large; if galvanized Layer is still good, but in the 3% nitric acid in the light, the coating has a shadow, the film appears when the passivation tan, zinc plating solution may be copper or lead and other metal impurities.

When such failures occur, first check the temperature and current density, and then through the bath analysis, determination and adjustment of zinc and sodium hydroxide bath content, DPE content is low, you can Hull groove test to understand. If the rough coating is not caused by the above reasons, it may be caused by the impurities in the bath, you can take a small amount of plating solution, after filtration I test, and then take a small amount of bath, with zinc powder after the test, check the failure Whether it is solid particles or copper, lead and other impurities caused by different metals, one by one test, the cause of the malfunction is not difficult to find.

(2) coating blistering, bonding is not good. Pre-plating is not good; bath temperature is too low; poor quality additives or additives and organic impurities will result in poor bonding.

Alkaline zincate zincate degreasing and activation ability than cyanide zinc plating solution, so the pre-treatment requirements than cyanide zinc, oil is not completely over or acid corrosion will cause the coating blister. However, the main reason for the foaming of such baths is that the adsorption of organic additives on the crystal plane of the plated layer causes the original crystals or some crystal planes to not grow normally, resulting in the abnormal development of new crystal nuclei or some crystal planes. Organic additives in the coating in the folder, hinder the normal lattice arrangement, causing lattice distortion, thereby increasing the internal stress coating, resulting in coating blistering. Generally, the more the amount of additives is, the lower the temperature of the plating solution is, the larger the amount of the additive adsorbed on the electrode surface is. Therefore, the plating layer has greater internal stress and is more likely to foam.

The quality of the additive also affects the blistering of the coating. Some of the additives are incompletely synthesized during their synthesis and continue to polymerize during long-term storage or long-term use. Such additives may easily strain the lattice and cause stress, causing the coating to bubble.

When the coating bubbles, first check the bath temperature. If the bath temperature is not low, then strengthen the degreasing before plating to prevent the matrix metal in the acid corrosion. If you pay attention to these issues, blistering still exists, then you should pay attention to the amount and quality of additives, then you can stop adding additives, with a high current electrolysis for a period of time, to reduce the content of additives to observe the foaming is improved. If there is still no improvement, you should check whether the additive storage period is too long, or additives, excessive impurities. A factory has done tests, using industrial grade raw materials synthesized DPE-Ⅰ, added to the plating bath plating, plating blister, and with high purity raw materials synthesized DPE-Ⅰ, added to the bath plating, the coating on Not blistering Then they will industrial grade raw materials synthesized DPE-Ⅰ, electrolysis before use for some time, and then use, blistering basically did not occur, the plant's test proved that the purity of the additives affect the coating blister should be Attract attention

If the coating blister is caused by the poor quality of the additive, then on the one hand, the plating solution should be treated, the treatment method available hydrogen peroxide-activated carbon or potassium permanganate-activated carbon. There are information on the use of lg / LBa (OH) 2 and 1g / LCa (ocl) 2 treatment bath, the effect is good; the other hand, unused additives, electrolysis before use to reduce the additive impurities in the coating Bubble effect.

(3) slow deposition rate, poor plating ability. Low bath temperature, low cathodic current density, too low sodium hydroxide or DPE content in the plating bath, or other types of impurities such as turbidity or lead in the plating solution.

In fact, low bath temperature, low sodium hydroxide or low DPE levels lower the cathodic current density limit, slow the deposition rate due to the reduced cathodic current density, and lower current densities in the deep pockets of the part. Zinc layer. Bath temperature is low only in the winter, is generally not common. Low sodium hydroxide in the bath can also cause the coating to be rough, which can be adjusted as analyzed. Low DPE content Low cathodic current density decreases, poor gloss coating. It can be adjusted by Hull slot test.

The bath is turbid, the naked eye is able to identify and found the bath turbid, the first analysis should be adjusted bath composition, and then filtered bath to remove solid impurities.

If lead impurities exist in the bath, sometimes the cathode is not plated with zinc, sometimes even if the coating is obtained, the coating immersed in 3% nitric acid in the light, not only does not shine, but there will be dark shadow flowers Blot, and passivation will not be satisfied with the passivation film, where there is such a phenomenon should be found in the source of lead impurities (lead impurities mostly from impure zinc oxide or impure anode), and then with zinc Dispose of plating solution, remove lead and other dissimilar metals.

(4) coating dark, passivation film is not bright. Too little additive in the plating solution or too much impurity of the dissimilar metal will make the coating dark, and can not obtain the bright passivation film when passivation.

Whether the additive content is too small can be determined by Hull cell test. Different metal impurities can also be Hull slot or other small test to be checked, the fault solution with zinc powder after the test, if the coating color is significantly improved, that is the impact of different metal impurities, then the bath should be electrolysis or zinc Powder treatment to eliminate the influence of different metal impurities.

(5) Parts of the tip or edge easy to scorch. Plating bath of zinc and sodium hydroxide content or additive content is too low, the bath temperature is too low or the cathode current density is too large, etc. will make the coating easy to scorch.

When such phenomena occur, first check the temperature and current density, and then analyze the bath composition, if there are problems, according to the inspection and analysis to be corrected. In fact, the tip and edge of the part are easily burned, which means that the current density of the plating solution is relatively narrow. This test can better reflect the situation by using the Hull cell test. For example, The current model cathode current range of the most wide, coupled with the bath analysis, so that failure is not difficult to rule out.

Article from NdFeB Industry Network

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