Why does nitric acid turn black after galvanizing out of the groove?

The concentration of nitric acid is too high, pure white electroplating is the most difficult technology to master in cold galvanizing, you have to find a professional galvanizing master to mix nitric acid passivation solution.

In fact, the most important thing is caused by other metal impurities in the coating, especially copper and lead, and the source of pollution in the production line is investigated. The bath solution can be treated with impurity remover, zinc powder, or low-current electrolysis.

For this phenomenon, the location of the blackening should be observed first. If most of the blackened parts appear in the corners with large currents, if black spots appear on the plated parts during barrel plating, it means that there are more iron impurities in the plating solution. If it is not in the corners, but it is irregular, it is likely that the surface of the coating before and after passivation is contaminated with acid.

The blackening of the plating after passivation caused by iron impurities in the plating solution is a common symptom in the potassium chloride galvanizing process. So where does iron come from? In the hanging plating tank, it is mainly caused by the parts that fall into the tank and are taken out in time and immersed in the plating solution for corrosion.

In addition to the corrosion caused by the parts falling into the tank, there are two other important ways during barrel plating: one is to bring in iron ions during pickling (caused by unclean washing). Another way is caused by chemical corrosion after the parts are slotted.

There are two valence states when iron ions enter the bath: ferric iron and ferric iron. Divalent iron can be co-deposited with zinc ions to plated onto parts.

Although the precipitation potential of iron is more positive than that of zinc, it tends to be enriched in the high-current region in the coating, rather than evenly dispersed. When the nitric acid is released, the iron in the high-current region will turn into a brown-black coating. In the galvanized color passivation pair, this phenomenon is not very obvious, and it is very obvious in the blue and white passivation, resulting in unqualified appearance.

This phenomenon is often similar to the common barrel eye in barrel plating. But a closer look is different. If the current is too large or the main salt content is too low, it will be black before the groove, and after the light is out, the black spot will become a white spot, but it will not be bright.

The black spots caused by iron impurities do not appear black when they are out of the groove, and sometimes they are shiny here, but once the nitric acid is lightened, black spots appear immediately.

The most effective and economical way to remove iron impurities is to treat it with hydrogen peroxide: first dilute the hydrogen peroxide 10 times with clean water and pour it into the plating solution with stirring. Then check the pH value of the plating solution, and adjust the pH value with 10% caustic soda solution.

Then let it sit overnight and finally filter. Then adjust the pH value with 10% dilute hydrochloric acid to between. A small amount of cylinder opener can be added.

The specific amount of hydrogen peroxide to add depends on the content of iron impurities in the plating solution. Under normal conditions, it is about 1mul. Hydrogen peroxide can also destroy brighteners.

When adding hydrogen peroxide, we should pay attention to two points: one is to dilute and add (and add while stirring). Second, don't overdo it.

If it is found that it is not bright after hot-dip galvanizing, you can spray a bright silver galvanizing colorant, which can achieve a good appearance repair effect.