The total acidity of phosphating solution is the sum of the content of dihydrogen phosphate [Me(H2PO4)2] and free acid in phosphating solution. The total acidity is too high, the phosphating film is rough, and the phosphating sediment will increase naturally; The total acidity is too low, the phosphating film formation speed is slow, it is difficult to form high-quality phosphating film. Only by maintaining proper total acidity can the normal film formation of phosphating film be ensured and a large amount of phosphating sediment be prevented.
1. Accelerator
In order to promote the phosphating reaction smoothly, oxidants (accelerators) need to be added to the phosphating treatment solution. The usual accelerator used is sodium nitrite. In the low-load production line, due to the continuous addition of accelerators will increase the consumption of phosphating agents, there will be anisol residue, so that the index parameters of the phosphating solution will be out of balance (often free acid decline). Therefore, it is recommended that the pre-treatment line concentrate production as much as possible, and do not add accelerators when stopping the line to prevent excessive waste and unnecessary sediment generation.
2. Neutralizer
The neutralizer is usually a strong base (NaOH), which is added when the free acid is adjusted, resulting in a large amount of zinc phosphate precipitation. NaOH+Zn(H2PO4)2→Zn3(PO4)2(precipitation)↓+H2O+Na3PO4 For this purpose, the neutralizer is diluted 10 times and slowly added to the tank to avoid the excessive neutralization reaction and a large amount of sediment due to the high concentration of the neutralizer.
Under the same total acidity condition, the acid ratio is too low, the free acid in the tank is high, resulting in the phosphating film is not easy to form, affecting the phosphating quality. Therefore, only by controlling the acid ratio of the phosphating tank within an appropriate range (should be maintained at 20 ~ 30), can the residue be minimized and the quality of the phosphating film be ensured.
3.Phosphating temperature
When the temperature of phosphating solution increases, the dissociation degree of dihydrogen phosphate increases, and the concentration of film-forming ions increases accordingly. However, too high phosphating temperature will lead to excessive hydrolysis of the active ingredient (Zn(H2PO4)2) in the phosphating tank, resulting in a large amount of sediment. In addition, the local temperature of phosphating solution is too high, which will also produce excessive sediment. Therefore, the phosphating liquid heating should avoid direct steam heating, requiring the use of secondary heating outside the tank, and usually the heat exchanger control temperature does not exceed 60°C.
4. Cycle
In order to ensure that the concentration of the phosphating tank is uniform in each region, the phosphating tank requires full circulation (usually 2 to 3 times /h). Otherwise, the local circulation amount is small or no circulation is easy to lead to the uneven mixing of phosphating main agents and additives (such as promoters, neutralizing agents), resulting in high local sediment Phosphating process is a multi-process process, there are many factors affecting the quality of phosphating, some unreasonable process, non-standard operation will affect the quality of phosphating, in the process of the use of phosphating liquid, there are some misunderstandings.
[Misunderstanding 1, the thicker the phosphating film, the better?]
The main function of the phosphating film is to protect the material during drawing and cold heading. The effect of protection depends not only on the thickness of the film, but also on the binding force between the film and the substrate.
When the binding force of the film layer is insufficient, although the phosphating film formed is very thick and the surface looks very black, it is also easy to fall off during the drawing and cold heading process, resulting in ringing, whitening, scratching and other phenomena, which cannot achieve the effect of protecting the material. In addition, the thickness of the film layer is also determined according to the needs of subsequent drawing and cold heading, and too thick film layer will affect the cold heading effect of some small diameter products.
Therefore, the pursuit of film thickness by making the total acidity too high or delaying the phosphating time does not necessarily achieve the ideal effect. It is better to control the total acidity and phosphating time within the standard range, and ensure the binding force of the film layer while the film thickness is sufficient.
[Misunderstanding 2, long time without adding medicine, a large amount of one-time supplement]
The phosphating operation without adding agents for a long time is easy to cause the total acid of the tank to be too low, the acid ratio to be larger, and the phosphating film formed is too thin (or even unable to form a film), resulting in unstable phosphating quality. A large number of agents are added at one time, resulting in excessive fluctuations in the tank data, which may be difficult to restore the optimal tank performance.
The correct approach should be to regularly detect the data of phosphating agents (total acid, free acid, accelerator, temperature), and add it in time according to the consumption, so as to keep the data of the tank liquid always within the standard range of the agent, so as to ensure the stability of the quality.
【 Misunderstanding 3, the use of groundwater, river water, recycled water, etc., to open tanks or supplement tank water 】
Groundwater, river water generally contains a large number of calcium ions, magnesium ions and other metal ions, and the recovered water generally contains high chloride ions, these components into the phosphating tank, will make the tank sludge a large increase, phosphating quality decline, easy to rust yellow. Serious will make the whole tank liquid data imbalance scrap, resulting in a great waste.
The correct approach is to use tap water for the grooving and water replenishment of phosphating.
[Misunderstanding four, the quality of the phosphating film is only related to the phosphating tank]
Pickling phosphating process has many processes, the quality of phosphating film is affected by the material properties, the surface condition of the material annealing, the surface condition of the material pickling, the water quality of the washing and other factors.
For example, if the material itself has defects (such as poor organization, non-metallic inclusions), it will make the material prone to surface cracking during the drawing or cold heading process, which cannot be solved by phosphating. For example, excessive pickling of the material will make the surface of the material white, lack of reactive points, difficult to film during phosphating, resulting in too thin film layer, and insufficient binding force, and the film layer is easy to fall off during subsequent drawing and cold heading. Analyzing different factors from multiple aspects helps to solve problems more efficiently.
[Misunderstanding five, water washing using groundwater, river water, recycled water for washing, and do not pay attention to replacement]
Some companies use groundwater, river water, or recycled water for water washing. These waters generally contain more impurity ions (such as calcium ions, magnesium ions, chloride ions, etc.). When using this water, it is necessary to ensure that its pH value and TDS value (dissolved solid matter) are within the standard range, maintain sufficient overflow, and when the water data exceeds the standard, it needs to be replaced in time. Impurity ions into the phosphating tank will cause a large increase in phosphating sludge, acid ratio is unstable, and greatly affect the quality of phosphating.
At present, more and more enterprises are using recycled water, which generally contains a high concentration of chloride ions. Chloride ions into the phosphating tank will affect the stability of the tank, and the material is easy to yellow and rust, so it is more important to pay attention to the regular detection of the pH value and TDS value of the water to ensure the water quality.
[Misunderstanding six, do not detect the tank liquid data, add drugs by feeling]
The use of phosphating liquid has a standard data range, which is determined by the composition ratio of phosphating liquid, produced within the standard data range, in order to ensure the quality of phosphating film.
Different diameters, different materials, different phosphating time will have different consumption of agents, so it is easy to add agents by feeling too high or too low data, which will greatly affect the stability of the phosphating quality, so the data of the tank should be regularly tested and recorded, supplemented according to the actual consumption, to ensure the stability of the data.
[Misunderstanding 7, different manufacturers of phosphating solution mixed use]
The composition ratio, ion type, and use conditions of phosphating solution from different manufacturers may be different, and different agents have different standard data ranges.
Mixed phosphating agents are easy to destroy the composition ratio of the tank, resulting in an increase in sediment and a decline in phosphating quality, which will destroy the balance of the tank and cause the tank to be scrapped. In order to ensure the stability of quality, it is necessary to avoid the use of mixed tanks of phosphating solution.