In order to master and manage the degreasing process, it is necessary to correctly master the principle of the combination of the coating and the metal matrix. This point is often ignored, which brings difficulties to practice.
Relevant data point out that the mechanical bonding between the coating and the substrate due to the micro unevenness of the surface is only when the intermolecular and intermetallic forces are bonded between the coating and the metal body, the bonding between the coating and the substrate is firm, and the intermolecular and intermetallic forces can only be shown in a very small distance.
When the distance between molecules exceeds 5 μ M U.M, the intermolecular force will not work. Therefore, the thin oil film and oxide film on the surface of the substrate can also hinder the intermolecular or metal bonding.
In order to achieve the above combination, the oil stain, rust and oxide scale on the products must be removed quite thoroughly. By "quite thorough", we do not mean that the surface is required to be absolutely clean after pre-treatment, but only that it has a qualified surface. The so-called qualified surface actually means that the film that hinders electroplating after pre-treatment must be removed and replaced by a film that is suitable for electroplating.
At the same time, the metal surface is required to be absolutely flat through pre plating treatment, and obvious scratches, burrs and other defects are removed from the surface after mechanical treatment such as polishing, polishing or rolling, sandblasting, etc., so that the base surface meets the requirements of base leveling and finish of plated parts, and then the oil and rust are removed.
This point must be clear. Only when this point is clear can the process flow and formula of pretreatment be correctly selected in the same formula of pretreatment.
How to apply degreasing process in production?
Alkaline degreasing is usually used. The composition and process conditions of the degreasing solution are selected according to the state of oil contamination and the type of metal materials.
When there is a large amount of grease on the surface, that is, the oil layer is very thick, slippery and sticky, it is not easy to remove the oil only by alkali. It is necessary to remove the oil by other methods such as brushing solvent before alkaline oil removal. The alkaline oil removal liquid is strongly alkaline. It reacts with some metals to cause obvious corrosion.
Therefore, the degreasing of aluminum and zinc plated parts should be carried out under low temperature and low alkali conditions as far as possible. It is generally possible to treat steel parts with higher alkalinity. However, when treating non-ferrous metal parts, the pH of the degreasing solution should be adjusted to an appropriate range. For example, the pH of aluminum, zinc and their alloys should be controlled below 11, and the oil removal time of such products should not exceed 3min.
From the perspective of cost, some people advocate low-temperature degreasing, but it is contradictory to reduce the temperature and improve the efficiency. The higher the temperature, the faster the physicochemical reaction between the oil adhered to the surface and the cleaning agent, and the easier the degreasing.
Practice has proved that the viscosity of oil will decrease with the increase of temperature, so oil removal is easy, but low temperature has no such effect. Therefore, considering the use of emulsifiers and surfactants, whether high-temperature degreasing is good or not and what temperature is appropriate to control, the author's experience is that it is better to control it at 70 ~ 80 ℃. This is also conducive to eliminating the residual stress of the base metal due to machining, and is beneficial to improving the bonding strength of the plating, especially multi-layer nickel.
General Iron and steel parts can be degreased in combination, such as cathode degreasing for 3-5min, then anode degreasing for 1-2min, or anode degreasing for 3-5min, and then cathode degreasing for 1-2min. This can be achieved by twice degreasing or by power supply with reversing device.
For high-strength steel, spring steel and thin parts, in order to prevent hydrogen embrittlement, the anode is only degreased for a few minutes. However, for non-ferrous metal parts such as copper and copper alloy, anode degreasing is not allowed, and cathode degreasing is only allowed for 1-2min.
Preparation and maintenance of degreasing solution. The preparation of chemical and electrolytic degreasing solutions is relatively simple. First, dissolve other materials except surfactants with 2 / 3 of the tank volume of water, and stir at the same time (to prevent drug caking). Since these drug materials are exothermic when dissolving, there is no need to warm them. The surfactants should be dissolved with hot water before adding. If they can't be dissolved at one time, pour out the supernatant and add water to dissolve them. Add them to the specified volume and stir them evenly.
Attention shall be paid to the management of degreasing fluid:
① Test the supplementary materials regularly, and add 1 / 3 ~ 1 / 2 of the original dosage of surfactant weekly or half a month according to the production capacity.
② The iron plate used shall not contain too much heavy metal impurities to prevent them from being brought into the coating. The current density shall be kept at 5 ~ 10A / DM2, and the selection shall ensure that sufficient bubbles can be precipitated. This not only ensures that the oil beads are mechanically torn off the electrode surface, but also stirs the solution. When the oil stain on the surface is constant, the higher the current density, the faster the oil removal speed.
③ The floating oil in the tank shall be removed in time.
④ Regularly clean the sludge and dirt in the tank and replace the tank liquid in time.
⑤ Low foam surfactant shall be selected as far as possible in the electrolyte, otherwise the quality will be affected when it is brought into the electroplating bath.
How to master and manage the acid etching (pickling) process?
Like the degreasing process, acid etching (pickling) plays an important role in the pre plating process. The two are used in the production of pre plating process in cooperation with each other, and their main purpose is to remove the rust and oxide scale of metal plating products.
Generally, strong etching is used to remove a large number of oxides, and weak etching is used to remove thin oxide films that are difficult to be detected by naked eyes, which can be divided into chemical etching and electrochemical etching. Weak etching is used for the last treatment process after strong etching, that is, before the workpiece enters the plating process. It is the activation process of the metal surface, which is easy to be ignored in production, which is precisely one of the reasons for plating peeling.
If the weak etching solution is one of the components in the next plating solution, or its introduction will not affect the plating solution, it is better to directly enter the activated plating part into the plating bath without cleaning.
For example, for the dilute acid activation solution used before nickel plating, in order to ensure the smooth etching process, the oil must be removed before etching, otherwise, the acid and metal oxide will not be in good contact, and the chemical dissolution reaction will be difficult.
Therefore, to master acid etching well, it is also necessary to understand these basic principles theoretically.
Usually, sulfuric acid and hydrochloric acid are used to remove the oxide scale of steel parts. The method is simple, but if we don't pay attention to it in actual production, it is difficult to achieve the expected purpose.
The selection criteria of sulfuric acid etching process conditions are usually identified by experience based on the appearance of the workpiece after pickling, which can not be controlled quantitatively after all. Practice has proved that the effect of removing oxide scale at 40 ℃ is much greater than that at 20 ℃ in sulfuric acid pickling, but the peeling effect is not proportional to the further increase of temperature.
At the same time, in sulfuric acid with a concentration below 20%, the acid etching speed increases with the increase of the concentration, but decreases when the concentration exceeds 20%. For this reason, we think that the standard process conditions of 10% ~ 20% sulfuric acid concentration and etching below 60 ℃ are more suitable. Attention should also be paid to the aging degree of sulfuric acid solution. Generally, when the iron content in the acid washing solution exceeds 80g / L and the ferrous sulfate content exceeds 2.5g/l, the sulfuric acid solution can not be reused.
At this time, the solution should be cooled to remove the excessive ferrous sulfate crystals, and then new acid should be added until the process requirements.
The standard for selection of acid etching process conditions of hydrochloric acid is that the concentration should be generally controlled at 10% ~ 20% and carried out at room temperature. Compared with sulfuric acid, under the same concentration and temperature, the etching speed of hydrochloric acid is 1.5-2 times faster than that of sulfuric acid.
Whether sulfuric acid or hydrochloric acid is the best choice for acid etching depends on the actual production situation. For example, sulfuric acid or hydrochloric acid is commonly used in strong etching of ferrous metals, or "mixed acid" in which the two are mixed in a certain proportion is used.
However, what kind of acid is selected for strong chemical etching depends on the composition and structure of oxides on the surface of steel parts. At the same time, the etching speed should be fast, the production cost should be low, and the dimensional deformation and hydrogen permeation of metal products should not occur as far as possible. However, it must be understood that the removal of oxide scales in hydrochloric acid mainly depends on the chemical dissolution of hydrochloric acid, and the mechanical stripping effect of hydrogen is much smaller than that in sulfuric acid. Therefore, the consumption of acid when hydrochloric acid is used alone is larger than that when sulfuric acid is used alone.
When the rust and oxide scale on the surface of the plated part contain more oxides of high valent iron, mixed acid etching can be used, which not only plays the tearing role of hydrogen on the oxide scale, but also accelerates the chemical dissolution of its oxides. However, if the metal surface only contains loose corrosion products (mainly Fe2O3), it can be etched with hydrochloric acid alone, because its etching speed is fast, the matrix dissolution is less, and the hydrogen permeation is also less.
However, when the metal surface is a close scale, hydrochloric acid alone consumes a lot of energy and costs a lot. At the same time, the peeling effect of hydrochloric acid on the scale is worse than that of sulfuric acid, which is better than that of sulfuric acid.
Electrolytic etching (electrolytic acid, electrochemical etching) can be carried out in 5% ~ 20% sulfuric acid solution, whether it is cathode electrolysis or anode electrolysis PR electrolysis (periodically changing the positive and negative electrodes of the workpiece).
Compared with chemical etching, electrolytic etching is easier to quickly remove the oxide scale that is firmly bonded, has less corrosion to the base metal, and is easy to operate and manage. It is suitable for automatic electroplating lines. PR electrolysis is widely used in Japan to remove stainless steel scale.
In China, cathode and anode electrolytic pickling, combined with electrolytic degreasing, are used for pretreatment. Ferrous metal anode electrolytic acid is suitable for treating metal parts with large scale and rust. Most of it can be carried out at room temperature. Raising the temperature can improve the acid etching speed, but it is not as large as chemical acid etching. The current density increases, and the acid etching speed increases. However, when it is too high, the base metal will be passivated.
At this time, the chemical and electrochemical dissolution of the base metal basically disappears, leaving only oxygen to peel off the oxide scale. Therefore, the etching speed increases very little, which must be mastered well. Generally, the current density of 5 ~ 10A / DM2 is appropriate. O-xylene thiourea or sulfonated wood glue can be used as corrosion inhibitor for anodic acid etching, and the dosage is 3-5g / L; The electrolytic acid for ferrous metal cathode can be sulfuric acid solution, mixed acid of about 5% sulfuric acid and hydrochloric acid, and proper amount of sodium chloride. Because it has no obvious chemical and electrochemical dissolution process of metal matrix (iron), properly adding compounds containing Cl - can help loosen the oxide skin on the surface of parts and accelerate the etching speed. At the same time, formaldehyde or Urotropine can be used as corrosion inhibitor.
In short, sulfuric acid is widely used for acid etching of steel, copper and brass. In addition to the above, sulfuric acid, together with chromic acid and dichromate, is used as aluminum deoxidizer and desalting agent.
It is used to remove the scale of stainless steel together with hydrofluoric acid or nitric acid or both. The advantage of hydrochloric acid is that it can effectively pickle many metals at room temperature. One of its disadvantages is that it must pay attention to the prevention and control of HCl vapor acid mist pollution.
In addition, nitric acid and phosphoric acid are also commonly used in manual plating pretreatment. Nitric acid is an important component of many bright etchants. It is mixed with hydrofluoric acid to remove the oxide scale of heat treatment on aluminum, stainless steel, nickel based and iron-based alloys, titanium, zirconium and some cobalt based alloys.
Phosphoric acid is used for rust removal of steel parts, and can also be used in special bath liquids of stainless steel, aluminum, brass and copper. Phosphoric acid nitric acid acetic acid mixed acid is used for the pretreatment of bright anodizing of aluminum parts. Fluoroboric acid has been proved to be the most effective acid solution for lead based alloys or copper or brass parts with solder joints.
It has been reported that the removal of metal oxide scales and oxides consumes 5% of the world's sulfuric acid output, 25% of hydrochloric acid, most of hydrofluoric acid and a large amount of nitric acid and phosphoric acid.
Therefore, correctly mastering these acids for acid etching is obviously an important subject in the application technology of pre plating treatment, but it is not difficult to use them well, save time and reduce consumption.
