Aluminium base plate for PS plate needs to have hydrophilicity and water retention and adsorption to the photosensitive resin layer. In the PS plate production, the electrolytic plate-making process is to achieve this purpose, so the electrolytic plate is undoubtedly a PS plate. The key to production.
Each manufacturer has its own process conditions and control methods, but it must follow a basic principle that the acid concentration must match the current density. Once the process conditions are deviated (mechanical or man-made) and the two do not match well, the product quality will be unqualified and the plant will suffer economic losses. In general, the mismatch between the acid concentration and the current density can be judged from the appearance of the PS plate after the machine is down. The following is an example of electrochemical graining in hydrochloric acid (HCl) solution.
Black-printing phenomenon
1) After the production line is started, the electrolyte is injected into the electrolytic cell. During the process when the electrolysis current rises from zero to normal, the plate base is always black; when the production line is ready to stop, the electrolytic current is reduced from the normal value to zero. In the middle, the phenomenon of darkening of the base plate will also be observed. We call the phenomenon of black printing of the base plate as "burning."
2) During the operation of the production line, observing the running plate before coating reveals that there are some irregularly-shaped, dark-colored sheets on the plate surface.
3) Sometimes the PS version is very bright after the machine is down. Adjusting the angle of view under the security light will reveal that the phenomenon of darkening the plate base is particularly obvious at a certain angle.
The above three kinds of phenomena, the causes of which can be attributed to the acid concentration and current density do not match, of which the first two are the hydrochloric acid concentration is too high, the latter is the hydrochloric acid concentration is too low. Why doesn't the concentration of hydrochloric acid and current density match the black and white variation of the printing plate? We need to understand the mechanism of electrochemical graining first.
Electrolysis Mechanism The power supply we use is generally three-phase alternating current, graphite electrode is energized, and the aluminum plate is not energized. Under the action of symmetrical three-phase alternating current, the potential of the aluminum plate is zero. Actual three-phase AC may not be completely symmetrical, but its potential is near a certain value of zero, when the graphite-pass sine wave AC, the positive semi-period graphite potential is higher than the aluminum plate potential, graphite is the anode, and aluminum is the cathode; negative half of the graphite The potential is lower than that of the aluminum plate, graphite is the cathode, and the aluminum plate is the anode.
In the case of aluminum plates, dissolution of aluminum occurs mainly when used as an anode:
Al-3e=Al3+
When a cathode is used, hydrogen evolution occurs: 3H++3e=3/2H2.
The overall reaction is: Al+3H+=Al3++3/2H2↑
1. When the aluminum plate is used as a cathode
When the aluminum plate is used as a cathode, due to the hydrogen evolution, the pH of the electrolyte rises (more than 14), and cathodic corrosion occurs. The reaction formula is: Al+4OH--3e=Al(OH)4-, so that the anode current will change. If the anode current efficiency is greater than 100%, about 130%. If the anode current is only passed through the aluminum plate and the cathode current is filtered out, it will be found that in the same period of time, the amount of dissolved aluminum in the absence of the cathode current is less than the amount of dissolved aluminum in the presence of the cathode current (ie, a normal sine wave).
Cathodic corrosion occurs more because of chemical rather than electrochemical effects because the cathode is protected under electrochemical action and aluminum anode reactions do not occur; under chemical action, as long as the alkali around Al is sufficiently strong , it will react anytime, anywhere. Its chemical reaction can be expressed as:
Al+4OH--3e= Al(OH)4-
3H++3e=3/2H2↑
The overall reaction is: Al+4OH-+3H+=Al(OH)4-+3/2H2↑
The H2 precipitation is partly due to the electrochemical action, that is, H+ directly on the electrode to become H2. It is precisely because of the electrochemical action that causes the pH value of the electrolyte to increase, which causes Al to be eroded by OH-, and the result that the aluminum plate is eroded further increases the pH value of the electrolyte.
Al(OH)4- only existed stable at pH>9; existed as metastable colloids at pH 4-9, and precipitated at pH<4. The pH of the electrolyte itself is approximately 1. When the aluminum plate is not used as a cathode, the pH immediately returns to its original level and the following reaction occurs:
Al(OH)4-+H+=Al(OH)3↓+H2O
Thus deposited on the surface of the aluminum plate, which is apparently white.
When the aluminum plate is used as an anode
In the process of Al→Al3+ occurring in the anode of aluminum plate, the occurrence of corrosion point is often caused by chemical action. Among them, Cl- is extremely aggressive, and erosion preferentially occurs on the grain boundary of aluminum alloy, resulting in pit-like Corrosion point. Since the PS version uses aluminum as commercial pure aluminum, its activity is higher than that of ultra-pure aluminum (99.99% Al), and it is highly susceptible to Cl-erosion and grain boundary corrosion, resulting in the formation of some Al clusters and some intergranular particles from the matrix. Fall off and mix in the Al (OH) 3 precipitate, as shown in Figure 1. Because the aluminum swarf is highly dispersed and strongly absorbs light, the substrate is black.
The explanation of the black version
From the above analysis, the electrolysis graining process is the result of a combination of chemical and electrochemical reactions. The "gray matter" produced by the electrolysis is mainly composed of H2O, Al(OH)3 and Al chips. Among them, Al(OH)3 is white and aluminum chips are black. The "gray matter" produced by normal electrolysis is gray, which is the result of the joint action of Al(OH)3 and aluminum chips.
For one reason, in the alkaline cleaning process after electrolysis, the "gray matter" was washed away, but it remained black after the alkali washing. Because aluminum flakes are dispersed in Al(OH)3 under normal conditions, they are easy to wash away, but when the amount of aluminum flakes in Al(OH)3 is increased, there is an increased chance that aluminum flakes will polymerize. Not easy; two "gray" and more, requiring increased alkali washing intensity or time. Therefore, the normal caustic washing conditions cannot completely remove it.
Second reason, the effect of HCl concentration and current density on the appearance of electrolytic graining. As shown in Fig. 2, when the current density is constant, the concentration of HCl is too low, because the aggressive Cl- decreases, the number of corrosion pits per unit area decreases, resulting in too much electrolysis platform, there are many reflections of light, Therefore, the base shines. If the concentration of HCl is too high, the number of corrosion sites per unit area increases due to the increase of the aggressive Cl-concentration, and the amount of aluminum swarf caused by Cl-erosion increases. At this time, if the current density is increased, the appearance of the substrate will return to normal. Due to the increase of current density, Al atoms of Al→Al 3+ will increase in the unit area of ​​the printing plate, and the aluminum swarf caused by the erosion of Cl − will fall off from the substrate in the future. It reacts to produce Al3+, which reduces aluminum scrap.
The third reason is that the electric current continues to rise, making the sand grains bigger, and the sand holes and the sand holes are connected together to form pits. The shape of the sand holes is no longer hemispherical and becomes irregular, but the plate base will not be black. This is inconsistent with some reports in the literature and it is not consistent with what we imagined. We think from the "burned version" that the higher the current density, the more heat is generated and the easier it is to "burn". However, the direct cause of “burning†we usually say is not heat, but aluminum swarf caused by the erosion of Cl-. When the concentration of HCl is constant, the current density is too high and the substrate does not turn black. Its correctness has been confirmed by experiments and production.
in conclusion
From the above analysis, we can draw the following conclusions:
1) Electrochemical graining of commercial pure aluminum is the result of electrochemical and chemical interactions under the action of alternating current in HCl solution.
2) The chemical reaction determines the formation of corrosion sites, and the electrochemical action determines that the corrosion sites grow into hemispherical sands.
3) A good electrolysis graining effect must be a balance between chemical action and electrochemical action. If the chemical action is too strong, it will cause too aggressive, so-called "tunnel corrosion", and the substrate becomes black. If the electrochemical effect is too strong, the corrosion point will be incomplete, and there will be many electrolysis platforms and coarse sand. This and the HCl concentration must match the current density is the relationship between the nature and the phenomenon.
4) When the concentration of HCl is appropriate, the corrosion point per unit area will occur more fully. When the current density is higher than its proper value, it will only result in high Rz value and will not cause the plate base to shine or darken. .
Source: Ke Yin Media
Each manufacturer has its own process conditions and control methods, but it must follow a basic principle that the acid concentration must match the current density. Once the process conditions are deviated (mechanical or man-made) and the two do not match well, the product quality will be unqualified and the plant will suffer economic losses. In general, the mismatch between the acid concentration and the current density can be judged from the appearance of the PS plate after the machine is down. The following is an example of electrochemical graining in hydrochloric acid (HCl) solution.
Black-printing phenomenon
1) After the production line is started, the electrolyte is injected into the electrolytic cell. During the process when the electrolysis current rises from zero to normal, the plate base is always black; when the production line is ready to stop, the electrolytic current is reduced from the normal value to zero. In the middle, the phenomenon of darkening of the base plate will also be observed. We call the phenomenon of black printing of the base plate as "burning."
2) During the operation of the production line, observing the running plate before coating reveals that there are some irregularly-shaped, dark-colored sheets on the plate surface.
3) Sometimes the PS version is very bright after the machine is down. Adjusting the angle of view under the security light will reveal that the phenomenon of darkening the plate base is particularly obvious at a certain angle.
The above three kinds of phenomena, the causes of which can be attributed to the acid concentration and current density do not match, of which the first two are the hydrochloric acid concentration is too high, the latter is the hydrochloric acid concentration is too low. Why doesn't the concentration of hydrochloric acid and current density match the black and white variation of the printing plate? We need to understand the mechanism of electrochemical graining first.
Electrolysis Mechanism The power supply we use is generally three-phase alternating current, graphite electrode is energized, and the aluminum plate is not energized. Under the action of symmetrical three-phase alternating current, the potential of the aluminum plate is zero. Actual three-phase AC may not be completely symmetrical, but its potential is near a certain value of zero, when the graphite-pass sine wave AC, the positive semi-period graphite potential is higher than the aluminum plate potential, graphite is the anode, and aluminum is the cathode; negative half of the graphite The potential is lower than that of the aluminum plate, graphite is the cathode, and the aluminum plate is the anode.
In the case of aluminum plates, dissolution of aluminum occurs mainly when used as an anode:
Al-3e=Al3+
When a cathode is used, hydrogen evolution occurs: 3H++3e=3/2H2.
The overall reaction is: Al+3H+=Al3++3/2H2↑
1. When the aluminum plate is used as a cathode
When the aluminum plate is used as a cathode, due to the hydrogen evolution, the pH of the electrolyte rises (more than 14), and cathodic corrosion occurs. The reaction formula is: Al+4OH--3e=Al(OH)4-, so that the anode current will change. If the anode current efficiency is greater than 100%, about 130%. If the anode current is only passed through the aluminum plate and the cathode current is filtered out, it will be found that in the same period of time, the amount of dissolved aluminum in the absence of the cathode current is less than the amount of dissolved aluminum in the presence of the cathode current (ie, a normal sine wave).
Cathodic corrosion occurs more because of chemical rather than electrochemical effects because the cathode is protected under electrochemical action and aluminum anode reactions do not occur; under chemical action, as long as the alkali around Al is sufficiently strong , it will react anytime, anywhere. Its chemical reaction can be expressed as:
Al+4OH--3e= Al(OH)4-
3H++3e=3/2H2↑
The overall reaction is: Al+4OH-+3H+=Al(OH)4-+3/2H2↑
The H2 precipitation is partly due to the electrochemical action, that is, H+ directly on the electrode to become H2. It is precisely because of the electrochemical action that causes the pH value of the electrolyte to increase, which causes Al to be eroded by OH-, and the result that the aluminum plate is eroded further increases the pH value of the electrolyte.
Al(OH)4- only existed stable at pH>9; existed as metastable colloids at pH 4-9, and precipitated at pH<4. The pH of the electrolyte itself is approximately 1. When the aluminum plate is not used as a cathode, the pH immediately returns to its original level and the following reaction occurs:
Al(OH)4-+H+=Al(OH)3↓+H2O
Thus deposited on the surface of the aluminum plate, which is apparently white.
When the aluminum plate is used as an anode
In the process of Al→Al3+ occurring in the anode of aluminum plate, the occurrence of corrosion point is often caused by chemical action. Among them, Cl- is extremely aggressive, and erosion preferentially occurs on the grain boundary of aluminum alloy, resulting in pit-like Corrosion point. Since the PS version uses aluminum as commercial pure aluminum, its activity is higher than that of ultra-pure aluminum (99.99% Al), and it is highly susceptible to Cl-erosion and grain boundary corrosion, resulting in the formation of some Al clusters and some intergranular particles from the matrix. Fall off and mix in the Al (OH) 3 precipitate, as shown in Figure 1. Because the aluminum swarf is highly dispersed and strongly absorbs light, the substrate is black.
The explanation of the black version
From the above analysis, the electrolysis graining process is the result of a combination of chemical and electrochemical reactions. The "gray matter" produced by the electrolysis is mainly composed of H2O, Al(OH)3 and Al chips. Among them, Al(OH)3 is white and aluminum chips are black. The "gray matter" produced by normal electrolysis is gray, which is the result of the joint action of Al(OH)3 and aluminum chips.
For one reason, in the alkaline cleaning process after electrolysis, the "gray matter" was washed away, but it remained black after the alkali washing. Because aluminum flakes are dispersed in Al(OH)3 under normal conditions, they are easy to wash away, but when the amount of aluminum flakes in Al(OH)3 is increased, there is an increased chance that aluminum flakes will polymerize. Not easy; two "gray" and more, requiring increased alkali washing intensity or time. Therefore, the normal caustic washing conditions cannot completely remove it.
Second reason, the effect of HCl concentration and current density on the appearance of electrolytic graining. As shown in Fig. 2, when the current density is constant, the concentration of HCl is too low, because the aggressive Cl- decreases, the number of corrosion pits per unit area decreases, resulting in too much electrolysis platform, there are many reflections of light, Therefore, the base shines. If the concentration of HCl is too high, the number of corrosion sites per unit area increases due to the increase of the aggressive Cl-concentration, and the amount of aluminum swarf caused by Cl-erosion increases. At this time, if the current density is increased, the appearance of the substrate will return to normal. Due to the increase of current density, Al atoms of Al→Al 3+ will increase in the unit area of ​​the printing plate, and the aluminum swarf caused by the erosion of Cl − will fall off from the substrate in the future. It reacts to produce Al3+, which reduces aluminum scrap.
The third reason is that the electric current continues to rise, making the sand grains bigger, and the sand holes and the sand holes are connected together to form pits. The shape of the sand holes is no longer hemispherical and becomes irregular, but the plate base will not be black. This is inconsistent with some reports in the literature and it is not consistent with what we imagined. We think from the "burned version" that the higher the current density, the more heat is generated and the easier it is to "burn". However, the direct cause of “burning†we usually say is not heat, but aluminum swarf caused by the erosion of Cl-. When the concentration of HCl is constant, the current density is too high and the substrate does not turn black. Its correctness has been confirmed by experiments and production.
in conclusion
From the above analysis, we can draw the following conclusions:
1) Electrochemical graining of commercial pure aluminum is the result of electrochemical and chemical interactions under the action of alternating current in HCl solution.
2) The chemical reaction determines the formation of corrosion sites, and the electrochemical action determines that the corrosion sites grow into hemispherical sands.
3) A good electrolysis graining effect must be a balance between chemical action and electrochemical action. If the chemical action is too strong, it will cause too aggressive, so-called "tunnel corrosion", and the substrate becomes black. If the electrochemical effect is too strong, the corrosion point will be incomplete, and there will be many electrolysis platforms and coarse sand. This and the HCl concentration must match the current density is the relationship between the nature and the phenomenon.
4) When the concentration of HCl is appropriate, the corrosion point per unit area will occur more fully. When the current density is higher than its proper value, it will only result in high Rz value and will not cause the plate base to shine or darken. .
Source: Ke Yin Media
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