Galvanizing needs passivation is a special case, that is to say, in fact, basically only galvanizing must be passivation after plating, plating other metals, such as copper plating, nickel plating, silver plating, etc., does not necessarily need passivation, there is no problem affecting electrical conductivity.
The fundamental reason for the need for passivation is that zinc corrosion resistance is poor, although zinc can form a protective layer with basic zinc carbonate as the main component in the air, the corrosion resistance of this protective layer is not strong, and it is easy to be destroyed in the industrial environment often containing acid and alkaline waste gas, as well as in the Marine environment containing salt spray, especially galvanized parts are used in the industrial environment. This shortcoming must be addressed.
Zinc and aluminum are both amphoteric metals, but the corrosion resistance of zinc surface protective layer is far less than that of aluminum oxide protective layer on aluminum surface.
So although aluminum is more reactive than zinc, aluminum is in many cases more resistant to corrosion than zinc.
The unpolished zinc granules and aluminum sheets are respectively put into cold dilute hydrochloric acid and dilute sulfuric acid, and it can be seen that zinc begins to react quickly in both dilute hydrochloric acid and dilute sulfuric acid, and bubbles emerge, which is more violent in the beginning of the reaction in dilute hydrochloric acid, because the destruction of the protective layer on the zinc surface is faster.
And aluminum in the cold dilute sulfuric acid basically does not react, in the cold dilute hydrochloric acid in the beginning of the reaction is not obvious, but after a few minutes of aluminum surface bubbles began to increase, the reaction gradually accelerated, and finally violent reaction, a large number of bubbles, this is because chloride ions will eventually destroy the protective layer of aluminum surface, but sulfate ions damage the protective layer of aluminum surface is very weak caused by.
The basic zinc carbonate protective layer on the surface of zinc has a poor ability to resist acid, alkali and chloride ion damage, so the galvanizing layer needs to be passivated to enhance the corrosion resistance. Taking hexavalent chromium (passivated solution prepared with chromate anhydride as the main raw material and chromate as the main component) as an example, zinc is oxidized during passivation, while hexavalent chromium is reduced to trivalent chromium. A protective layer with chromium oxide as the main component is formed on the zinc surface, and the corrosion resistance of this protective layer is greatly enhanced compared with the original protective layer on the zinc surface. Since the corrosion resistance of zinc coating is not strong, why galvanized parts are commonly used in industry?
First, because the cost of galvanizing is low, even if coupled with passivation treatment cost is also very low;
Second, because galvanized has its special advantages, zinc is more active than iron, galvanized on steel, once the galvanized layer is damaged, because zinc is more active, the cathodic protection of the iron at the sacrifice of the anode, zinc is corroded, protecting the iron from corrosion, so galvanized steel parts even if the coating is damaged can also protect the iron from corrosion for a period of time, or in electroplating terms, The galvanized layer on the surface of iron is a rare anode coating, which can play a cathodic protection role on iron.
Therefore, galvanized requires post-plating treatment (passivation) is a special case, can not be generalized, that electroplating any metal must be passivation.
Of course, when necessary, other metal electroplating in addition to galvanized can also be the necessary post-plating treatment, including passivation, to meet some special requirements, but this and galvanized almost must be specialized passivation treatment to enhance the corrosion resistance of the coating is not exactly the same thing. High school chemistry teaching electroplating, choose galvanizing as a teaching case, there are special reasons, zinc is more active than most plated metal, galvanizing does not have to consider in the absence of power, plating metal and plating solution replacement reaction problem, even if the direct use of zinc chloride solution as plating solution can barely complete the theory and experiment teaching, the principle is simple, can simplify teaching, It is easy for middle school students to understand, if you choose copper plating, silver plating, etc. as a teaching case, then the electroplating solution must generally add a complexing agent such as cyanide, otherwise the replacement reaction may occur without power, which increases the difficulty of teaching and is not easy for middle school students to understand.