Hot dip galvanizing line outside annealing
Hot dip galvanizing line outside annealing, be cold rolled steel sheet enters hot galvanizing work line before, undertake recrystallization annealing is carried out in continuous type annealing furnace or cover fire type annealing furnace first, such do not exist inside galvanizing line annealing process.Before galvanizing, the steel plate must maintain a clean surface of pure iron without oxide or other dirt.The galvanization method involves removing the surface of the annealed steel sheet by pickling and then coating it with a solvent containing either zinc chloride or a mixture of ammonium chloride and zinc chloride to prevent the steel from being oxidized.If the solvents on the surface of the steel plate are not dried (that is, the surface is still wet), they will enter the zinc liquid covering the surface of the molten solvent to conduct hot galvanizing. This method is called wet hot galvanizing.To reduce the galvanizing time, to reduce the leaching of the zinc pot and to easily catch the zinc residue, this method is usually filled with a large amount of lead liquid at the bottom of the zinc pot.When the steel plate enters the zinc pot, it first contacts the melting solvent, and then enters the zinc layer. Only at the exit of the zinc pot, the steel plate contacts the zinc liquid for a short time.
Wet galvanization does not allow the addition of aluminum to the zinc solution as it is very easy to undergo the following chemical reaction with the molten solvent:
2 al + zncl2 alcl3 = = 2 + 3 zinc
The less reactive zinc metal is replaced from its compound by the active metal aluminum, but the resulting Alcl3 can boil even at a low temperature of 123 degrees, so it quickly evaporates from the solvent and is consumed.Alcl3 may also react with ammonium chloride to form alcl3.nh3, which boils at 400 degrees.Because wet galvanizing can only be done without aluminium, the coating alloy is very thick and has poor adhesion.In addition, the resulting zinc slag is deposited at the interface between the zinc and the lead liquid and cannot be deposited at the bottom of the pot (because the density of the zinc slag is greater than that of the zinc liquid and less than that of the lead liquid), so that the steel plate is contaminated by the surface through the slag layer, so this galvanizing method has been basically used at present.
As a result of a series of reforms to the original process, the sheet steel dry hot galvanizing unit was included in the improvement of cleaning and drying transmission mode.