How to eliminate the possible failure in the process of galvanizing?

Zinc dissolves easily in acids and also in alkalis, so it is called an amphoteric metal. Zinc changes little in dry air. In moist air, the zinc surface will form an alkaline zinc carbonate film. In sulfur dioxide, hydrogen sulfide and Marine atmosphere, zinc corrosion resistance is poor, especially in high temperature and high humidity atmosphere containing organic acid, zinc coating is easy to be corroded.
The standard electrode potential of zinc is -0.76V, for the steel substrate, zinc coating belongs to the anodic coating, it is mainly used to prevent the corrosion of steel, its protective performance and the thickness of the coating is very important.
It is not easy to do a good job of galvanizing that some people think is very simple. Here are some examples to illustrate.
Case 1: A factory is a new location of the new relocation, mainly electric galvanized mallet steel (cast iron) and cold-rolled sheet stamping parts, galvanized process is potassium chloride, one of the rolling galvanized line (three slots), rolling galvanized pass rate is low. 10% unqualified circular workpiece for high area dark, thin coating; Malleable steel workpiece 80% unqualified products for high area is not bright, low area dark, thin coating. The author was invited to the site for processing, the barrel is newly manufactured, and many improvements have been made, such as lengthening, strengthening, cathode into the barrel side plate hole to reduce, avoid small parts falling out, the barrel speed is 7r/min, carefully examine the conduction in the barrel, found that the conductive braid because of the small gap with the barrel side plate hole and the barrel rotation and tilt, rather than droop into the workpiece, conductive braid head is too light, And cylindrical, outer diameter is smaller than the inner diameter of the circular ring workpiece, it is possible that some pieces into the conductive head, resulting in these pieces have been plated, at the same time to check the pH value of 6.2, 6.4, 6.3. The conducting separator was lengthened by 100mm immediately, and the conducting separator was strengthened. The pH value was reduced to 5.4 after dilution with pure hydrochloric acid and water. At the same time, the tank liquid was taken with A clean beaker and brightener A and B were added to observe whether there was precipitation and determine whether there was excessive brightener. The optimal brightener ratio was tested by Hall chamber, and it was found that brightener A and B were less. Replace the lengthened and weighted conductive braid, add brightener and apply refractory malleable steel workpiece to achieve all qualified. Point of this example: Observe carefully, conduction is key.
Case 2: A factory is a simple electrogalvanizing workshop, produced by itself for self-use, passivated fog after cyanidation roll-galvanizing. The author was invited to the site for processing. The workpiece was a shaft with a diameter of 10~15mm and a length of 130mm, with a lot of oil on the surface and no oil removal process. The whole basket of 50kg workpiece was immersed in concentrated hydrochloric acid for 10min and then washed into the tank for cyanidation and galvanizing. Immediately find washing powder instead of oil removal agent dissolved in warm water bubble will be the workpiece after oil removal, washing, pickling, washing into the tank galvanized, after light fog phenomenon reduced a lot, that oil is the main cause, before into the cyanide galvanized tank oil is still a lot, the removal of cyanide tank oil is difficult, the author tried CK-778 galvanized impurity removal agent, The aluminum powder was used to react with the tank to lift up the oil, and the floating matter was constantly removed to remove most of the oil. At the same time, the sampling and analysis were as follows: zinc oxide 30g/L, sodium cyanide up to 100g/L, too much brightener. Ask the operator answer: because of fog, mistaken sodium cyanide is too small, less brightener, sodium cyanide and brightener repeatedly added. Immediately take out part of the tank with water dilution, add sodium hydroxide, zinc oxide, according to the normal process, all qualified. A factory for mala steel (cast iron) galvanizing, the process is potassium chloride roll-galvanizing, product export. Large output, the lowest required cost. The author was invited to discuss improving the process to reduce the cost. The workpiece of the factory is cylindrical shape, a single weight of about 200~500g, it is recommended to use spiral shell type roller plating bucket, large opening, no drum door cover, no loading and unloading of the drum door cover of the tedious process, the inside and outside of the tank update fast, reduce the plating time, positive direction plating, reverse the workpiece poured out, just like the olive in the clay pot, no one left. Immediately use cardboard to make a spiral shell type roller plating bucket simulation, with small stones instead of the workpiece to observe the motion of the workpiece positive and negative rotation. The factory immediately arranged production according to the drawing, after the use of efficiency increased by 30%, less brightener, cost reduced by 10%. The author also formulated the main points of dosing: potassium chloride, zinc chloride, boric acid are almost taken out of consumption, only a small amount of drugs, more hanging zinc plate; Boric acid does not need to be dissolved in boiling water. Sew two 500mm long and 220mm wide cloth bags with polyester cloth, add about half of the bag of boric acid, tie it up and hang it at both ends of the anode rod to dip into the tank. Brightener should be diluted with water or tank before slowly adding; Adjust pH value with diluted hydrochloric acid per shift; PP pipe as cooling pipe through the well water cooling tank, reduce brightener consumption; The small zinc block is connected to the long zinc plate with titanium hook; Hall chamber tests are often done. If there is not good plating bath, it will be transferred to pre-galvanized liquid: high zinc chloride, moderate potassium chloride, less brightener, 3min after impact plating, then change the groove and normal plating; The rectifier is changed to switching power supply to reduce energy consumption and improve coating uniformity.