How to eliminate the possible failure in the galvanizing process?

Zinc is soluble in acids and alkali, so it is called an amphoteric metal. Zinc changes little in dry air. In humid air, an alkaline zinc carbonate film is formed on the surface of zinc. In the atmosphere containing sulfur dioxide, hydrogen sulfide and Marine atmosphere, zinc corrosion resistance is poor, especially in the high temperature and high humidity atmosphere containing organic acid, zinc coating is easily corroded.
The standard electrode potential of zinc is -0.76V, for the steel substrate, zinc coating belongs to the anodic coating, which is mainly used to prevent the corrosion of steel, and its protective performance is greatly related to the thickness of the coating.
To be considered by some people to be very simple galvanized really well, it is not easy, here are some examples.
Case 1: A factory for the new relocation of the new address, the main electric galvanized malleable steel (cast iron) and cold-rolled sheet stamping parts, galvanized process is potassium chloride, one of the galvanized line (three slots), galvanized pass rate is low. 10% of the non-conforming products of the circular workpiece are high dark area and thin coating; 80% of malleable steel workpieces are not bright in the high area, dark in the low area, and 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, the cathode into the barrel side plate hole to reduce the size to avoid small parts falling out, the barrel rotating speed of 7r/min, carefully looking at the conduction in the barrel, and found that the conductive braid is tilted with the barrel rotation because the gap between the barrel side plate hole and the barrel is too small, rather than droop into the workpiece, the conductive braid head is too light. And cylindrical, the outer diameter is smaller than the inner diameter of the circular ring workpiece, it is possible that some parts are set into the conductive head, resulting in these pieces have been plated, while checking the pH value of 6.2, 6.4, 6.3. Immediately make A 100mm longer conductive separator, increase the conductive separator, dilute with chemical pure hydrochloric acid and lower the pH value to 5.4, at the same time, use a clean beaker to take the tank liquid and add brightener A and B, observe whether there is precipitation, and determine whether there is excessive brightener; The optimum proportion of brightener was tested by Hall cell, and it was found that brightener A and B were less. Replace the lengthened and weighted conductive braid, add brightener, and then apply refractory malleable steel workpieces to achieve all qualified. Point of this example: Careful observation, conduction is the key.
Case 2: A factory is a simple electrogalvanizing workshop, self-produced and self-used, passivation fog after cyanide roll-galvanizing. The author was invited to the site to deal with the workpiece for extrusion and pressing processing of 10~15mm diameter, 130mm long shaft, a lot of surface oil, no oil removal process, the whole basket of about 50kg workpiece immersed in concentrated hydrochloric acid for 10min, washed into the tank cyanide galvanized, fog after light, no flange white passivating. Immediately find a washing powder instead of degreasing agent with warm water bubble to dissolve the workpiece after degreasing, washing, pickling, washing into the tank galvanized, the fog phenomenon after the light is reduced a lot, indicating that the oil is the main cause, before taking into the cyanide galvanized tank liquid oil is still a lot, remove the cyanide tank liquid oil is difficult, the author tried CK-778 galvanized impurity removal agent, The aluminum powder was used to react with the tank liquid to lift up the oil, and the floating matter was continuously fused out to remove most of the oil. At the same time, the sampling and analysis were as follows: 30g/L zinc oxide, 100g/L sodium cyanide, and too much brightener. Ask the operator to answer: because of fog, mistaken sodium cyanide is too small, less brightener, sodium cyanide and brightener are added many times. Immediately take out part of the tank liquid and dilute with water, add sodium hydroxide, zinc oxide, according to the normal process, all qualified. A factory for malleable steel (cast iron) electrogalvanizing, the process is potassium chloride roll-galvanizing, products exported. Large output, minimum cost required. 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 roller barrel door cover, no loading and unloading of the roller barrel door cover of the tedious process, the tank inside and outside the barrel update fast, reduce the plating time, the direction of plating, reverse the workpiece poured out, just like the olive in the pottery pot, no one is left. A simulation of a spiral shell barrel was made with cardboard, and the motion of the workpiece was observed with small stones instead of the workpiece in positive and negative rotation. The factory immediately arranged production according to the drawing, and the efficiency was increased by 30% after operation, the brightener was used less, and the cost was 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 on the anode rod and dip it into the tank. Brightener should be diluted with water or tank solution before slowly adding; Adjust pH value with diluted hydrochloric acid per shift; PP pipe is used as cooling pipe to cool the well water and reduce the consumption of brightener. The small zinc block is connected to the long zinc plate with a titanium hook; Hall chamber tests are often done. If there is a tank that is not easy to plate, it is adjusted to pre-galvanizing liquid: high zinc chloride, moderate potassium chloride, less brightener, 3 minutes after impact plating, then change the tank and normal plating; The rectifier is changed to switching power supply to reduce energy consumption and improve the uniformity of coating.