TECHNOLOGY AND EQUIPMENT FOR MAKING PROTECTIVE ZINC COATING ON STEEL
AND CAST IRON ITEMS BY THERMO-DIFFUSION DINAMIC ZINC GALVANIZING METHOD
Zinc coating being of anode type with regard to cast iron and steel has been used for corrosion protection over 100 years. Thermo-diffusion galvanizing is aimed at forming of coating on the product by means of zinc diffusion to the product's surface from the heated zinc powder charge.
This method was invented by the English engineer Sherard in 1907 and is known in the world as the process of sherardization. It makes possible to get the surface with remarkable proprieties: with life time 2- 3 times exceeding the coating treated by hot zinc method along with ideal following the complex shape.
The process of thermo-diffusion dynamic zinc galvanizing takes place in hermetically closed rotary retort with installed processing items and zinc charge at approximate temperature of 400C. Zinc mixture ( charge) consists in addition to zinc powder of filling stuffs and activating additives, while the process itself is performed in the protective gas medium, that evolves in a course of charge heating whilst air forcing out. Thickness of the coating achieved by thermo-diffusion method depends from the process duration and can be adjusted accordingly.
Owing to dynamic nature of the process and to the fact, that the diffusion is realized partially from vapor phase, the quality of the coating advises exceptionally even surface, also for the parts revealing complex shape with holes and hollows.
Thermo-diffusion process dynamic galvanizing is an ideal application for coating of different small metallic parts, springs and other small and medium sized items.
Special advantage of the thermo-diffusion galvanizing method is its complete stableness in terms of hydrogen metal short, that is especially important for the quality and reliability of springs and hardware.
Basic advantage of the thermo-diffusion coating is its structure, since the coating consists of iron and zinc alloy with zinc content rate increasing as closer to the surface and only the surface of the item advises pure zinc layer. This kind of structure assigns the coating high thermo-stability, wear-proof and antirust proprieties, geometric dimensions of the items treated undergo insignificant changes and preserve exact shape contours.
Color of the galvanized coating made by thermo-diffusion method range from light-gray to dark-gray, coating quality itself is mat, sometimes with minor sparkling shining.
Termo-diffusion galvanizing method makes it possible to create coatings 6-150mkm thick.
Surface of the items subject to thermo-diffusion galvanizing should be freed from slag and degreased, a thin layer of rust is allowed. Oil and cooling liquid rests influence coating quality insignificantly, but still decrease life-time of zinc mixture (charge), slag must be removed at any rate in a way of mechanical cleaning or etching with further drying.
After galvanizing, the items, depending on demand are treated by blowing on with compressed air, cleansed by water, phosphatized, treated by passivation or chrome, and then dried.
Zinc mixture (charge) is used repeatedly (from 5 to 25 cycles), during operation from cycle to cycle to the charge will be added consumption materials: zinc and activating additives. Charge waste lies under dander class utilization number four.
Thus, thermo-diffusion dynamic galvanizing method makes it possible to get coatings :
The items coated using this method:
The process itself is;
Comparative analysis of different methods used for zinc galvanizing surfaces
||High (owing to TD)
||Low (3-5 times worse compared with TD
||Medium (1,5-2 times worse compared with TD
||2,5 - 3 hours
||1,0 -1,5 hours
||Rust stains allowed. Slag removal by pellet-sand cleaning
||Thorough preparation required, incl. mechanical cleaning, etching, cleansing etc.
|Thickness of coating surface
||From 10 to 100mm per Customer's wish
|Gas and liquid wastes
||Alkali, acid. Wastes' rate in air exceeds the Norm. Cleaning utilities are needed.
||Manufacturing waters need utilization
|Required production site area
||50-80 sq m
||100 sq m
||more, than 100 sq m
|Relative prime cost
Quality parameter for the coating are standardized
according to TU U 29.2-14311548.003-2002
"Zinc thermo-diffusion coatings"
Thermo -diffusion dynamic galvanizing device CБЦ-300/400
Thermo-diffusion dynamic galvanizing device presents itself a special drum type of furnace with electrical heating, provided with hermetically closing retort and bunker for zinc material ( charge). Electrical operated drive ensures retort rotation to achieve homogeneous distribution of zinc mass (charge) and treated items as well. Overturning mechanism ensures distribution of the charge and items upon galvanizing process completion ( with charge placing in bunker), as well as items' uploading from the retort. Prior to uploading retort is cooled down by the fan blast coming thorough the distribution header, for some models retort is cooled down by the compressed air blast.
Control unit comprises start- protection devices, CPU - thermo-regulator, power-key, technological cycle timer, indication lights and control panel.
|Installed electric power, kWt
|Power supply, V
|Compressed air tension, mPa (for the case )
|Retort operating temperature, C
|Average duration of galvanizing, minutes
|Device overall dimensions, mm, LxWx H
|Retort operation surface size, mm
Demands to the production site:
||of general industrial type
||locally placed over the equipment with cyclone purification
||as per existing standards, of general and local type
||category "Г" ( 2.09.02-85)
Equipment is produced in accordance with TU U 29.2-14311548.009-2002