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Thermal Diffusion Galvanizing A Cost Effective, Environment Friendly Technology By: Moshe Moked Unstain North America 617-566 0058 email@example.com • First: • Why am I here????? • What is Thermal Diffusion Galvanizing? It is a method of applying a metallic (Zinc alloys) coating to Steel, using a metallurgical diffusion process rather than hot dip. • How is it applied? • It is applied by tumbling the parts to be treated in a cylinder which rotates inside a furnace, with a powder composed of Zinc + alloying elements. The process takes place at elevated temperatures, between 3000C and 5000C (5700F- 9300F), over a period of 2-4 Hrs, depending on process requirements. • Illustration of the process: Step 1 • Pre treatment – mainly light cleaning. – Light Oils Acceptable – Light Corrosion Acceptable Step 1-Pretreatment Step 2-Loading Step 3- Treatment in a slow rotating cylinder in the oven Step 4 – Cooling Step 5-Unloading parts –Filtration system absorbs dust and excess powder Step 6 - Rinse/Polish/Passivation Step 7 - dry Step 8 – Second passivation/sealer Step 9 – Final drying Characteristics of the coating • The Thermo-Diffusion zinc coating is anodic in relation to ferrous metals and provides for electrochemical protection of steel against corrosion (Sacrificial protection – Galvanic action). • One of the main advantages of this method, in comparison with the Hot Dip process, is the absence of hydrogen embitterment. •The coating obtained follows exactly the contours of the article. It’s thickness is uniform over the whole surface of the part, including complex-shaped articles (threaded connections, etc.) • When applying the coating, the parts are placed in a container. Since the size of the parts is restricted by the size of the container, this process is most economic for small-sized articles which can be packed closely in the container. For long and relatively narrow parts, the process works well by providing long containers. • The coating has strong adhesion to base metal due to mutual diffusion of zinc and iron. Zinc penetrates to the base metal to about 1/3 of the coating thickness. Typical cross section 5. FeZn7 + Zn 4. FeZn7 3. Fe11Zn40 zinc rich layer 2. Solid solution Zn in Fe 1. Base Steel US 14-09 Elemental Analysis Shank Diffusion Section 120 100 80 Fe 60 Zn 40 20 0 171 175 179 183 187 191 195 199 203 207 211 215 219 • The corrosion resistance of the Thermo-Diffusion zinc coating is significantly higher than that of the Electro plating and higher than that of Hot Dip Galvanizing as well, in exposure to industrial and marine atmospheres. • The coating consists mainly of the iron-zinc d1-phase containing 7 to 10 % Iron. • The coating is harder than other zinc coatings and offers a good resistance to abrasive wear. • The coating provides an excellent substrate for additional surface treatment (painting, rubberizing, etc.). • The processing temperature is not high, and therefore the coating can be applied to parts made of spring or other special steel. • The process can be used for coating parts made by powder metallurgy, castings, porous parts, and also to preassembled units. Where does the process fit in your Industry? • The process is cost competitive with HDG. • Following are some samples of typical parts that are coated with thermal diffusion. Many similar parts are found on towers, poles and various hardware.
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