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An overview of the Thermal Diffusion Galvanising Process and its application in Electrical Line and Substation Hardware.
Do you have corrosion problems in Line and Substation hardware that is prematurely limiting the life of your electrical equipment?
Metal corrosion on galvanised parts and equipment - particularly in coastal areas is a significant problem in the utility industry. Lifespan of Hot Dipped Galvanised (HDG) parts is typical 5-15 years which is generally significantly less than the mechanical life expectancy of the parts or equipment.
Inspection, maintenance and preventive measures are limited in application and have marginal results in prolonging the coating protection on metal surfaces.
The generally accepted alternative to HDG for parts that are severally corroding is to use stainless steel – usually SS 304 but in some cases SS 316 (marine grade) is also used. The obvious downside of using stainless steel is the significant cost premium incurred.
Thermal Diffused Galvanising (TDG) has been around for a little while but is mow coming available in line hardware components. (Up to and including a DDO). TDG is a method of applying uniform, sacrificial, Zinc Iron alloy coating using a metallurgical vapour infusion process. Applied by tumbling metal parts within a patented zinc powder in a cylinder which is slowly rotated inside an oven at around 427C (800F) for two hours. The zinc iron alloy is infused into the surface of the metal as opposed to the HDG process which lays a coating on the surface.
The benefits of TDG over HDG are:
- Significant improved corrosion prevention performance – extends life up to 10 times
- Negligible changes in base material properties
- More uniform coating thickness
- Nuts & threads comparable to HDG
- No Hydrogen embrittlement
- More environmentally friendly with a non-toxic coating and virtually zero waste process
- Acid free service preparation (no pickling acid)
- Meets ASTEM 1059
- Exceeds ASTM 123 & 153
- Abrasion resistant
- Increased surface hardness
- Can be painted over
Pictorial representation of a surface after Hot Dipped Galvanised (HDG) process. Most of HDG layers are Eta or Zeta phase, very thin Delta and Gamma phases.
By contrast a view below shows the structure after Thermal Diffused Galvanising (TDG) – in this case courtesy of the ArmorGalv brand (by Hubbell).
TDG forms no Eta (pure zinc) layer, thick Gamma and Delta phases = much better harder and more durable finish
Testing here after around 5,000 hours in salt fog shows improved performance of TDG over HDG.
HDG after 3,000 hours in salt fog Armorgalv® after 3,000 Hours in Salt Fog
TDG is a cost effective solution for metal parts in medium to high corrosion situations. Value as shown below is derived from longer life for the part or component, with performance comparable to SS304.
In summary Line hardware and steel components which are treated by the Thermal Diffused Galvanising (TDG) process instead of Hot Dipped Galvanised are significantly more cost effective. The TDG treated steel has a harder, more uniform and more durable protection layer. The TDG process sublimates zinc powder into a vapour, which then diffuses into the surface of steel parts to forma a sacrificial, protective layer. In a salt fog performance test TDG is approximately 10 times better than HDG and is approximately equal in corrosion prevention performance to stainless steel – at significantly lower cost.
Want to discuss now? Please contact:
Bruce Blundell
P: +64 9 274 4280
M: +64 21 427 774