Hot-dip galvanizing is a process of applying a protective zinc coating by dipping product in bath of molten zinc. Galvanizing is a favored method of protective coating due to its low cost, ease of application, and long maintenance-free service life. Generally applied at 830 to 870 degrees, the zinc will bond with the steel. It is the most common outdoor protective coating in use. When used on bolts, due to the thickness of the zinc coating, all galvanized nuts must be tapped oversize.
Zinc plating or electroplating is a process where zinc is applied by using a current of electricity. It is a thinner coating than hot-dip galvanizing making it unsuitable for outdoor applications. Its advantages are its brightness and uniform color making it more aesthetically appealing.
The ASTM specification covering hot-dip galvanized fasteners is F2329, while the specification covering zinc plated fasteners is F1941.
Hot dip galvanizing is always done after all the machining/ cutting operations are done on the parts,to ensure no machined surface is left uncovered.
There is no exception to threaded fasteners.If you retap or re – chase threads,it amounts to scraping the zinc deposit and the very purpose of dip galvanizing is lost.
Due precaution needs to be taken by process control during HDG or some resort to centrifugal cleaning after HDG,to even out buildup.
@A.S. – you are correct that the hot dip galvanizing process should be carried out after machining and cutting. Chasing the threads on external threads is prohibited by A153 and F2329, but the re-tapping of internal threads is required in order for the bolt and nut to mate properly. The sacrificial properties of the zinc on the external threads will help to protect the internal threads even if the zinc has been removed.
I have M20×40 bolts din 912 of 8.8 .I want to do galvanised or zinc plating.which is the best way.I did not want to over tapping the threads after plating.please give the suggestion.
@Anil- hot dip galvanizing is a better coating from a corrosion resistance standpoint, but it would require overtapping the nuts. Zinc plating provides minimal corrosion protection – it is mostly aesthetic, and requires no overtapping. As for which is “better”, that is subjective and would be up to you or your engineer.
Does anyone know about the effectiveness of cadmium plated bolts vrs. galvanizing? I have an outdoor application that must be protected from corrosion and the structure requires high strength bolts grade eight. Any advise?
@R Elgert- We don’t have any technical information regarding cadmium plating and its corrosion properties. We can say that zinc plated grade 8 bolts are quite commonly available if those might work for your application.
HDG affects the thread tolerance , what type of protection can i use for M52 and M56 grade 8.8 bolts to connect valves flanges in a water pipe line ?
Also is that related to the note at table 9 in Bs 4190 .
Table 9 — Recommended bolt and nut combinations
( * When a thick protective coating is applied to a bolt of grade 8.8 or 10.9, which requires the nut thread to be overtapped, the next higher grade of nut should be used.)
@Ahmed- Apologies, but other than hot dip galvanizing or electro-zinc plating, we do not have experience with many available coatings.
What details should be required for floating inquiry for quote of galvanize on leaf spring . Pl suggest sir
@Hardik- We are not familiar with the details of galvanizing automotive parts. You would probably want to check with the part manufacturer and see if they have any recommendations regarding galvanizing or coating.