How does hot-dip galvanizing differ from electrogalvanizing?

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.

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62 comments

    Dear Babu,Galvanizing is done on steel parts prone to rusting & corrosion.Where is the need to galvanize SS which are corrosion resistant by nature?

    @A.S.- Not only is it unnecessary to galvanize stainless steel materials, the zinc will not properly adhere to the stainless steel, so it is impossible.

    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.

    I use grade 8.8 threaded rods connecting with metric threaded clevis ends (internal threads).HDG affects the thread tolerance.Would electroplating provide acceptable corrosion protection & thread fit?

    @David- Electroplating will offer a minimal amount of corrosion protection. Hot dip galvanizing is a far superior coating, but does affect the thread fit.

    Hi
    Can we do zinc plating instead of zinc galvanising for solar mounting structures
    Can we get 80um coating in zinc plating

    @Sandeep- You would need to talk to the project engineer about any plating substitutions, but normally zinc plating is thinner and less corrosion resistant than galvanizing. Zinc plating cannot be applied with 80um thickness, 25um is the thickest we are aware of.

    Hello Dean,

    Please i am hoping you could help me.

    I purchase some stud bolts and nuts for one of my client which is suppose to be Hot dipped galvanized.

    Upon receipt the client rejected them saying they are electroplated without running any test what so ever.

    Since you do a lot of HDG i thought i ask if you could advice me of any TEST that i could carry out to determine if the bolt and nuts are actually electroplated or hot dipped galvanized.

    I await your response.

    Many thanks for your help.

    Kind Regards,

    Timothy Nunu

    @Timothy- An accurate test would be to use a coating thickness tester, that can give you the thickness, and you can tell by the thickness which coating it is. Hot dip galvanizing is 2-5 mil (0.002″ to 0.005″)thick, whereas electroplated zinc is less than one mil (0.0001″). Another, but less scientific method would be to try a plain, standard nut on the bolt. If it threads on, then the bolt is electroplated. If not, it is likely galvanized. Hot dip galvanized nuts are tapped oversize to accommodate the thicker coating, so not-tapped oversize nuts should not be able to thread on.

    Dear Sir, What is the different between zinc plated bolt and GI bolt, which is suitable for Oil and Gas industry?

    I am getting iron railings for indoor stairs . Some of the spindles have a little rust on them ( I had them for a while ) . I want to put good protective coating on the railings . The place that will do the coating offers both Zinc dichromate plating or galvanizing and then powder coating . Which one would be better ?

    @Ted- For an indoor application, I wouldn’t think it would matter all that much. Once you get the current rust off, if they are indoors they should not continue to rust, and either plating would likely do a sufficient job.

    @Alex- Sometimes it can be tricky, but typically the electro zinc is more shiny and more even. The hot dip galvanizing is typically more gray and dull and a bit uneven.

    Another issue is that there is a possibility of shape distortion due to hot dipping on the sheet metal of the manufactured cabinet. What is the minimum sheet thickness in order not to have any distortion problem?

    Thanks for your help.

    @Abdullah- We do not typically galvanize large objects like your cabinet, so I really can’t be certain. You might need to contact a large kettle galvanizer who is more familiar with objects like this.

    What is the minimum thickness for hot-dip galvanizing? We need an outdoor cabinet for about 25 years and dont want to renew it. We thought that building it with galvanized sheet metal is meaningless because zink will be gone from the welding points. Is it the best; hot-dip galvanizing after building the whole cabinet?

    Thanks for help

    @Abdullah- The minimum thickness for hot dip galvanizing of bolts and hardware, which is what we do, is 2.0 mils. For structural members the thickness can vary from 1.4 to 3.9 mils, depending on the class of galvanizing specified. There are different standards for sheet metal, and I am less familiar with what those requirements are. Galvanizing the cabinet after fabrication might be a good idea, but you would need to discuss dimensions and details with your galvanizer and make sure it is within their capabilities.

    @Khozema- No, hot dip galvanizing is a completely different coating than a zinc chromate. Chromates are simply a sealer on top of the metallic coating, whereas hot dip galvanizing actually bonds with the steel in order to protect it.

    Where can I get a swing set hot dipped galvanized again? It is about 60 years old and some places have rust showing. I am in the Houston , Texas area.

    @Robert- You would need to call around to some local galvanizers and find out what their largest item dimensions are. Your swing set would be much too large for our tank.

    @Kailas- Hot dip galvanized and swedged anchor bolts are not mutually exclusive – you can have both. Additionally, one doesn’t have anything to do with the other, Swedging helps to increase the bolt’s pull out strength in the concrete, whereas hot dip galvanizing is simply a corrosion protection coating that has nothing to do with the strength of the material.

    I noticed that ‘wedge’ type expansion anchors can be supplied with what they call a
    “mechanical’ galvanized finish, which actually looks just like ‘hot-dipped’.
    What’s the difference? Does it meet the same specs?

    @Dan- Mechanical galvanizing is performed per ASTM B695, and is tumbled with zinc dust and glass beads in a liquid medium, so the zinc is cold fused onto the steel. Hot dip galvanizing is just that, dipped into hot molten zinc. Hot dip galvanizing is performed per ASTM A153 or F2329 and is a thicker and longer lasting coating. For wedge anchors mechanical galvanizing is more commonly available, although I believe some manufacturers make hot dip galvanized anchors.

    Dear Sir
    now i planed to purchase the electroplated Swivel coupler for scaffolding purchase.

    But i want to know which is give better life & corrosion resistance ELECTROPLATED or GALVANIZED.

    Because out power plant near Sea.

    Please

    @Rabindra- Hot dip galvanizing is a much thicker coating and will provide a longer service life than electroplating.

    Your descriptions above indicate that electroplating is not suitable for outdoor applications. Would that include underwater applications?

    @Ramsey- Electroplating is a very thin coating (0.0003 average) that will offer limited corrosion protection regardless of the environment. Hot dip galvanizing is roughly 10 times that thick (approx 0.002 min) and offers more corrosion protection. We cannot make any recommendations for your specific application, but caution should be used with electroplating if corrosion protection is a concern.

    During Electro Galvanizing of CS fittings why the coating thickness on the outer surface is more than the inner surface.? Is it not possible to achieve same coating thickness on both outer and inner surfaces?

    @Jabin- I am sorry, but we are not experts in electro plating, you will need to contact an electro plater in order to determine why that is.

    When using HDGAF 1/2″ threaded rod, is it acceptable to use EG nuts? Is there a dissimilar issue when using different kinds of galvanizing? Thanks, Danny

    @Danny- Normally, you don’t want to mix plating types. For many grades of bolt, ASTM specifies that the plating types be the same. In addition, nuts for use with HDG bolts are tapped oversize to accommodate the thickness of the zinc. For electro plated nuts, this is not the case, so you could run into thread fit issues.

    @Mahamuka – 85um=3.4 mils. We should be able to get that thickness on our fasteners as an average thickness, but we could not guarantee that there would not be spots that were thinner than 85um. The minimum allowable thickness per F2329 and A153 is 43um=1.7 mils.

    Can electroplating be done after hot dipped galvanizing for shinier surface? Will the galvanizing still hold after the plating top coat?

    @Akhilesh – I don’t think electro-zinc will adhere over the top of hot dipped zinc, you might need to contact an electro-plater to see if they have other options that would get you the result you desire.

    Hi,
    I’ve sent hexagon nut for zinc plating.
    The zinc plating covered only the outside surface but not the threaded area.
    Why

    @Rita – electro-zinc plating should be on all surfaces of the nut, including the threads. Thicker coatings, like hot dip galvanizing require that the threads be overtapped after coating, so those threads are bare.

    Is it possible to electroplate / galvanize SS 321 ?
    If so, what are the possible thickness can be coated ?
    What about their lives ?
    Can we heat the coated SS upto 500 deg C ?

    @Babu – I don’t believe the galvanizing will adhere to the stainless, so I wouldn’t think it’s possible. However, there may be alternative coatings that could be used, however I don’t have any familiarity with them. Any heating/service life will depend on the coating and you would need to get that information from the coater.

    @Bill Rush – Sheet metal is plated via a continuous feed process, which is a different process than finished parts are put through. The problem you may run into in trying to electro-plate beam, channel, etc is that they may be too large. You will need to contact an electro-plater and find out their length and size restrictions.

    @Anoop – If you use the torque equation T=KDP as explained on our torque page, the friction coefficient for zinc plated bolts is 0.22 vs 0.25 for galvanized. If you plug in this value to the equation, you will get torque values for zinc fasteners that are roughly 12% lower than galvanized fasteners to achieve the same tension. This is an estimate only, the actual numbers can vary.

    @Morad – We do not do much with electro-zinc and do not have any technical information about it. You will need to contact someone who does electro plating to obtain this information.

    hi,
    When used on A-36 plate with threaded holes and thue to the thickness of the zinc coating how much bigger I would have to drill the holes in order to screw the SS 304 bolts?
    Typically without galvanizing, If I use a SS 304 3/8″ 16 UNC bolt then I bore the plate with a 21/64″ drill in order to tap(3/8″ 16 UNC) the hole.
    Thanks for your help.just in this case the thread on the plate is not galvanized.

    @Ferney- The holes would need to be tapped the same regardless of the coating on the plate. If the hole is tapped after galvanizing, nothing additional would need to be done. However, if the hole is tapped prior to galvanizing, then you would need to chase the holes again with the same tap after the coating has been applied, in order to clean out any zinc buildup in the threads.

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