Galvanising v Micro Zinc Processing 6

[deco0404]

Hi Guys,

we are buying some Hang-off systems for the offshore sector. Our supplier usually sells these as galvanised finish. Now they are trying to sell us Micro zinc processed finish. We are not too familiar with this system so would like to know if anyone is familiar with it, know of comparisons or in particular, any problems concerning it.

The supplier we have is fairly reputable, but will only give us the sunny side of the story. I'm not sure if there is even a dark side to the story, but would be very interested to hear if there is one

regards

Declan

 

[TomDOT]

I have not yet seen a zinc coating which matches hot-dip galvanizing. Possibly thermal spray plus clearcoat/pore filler, but I haven't seen enough performance examples yet, and the cost is typically higher. Electrogalv/plating/sherardizing all suck compared to hot-dip for a medium to high corrosion environment.

With any zinc coating in a high corrosion environment, lifespan is proportional to thickness until you exceed the material limits and start having cracking/disbonding/etc shortly after application due to overthickness. 4 mils of hot-dip is much better than 2 mils.

 

[deco0404]

Hi Tom,

we usually use hot-dip with a coating of circa 100micron. The environment would in fact be medium-high corrosion

 

[TomDOT]

deco,

That is the same thickness we use, though we call it 4 mils, since our metric conversion project failed in 1995 and was rolled back.

Has the vendor given you thorough technical data and independent* exposure site testing for his Micro Zinc, using 100 micron hot dip as a control? What's the thickness? How did it perform in the real world compared to hot dip in an environment similar to yours?

*Somewhere like the Kennedy Space Center (KSC) exposure racks. Although they're about as far inland (up the beach) as I would trust. My own exposure racks are on a platform over the water. Do NOT trust salt fogs, particularly ASTM B117 salt fogs.

 

[TVP]

When you say micro zinc, are you referring to non-electrolytically applied zinc flake coatings (ISO 10683) such as those provided by NOF, Dörken, Magni, etc.? These are starting to appear in some offshore applications like fasteners for wind turbines.

 

[CoryPad]

Pursuant to TVP's comment, those types of coatings are referred to as microlayer corrosion protection systems (or Mikroschicht-Korrosionsschutz-Systeme auf Deutsch). They are much thinner, closer to 10 micrometers rather than 100 micrometers.

 

[deco0404]

Hi guys,

thanks for the comments.

Tom, we have been given salt spray test results which from the documentation supplied would suggest 2500-3000hrs until rusting, whereas hot-dip for the same material would only be 400-500hrs.

TVP, I dont think its a flake deposit (might be wrong though), they call it a galvanising process, i'm assuming some sort of electrolytic system...good call on the offshore wind turbines...

What concerns me most is the thickness, as discussed the hot-dip is roughly 100micron(4mil), but this process is 5micron(0.2mil)

They are quoting ISO1461 as their acceptance spec which is for hot-dip galvanising. I will run through this and see whether or not this makes things any clearer

Declan

 

[TomDOT]

Which type of salt fog? Galv does poorly in accelerated weathering and particularly poorly in a constant-wet condition like ASTM B117. It is much more corrosion resistant when it has time to allow zinc carbonates to form on the surface, instead of mostly hydroxides. Wet/dry (cyclic) salt fog is typically not quite as bad - but galv really shines (ha!) in real world exposures. Admittedly, it does more poorly with higher time-of-wetness in the real world as well.

 

[stevenal]

Electroplating provides a completely different (inferior) product than one that is hot dip galvanized. It's not just the zinc thickness, it's the bond between the metals.

 

[TomDOT]

Yep, metallurgical alloying giving you 4 distinct layers in galvanizing: Eta, Zeta, Delta, Gamma.

Good images here, admittedly from a trade association - but you can see the same thing in reference books. Just not for free.

http://www.galvanizeit.org/about-hot-dip-galvanizing/what-is-hot-dip-galvanizing/the-hdg-coating

 

[deco0404]

Thats great & very helpful Tom, thank you.

Steven, do you have any examples to support your comments.

I'm in a bit of a rush here and need to travel to Germany next week to view these items, but as I am unfamiliar with this type of process (microzinq), i need all the help I can get. I had a look at the salt spray tests, and while they looked ok, my concern was overall coverage, and in particular, difficult to get at locations such as bolt holes, surface cavities and sharp section changes or sharp edges, basically all areas with which I would consider to be active corrosion points or locations. The sample they used for the tests was some form of casting, but you could see that the initial breakdown of the coating was in these areas. The most pronounced area was where the serial number of the casting is embossed (embossed is probably not the right word, but cant think of an alternative, anyone who knows about castings will know what I mean, in that the serial no., cast no etc is raised rather than stamped in..)

Declan

 

[TVP]

Declan,

microZINQ® is a modified hot dip galvanizing process that uses a binary Zn-Al alloy (5% Al). You can find some additional English-language information by using the following links:

http://www.fontaine-technologie.net/index.php/next-level-hdg-technologies

http://www.thyssenkrupp.com/en/presse/art_detail.html&eid=tk_pnid928

 

[stevenal]

Examples? Just experience seeing platings flake off. TomDOt provided the technical links showing the HDG bond. Sounds like the process in question is not a simple plating, though.