Hot dip galvanizing of carbon steel 2

[pipexp]

Dear Experts,

I am little confused to understand that ZINK being anodic with respect to carbon steel how it will save carbon steel from corrosion in case of hot dip galvanizing. Rather, the zink layer will get corroded.

Please clarify my understanding on the issue,

Regards

 

[SJones]

Precisely, the zinc is corroded and the resulting electrochemical current goes toward protecting the carbon steel substrate. Try looking up 'sacrificial anode cathodic protection'.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Lizard7709]

http://www.galvanizeit.org/aga/about-hot-dip-galvanizing/the-corrosion-problem/corrosion-process

I would recomend checking out this site. Rust is caused by electricy moving around within the part. The zinc coating controls the direction of the current and thus is able to sacrifice itself to save the steel.

 

[dcfrush]

For corrosion to occur you need an anode, a cathode, and an electrolytic solution; remove any one of these from the equation and you will eliminate the corrosion mechanism. The anode will experience an oxidation reaction and the cathode will experience a reduction reaction, or more simply the anode will lose electrons and the cathode will gain electrons. Whether a metal is anodic or cathodic is relative to the metal it's in contact with and their distance from each other in the galvanic series. Zinc is anodic to carbon steel, so the zinc layer will give up its electrons before the carbon steel will (sacrificial anode). This is also why a lot of coatings systems will call for an inorganic zinc primer.

Hope this helps!

 

[Sgalb]

dcfrush provided a good explaination. My question to dcfrush is with the zinc rich primer. The inorganic zinc (i.e. metallic particles) that is added to primers would seem to me to be fully encapsulated by the polymer matrix in the primer, expecially with a low VOC or two part epoxy system. But the zinc has to make electrical contact with the substrate in spite of the fact that it's encapsulated with a dielectric insulator. What's the story hear?

 

[SJones]

The story here is that the coating should be so highly loaded with zinc to the extent that it can form a continuous conducting network. That is why you will see specifications demanding a minimum amount of zinc to ensure this characteristic.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[hokie66]

The zinc in an inorganic zinc primer is not encapsulated in resin. The zinc lies hard against the bare steel. Thus it provides superior protection (as a single coat, shop spray applied), to the organic zinc rich primers, where the zinc is surrounded by a resin, usually epoxy.

 

[unclesyd]

A Zinc rich primer will lay dormant until it damaged, like a scratch, which exposes the Zinc. At this point the Zinc will do it's thing.
I have some ZRC+epoxy+urethane that have been in service at the fish house since the eighties. There are a few cases where ZRC+chlorinated rubber have lasted even longer.

 

[SJones]

Not strictly true. At initial application, the same mechanism, direct electrical continuity, can apply to both types of zinc rich primer provided that the necessary pigment volume concentration:critical pigment volume concentration ratio for the zinc is met or exceeded in the organic system.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[hokie66]

Steve,
How is the critical concentration of zinc determined? I think most inorganic zinc silicate coatings are about 75-80% zinc, and the zinc filled epoxies are in the order of 60% zinc.

 

[Sgalb]

SJones, thank you. That was my suspicion, now confirmed. I guess unfortunately the trade-off is that there is little binder left for adhesion.

 

[Sgalb]

Good discussion all. Would somebody versed in the art provide a brief explanation on:

(1) Organic zinc: +/- attributes (metallic?)
(2) Inorganic zinc: +/- attributes and what is it.
(3) Why zinc and why not something more electronegative such as magnesium or something less electronegaive depending on the electronegativity of the substrate.

Thank you in advance.

 

[SJones]

The file was this paper:

http://ppgamercoatus.ppgpmc.com/techcenter/docs/Inorganic_organiccoatings.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[hokie66]

Thanks, Steve. This paper confirms that inorganic zinc silicate provides greatly superior protection when compared to the zinc filled epoxy. Decisions to use epoxy primers are driven by commercial first costs rather than life cycle analysis. This short sighted thinking seems to be more prevalent in Europe than elsewhere.

This all applies to new construction, not to maintenance, where inorganic zinc cannot be used.

Hot dipped galvanizing is still the "Cadillac" protection, as there is much more zinc than with the applied coatings.

 

[SJones]

Well, I wouldn't quite go as far as that myself. All three coatings have their advantages and drawbacks under different circumstances. The key is to know those circumstances and select the coating accordingly.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04