How big is the piece you wish to protect? You might get by much better with a WC-Co or a ceramic coating.

Keep in mind that zinc is often used as a "sacrificial" coating. This means that you expect it to corrode because in doing so, it prevents corrosion of the steel that it is laying on.

You may want a "barrier" coating. This would be a layer which is inherently resistive to corrosive attack. It requires, however, that it be more hermetic than a zinc plate layer.

The zinc-fertilizer reaction may be organic in nature, but it could very well be a simple acid etching reaction (many pesticides are acetic acid based) or an oxidation reaction. For many applications, a sacrificial zinc coating is the way to go. It's cheap and is appropriate for complex (e. g., agricultural) environments. You simply need to optimize the coating thickness and integrity. You also need to periodically inspect the surface. Zinc paints are a cheap, easily replaceable option.

www.zingacoating.com

If you want a barrier coating, thermal spray WC-Co  or pure ceramics may work well. If the part size, geometry and thermal cycling complicate things, then you might switch to a thermal spray metal. I assume there has been research into which alloys work particularly well with agricultural chemistries. Possibly a high nickel alloy, possibly an MCrAlY or Monel. USDA or www.scirus.com may have some listed.

You also can try other platings such as nickel (both electrolytic and electroless) or chromium.

Another factor may be physical abrasion of the zinc. If this is a factor, then you want a harder coating material. ZnCo is harder than pure Zn.

Zinc is a good Anode as compared to most of the common material. In fact we use zinc as sacrificial anodes when we want to protect other structures especially when galvanic corrosion is envisaged. Probably the component you are using is getting corroded due to Galvanic corrosion.

How to prevent this? You may check with a galvenic corrosion expert.

You may either provide some other sacrificial anode (Which is more Anodic than Zinc, Check the electrochemical series in Perry) or make the Zinc component cathodic by connecting to a negative charge(DC, Full details experts could give after survey of the fluid in contact with the component)

Is the Zinc plating falling off? as a pose to corroding.

The problem we are having relates to the corrosion of hydraulic couplings, these are attached to machinery in the field and are not protected.

I am concerned that Zinc oxide and zinc salt (formed from possible nitric acid in fertiliser) are easily soluble in aqueous fertiliser (NO2 (aq), NH3 (aq).

The couplings arte approximately 50mm (2") in diameter and are 100mm (4") long.

I would be interested in getting samples of an appropriate plating that can initially be applied for a trial but that can (preferably one that can be electrochemically deposited for serial production)

Jon Swift

Can anybody put me in contact with someone who can consult on the reactions of Zink with:

Fertilisers, example chemicals include.

I.e. Amonium Nitrate
     Amonium sulphate,... etc