Fertiliser resistance

Farmers regularly fertilise their fields. This may involve adding phosphate or nitrogen-based mineral fertiliser or organic fertilisers such as slurry or dung. However, some substances in fertilisers can increase corrosion of vehicles, machinery and components. These include ammonia (HN3), ammonium nitrate (NH4NO3), urea ((NH2)2CO), potassium chloride (KCl) or calcium carbonate (lime, CaCO3). The result: rust on harrows, gates or tractors is commonplace. And in a short time bolts or chassis begin to rust even in new vehicles.

Corrosion protection in agriculture is therefore faced with a particular challenge: not only is it necessary to protect against aggressive agents such as oxygen, but also against substances in fertilisers. In addition, protective coatings also need to withstand high mechanical stresses and be temperature resistant in the vicinity of engines. Premium manufacturers in particular are also placing increasing emphasis on the long-term high-grade appearance of their products. There is no standard or other generally-recognised regulation for the testing of fertiliser resistance of coatings.

Today, a large number of coatings with multi-layer structure exist for components of vehicles and machinery used in agriculture. Phosphate and varnish are applied, electroplating and hot-dip galvanisation are employed and powder coatings added. In vehicles, the protective coating usually consists of two coats: an electrolytically-applied coating (keyword: cathodic dip coating, KTL) and a powder coating. This combination provides passive protection against corrosion. If these coats are damaged, components can rapidly begin to rust.

There is also another approach: studies show that multi-coat systems containing zinc flakes provide better protection against corrosive agents such as fertilisers than the combination of KTL coating and powder coating. This multi-layer system with good fertiliser resistance usually comprises two layers: the base coat contains zinc and aluminium flakes. These flakes provide active protection to the component against corrosive agents such as oxygen and moisture thanks to active cathodic corrosion protection. Their scale-like arrangement also results in a barrier effect that slows attacks by corrosive media.

The top coat consists of an organic or inorganic material, depending on application. This top coat protects against chemicals such as acids and lyes, as well as aggressive substances such as fertiliser.