Iron phosphate is a chemical compound which contains iron, phosphorous, and oxygen. It is used mainly as an ingredient in garden pesticides as well as for a coating on industrial metal surfaces. Because of its relatively low toxicity, it can be used in so-called “organic” farming, where pesticides are typically avoided.
Iron phosphate is particularly useful in combating garden slugs and snails. Before they were used in the United States, iron phosphate slug baits had been successful in Europe. These baits, usually in the form of pellets, have proven in many cases to be as effective as their predecessors, which were made of a somewhat more dangerous chemical called metaldehyde. Iron phosphate, unlike metaldehyde, is safe for use around birds, pets, and other wildlife.
The effectiveness of iron phosphate baits depends in part on how many slugs and snails encounter and ingest the bait. Since slugs are underground most of the time, many may not come in contact with the bait, so total elimination is impossible to achieve with any pesticide. A good kill rate when using phosphate slug bait is considered to be around 60%.
The other principal use of iron phosphate is as an industrial coating applied to metals, in order to give the metal surface certain properties. Iron phosphate provides a stable, inert coating on the surface of the metal, which helps to inhibit the spread of rust if it begins to form. As a coating, it also makes it easier for paint to adhere to the metal. In an industrial context, it is usually applied in a series of stages, which include thoroughly washing the metal to remove any surface impurities. The presence of any contaminants will otherwise keep the iron phosphate from sticking to the metal.
Phosphate chemicals can be applied over a wide range of temperatures and in varying quantities. The weight of a finished phosphate coating can be anywhere between 0.00053 ounces to 0.0035 ounces per square foot (49.2 mg to 328 mg per square meter). When iron phosphate is applied to a metal surface, it actually etches the surface slightly. A very small amount of metal is removed from the surface and then re-deposited in an irregular fashion. Since the surface characteristics are varied in this way after phosphate is applied, there is more surface area for paint to adhere to. However, this etching also has the side effect of permanently removing some of the metal, since not all of it is re-deposited.