Iron Phosphate

Iron phosphate coatings have been in use for more than half a century because they improve paint adhesion and reduce rust on metal. There are two types of iron phosphate coatings used by industry today. They include straight iron phosphate and what are characterized by chemical companies like Vanchem as cleaner/coaters. Deciding to use a cleaner/coater or straight iron phosphate pretreatment before paint depends on your customer corrosion performance requirements, more on this below.

Cleaner/coater iron phosphate is done in two steps. Cleaning and coating metal parts is done simultaneously in the first step, hence the name cleaner/coater, and then the parts are rinsed in the second step. Straight iron phosphating requires multiple steps, a separate cleaner, rinse stages, iron phosphate stage, sealer, often followed by DI or RO water to enhance field performance.

A sealer can be added to a cleaner/coater system as well. However, a minimum of three separate stages are required and are normally configured as follows: cleaner/coater, rinse, sealer. There is the special case of steam iron phosphate, which is a dilute cleaner/coater that doesn’t necessarily need rinsing depending on the formulation. This technology is employed where large parts are painted and it would be impractical to pretreat them in a spray washer or immersion system.

Cleaner/coater formulations are primarily utilized where painted items do not require high levels of corrosion protection and are typically not exposed to outdoor weather conditions. Straight iron phosphate formulations produce higher coating weights and can be used on metal parts that are exposed to exterior weather conditions providing there is quality paint on top of it. There are cases where a clean/coater is used in a multi-stage pretreatment system where extra cleaning is required.

There is a cost impact to this. Larger systems require separate cleaning, iron phosphate, sealer, and multiple rinse stages. The cost to build and operate larger pretreatment systems has to be considered in the design phase. A typical system might look like this: cleaner, rinse, straight iron phosphate or cleaner/coater if extra cleaning is required, rinse, sealer, DI or RO water. If the end user is cost sensitive and high levels of corrosion protection are not necessary, cleaner/coater iron phosphate with a minimum of two stages is recommended.

With technological and chemical advancements over the past two decades, it’s now possible to use iron phosphate pretreatment in place of zinc phosphate to achieve high corrosion resistance and paint adhesion on a large variety of metal substrates. This was not the case in the past. Historically, industry relied on zinc phosphate to achieve high corrosion performance. Zinc phosphate pretreatment is more complicated, more expensive to run, and has a greater impact on the environment, but in many cases, it’s the go-to technology for high corrosion protection, like the automotive sector.

Iron phosphate products are formulated with different accelerating agents (catalysts) and work under different operating parameters. There are two types of iron phosphate catalysts, non-metal-accelerated and metal-accelerated. Non-metal-accelerated iron phosphate coatings produce higher coating weights, resulting in better paint adhesion and corrosion resistance. This type of coating usually has a matt-grey-to-brown colour whereas metal-accelerated iron phosphate (molybdate is commonly used), has an iridescent-to-purple-to-blue colour. The accelerator package can vary between straight iron phosphate and cleaner/coater depending on customer requirements.

The so-called moly blue colour is easy for the operator to see without testing for total acid or pH. Moly accelerated iron phosphate coatings are much easier to run because the operator can “see” that the chemical parameters are in specification just by the colour of the coating. This is not true for non-metal accelerated iron phosphate coatings where we recommend that the chemical parameters be monitored regularly.

Prior to painting steel parts, iron phosphate coating is applied by either spray or immersion. Metal corrosion prevention is very important to the general market place. It’s in everyone’s interest that a properly pretreated and painted metal item should last for many years without rusting. Iron phosphating is an important first step to achieve high quality paint field performance.

Vanchem Performance Chemicals is a Burlington based company offering amongst other things, pre-paint (pretreatment) application services which include iron phosphating.

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