Phosphorus passivators

Phosphorus passivators are water-soluble synthetic polymers with phosphorus-based functional groups, suitable for reacting on surfaces (multi-metal) to create passivating nano-coatings capable of promoting paint adhesion and significantly increasing corrosion resistance.

Phosphorus passivators have entered the market a few years ago and are gradually spreading to replace or parallel to silanes. They are supplied in a concentrated form at around 50% and in different versions, based on the substrate to be treated (multi-metal, aluminium, galvanized, etc.). The active material of some of the main types is represented by:

• Acrylic acid, polymer with vinylphosphonic acid
• Sodium acrylate/methacryloylethyl phosphate copolymer

Process

To be able to use them, simple dilution formulas and addition of ceramic mineral acids that catalyze the passivation reaction with the substrates are sufficient.

Unlike silanes, they operate at much lower concentrations (typically 200 ppm) and the bonds created provide better resistance to hydrolysis in corrosive conditions, making baths easier to manage and without the creation of sludge.

The product greatly facilitates the management of treatment tunnels, allowing the number of washing tanks to be reduced and almost completely eliminating the control of the passivation bath. The only fundamental requirement is to obtain completely degreased and uniform surfaces before final passivation.

A typical pre-treatment system will therefore be able to count on just 4 steps: alkaline or acid degreasing/demi-rinse/demi-rinse/passivation at room temperature. There is no rinsing after passivation, but only a passage in hot ventilated air (80°C) for 5 minutes. The passivation bath can be set up for immersion or disposable spraying. Nebulization allows for greater quality, constant over time (obviously to the detriment of product waste), while immersion treatment must be monitored via the level of acidity and conductivity. If used appropriately, they can achieve the same performance as a tricationic phosphating.

Pros and cons

This new technology has a very high cost and in order to be used in competitive formulations it is necessary to create extremely diluted products; if on the one hand we can consider it a clean technology with low environmental impact, on the other hand we have a useless movement of products made up of up to 98% water.

Another negative note is the use of toxic acids such as hexafluorozirconic acid which, although present in minimal percentages, has a very high environmental impact. Due to the low percentage required for the formula (<1%), it may not be indicated in the safety data sheet, this means that the wastewater generated by customers is then mistakenly managed as non-hazardous aqueous waste.

The special polymer combines two functional groups, reacting with the metallic substrate (green) and with an organic layer (red).